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African Plasmodium vivax: Distribution and origins
International Journal for Parasitology 42 (2012) 1091–1097
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International Journal for Parasitology journal homepage: www.elsevier.com/locate/ijpara
Invited Review
African Plasmodium vivax: Distribution and origins Richard Culleton a,⇑, Richard Carter b,⇑ a b
Malaria Unit, Institute of Tropical Medicine (NEKKEN), Nagasaki University, Nagasaki, Japan Institute of Immunology and Infection Research (IIIR), University of Edinburgh, Edinburgh, UK
a r t i c l e
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Article history: Received 7 May 2012 Received in revised form 22 August 2012 Accepted 23 August 2012 Available online 24 September 2012 Keywords: Plasmodium vivax Malaria Africa Duffy negativity Phylogenetics
a b s t r a c t There is increasing evidence that the malaria parasite, Plasmodium vivax, is endemic in west and central Africa, a region from which it was previously thought to be almost completely absent due to the very high prevalence of the Duffy negative phenotype in the local human populations. Furthermore, P. vivax, or very closely related parasites, has been identified in both chimpanzees and gorillas from this region. In this review, we discuss the implications of these findings for the current understanding of the origins of P. vivax as a human parasite. With the support of new evidence from mitochondrial genome sequencing, we propose that the evidence is consistent with current, extant P. vivax populations having their origins in Africa. Ó 2012 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction Plasmodium vivax is the most geographically widespread of the malaria parasite species that cause disease in humans. Because of its ability to transmit at temperatures as low as 16 °C, and to overwinter by virtue of its latent hypnozoite stages in the liver, the natural range of P. vivax extends throughout the temperate world and into most of the tropics (Coatney et al., 1971; Fig. 1). In west and central tropical Africa, however, where the transmission intensities of other human malarias, including the tropically and subtropically restricted malignant tertian malaria, Plasmodium falciparum, are at their highest, P. vivax has been considered exceedingly rare or even totally absent (Culleton et al., 2008). This was first noted in the literature by Brumpt (1939) and has since been extensively commented upon and debated, e.g. by Garnham (1956), Coatney et al. (1971) and others. It is the issue from which the present review of P. vivax in Africa begins.
2. Duffy negativity and P. vivax in Africa It is a striking fact that the same human populations in central and west Africa from which P. vivax transmission seemed to be so ⇑ Corresponding authors. Address: Malaria Unit, Institute of Tropical Medicine, Nagasaki University, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan. Tel.: +81 95 8197903; fax: +81 95 8197805 (R. Culleton). Address: Institute of Immunology and Infection Research (IIIR), University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK. Tel.: +44 131 6505558; fax: +44 131 6513605 (R. Carter). E-mail addresses: [email protected] (R. Culleton), [email protected] (R. Carter).
mysteriously absent are characterized by the almost universal absence of the Duffy antigen from the surface of their erythrocytes (Cavalli-Sforza et al., 1994; Howes et al., 2011). This ‘‘Duffy negative’’ state is extremely rare outside African populations and their descendants but reaches frequencies of 95–100% in west and central Africa (Cavalli-Sforza et al., 1994; Howes et al., 2011; Fig. 2). The phenotype is due to a single nucleotide polymorphism in the erythroid-specific promoter region of the DARC (Duffy Antigen Receptor for Chemokines) gene. In the homozygous state, the Duffy antigen is not transcribed in erythrocyte precursor cells and is completely absent from the erythrocyte surface (Tournamille et al., 1995). In 1976, Miller and colleagues showed that P. vivax relies absolutely on the presence of the Duffy antigen on the surface of erythrocytes in order to invade these cells and establish a blood-stage infection (Miller et al., 1976). In their study, ‘‘Duffy negative’’ individuals were totally refractory to blood infection with P. vivax. The finding appeared to explain why, among the almost universally Duffy negative central and west African populations, P. vivax appeared to be almost completely absent. The invasion of P. vivax into Duffy positive erythrocytes was subsequently found to be mediated through the ligand–receptor interaction between the P. vivax Duffy Binding Protein (PvDBP) and the Duffy antigen (Horuk et al., 1993). PvDBP is a type 1 membrane protein that is secreted by the merozoite from the micronemes during the early stages of erythrocyte invasion. The interaction between PvDBP and the Duffy antigen results in the irreversible formation of a junction between the parasite and the host cell immediately prior to invasion (Adams et al., 1990). PvDBP is one of a family of related proteins – the Duffy binding-like
0020-7519/$36.00 Ó 2012 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijpara.2012.08.005
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Fig. 1. The distribution of P. vivax in 2009. Pink areas show unstable transmission (defined as areas with P. vivax annual parasite incidences (PvAPI) of less than 0.1 per 1,000 people per year) and red areas show regions of stable transmission (PvAPI P 0.1 per 1,000 people per year). Hatched areas indicate regions in which Duffy negativity prevalence was estimated as P90%. This map has been reproduced from Guerra et al. (2010).
erythrocyte binding proteins (DBL–EBPs) – utilized by malaria parasites for the invasion of red blood cells (RBCs). Many malaria parasite species carry multiple copies of these genes, which enables them to utilize different invasion pathways based on different receptors (Iyer et al., 2007). Plasmodium vivax however, possesses only a single copy of this gene, and has, therefore, been presumed to be unable to utilize any invasion pathways alternative to that via the Duffy antigen (Ntumngia et al., 2012).
3. Duffy negativity and the case for an ancient prevalence of P. vivax in west and central Africa Based on the current prevalence of the Duffy negative phenotype in African populations, and the apparent dependence of P. vivax upon the Duffy antigen to infect humans, we have elsewhere presented the case that P. vivax may previously have been highly prevalent in west and central Africa (Carter and Mendis, 2002; Carter, 2003) within the period of the past 100,000 years. The essence of the case is that P. vivax itself gradually selected for individuals who were Duffy negative as this increased their likelihood of survival and fecundity. Since the Duffy negative state is itself without known significant detrimental effect, the process of selection can continue towards fixation of the Duffy negative allele in a human population. As it moves towards this, however, the prevalence of P. vivax itself must also decline towards extinction in the human population as it becomes increasingly refractory to malaria. The rise in prevalence of Duffy negativity would stop, so the argument goes, at the point that P. vivax itself becomes extinct in that population. Because the Duffy negative phenotype depends upon the underlying point polymorphism to be present in the homozygous state (see above), selection for this condition would tend to take much longer than selection for a gene that is dominantly expressed in
the heterozygous state. This is because an initially rare mutation will be extremely rare in homozygous combination and will remain so until the mutation itself reaches high frequency. If selection is solely on the homozygous combination this will be an exceedingly long process. If, as is generally estimated, selection for malaria protective human genes such as the sickle gene and glucose-6-phosphate dehydrogenase (G6PD) deficiency, which are protective in the heterozygous state, take hundreds to one or two thousands of years to be selected under malaria to a population equilibrium, selection for a protective condition that largely depends for its effect upon the protective gene in the homozygous state, could be expected to take a great deal longer, e.g. tens to hundreds of thousands of years. It has, however, been shown that even heterozygotes for the mutation for Duffy negativity (which have reduced levels of the Duffy antigen on their erythrocyte surface) have some degree of resistance to P. vivax infection (Zimmerman et al., 1999). This could be sufficient for the process of selection for high frequencies of Duffy negativity to be significantly faster than it would be by selection only on the homozygous state. Fortunately there exists data that indicates the approximate time period that may have been involved for the selection for Duffy negativity in African populations. Human population genetic analysis has indicated that the selection for the Duffy negative genes in Africa took place, with 95% confidence, between around 5,000 and 100,000 years ago (Hamblin and Di Rienzo, 2000). If P. vivax was, indeed, the selective agent, it follows that this parasite was prevalent in the ancestors of today’s west and central African populations within this period of time. If P. vivax was able to select for the Duffy negative condition so effectively in Africa, it may be asked why has it not happened to the same extent wherever P. vivax is, and has been, prevalent? As will be further discussed, P. vivax may only have entered modern human populations outside Africa in the latter part, or after the
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Fig. 2. The frequency of Duffy negativity in Africa, and the locations of surveys reporting evidence for the transmission of P. vivax in local human populations (red stars) from Rubio et al. (1999), Culleton et al. (2008, 2009), Dhorda et al. (2011); reports of P. vivax infecting Duffy negative individuals (yellow stars) from Ryan et al. (2006), Menard et al. (2010), Mendes et al. (2011), Wurtz et al. (2011); and reports of P. vivax or P. vivax-like parasites isolated from samples collected from great apes (green triangles) from Krief et al. (2010), Kaiser et al. (2010) and Liu et al. (2010). Duffy negativity data from Howes et al. (2011).
end, of the last glacial period, possibly within the past 10,000 years, leaving perhaps insufficient time for P. vivax-mediated selection for high prevalence of the Duffy negative condition to have taken place. In brief, the case has been put that the high prevalence of Duffy negativity in sub-Saharan Africa, and its virtual absence elsewhere, reflects an ancient presence of P. vivax in sub-Saharan Africa within the past 5,000–100,000 years.
4. Evidence for the presence of P. vivax among the largely Duffy negative populations of central and west Africa As already noted, it has been a matter of debate as to whether P. vivax occurs at all in west and central Africa. If it does, how is its transmission maintained in the almost entirely Duffy negative human populations? Several lines of evidence indicate that P. vivax is, indeed, endemic in Duffy negative Africa. Firstly, there are numerous reports of travelers to the region returning infected with parasites that are diagnosed as P. vivax (Muhlberger et al., 2004). Secondly, several surveys carried out within local populations in central and west Africa have shown evidence for the presence of P. vivax in human and mosquito populations. Sensitive and accurate PCR species diagnosis identified the presence of P. vivax parasites in the blood of four Duffy positive children from Equatorial Guinea, a country
with an otherwise almost entirely Duffy negative population (Rubio et al., 1999). In 2009, we demonstrated that approximately 10% of the (Duffy negative) population surveyed in Pointe-Noire, Republic of Congo, were positive for antibodies against the preerythrocytic stages of P. vivax (Culleton et al., 2009). In 2011, a report from Uganda described P. vivax parasites in the blood of three Duffy positive pregnant women, from a predominantly Duffy negative region (Dhorda et al., 2011). These reports are summarized in Fig. 2. The assumption that erythrocytes of the Duffy negative phenotype are always refractory to P. vivax infection has itself been challenged. There are reports of P. vivax in the blood of Duffy negative individuals in western Kenya (Ryan et al., 2006), the Brazilian Amazon (Cavasini et al., 2007), Madagascar (Menard et al., 2010), Mauritania (Wurtz et al., 2011) and Angola and Equatorial Guinea (Mendes et al., 2011) (Fig. 2). In the latter two studies, however, caution should be applied in concluding that these are of P. vivax growing in the erythrocytes of Duffy negative individuals. Plasmdoium vivax diagnosis was performed by PCR and was not confirmed by microscopy. Plasmodium vivax sporozoites are able to invade and grow in the liver cells of Duffy negative individuals releasing tens of thousands of merozoites into the blood stream. These, even if unable to invade the Duffy negative erythrocytes, could be detectable by PCR. It cannot be ruled out, therefore, that DNA from non-erythrocytic stage parasites was being amplified by the sensitive PCR techniques employed by Wurtz et al. (2011) and Mendes
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et al. (2011). Consequently it remains to be confirmed if these reports are of P. vivax invading Duffy negative erythrocytes. Notwithstanding, the report of Mendes et al. (2011), as is that of Rubio et al. (1999), is evidence of P. vivax transmission in deep central west Africa in a human population that is almost 100% Duffy negative. It has been speculated that the high rates of inoculation of P. vivax into Duffy negative individuals that occurs in populations which retain high proportions of Duffy positive individuals, selects for parasites that can invade Duffy negative erythrocytes (Menard et al., 2010). The question arises, are the selected parasites genetically mutated to be able to invade Duffy negative erythrocytes, or are they transient phenotypic adaptations to the same end? If the phenotype is due to a stably inherited genetic mutation(s) it should surely by now have reached high prevalence in Duffy negative human populations throughout Africa even if P. vivax reached Africa only within the past few centuries. Yet P. vivax of any phenotype remains very rare in Duffy negative populations. The phenotype could, on the other hand, represent the expression of an alternative invasion mechanism that is not used in the presence of Duffy positive erythrocytes but which can be expressed when these are not available. Given the normal high dependence of P. vivax upon the Duffy antigen, such an alternative invasion phenotype is likely to be much less efficient than the Duffy-dependent pathway, and may result in the infection of Duffy negative individuals only occasionally, in the presence of a high degree of P. vivax transmission intensity. This would be consistent with the rarity of P. vivax in predominantly Duffy negative human populations, but with the occasional occurrence of infection of Duffy negative individuals in regions where there are sufficiently high numbers of Duffy positives to maintain a high P. vivax transmission rate. Until it has been determined if the Duffy negative-invading P. vivax phenotype has a stable genetic or transient physiological basis, or indeed, is due to an as yet uncharacterised human RBC polymorphism, further speculation is difficult. Genetically based or not, P. vivax blood infections in Duffy negative individuals could account, at least in part, for the presence of this parasite among the largely Duffy negative human populations of west and central Africa. Other sources of P. vivax in these regions may also be significant. As we have argued previously (Culleton et al., 2009), the capacity of these environments to transmit malaria is typically so high that even a very small proportion of the human population of the Duffy positive phenotype could sustain the presence of P. vivax.
5. Plasmodium vivax–like parasites in apes in west and central Africa It has long been known that the African great apes, the chimpanzees and gorillas, are naturally infected in the wild in Africa with malaria parasites that appear closely similar to species that infect humans in Africa (Garnham, 1956). Based upon microscopic examination, parasites from the blood of wild caught apes were described and their identities interpreted from their morphology. Those indistinguishable from human P. falciparum (Reichenow, 1920; Blacklock and Adler, 1922; Adler, 1923) were to become named Plasmodium reichenowi (Bray, 1956; Coatney, 1971), now confirmed by molecular genetic analysis as a species separate from, but closely related to, human P. falciparum (e.g. Liu et al., 2010). Those resembling the human parasite, Plasmodium malariae, were named Plasmodium rhodaini (Rodhain, 1938; Coatney, 1971), a third morphological type was named Plasmodium schwetzi, and was considered to be P. vivax- or Plasmodium ovale-like (Schwetz, 1933; Bray, 1958a; Coatney, 1968, Coatney, 1971) having the stippling of the infected erythrocytes which characterizes these two parasite species in human infections.
These reports and observations were all made in the early to mid 20th century. With the exception of a single line of P. reichenowi maintained as infections in simians in the laboratory and, thereby, making it available for molecular analysis, all other laboratory material representing these parasites has long since been lost or was never maintained in the laboratory. For legal, ethical and practical reasons it has, in more recent times, been effectively unrealistic to attempt to make extensive surveys of parasites in blood and tissues from wild populations of African apes. Nevertheless, within these constraints, analyses of malaria parasites in tissues and excreta from these animals have been recently reported. An informative, if often-controversial literature is emerging from these studies. There is consensus, however, that a rich diversity of malaria parasites related to those endemic in human populations in Africa is being revealed in African apes (see Rayner et al., 2011). Using PCR amplification of the Plasmodium 18s ssrRNA gene for their detection and mitochondrial genome amplification to characterize the parasites, Kreif et al. (2010) reported P. vivax-like parasites in one of eight young orphaned chimpanzees, Pan troglodytes troglodytes, rescued in the Democratic Republic of Congo (DRC) in 2003 and 2006, and in one of three dead or wounded wild Pan troglodytes schweinfurthii from western Uganda, sampled opportunistically in 2006 and 2007. The P. vivax-like parasite from the DRC is from deep into the territory of nearly 100% prevalence of Duffy negativity in the human populations, while that from Uganda is closer to where there are significant proportions of Duffy positives. Kaiser et al. (2010), using PCR of the Plasmodium mitochondrial Cytb gene, identified P. vivax-like parasites in one of six wild chimpanzees that had died from natural causes in the Taï National Park, Côte d’Ivoire, also in human Duffy negative territory (see Fig. 2). Chimpanzees and gorillas are Duffy positive. It has recently been found possible to detect the presence of malaria parasites in fecal samples collected from the ground in the locations inhabited by these primates using PCR amplification of their mitochondrial genes (Liu et al., 2010). The material represents sites across much of central and west Africa. Among approximately 3,000 samples amplified from fecal specimens from gorillas and chimpanzees, parasites representing, or related to, all four of the human malaria parasite species found in Africa, have been identified. In addition to a complex of P. falciparum-related species, parasites genetically indistinguishable from P. malariae, P. ovale and, notably in the present context, P. vivax, were found. The P. vivax-like parasites were in one chimpanzee, P.t. schweinfurthii, from eastern DRC, and in three gorillas, Gorilla gorilla, from the western Congo. All were from regions of near 100% prevalence of Duffy negativity in the human populations (see Fig. 2). These reports, also reviewed by Rayner et al. (2011), are the first time that it has been possible to conclude that parasites identical, or very closely related, to P. vivax of humans are naturally prevalent in populations of non-human hosts in deep central west Africa. Do these parasites represent an ancient presence of P. vivax-like parasites in gorillas and chimpanzees in west and central Africa, or have they been introduced from elsewhere, e.g. from humans either recently or in more ancient times? Further, and related to this question, do these parasites represent a population of parasites that is shared with the human populations in the region, or are they separate stocks of parasites infecting only their ape hosts? Until more information is available on the genetic and phylogenetic relationships between the various P. vivax-like parasites in the human and animal hosts of west and central Africa it may be impossible to begin to answer these questions. The answers are all likely to be relevant to the matter which we will now discuss: where does P. vivax in Africa come from and where, if anywhere, may it have spread to?
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6. Where does P. vivax in Africa come from? We have reviewed evidence concerning P. vivax in Africa in the present time. This, we suggest, strongly implies that P. vivax and/or P. vivax-like parasites are present in both the largely Duffy negative human populations of central and west Africa and in apes in the same regions. What is completely unclear to this point, is whether these parasites are related or are of independent origins in humans and animals, and whether either or both are of ancient presence or recent introduction. In considering these questions we note the speculations in the literature concerning the evolutionary origin of P. vivax malaria. There is consensus on the close relationship among a group of parasites that includes P. vivax itself, and a number of species of malaria parasite that are today known only from monkeys of southern, south east and far eastern Asia and the western Pacific. These include Plasmodium cynomolgi, that most closely related and similar to P. vivax, and Plasmodium knowlesi, Plasmodium fieldi, Plasmodium fragile, Plasmodium coatneyi, Plasmodium simiovale and several others. On the basis of the uncontested close relationship between these parasites of Asian monkeys, within which all molecular phylogentic analyses show P. vivax to be strongly rooted (reviewed in Cornejo and Escalante, 2006), it has been speculated that P. vivax is itself a parasite of Asian origin. Another point of view holds that there is no reason to suppose that a place where the ancestors of P. vivax separated from those of the parasites found today in monkeys in the region of south and eastern Asia, must be where these parasites and their hosts find themselves today. The time of the separation of the vivax line, has been estimated to be in the order of 1 million years ago (Cornejo and Escalante, 2006), and before the start of the recent period of major glaciations in the northern hemisphere when the ancestors of the hosts of the malaria parasites of Asian monkeys were spread across Asia, Europe and North Africa. The ancestors of modern macaques (subtribe Macacina) and drills (subtribe Papionina) first
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appeared in Africa around 13–10 million years ago, macaques spreading to Europe by 5.5 million years ago, and to China and India 4–3 million years ago. In Europe, their descendants survived until at least the beginning of the last ice age 100,000 years ago, and possibly until as recently as 20,000 years ago (reviewed in Fooden, 2007). In Africa, their descendants remain in the form of the Barbary macaques of the Atlas mountains, Macaca sylvanus. There is no obvious reason to rule out that, at the time in which P. vivax speciated, macaques shared their ranges with other African primates including humans and apes. It is as likely as not that the ancestral P. vivax was a parasite of ancestors of today’s African primates as of those of south and east Asia. If this were so, the P. vivax-like parasites endemic in African apes in central and west Africa could be in direct line of descent from the ancestral P. vivax stock. By the same token, the P. vivax of humans in central and west Africa could itself represent not just an ancient population of these parasites on this continent, but the original and most ancient P. vivax stock upon the planet. The reason that the P. vivax-related parasites that are found today in Asian monkeys do not occur in Africa, would be that only ancestral vivax made the host switch to African apes/humans. As the Ice Ages descended around 1 million years ago, the monkey populations carrying the parasites that had not made this switch were driven south and east through Asia to where they are found today, totally isolated from Africa. What, if any, further information might test the above speculations? 7. The mitochondrial genome of P. vivax Comparisons of mitochondrial genome sequences have been made for isolates of P. vivax collected from locations around the world (Fig. 3; Jongwutiwes et al., 2005; Mu et al., 2005; Culleton et al., 2011). Although relatively non-polymorphic, the advantages of using the mitochondrial genome for this type of analysis are that it is uniparentally inherited, does not undergo recombination, is
Fig. 3. Haplotype network incorporating 320 P. vivax mitochondrial genome sequences. Node sizes are proportional to haplotype frequency. Node colors indicate the geographic origin of the isolates and are coded as follows: red; Africa, blue; South America, green; Asia, pink; Melanesia, orange; the Indian subcontinent, brown; Turkey and Iran. Hypothetical intermediates are indicated by small black dots. Out-group probabilities as calculated by TCs 1.21 (Clement et al., 2000) are given for the two haplotypes with the highest probabilities of being the ancestral type. All other haplotypes had out-group probabilities of less than 0.06. Figure reproduced from Culleton et al. (2011).
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‘‘neutral’’, in that there is no evidence for the action of directional selection on any of its genes, is relatively easy to amplify and sequence due to multiple copy numbers within parasites, and there is a large dataset available publicly. Interestingly the P. vivax mitochondrial haplotype identified as most likely to be ancestral is that found in South America (Fig. 3) (Culleton et al., 2011). The following hypothetical account (see also Fig. 4) is compatible with the network of mitochondrial genome relationships shown in Fig. 3. Over 1 million years ago the ancestral stock of P. vivax arose by a host transfer event somewhere across the expanses of northern Africa and the Eurasian content where ancestral Macaque and other monkeys came into contact with the habitats of humans and/or apes. As the Ice Ages encroached, two groups of host populations of the ancestral P. vivax were isolated either in southern parts of Asia or in Africa and the edges of the Mediterranean. It has been suggested that human populations in eastern and southern Asia, represented by Homo erectus, died out around 100,000 years ago (Jin and Su, 2000). This could have left only Homo neanderthalis, around the Mediterranean and the near east, and Homo sapiens and the anthropoid apes in Africa, as the only surviving host populations for P. vivax (Fig. 4A). Modern humans began to disperse out of Africa from about 60,000 years ago to re-populate southern and far eastern Asia and the western Pacific. They may have carried their malaria parasites with them through the temperate regions around the Arabian Peninsula and on through the warm Indian subcontinent, or the parasites may have followed later, perhaps only after the end of the last glacial episode around 10,000 years ago (Fig. 4B). Sometime during the same period the rise of Duffy negativity in African humans led to the drastic reduction of the prevalence of P. vivax in them while its prevalence in the African apes remained high.
By early historical times, around 5,000 years ago, P. vivax from the African/Mediterranean stock of these parasites would have re-entered much of the temperate and tropical regions of the world excepting the Americas and the Pacific, other than the western Pacific. This stock also became widespread throughout medieval and modern Europe, crossing the Atlantic to the Americas post Columbus (Fig. 4C), only to become extinct in Europe and North America in the last half of the 20th century (Fig. 4D). The mitochondrial genome signature of P. vivax from eastern Africa today relates most closely to that from India (Fig. 3) and indicates the re-introduction of vivax malaria into east Africa by human immigration from southern Asia, probably within the past few thousand, or even few hundred, years. This process could also have re-introduced the Duffy positive phenotype into the east African human populations, thereby supporting the re-introduction of P. vivax. Evidence that could have illuminated these speculations is missing from two populations of P. vivax. We have no mitochondrial genome data from the now extinct European P. vivax. This could have confirmed or refuted the genetic origin of South American vivax from Europe. Nor do we have the equivalent data from human P. vivax from deep west and central Africa. This, however, should be obtainable both for parasites capable and incapable of infecting Duffy negative human erythrocytes. Data from such material would be informative to the speculation that P. vivax in this part of Africa is, or is most closely related to parasites which were, ancestral to extant global populations of P. vivax. Perhaps most significant of all would be genomic data from the P. vivax-like parasites of west and central African apes. With this, together with the above, the relationships between surviving global stocks of P. vivax could be determined. Then, perhaps, we might
Fig. 4. Hypothetical world-wide spread of P. vivax. (A) More than 50,000 years ago (y.a), the original Eurasian–African stock of P. vivax was endemic in the human populations of Africa and Eurasia, and circulated freely between them; (B) some time following the end of the last ice age, African P. vivax spread east, possibly along with modern humans, through the Indian sub-continent and into east Asia. Independently, although perhaps contemporaneously, P. vivax also moved into the western Pacific region from Africa. By this time, Duffy negativity may have appeared in the human populations of Africa, leading to a reduction in the prevalence of the parasite in this region; (C) in postColumbus times, P. vivax entered the new world from Europe/Africa. By this time, the parasite would have been extinct (or very rare) in African humans, due to selection for Duffy negativity; (D) P. vivax distribution today, following its eradication from North America and Europe in the latter half of the 19th Century.
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know where the ancestral stocks of global P. vivax may reside, if, indeed, they still survive. In summary, we have reviewed evidence that suggests that P. vivax and/or P. vivax-like parasites are present in both human and ape populations in west and central Africa. Furthermore, we suggest that the origin of P. vivax as a human parasite may lie in Africa. Acknowledgements We thank Brian Cooke for the invitation to submit this review to IJP, and the participants of the Singapore Malaria Network Meeting 2012 for insightful discussion of this topic. We also thank Pedro Ferreira, Paul Sharp and Douglas Brandon-Jones for helpful discussion. References Adams, J.H., Hudson, D.E., Torii, M., Ward, G.E., Wellems, T.E., Aikawa, M., Miller, L.H., 1990. The Duffy receptor family of Plasmodium knowlesi is located within the micronemes of invasive malaria merozoites. Cell 63, 141–153. Adler, S., 1923. Malaria in chimpanzees in Sierra Leone. Ann. Trop. Med. Parasitol. 17, 13–19. Blacklock, B., Adler, S., 1922. A parasite resembling Plasmodium falciparum in a chimpanzee. Ann. Trop. Med. Parasitol. 16, 99–107. Bray, R.S., 1956. Studies on malaria in chimpanzees. I. The erythrocytic forms of Plasmodium reichenowi. J. Parasitol. 42, 588–592. Bray, R.S., 1958a. Studies on malaria in chimpanzees. V. The sporogonous cycle and mosquito transmission of Plasmodium vivax schwetzi. J. Parasitol. 44, 46–51. Brumpt, E., 1939. Les parasites du paludisme des chimpanzes. C. R. Soc. Biol. 130, 837–840. Carter, R., 2003. Speculations on the origins of Plasmodium vivax malaria. Trends Parasitol. 19, 214–219. Carter, R., Mendis, K.N., 2002. Evolutionary and historical aspects of the burden of malaria. Clin. Microbiol. Rev. 15, 564–594. Cavalli-Sforza, L.L., Menozzi, P., Piazza, A., 1994. The History and Geography of Human Genes. Princeton University Press, Princeton, N.J.. Cavasini, C.E., Mattos, L.C., Couto, A.A., Bonini-Domingos, C.R., Valencia, S.H., Neiras, W.C., Alves, R.T., Rossit, A.R., Castilho, L., Machado, R.L., 2007. Plasmodium vivax infection among Duffy antigen-negative individuals from the Brazilian Amazon region: an exception? Trans. R. Soc. Trop. Med. Hyg. 101, 1042–1044. Clement, M., Posada, D., Crandall, K.A., 2000. TCS: a computer program to estimate gene genealogies. Mol. Ecol. 9, 1657–1659. Coatney, G.R., Collins, W.E., Warren, M., Contacos, P.G., 1971. The Primate Malarias. US Government Printing Office, Washington DC. Coatney, G.R., 1968. Simian malarias in man: facts, implications, and predictions. Am. J. Trop. Med. Hyg. 17, 147–155. Cornejo, O.E., Escalante, A.A., 2006. The origin and age of Plasmodium vivax. Trends Parasitol. 22, 558–563. Culleton, R., Coban, C., Zeyrek, F.Y., Cravo, P., Kaneko, A., Randrianarivelojosia, M., Andrianaranjaka, V., Kano, S., Farnert, A., Arez, A.P., Sharp, P.M., Carter, R., Tanabe, K., 2011. The origins of African Plasmodium vivax; insights from mitochondrial genome sequencing. PLoS ONE 6, e29137. Culleton, R., Mita, T., Ndounga, M., Unger, H., Cravo, P., Paganotti, G., Takahashi, N., Kaneko, A., Eto, H., Tinto, H., Karema, C., D’Alessandro, U., do Rosário, V., Kobayakawa, T., Ntoumi, F., Carter, R., Tanabe, K., 2008. Failure to detect Plasmodium vivax in West and Central Africa by PCR species typing. Malar. J. 7, 174. Culleton, R., Ndounga, M., Zeyrek, F., Coban, C., Casimiro, P., Takeo, S., Tsuboi, T., Yadava, A., Carter, R., Tanabe, K., 2009. Evidence for the transmission of Plasmodium vivax in the Republic of the Congo, West Central Africa. J. Infect. Dis. 200, 1465–1469. Dhorda, M., Nyehangane, D., Renia, L., Piola, P., Guerin, P.J., Snounou, G., 2011. Transmission of Plasmodium vivax in south-western Uganda: report of three cases in pregnant women. PLoS ONE 6, e19801. Fooden, J., 2007. Systematic Review of the Barbary Macaque, Macaca sylvanus (Linnaeus, 1758) Fieldiana. Zoology 113, 1–60. Garnham, P.C., 1956. Some observations on malaria parasites in a chimpanzee, with particular reference to the persistence of Plasmodium reichenowi and Plasmodium vivax. Ann. Soc. Belg. Med. Trop. 36, 811–821. Guerra, C.A., Howes, R.E., Patil, A.P., Gething, P.W., Van Boeckel, T.P., Temperley, W.H., Kabaria, C.W., Tatem, A.J., Manh, B.H., Elyazar, I.R., Baird, J.K., Snow, R.W., Hay, S.I., 2010. The international limits and population at risk of Plasmodium vivax transmission in 2009. PLoS Negl. Trop. Dis. 4, e774. Hamblin, M.T., Di Rienzo, A., 2000. Detection of the signature of natural selection in humans: evidence from the Duffy blood group locus. Am. J. Hum. Genet. 66, 1669–1679. Horuk, R., Chitnis, C.E., Darbonne, W.C., Colby, T.J., Rybicki, A., Hadley, T.J., Miller, L.H., 1993. A receptor for the malarial parasite Plasmodium vivax: the erythrocyte chemokine receptor. Science 261, 1182–1184.
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Wild chimpanzees infected with five Plasmodium species. Emerg. Infect. Dis. 16, 1956–1959. Krief, S., Escalante, A.A., Pacheco, M.A., Mugisha, L., André, C., Halbwax, M., Fischer, A., Krief, J.M., Kasenene, J.M., Crandfield, M., Cornejo, O.E., Chavatte, J.M., Lin, C., Letourneur, F., Grüner, A.C., McCutchan, T.F., Rénia, L., Snounou, G., 2010. On the diversity of malaria parasites in African apes and the origin of Plasmodium falciparum from Bonobos. PLoS Pathog. 6 (2), e1000765. Liu, W., Li, Y., Learn, G.H., Rudicell, R.S., Robertson, J.D., Keele, B.F., Ndjango, J.B., Sanz, C.M., Morgan, D.B., Locatelli, S., Gonder, M.K., Kranzusch, P.J., Walsh, P.D., Delaporte, E., Mpoudi-Ngole, E., Georgiev, A.V., Muller, M.N., Shaw, G.M., Peeters, M., Sharp, P.M., Rayner, J.C., Hahn, B.H., 2010. Origin of the human malaria parasite Plasmodium falciparum in gorillas. Nature 467, 420–425. Menard, D., Barnadas, C., Bouchier, C., Henry-Halldin, C., Gray, L.R., Ratsimbasoa, A., Thonier, V., Carod, J.F., Domarle, O., Colin, Y., Bertrand, O., Picot, J., King, C.L., Grimberg, B.T., Mercereau-Puijalon, O., Zimmerman, P.A., 2010. Plasmodium vivax clinical malaria is commonly observed in Duffy-negative Malagasy people. Proc. Natl. Acad. Sci. USA 107, 5967–5971. Mendes, C., Dias, F., Figueiredo, J., Mora, V.G., Cano, J., de Sousa, B., do Rosario, V.E., Benito, A., Berzosa, P., Arez, A.P., 2011. Duffy negative antigen is no longer a barrier to Plasmodium vivax-molecular evidences from the African West Coast (Angola and Equatorial Guinea). PLoS Negl. Trop. Dis. 5, e1192. Miller, L.H., Mason, S.J., Clyde, D.F., McGinniss, M.H., 1976. The resistance factor to Plasmodium vivax in blacks. The Duffy-blood-group genotype FyFy. N. Engl. J. Med. 295, 302–304. Mu, J., Joy, D.A., Duan, J., Huang, Y., Carlton, J., Walker, J., Barnwell, J., Beerli, P., Charleston, M.A., Pybus, O.G., Su, X.Z., 2005. Host switch leads to emergence of Plasmodium vivax malaria in humans. Mol. Biol. Evol. 22, 1686–1693. Muhlberger, N., Jelinek, T., Gascon, J., Probst, M., Zoller, T., Schunk, M., Beran, J., Gjorup, I., Behrens, R.H., Clerinx, J., Bjorkman, A., McWhinney, P., Matteelli, A., Lopez-Velez, R., Bisoffi, Z., Hellgren, U., Puente, S., Schmid, M.L., Myrvang, B., Holthoff-Stich, M.L., Laferl, H., Hatz, C., Kollaritsch, H., Kapaun, A., Knobloch, J., Iversen, J., Kotlowski, A., Malvy, D.J., Kern, P., Fry, G., Siikamaki, H., Schulze, M.H., Soula, G., Paul, M., Gomez i Prat, J., Lehmann, V., Bouchaud, O., da Cunha, S., Atouguia, J., Boecken, G., 2004. Epidemiology and clinical features of vivax malaria imported to Europe: sentinel surveillance data from TropNetEurop. Malar. J. 3, 5. Ntumngia, F.B., Schloegel, J., Barnes, S.J., McHenry, A.M., Singh, S., King, C.L., Adams, J.H., 2012. Conserved and variant epitopes of Plasmodium vivax Duffy binding protein as targets of inhibitory monoclonal antibodies. Infect. Immun. 80, 1203– 1208. Rayner, J.C., Liu, W., Peeters, M., Sharp, P.M., Hahn, B.H., 2011. A plethora of Plasmodium species in wild apes: a source of human infection? Trends Parasitol. 27, 222–229. Reichenow, E., 1920. Über das Vorkommen der Malariaparasiten des Menschen bei den Afrikanischen Menschenaffen. Centralbl. f. Bakt. I. Abt. Orig. 85, 207–216. Rodhain, J., 1938. Les plasmodium des anthropoids de l’Afrique centrale et leur relation avec les plasmodium humains. Acta Convent Tertii de Trop. Atqu Malar. Morbis. 2, 539–544. Rubio, J.M., Benito, A., Roche, J., Berzosa, P.J., Garcia, M.L., Mico, M., Edu, M., Alvar, J., 1999. Semi-nested, multiplex polymerase chain reaction for detection of human malaria parasites and evidence of Plasmodium vivax infection in Equatorial Guinea. Am. J. Trop. Med. Hyg. 60, 183–187. Ryan, J.R., Stoute, J.A., Amon, J., Dunton, R.F., Mtalib, R., Koros, J., Owour, B., Luckhart, S., Wirtz, R.A., Barnwell, J.W., Rosenberg, R., 2006. Evidence for transmission of Plasmodium vivax among a Duffy antigen negative population in Western Kenya. Am. J. Trop. Med. Hyg. 75, 575–581. Schwetz, J., 1933. Sur les parasites malariens (Plasmodium) des singes superieurs (Anthropoides) Africains. C. R. Soc. Biol. 112, 710–711. Tournamille, C., Colin, Y., Cartron, J.P., Le Van Kim, C., 1995. Disruption of a GATA motif in the Duffy gene promoter abolishes erythroid gene expression in Duffynegative individuals. Nat. Genet. 10, 224–228. Wurtz, N., Mint Lekweiry, K., Bogreau, H., Pradines, B., Rogier, C., Ould Mohamed Salem Boukhary, A., Hafid, J.E., Ould Ahmedou Salem, M.S., Trape, J.F., Basco, L.K., Briolant, S., 2011. Vivax malaria in Mauritania includes infection of a Duffynegative individual. Malar. J. 10, 336. Zimmerman, P.A., Woolley, I., Masinde, G.L., Miller, S.M., McNamara, D.T., Hazlett, F., Mgone, C.S., Alpers, M.P., Genton, B., Boatin, B.A., Kazura, J.W., 1999. Emergence of FY⁄A(null) in a Plasmodium vivax-endemic region of Papua New Guinea. Proc. Natl. Acad. Sci. USA 96, 13973–13977.
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International Journal for Parasitology journal homepage: www.elsevier.com/locate/ijpara
Invited Review
African Plasmodium vivax: Distribution and origins Richard Culleton a,⇑, Richard Carter b,⇑ a b
Malaria Unit, Institute of Tropical Medicine (NEKKEN), Nagasaki University, Nagasaki, Japan Institute of Immunology and Infection Research (IIIR), University of Edinburgh, Edinburgh, UK
a r t i c l e
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Article history: Received 7 May 2012 Received in revised form 22 August 2012 Accepted 23 August 2012 Available online 24 September 2012 Keywords: Plasmodium vivax Malaria Africa Duffy negativity Phylogenetics
a b s t r a c t There is increasing evidence that the malaria parasite, Plasmodium vivax, is endemic in west and central Africa, a region from which it was previously thought to be almost completely absent due to the very high prevalence of the Duffy negative phenotype in the local human populations. Furthermore, P. vivax, or very closely related parasites, has been identified in both chimpanzees and gorillas from this region. In this review, we discuss the implications of these findings for the current understanding of the origins of P. vivax as a human parasite. With the support of new evidence from mitochondrial genome sequencing, we propose that the evidence is consistent with current, extant P. vivax populations having their origins in Africa. Ó 2012 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction Plasmodium vivax is the most geographically widespread of the malaria parasite species that cause disease in humans. Because of its ability to transmit at temperatures as low as 16 °C, and to overwinter by virtue of its latent hypnozoite stages in the liver, the natural range of P. vivax extends throughout the temperate world and into most of the tropics (Coatney et al., 1971; Fig. 1). In west and central tropical Africa, however, where the transmission intensities of other human malarias, including the tropically and subtropically restricted malignant tertian malaria, Plasmodium falciparum, are at their highest, P. vivax has been considered exceedingly rare or even totally absent (Culleton et al., 2008). This was first noted in the literature by Brumpt (1939) and has since been extensively commented upon and debated, e.g. by Garnham (1956), Coatney et al. (1971) and others. It is the issue from which the present review of P. vivax in Africa begins.
2. Duffy negativity and P. vivax in Africa It is a striking fact that the same human populations in central and west Africa from which P. vivax transmission seemed to be so ⇑ Corresponding authors. Address: Malaria Unit, Institute of Tropical Medicine, Nagasaki University, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan. Tel.: +81 95 8197903; fax: +81 95 8197805 (R. Culleton). Address: Institute of Immunology and Infection Research (IIIR), University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK. Tel.: +44 131 6505558; fax: +44 131 6513605 (R. Carter). E-mail addresses: [email protected] (R. Culleton), [email protected] (R. Carter).
mysteriously absent are characterized by the almost universal absence of the Duffy antigen from the surface of their erythrocytes (Cavalli-Sforza et al., 1994; Howes et al., 2011). This ‘‘Duffy negative’’ state is extremely rare outside African populations and their descendants but reaches frequencies of 95–100% in west and central Africa (Cavalli-Sforza et al., 1994; Howes et al., 2011; Fig. 2). The phenotype is due to a single nucleotide polymorphism in the erythroid-specific promoter region of the DARC (Duffy Antigen Receptor for Chemokines) gene. In the homozygous state, the Duffy antigen is not transcribed in erythrocyte precursor cells and is completely absent from the erythrocyte surface (Tournamille et al., 1995). In 1976, Miller and colleagues showed that P. vivax relies absolutely on the presence of the Duffy antigen on the surface of erythrocytes in order to invade these cells and establish a blood-stage infection (Miller et al., 1976). In their study, ‘‘Duffy negative’’ individuals were totally refractory to blood infection with P. vivax. The finding appeared to explain why, among the almost universally Duffy negative central and west African populations, P. vivax appeared to be almost completely absent. The invasion of P. vivax into Duffy positive erythrocytes was subsequently found to be mediated through the ligand–receptor interaction between the P. vivax Duffy Binding Protein (PvDBP) and the Duffy antigen (Horuk et al., 1993). PvDBP is a type 1 membrane protein that is secreted by the merozoite from the micronemes during the early stages of erythrocyte invasion. The interaction between PvDBP and the Duffy antigen results in the irreversible formation of a junction between the parasite and the host cell immediately prior to invasion (Adams et al., 1990). PvDBP is one of a family of related proteins – the Duffy binding-like
0020-7519/$36.00 Ó 2012 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijpara.2012.08.005
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Fig. 1. The distribution of P. vivax in 2009. Pink areas show unstable transmission (defined as areas with P. vivax annual parasite incidences (PvAPI) of less than 0.1 per 1,000 people per year) and red areas show regions of stable transmission (PvAPI P 0.1 per 1,000 people per year). Hatched areas indicate regions in which Duffy negativity prevalence was estimated as P90%. This map has been reproduced from Guerra et al. (2010).
erythrocyte binding proteins (DBL–EBPs) – utilized by malaria parasites for the invasion of red blood cells (RBCs). Many malaria parasite species carry multiple copies of these genes, which enables them to utilize different invasion pathways based on different receptors (Iyer et al., 2007). Plasmodium vivax however, possesses only a single copy of this gene, and has, therefore, been presumed to be unable to utilize any invasion pathways alternative to that via the Duffy antigen (Ntumngia et al., 2012).
3. Duffy negativity and the case for an ancient prevalence of P. vivax in west and central Africa Based on the current prevalence of the Duffy negative phenotype in African populations, and the apparent dependence of P. vivax upon the Duffy antigen to infect humans, we have elsewhere presented the case that P. vivax may previously have been highly prevalent in west and central Africa (Carter and Mendis, 2002; Carter, 2003) within the period of the past 100,000 years. The essence of the case is that P. vivax itself gradually selected for individuals who were Duffy negative as this increased their likelihood of survival and fecundity. Since the Duffy negative state is itself without known significant detrimental effect, the process of selection can continue towards fixation of the Duffy negative allele in a human population. As it moves towards this, however, the prevalence of P. vivax itself must also decline towards extinction in the human population as it becomes increasingly refractory to malaria. The rise in prevalence of Duffy negativity would stop, so the argument goes, at the point that P. vivax itself becomes extinct in that population. Because the Duffy negative phenotype depends upon the underlying point polymorphism to be present in the homozygous state (see above), selection for this condition would tend to take much longer than selection for a gene that is dominantly expressed in
the heterozygous state. This is because an initially rare mutation will be extremely rare in homozygous combination and will remain so until the mutation itself reaches high frequency. If selection is solely on the homozygous combination this will be an exceedingly long process. If, as is generally estimated, selection for malaria protective human genes such as the sickle gene and glucose-6-phosphate dehydrogenase (G6PD) deficiency, which are protective in the heterozygous state, take hundreds to one or two thousands of years to be selected under malaria to a population equilibrium, selection for a protective condition that largely depends for its effect upon the protective gene in the homozygous state, could be expected to take a great deal longer, e.g. tens to hundreds of thousands of years. It has, however, been shown that even heterozygotes for the mutation for Duffy negativity (which have reduced levels of the Duffy antigen on their erythrocyte surface) have some degree of resistance to P. vivax infection (Zimmerman et al., 1999). This could be sufficient for the process of selection for high frequencies of Duffy negativity to be significantly faster than it would be by selection only on the homozygous state. Fortunately there exists data that indicates the approximate time period that may have been involved for the selection for Duffy negativity in African populations. Human population genetic analysis has indicated that the selection for the Duffy negative genes in Africa took place, with 95% confidence, between around 5,000 and 100,000 years ago (Hamblin and Di Rienzo, 2000). If P. vivax was, indeed, the selective agent, it follows that this parasite was prevalent in the ancestors of today’s west and central African populations within this period of time. If P. vivax was able to select for the Duffy negative condition so effectively in Africa, it may be asked why has it not happened to the same extent wherever P. vivax is, and has been, prevalent? As will be further discussed, P. vivax may only have entered modern human populations outside Africa in the latter part, or after the
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Fig. 2. The frequency of Duffy negativity in Africa, and the locations of surveys reporting evidence for the transmission of P. vivax in local human populations (red stars) from Rubio et al. (1999), Culleton et al. (2008, 2009), Dhorda et al. (2011); reports of P. vivax infecting Duffy negative individuals (yellow stars) from Ryan et al. (2006), Menard et al. (2010), Mendes et al. (2011), Wurtz et al. (2011); and reports of P. vivax or P. vivax-like parasites isolated from samples collected from great apes (green triangles) from Krief et al. (2010), Kaiser et al. (2010) and Liu et al. (2010). Duffy negativity data from Howes et al. (2011).
end, of the last glacial period, possibly within the past 10,000 years, leaving perhaps insufficient time for P. vivax-mediated selection for high prevalence of the Duffy negative condition to have taken place. In brief, the case has been put that the high prevalence of Duffy negativity in sub-Saharan Africa, and its virtual absence elsewhere, reflects an ancient presence of P. vivax in sub-Saharan Africa within the past 5,000–100,000 years.
4. Evidence for the presence of P. vivax among the largely Duffy negative populations of central and west Africa As already noted, it has been a matter of debate as to whether P. vivax occurs at all in west and central Africa. If it does, how is its transmission maintained in the almost entirely Duffy negative human populations? Several lines of evidence indicate that P. vivax is, indeed, endemic in Duffy negative Africa. Firstly, there are numerous reports of travelers to the region returning infected with parasites that are diagnosed as P. vivax (Muhlberger et al., 2004). Secondly, several surveys carried out within local populations in central and west Africa have shown evidence for the presence of P. vivax in human and mosquito populations. Sensitive and accurate PCR species diagnosis identified the presence of P. vivax parasites in the blood of four Duffy positive children from Equatorial Guinea, a country
with an otherwise almost entirely Duffy negative population (Rubio et al., 1999). In 2009, we demonstrated that approximately 10% of the (Duffy negative) population surveyed in Pointe-Noire, Republic of Congo, were positive for antibodies against the preerythrocytic stages of P. vivax (Culleton et al., 2009). In 2011, a report from Uganda described P. vivax parasites in the blood of three Duffy positive pregnant women, from a predominantly Duffy negative region (Dhorda et al., 2011). These reports are summarized in Fig. 2. The assumption that erythrocytes of the Duffy negative phenotype are always refractory to P. vivax infection has itself been challenged. There are reports of P. vivax in the blood of Duffy negative individuals in western Kenya (Ryan et al., 2006), the Brazilian Amazon (Cavasini et al., 2007), Madagascar (Menard et al., 2010), Mauritania (Wurtz et al., 2011) and Angola and Equatorial Guinea (Mendes et al., 2011) (Fig. 2). In the latter two studies, however, caution should be applied in concluding that these are of P. vivax growing in the erythrocytes of Duffy negative individuals. Plasmdoium vivax diagnosis was performed by PCR and was not confirmed by microscopy. Plasmodium vivax sporozoites are able to invade and grow in the liver cells of Duffy negative individuals releasing tens of thousands of merozoites into the blood stream. These, even if unable to invade the Duffy negative erythrocytes, could be detectable by PCR. It cannot be ruled out, therefore, that DNA from non-erythrocytic stage parasites was being amplified by the sensitive PCR techniques employed by Wurtz et al. (2011) and Mendes
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et al. (2011). Consequently it remains to be confirmed if these reports are of P. vivax invading Duffy negative erythrocytes. Notwithstanding, the report of Mendes et al. (2011), as is that of Rubio et al. (1999), is evidence of P. vivax transmission in deep central west Africa in a human population that is almost 100% Duffy negative. It has been speculated that the high rates of inoculation of P. vivax into Duffy negative individuals that occurs in populations which retain high proportions of Duffy positive individuals, selects for parasites that can invade Duffy negative erythrocytes (Menard et al., 2010). The question arises, are the selected parasites genetically mutated to be able to invade Duffy negative erythrocytes, or are they transient phenotypic adaptations to the same end? If the phenotype is due to a stably inherited genetic mutation(s) it should surely by now have reached high prevalence in Duffy negative human populations throughout Africa even if P. vivax reached Africa only within the past few centuries. Yet P. vivax of any phenotype remains very rare in Duffy negative populations. The phenotype could, on the other hand, represent the expression of an alternative invasion mechanism that is not used in the presence of Duffy positive erythrocytes but which can be expressed when these are not available. Given the normal high dependence of P. vivax upon the Duffy antigen, such an alternative invasion phenotype is likely to be much less efficient than the Duffy-dependent pathway, and may result in the infection of Duffy negative individuals only occasionally, in the presence of a high degree of P. vivax transmission intensity. This would be consistent with the rarity of P. vivax in predominantly Duffy negative human populations, but with the occasional occurrence of infection of Duffy negative individuals in regions where there are sufficiently high numbers of Duffy positives to maintain a high P. vivax transmission rate. Until it has been determined if the Duffy negative-invading P. vivax phenotype has a stable genetic or transient physiological basis, or indeed, is due to an as yet uncharacterised human RBC polymorphism, further speculation is difficult. Genetically based or not, P. vivax blood infections in Duffy negative individuals could account, at least in part, for the presence of this parasite among the largely Duffy negative human populations of west and central Africa. Other sources of P. vivax in these regions may also be significant. As we have argued previously (Culleton et al., 2009), the capacity of these environments to transmit malaria is typically so high that even a very small proportion of the human population of the Duffy positive phenotype could sustain the presence of P. vivax.
5. Plasmodium vivax–like parasites in apes in west and central Africa It has long been known that the African great apes, the chimpanzees and gorillas, are naturally infected in the wild in Africa with malaria parasites that appear closely similar to species that infect humans in Africa (Garnham, 1956). Based upon microscopic examination, parasites from the blood of wild caught apes were described and their identities interpreted from their morphology. Those indistinguishable from human P. falciparum (Reichenow, 1920; Blacklock and Adler, 1922; Adler, 1923) were to become named Plasmodium reichenowi (Bray, 1956; Coatney, 1971), now confirmed by molecular genetic analysis as a species separate from, but closely related to, human P. falciparum (e.g. Liu et al., 2010). Those resembling the human parasite, Plasmodium malariae, were named Plasmodium rhodaini (Rodhain, 1938; Coatney, 1971), a third morphological type was named Plasmodium schwetzi, and was considered to be P. vivax- or Plasmodium ovale-like (Schwetz, 1933; Bray, 1958a; Coatney, 1968, Coatney, 1971) having the stippling of the infected erythrocytes which characterizes these two parasite species in human infections.
These reports and observations were all made in the early to mid 20th century. With the exception of a single line of P. reichenowi maintained as infections in simians in the laboratory and, thereby, making it available for molecular analysis, all other laboratory material representing these parasites has long since been lost or was never maintained in the laboratory. For legal, ethical and practical reasons it has, in more recent times, been effectively unrealistic to attempt to make extensive surveys of parasites in blood and tissues from wild populations of African apes. Nevertheless, within these constraints, analyses of malaria parasites in tissues and excreta from these animals have been recently reported. An informative, if often-controversial literature is emerging from these studies. There is consensus, however, that a rich diversity of malaria parasites related to those endemic in human populations in Africa is being revealed in African apes (see Rayner et al., 2011). Using PCR amplification of the Plasmodium 18s ssrRNA gene for their detection and mitochondrial genome amplification to characterize the parasites, Kreif et al. (2010) reported P. vivax-like parasites in one of eight young orphaned chimpanzees, Pan troglodytes troglodytes, rescued in the Democratic Republic of Congo (DRC) in 2003 and 2006, and in one of three dead or wounded wild Pan troglodytes schweinfurthii from western Uganda, sampled opportunistically in 2006 and 2007. The P. vivax-like parasite from the DRC is from deep into the territory of nearly 100% prevalence of Duffy negativity in the human populations, while that from Uganda is closer to where there are significant proportions of Duffy positives. Kaiser et al. (2010), using PCR of the Plasmodium mitochondrial Cytb gene, identified P. vivax-like parasites in one of six wild chimpanzees that had died from natural causes in the Taï National Park, Côte d’Ivoire, also in human Duffy negative territory (see Fig. 2). Chimpanzees and gorillas are Duffy positive. It has recently been found possible to detect the presence of malaria parasites in fecal samples collected from the ground in the locations inhabited by these primates using PCR amplification of their mitochondrial genes (Liu et al., 2010). The material represents sites across much of central and west Africa. Among approximately 3,000 samples amplified from fecal specimens from gorillas and chimpanzees, parasites representing, or related to, all four of the human malaria parasite species found in Africa, have been identified. In addition to a complex of P. falciparum-related species, parasites genetically indistinguishable from P. malariae, P. ovale and, notably in the present context, P. vivax, were found. The P. vivax-like parasites were in one chimpanzee, P.t. schweinfurthii, from eastern DRC, and in three gorillas, Gorilla gorilla, from the western Congo. All were from regions of near 100% prevalence of Duffy negativity in the human populations (see Fig. 2). These reports, also reviewed by Rayner et al. (2011), are the first time that it has been possible to conclude that parasites identical, or very closely related, to P. vivax of humans are naturally prevalent in populations of non-human hosts in deep central west Africa. Do these parasites represent an ancient presence of P. vivax-like parasites in gorillas and chimpanzees in west and central Africa, or have they been introduced from elsewhere, e.g. from humans either recently or in more ancient times? Further, and related to this question, do these parasites represent a population of parasites that is shared with the human populations in the region, or are they separate stocks of parasites infecting only their ape hosts? Until more information is available on the genetic and phylogenetic relationships between the various P. vivax-like parasites in the human and animal hosts of west and central Africa it may be impossible to begin to answer these questions. The answers are all likely to be relevant to the matter which we will now discuss: where does P. vivax in Africa come from and where, if anywhere, may it have spread to?
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6. Where does P. vivax in Africa come from? We have reviewed evidence concerning P. vivax in Africa in the present time. This, we suggest, strongly implies that P. vivax and/or P. vivax-like parasites are present in both the largely Duffy negative human populations of central and west Africa and in apes in the same regions. What is completely unclear to this point, is whether these parasites are related or are of independent origins in humans and animals, and whether either or both are of ancient presence or recent introduction. In considering these questions we note the speculations in the literature concerning the evolutionary origin of P. vivax malaria. There is consensus on the close relationship among a group of parasites that includes P. vivax itself, and a number of species of malaria parasite that are today known only from monkeys of southern, south east and far eastern Asia and the western Pacific. These include Plasmodium cynomolgi, that most closely related and similar to P. vivax, and Plasmodium knowlesi, Plasmodium fieldi, Plasmodium fragile, Plasmodium coatneyi, Plasmodium simiovale and several others. On the basis of the uncontested close relationship between these parasites of Asian monkeys, within which all molecular phylogentic analyses show P. vivax to be strongly rooted (reviewed in Cornejo and Escalante, 2006), it has been speculated that P. vivax is itself a parasite of Asian origin. Another point of view holds that there is no reason to suppose that a place where the ancestors of P. vivax separated from those of the parasites found today in monkeys in the region of south and eastern Asia, must be where these parasites and their hosts find themselves today. The time of the separation of the vivax line, has been estimated to be in the order of 1 million years ago (Cornejo and Escalante, 2006), and before the start of the recent period of major glaciations in the northern hemisphere when the ancestors of the hosts of the malaria parasites of Asian monkeys were spread across Asia, Europe and North Africa. The ancestors of modern macaques (subtribe Macacina) and drills (subtribe Papionina) first
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appeared in Africa around 13–10 million years ago, macaques spreading to Europe by 5.5 million years ago, and to China and India 4–3 million years ago. In Europe, their descendants survived until at least the beginning of the last ice age 100,000 years ago, and possibly until as recently as 20,000 years ago (reviewed in Fooden, 2007). In Africa, their descendants remain in the form of the Barbary macaques of the Atlas mountains, Macaca sylvanus. There is no obvious reason to rule out that, at the time in which P. vivax speciated, macaques shared their ranges with other African primates including humans and apes. It is as likely as not that the ancestral P. vivax was a parasite of ancestors of today’s African primates as of those of south and east Asia. If this were so, the P. vivax-like parasites endemic in African apes in central and west Africa could be in direct line of descent from the ancestral P. vivax stock. By the same token, the P. vivax of humans in central and west Africa could itself represent not just an ancient population of these parasites on this continent, but the original and most ancient P. vivax stock upon the planet. The reason that the P. vivax-related parasites that are found today in Asian monkeys do not occur in Africa, would be that only ancestral vivax made the host switch to African apes/humans. As the Ice Ages descended around 1 million years ago, the monkey populations carrying the parasites that had not made this switch were driven south and east through Asia to where they are found today, totally isolated from Africa. What, if any, further information might test the above speculations? 7. The mitochondrial genome of P. vivax Comparisons of mitochondrial genome sequences have been made for isolates of P. vivax collected from locations around the world (Fig. 3; Jongwutiwes et al., 2005; Mu et al., 2005; Culleton et al., 2011). Although relatively non-polymorphic, the advantages of using the mitochondrial genome for this type of analysis are that it is uniparentally inherited, does not undergo recombination, is
Fig. 3. Haplotype network incorporating 320 P. vivax mitochondrial genome sequences. Node sizes are proportional to haplotype frequency. Node colors indicate the geographic origin of the isolates and are coded as follows: red; Africa, blue; South America, green; Asia, pink; Melanesia, orange; the Indian subcontinent, brown; Turkey and Iran. Hypothetical intermediates are indicated by small black dots. Out-group probabilities as calculated by TCs 1.21 (Clement et al., 2000) are given for the two haplotypes with the highest probabilities of being the ancestral type. All other haplotypes had out-group probabilities of less than 0.06. Figure reproduced from Culleton et al. (2011).
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‘‘neutral’’, in that there is no evidence for the action of directional selection on any of its genes, is relatively easy to amplify and sequence due to multiple copy numbers within parasites, and there is a large dataset available publicly. Interestingly the P. vivax mitochondrial haplotype identified as most likely to be ancestral is that found in South America (Fig. 3) (Culleton et al., 2011). The following hypothetical account (see also Fig. 4) is compatible with the network of mitochondrial genome relationships shown in Fig. 3. Over 1 million years ago the ancestral stock of P. vivax arose by a host transfer event somewhere across the expanses of northern Africa and the Eurasian content where ancestral Macaque and other monkeys came into contact with the habitats of humans and/or apes. As the Ice Ages encroached, two groups of host populations of the ancestral P. vivax were isolated either in southern parts of Asia or in Africa and the edges of the Mediterranean. It has been suggested that human populations in eastern and southern Asia, represented by Homo erectus, died out around 100,000 years ago (Jin and Su, 2000). This could have left only Homo neanderthalis, around the Mediterranean and the near east, and Homo sapiens and the anthropoid apes in Africa, as the only surviving host populations for P. vivax (Fig. 4A). Modern humans began to disperse out of Africa from about 60,000 years ago to re-populate southern and far eastern Asia and the western Pacific. They may have carried their malaria parasites with them through the temperate regions around the Arabian Peninsula and on through the warm Indian subcontinent, or the parasites may have followed later, perhaps only after the end of the last glacial episode around 10,000 years ago (Fig. 4B). Sometime during the same period the rise of Duffy negativity in African humans led to the drastic reduction of the prevalence of P. vivax in them while its prevalence in the African apes remained high.
By early historical times, around 5,000 years ago, P. vivax from the African/Mediterranean stock of these parasites would have re-entered much of the temperate and tropical regions of the world excepting the Americas and the Pacific, other than the western Pacific. This stock also became widespread throughout medieval and modern Europe, crossing the Atlantic to the Americas post Columbus (Fig. 4C), only to become extinct in Europe and North America in the last half of the 20th century (Fig. 4D). The mitochondrial genome signature of P. vivax from eastern Africa today relates most closely to that from India (Fig. 3) and indicates the re-introduction of vivax malaria into east Africa by human immigration from southern Asia, probably within the past few thousand, or even few hundred, years. This process could also have re-introduced the Duffy positive phenotype into the east African human populations, thereby supporting the re-introduction of P. vivax. Evidence that could have illuminated these speculations is missing from two populations of P. vivax. We have no mitochondrial genome data from the now extinct European P. vivax. This could have confirmed or refuted the genetic origin of South American vivax from Europe. Nor do we have the equivalent data from human P. vivax from deep west and central Africa. This, however, should be obtainable both for parasites capable and incapable of infecting Duffy negative human erythrocytes. Data from such material would be informative to the speculation that P. vivax in this part of Africa is, or is most closely related to parasites which were, ancestral to extant global populations of P. vivax. Perhaps most significant of all would be genomic data from the P. vivax-like parasites of west and central African apes. With this, together with the above, the relationships between surviving global stocks of P. vivax could be determined. Then, perhaps, we might
Fig. 4. Hypothetical world-wide spread of P. vivax. (A) More than 50,000 years ago (y.a), the original Eurasian–African stock of P. vivax was endemic in the human populations of Africa and Eurasia, and circulated freely between them; (B) some time following the end of the last ice age, African P. vivax spread east, possibly along with modern humans, through the Indian sub-continent and into east Asia. Independently, although perhaps contemporaneously, P. vivax also moved into the western Pacific region from Africa. By this time, Duffy negativity may have appeared in the human populations of Africa, leading to a reduction in the prevalence of the parasite in this region; (C) in postColumbus times, P. vivax entered the new world from Europe/Africa. By this time, the parasite would have been extinct (or very rare) in African humans, due to selection for Duffy negativity; (D) P. vivax distribution today, following its eradication from North America and Europe in the latter half of the 19th Century.
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know where the ancestral stocks of global P. vivax may reside, if, indeed, they still survive. In summary, we have reviewed evidence that suggests that P. vivax and/or P. vivax-like parasites are present in both human and ape populations in west and central Africa. Furthermore, we suggest that the origin of P. vivax as a human parasite may lie in Africa. Acknowledgements We thank Brian Cooke for the invitation to submit this review to IJP, and the participants of the Singapore Malaria Network Meeting 2012 for insightful discussion of this topic. We also thank Pedro Ferreira, Paul Sharp and Douglas Brandon-Jones for helpful discussion. References Adams, J.H., Hudson, D.E., Torii, M., Ward, G.E., Wellems, T.E., Aikawa, M., Miller, L.H., 1990. The Duffy receptor family of Plasmodium knowlesi is located within the micronemes of invasive malaria merozoites. Cell 63, 141–153. Adler, S., 1923. Malaria in chimpanzees in Sierra Leone. Ann. Trop. Med. Parasitol. 17, 13–19. Blacklock, B., Adler, S., 1922. A parasite resembling Plasmodium falciparum in a chimpanzee. Ann. Trop. Med. Parasitol. 16, 99–107. Bray, R.S., 1956. Studies on malaria in chimpanzees. I. The erythrocytic forms of Plasmodium reichenowi. J. Parasitol. 42, 588–592. Bray, R.S., 1958a. Studies on malaria in chimpanzees. V. The sporogonous cycle and mosquito transmission of Plasmodium vivax schwetzi. J. Parasitol. 44, 46–51. Brumpt, E., 1939. Les parasites du paludisme des chimpanzes. C. R. Soc. Biol. 130, 837–840. Carter, R., 2003. Speculations on the origins of Plasmodium vivax malaria. Trends Parasitol. 19, 214–219. Carter, R., Mendis, K.N., 2002. Evolutionary and historical aspects of the burden of malaria. Clin. Microbiol. Rev. 15, 564–594. Cavalli-Sforza, L.L., Menozzi, P., Piazza, A., 1994. The History and Geography of Human Genes. Princeton University Press, Princeton, N.J.. Cavasini, C.E., Mattos, L.C., Couto, A.A., Bonini-Domingos, C.R., Valencia, S.H., Neiras, W.C., Alves, R.T., Rossit, A.R., Castilho, L., Machado, R.L., 2007. Plasmodium vivax infection among Duffy antigen-negative individuals from the Brazilian Amazon region: an exception? Trans. R. Soc. Trop. Med. Hyg. 101, 1042–1044. Clement, M., Posada, D., Crandall, K.A., 2000. TCS: a computer program to estimate gene genealogies. Mol. Ecol. 9, 1657–1659. Coatney, G.R., Collins, W.E., Warren, M., Contacos, P.G., 1971. The Primate Malarias. US Government Printing Office, Washington DC. Coatney, G.R., 1968. Simian malarias in man: facts, implications, and predictions. Am. J. Trop. Med. Hyg. 17, 147–155. Cornejo, O.E., Escalante, A.A., 2006. The origin and age of Plasmodium vivax. Trends Parasitol. 22, 558–563. Culleton, R., Coban, C., Zeyrek, F.Y., Cravo, P., Kaneko, A., Randrianarivelojosia, M., Andrianaranjaka, V., Kano, S., Farnert, A., Arez, A.P., Sharp, P.M., Carter, R., Tanabe, K., 2011. The origins of African Plasmodium vivax; insights from mitochondrial genome sequencing. PLoS ONE 6, e29137. Culleton, R., Mita, T., Ndounga, M., Unger, H., Cravo, P., Paganotti, G., Takahashi, N., Kaneko, A., Eto, H., Tinto, H., Karema, C., D’Alessandro, U., do Rosário, V., Kobayakawa, T., Ntoumi, F., Carter, R., Tanabe, K., 2008. Failure to detect Plasmodium vivax in West and Central Africa by PCR species typing. Malar. J. 7, 174. Culleton, R., Ndounga, M., Zeyrek, F., Coban, C., Casimiro, P., Takeo, S., Tsuboi, T., Yadava, A., Carter, R., Tanabe, K., 2009. Evidence for the transmission of Plasmodium vivax in the Republic of the Congo, West Central Africa. J. Infect. Dis. 200, 1465–1469. Dhorda, M., Nyehangane, D., Renia, L., Piola, P., Guerin, P.J., Snounou, G., 2011. Transmission of Plasmodium vivax in south-western Uganda: report of three cases in pregnant women. PLoS ONE 6, e19801. Fooden, J., 2007. Systematic Review of the Barbary Macaque, Macaca sylvanus (Linnaeus, 1758) Fieldiana. Zoology 113, 1–60. Garnham, P.C., 1956. Some observations on malaria parasites in a chimpanzee, with particular reference to the persistence of Plasmodium reichenowi and Plasmodium vivax. Ann. Soc. Belg. Med. Trop. 36, 811–821. Guerra, C.A., Howes, R.E., Patil, A.P., Gething, P.W., Van Boeckel, T.P., Temperley, W.H., Kabaria, C.W., Tatem, A.J., Manh, B.H., Elyazar, I.R., Baird, J.K., Snow, R.W., Hay, S.I., 2010. The international limits and population at risk of Plasmodium vivax transmission in 2009. PLoS Negl. Trop. Dis. 4, e774. Hamblin, M.T., Di Rienzo, A., 2000. Detection of the signature of natural selection in humans: evidence from the Duffy blood group locus. Am. J. Hum. Genet. 66, 1669–1679. Horuk, R., Chitnis, C.E., Darbonne, W.C., Colby, T.J., Rybicki, A., Hadley, T.J., Miller, L.H., 1993. A receptor for the malarial parasite Plasmodium vivax: the erythrocyte chemokine receptor. Science 261, 1182–1184.
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