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The evolution, ecological effects and health impact of parasites
Parasitology Today: vol. 6, no. 9, 1990
278
The Evolution, Ecological Effects and Health Impact of Parasites P. Schmid-Hempel and H. Tanner i
Basel, Switzerland: April 1990 Lions' claws and sharks' teeth rank high in capturing people's imagination and form an appreciation of how natural selection acts. Although it is conventional wisdom that predators normally eliminate the weak and sick among their prey, the implications for the role of parasites in ecological interactions or as selective factors affecting the course of evolution have not particularly attracted the imagination of researchers. Yet, just as Canadian hunters seem to hit parasiteinfected moose more often than unparasitized ones ~, so natural predators may simply benefit from successful parasites that have prepared their host for takeover by a potential intermediate host2, However, as a subject, parasitology has failed to address the unifying principles discussed by ecologists and evolutionary biologists. In turn though, the latter have lacked the detailed knowledge of the parasitologist needed for testing these hypotheses. This year's meeting of the Swiss Zoological Society (University of Basel, 6-7 April 1990) aimed to allow the exchange of ideas and approaches to the study of parasites and their roles in ecology and evolution, particularly between ecologists, evolutionary biologists, medical zoologists and parasitologists.
Parasitism and Mate Choice A major part of the meeting was devoted to an interdisciplinary exchange between epidemiology and medical zoology. The great variety of ecological and evolutionary processes in which parasites are involved made a deep impression and was reflected in the broad diversity of topics discussed throughout the meeting. Among them, Hamilton and Zuk's suggestion that parasites can exert negative frequencydependent selection to maintain the elaborate secondary sexual characteristics of many species has apparently attracted a great deal of research effort 3. A study on the influence of intestinal Abstracts will be published in the December issue of RevueSuissede Zoologie 97 (4).
roundworms on sexual ornamentation and mate choice in red jungle fowl by R. Thornhill (University of New Mexico, NM, USA) is a case in point. The degree of male ornamentation depends, among other things, on the presence of parasites, as they disproportionately adversely affect ornamental traits compared with non-ornamental male traits, and females prefer males with fewer indications of disease as revealed by their ornamentation. Similarly, in sticklebacks, the intensity of male breeding colouration is shown to be reduced if the male is infected by tapeworms (M. Milinski and T. Bakker, University of Bern, Switzerland). Like the jungle fowl, where sexual selection based on ornamentation and female choice is linked with selection through male-male competition, female sticklebacks may choose their mates not because they provide better diseaseresistance genes for offspring but because healthier males are superior fathers and can provide better parental care. Therefore, more needs to be known about the assumed genetic variance for parasite resistance before any firm conclusions can be reached. Although intraspecific studies have fared well, evaluating the role of parasites by comparative approaches has been more difficult. A. Read (Oxford University, UK) compared parasite load between polygynous and monogynous passerine birds. When controlling for the effects of phylogeny, body size, latitude and so forth, he found that the prevalence of blood parasites is lower in polygynous birds than in species of monogynous birds. This suggests that infection by parasites is somehow involved in avian mating systems, although it is unclear how. In a further glimpse of the overlooked importance of parasites, K. AIlander and R. Dufva (Uppsala University, Sweden) reported that large loads of ectoparasites reduce body weight and tarsus length of nestling great tits (Parus major). In itself, this may not be very surprising but it should be remembered that much of our knowledge about bird populations comes from studies on great tits, and such anatomical characteristics are commonly used as indicators of heritable traits.
Host Behaviour and Parasite Specificity Transmission to new hosts is a crucial step in the life cycle of parasites and it is often facilitated by certain host behaviours or status. For example, in England, juvenile oystercatchers (Haematopus ostralegus) are prone to helminth infection because their low dominance status means that they have to feed on cockles heavily infected with intermediate stages of parasites (C. Goater, University of Z0rich, Switzerland). Likewise for social animals, the spatial proximity or similar genetic constitution of hosts puts their parasites at an advantage. J. Shykoff and P. Schmid-Hempel (University of Basel, Switzerland) showed that the transmission of an intestinal trypanosome in bumblebees is facilitated if colonies of the insect consist of closely related individuals, in contrast to colonies composed of less related or unrelated individuals, which have smaller parasite burdens. This may explain why, contrary to expectations from kin selection theory, associations of full sibs in social groups are not the general rule. Several contributors concentrated on the role of parasites in mediating ecologi~ cal interactions of their hosts, a topic that was heralded by Park's 4 classical studies on flour beetles. For example, M. Bouletreau and P. Fouillet (University of Lyon, France), showed that in experimental systems using two sibling species of Drosophila, one species usually displaces the other. When a larval parasitold is added to the system both species of fruit fly will co-exist for prolonged periods, although the effect of the parasitoid is dependent on the ambient temperature. The modulation of parasite impact by environmental conditions was also pointed out by H. M011er(University of BaseL, Switzerland), who has shown that the damage done by root herbivores depends on the plant's environment. Whether parasites co-evolve with their hosts or simply track appropriate resources across host taxa is of considerable interest to evolutionary biologists in relation to host specificity. Tracking hosts that are taxonomically distinct (~) 1990, Elsevier Science Publishers Ltd, (UK) O1694707/90/$0200
Parasitology Today, voL 6, no. 9, 1990 but living in similar habitats may lead to sympatric speciation among parasites. T.. DeMee0s and his colleagues (University of Montpellier, France) suspect that this may have occurred in the copepod parasites of three species of Mediterranean fish. R. Colwell (University of Connecticut, CT, USA) pointed out that host specificity may have little to do with the nutritional or physiological characteristics of the host. He reported that the specificity of species of flower mites is selectively driven by the,. difficulties of finding a mate and does not relate to food. To improve their odds, different mite species therefore have become specialized at disembarking from their hummingbird transport hosts at specific plant species. Even well-known parasites harbour some surprises that may shed light on questions that have arisen in other disciplines. This is vividly demonstrated in work on the genetics of Trypanosoma brucei. L Jenni (Swiss Tropical Institute, Basel, Switzerland) reported that in T. brucei a fair amount of genetic exchange occurs. Recombination may explain why some phenotypic traits, such as the variable surface glycoprotein coat, drug resistance or pathogenicity, show variation and instability. Genetic exchange in parasites is of intrinsic interest to evolutionary biologists because of the increased potential of the parasite to undergo evolutionary change and because of its consequently increased ecological impact.
Models for Public Health The sessions on 'parasites and man' not only focused on the understanding of this host-parasite relationship, but also provided an outlook for the control of parasitic infections at the community level. A framework was provided by R.M. Anderson (Imperial College, UK) who discussed the various levels of genetic, social, behavioural and spatial heterogeneities found within and between host and parasite populations interactions that are non-linear in nature. Mathematical models have demonstrated their scope for predicting epidemiological scenarios and provide the basis for decision making in public health policy, eg. the necessary vaccination coverage for a certain level of protection, or the age group that can be immunized to the best effect in population-based programmes. At the level of an individual host, the dynamics of a parasite population is driven by rates of multiplication and
279 development. Similar factors can be incorporated in an 'ecological' approach for the analysis of the interactions between parasite populations and host immune cells at a molecular level. The concept of 'immuno-ecology' or 'ecoimmunology', as advocated by Anderson, should provide a new stimulus for immunologists. This could become a vital part of the interplay between the bench and the field and aid the translation of research findings into applications. The ubiquity of parasitic diseases and their significance to humans was illustrated by the reports of various field studies in Europe and the tropics. A case-control study of paediatric cryptosporidiosis has revealed the importance of this parasite in Europe and indicated that person-to-person rather than animal-to-human transmission is the main route for maintaining the disease (D. M~usezahl et al., Swiss Tropical Institute, Basel, Switzerland). A series of studies on Lyme borreliosis and on tickborne encephalitis in Switzerland was presented by A. Aeschlimann and his colleagues (University of Neuchatel, Switzerland). Their field studies have helped to form an understanding of how this disease spreads, but less encouragingly, laboratory experiments show that the protein profiles of different Borrelia burgdorferi strains can be altered after passage through its vector. Such variation may have epidemiological implications (C.M. Hu, University of Neuchatel, Switzerland). The pressing problem of malaria was highlighted by an intervention study that compared the effects of intradomicilliary spraying against mosquitoes on the incidence of malaria in children under ten years of age (Mnzava et al., Swiss Tropical Institute, Basel, Switzerland). Residual spraying was found not only to be effective at controlling populations of the endophilic Anopheles gambiae (sensu stricto) but also those of the exophilic A. arabiensis. Transmission of a parasite is not always straightforward and amenable to control measures. Complex transmission mechanics have been implicated as a problem in a study assessing the importance of intestinal parasitoses in a squatter area of Manila, Philippines (C. Auer, Swiss Tropical Institute, Basel, Switzerland): a high degree of multiparasitism seemed to stimulate the host's immune system, but inappropriately so. This study was carried out in the broader context of urban health, an area that has been neglected for a Iongtime despite its growing impact in the developing world. It will 15e a major challenge for health
planners and biomedical scientists in the years to come to solve the problem of urban health, and both parasitologists and epidemiologists will have to be involved in this task. Health intervention unfolds within the dynamic world of host-parasite relationships. To help unravel this complexity J. Koella (Swiss Tropical Institute, Basel, Switzerland) has developed mathematical models that aim to predict the effects of intervention, particularly in relation to age-dependent chloroquine resistance in Plasmodium falciparum. These ideas have been tested during the long-term monitoring of children living in an endemic area of Tanzania. It appears that immunity develops against the most frequent and persistent strains of parasite. If protective epitopes vary between P. falciparum strains, and the degree of immunity varies, then children under five will harbour more drug-resistant isolates than schoolchildren, a pattern that is actually observed. The evolution of drug resistance in helminths was discussed by S. Gupta (Imperial College, London, UK) who presented a model of genetic heterogeneity, formulated in terms of resistance and susceptibility of parasite alleles using simple Mendelian genetics. The model shows that although mean worm burden will recover rapidly from a single drug dose, the susceptible parasite population may become sufficiently suppressed to allow the fixation of a rare, recessive, resistant allele. When the frequency of drug application decreases, the relative advantage of the resistant over the susceptible population also decreases but the declining rates of heterozygote (susceptible) mortality may exert a positive effect on population growth. Such models suggest that controlling the intensity of disease may be more effective than eradication. Ultimately, field studies and field experiments are essential for understanding disease processes, particularly for vector-borne agents. C. Dye (London School of Hygiene and Tropical Medicine, UK) demonstrated the many subtleties of the interactions between selective host attraction for male sandflies and the subsequent female response. This will have obvious implications for sandfly control strategies involving pheromone traps.
Prospects Concluding the meeting, T. Freyvogel (Swiss Tropical Institute, Basel, Switzerland) emphasized the need to connect
280
Parasitology Today, vol. 6, no. 9, 1990
benchwork and fieldwork both within and between disciplines to design rational as well as cost-effective approaches and strategies to control parasitic infections. Although it has not been possible to describe all the work presented, the diversity of topics covered at this meeting has shown the potential for progress when researchers of diverse disciplines
join efforts, but at the same time, many gaps and misunderstandings have also become apparent. The prize at stake is learning about the role of some of the most important evolutionary and ecological agents in nature. References I Rau, M.E. and Caron, F.R. (1979)Can.J. Zool. 57, 2466-2468
2 Moore, J. (1984) Sci.Am. 250, 82-89 3 Hamilton, W.D. and Zuk, M. (1982) Science 218,384-387 4 Park, T. (1948) Ecol. Monogr. 18,265-307
Paul Schmid-Hempel is at the Zoologisches Institut der Universitdt, Rheinsprung 9, CH4051 Basel, Switzerland, and Marcel Tanner is at the Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland.
Helminths and Hydatid Disease Soils, Uruguay: December 1989 Following the very successful first workshop in 1987 (Ref. I), the International Workshop on Helminth Basic Research and Second Regional Workshop on Basic Research on Hydatid Disease set out to expand current research programmes between South American countries and promote new collaborating investigations with European laboratories. The International Scientific Cooperation Programme of the EC was the primary agency behind the meeting, and was represented by Alejandro Herrero (Belgium). More than 100 scientists from 13 countries in the Americas, Europe, Japan and Australia participated and over 50 papers on hydatid disease and other helminth infections were presented. In his opening address, Luis Yarzabal (University of Uruguay) stressed the impact of parasitic disease in the Third World, and the importance of collaboration between developing and developed countries in different areas of research.
Epidemiology The prevalence of asymptomatic hydatidosis in humans in Uruguay is in the range 1.37-4.05% (Carmelo Alvarez, University of Uruguay; Rosario Bonifacino, Instituto de Higiene, University of Uruguay; and Elbio Paolillo, Fundacion San Antonio de Padua, Durazno, Uruguay). These studies have revealed a considerable lack of awareness of the methods of transmission and control of hydatidosis by the population at risk, and have also highlighted the usefulness of ecography (ultrasound) as a diagnostic procedure. Furthermore, Perla Cabrera (University of Uruguay) drew attention to the low efficacy of current methods of examination at slaughter houses for the
detection of hydatid infections; it is estimated that 20% of offal passed fit for human consumption may be infected. Histopathological studies on hydatidosis in cattle and wild animals in Australia (Tsukasa Sakamoto, Iwate University, Japan) have shown that a sylvatic cycle involving the dingo ( Canis familiaris dingo) and macropods exists with cattle as the accidental host. Hector Alcaino (University of Chile) reported that in Chile Fasciola hepatica is the principal parasitic infection of veterinary importance, with up to 31% of cattle being infected (the human infection rate was found to be 0.64%). He also stressed the potential role of other domestic stock and wild animals in transmission. Elena Duran (University of Uruguay) found a high prevalence of precipitating antibodies to Toxocara canis in 246 children with eosinophilia (mean age 3.65 years). Reviews on the importance of socioeconomic development on the epidemiology of parasitic infections were provided by Ivo de Carneri (Universita degli Studi di Pavia, Italy) and Michael Gemmell (University of Cambridge, UK). Naftale Katz (Centro de Pequisas 'Rene Rachou', Belho Horizonte, Brazil) discussed similar aspects with regard to the distribution of schistosomiasis in the Americas. Over 95% of infected individuals live in Brazil, but the wider distribution of potential intermediate hosts raises questions about the future spread of the disease.
Biochemistry and Molecular Biology The biochemistry and molecular biology of hydatid disease was reviewed by Ricardo Ehrlich (University of Uruguay) and discussed by a number of participants. Ignacio Reisin (Centro de
Investigaciones M~dicas Albert Einstein, Buenos Aires, Argentina) described work on the ion transport mechanisms of Echinococcus granulosus in experiments using membranes reconstituted as planar lipid bilayers for the identification and characterization of specific K+ channels. An analysisof proteases and protease inhibitors in E. granulosus was reported by Marta Marco (University of Uruguay) who has identified a protease of 100 kDa in hydatid fluid. Anne-Fran~oise Petavy (Universite Claude Bernard, Lyon, France) reported on the lipid components of E. multilocularis and on enzymes involved in parasite glycogen metabolism, noting changes in the corresponding enzymes in the infected host. No doubt studies in vitro will be greatly assisted by the work of Gustavo Salinas (University of Uruguay), who described methods of isolatingviable cells from the protoscoleces of E. granulosus following disruption by sheared glass, collagenase digestion and purification over a percoll gradient', after six days in culture, 40% of the isolated cells remained viable. Guillermo Denegri (National University of La Plata, Argentina) reported on the establishment of the complete cycle of Spirometra erinacei in the laboratory, the cycle involving Paracyclops, an anuran intermediate host, with cats and dogs as final hosts. The impact of molecular biology on the study of hydatid disease was reviewed by Don McManus (Queensland Institute of Medical Research, Australia), and examples of cloned sequences described include genes encoding actin (Helio Ferreira and Arnaldo Zaha, Centro de Biotecnologia do Estado de Rio Grande do Sul, Brazil) and thymidylate synthetase (Liane Luz Livi and Arnaldo Zaha, Centro de Biotecnologia do Estado de Rio Grande do Sul, Brazil), repetitive sequences ~) 1990,ElsevierSciencePublishersLtd,(UK)0169~4707/90/$02.00
278
The Evolution, Ecological Effects and Health Impact of Parasites P. Schmid-Hempel and H. Tanner i
Basel, Switzerland: April 1990 Lions' claws and sharks' teeth rank high in capturing people's imagination and form an appreciation of how natural selection acts. Although it is conventional wisdom that predators normally eliminate the weak and sick among their prey, the implications for the role of parasites in ecological interactions or as selective factors affecting the course of evolution have not particularly attracted the imagination of researchers. Yet, just as Canadian hunters seem to hit parasiteinfected moose more often than unparasitized ones ~, so natural predators may simply benefit from successful parasites that have prepared their host for takeover by a potential intermediate host2, However, as a subject, parasitology has failed to address the unifying principles discussed by ecologists and evolutionary biologists. In turn though, the latter have lacked the detailed knowledge of the parasitologist needed for testing these hypotheses. This year's meeting of the Swiss Zoological Society (University of Basel, 6-7 April 1990) aimed to allow the exchange of ideas and approaches to the study of parasites and their roles in ecology and evolution, particularly between ecologists, evolutionary biologists, medical zoologists and parasitologists.
Parasitism and Mate Choice A major part of the meeting was devoted to an interdisciplinary exchange between epidemiology and medical zoology. The great variety of ecological and evolutionary processes in which parasites are involved made a deep impression and was reflected in the broad diversity of topics discussed throughout the meeting. Among them, Hamilton and Zuk's suggestion that parasites can exert negative frequencydependent selection to maintain the elaborate secondary sexual characteristics of many species has apparently attracted a great deal of research effort 3. A study on the influence of intestinal Abstracts will be published in the December issue of RevueSuissede Zoologie 97 (4).
roundworms on sexual ornamentation and mate choice in red jungle fowl by R. Thornhill (University of New Mexico, NM, USA) is a case in point. The degree of male ornamentation depends, among other things, on the presence of parasites, as they disproportionately adversely affect ornamental traits compared with non-ornamental male traits, and females prefer males with fewer indications of disease as revealed by their ornamentation. Similarly, in sticklebacks, the intensity of male breeding colouration is shown to be reduced if the male is infected by tapeworms (M. Milinski and T. Bakker, University of Bern, Switzerland). Like the jungle fowl, where sexual selection based on ornamentation and female choice is linked with selection through male-male competition, female sticklebacks may choose their mates not because they provide better diseaseresistance genes for offspring but because healthier males are superior fathers and can provide better parental care. Therefore, more needs to be known about the assumed genetic variance for parasite resistance before any firm conclusions can be reached. Although intraspecific studies have fared well, evaluating the role of parasites by comparative approaches has been more difficult. A. Read (Oxford University, UK) compared parasite load between polygynous and monogynous passerine birds. When controlling for the effects of phylogeny, body size, latitude and so forth, he found that the prevalence of blood parasites is lower in polygynous birds than in species of monogynous birds. This suggests that infection by parasites is somehow involved in avian mating systems, although it is unclear how. In a further glimpse of the overlooked importance of parasites, K. AIlander and R. Dufva (Uppsala University, Sweden) reported that large loads of ectoparasites reduce body weight and tarsus length of nestling great tits (Parus major). In itself, this may not be very surprising but it should be remembered that much of our knowledge about bird populations comes from studies on great tits, and such anatomical characteristics are commonly used as indicators of heritable traits.
Host Behaviour and Parasite Specificity Transmission to new hosts is a crucial step in the life cycle of parasites and it is often facilitated by certain host behaviours or status. For example, in England, juvenile oystercatchers (Haematopus ostralegus) are prone to helminth infection because their low dominance status means that they have to feed on cockles heavily infected with intermediate stages of parasites (C. Goater, University of Z0rich, Switzerland). Likewise for social animals, the spatial proximity or similar genetic constitution of hosts puts their parasites at an advantage. J. Shykoff and P. Schmid-Hempel (University of Basel, Switzerland) showed that the transmission of an intestinal trypanosome in bumblebees is facilitated if colonies of the insect consist of closely related individuals, in contrast to colonies composed of less related or unrelated individuals, which have smaller parasite burdens. This may explain why, contrary to expectations from kin selection theory, associations of full sibs in social groups are not the general rule. Several contributors concentrated on the role of parasites in mediating ecologi~ cal interactions of their hosts, a topic that was heralded by Park's 4 classical studies on flour beetles. For example, M. Bouletreau and P. Fouillet (University of Lyon, France), showed that in experimental systems using two sibling species of Drosophila, one species usually displaces the other. When a larval parasitold is added to the system both species of fruit fly will co-exist for prolonged periods, although the effect of the parasitoid is dependent on the ambient temperature. The modulation of parasite impact by environmental conditions was also pointed out by H. M011er(University of BaseL, Switzerland), who has shown that the damage done by root herbivores depends on the plant's environment. Whether parasites co-evolve with their hosts or simply track appropriate resources across host taxa is of considerable interest to evolutionary biologists in relation to host specificity. Tracking hosts that are taxonomically distinct (~) 1990, Elsevier Science Publishers Ltd, (UK) O1694707/90/$0200
Parasitology Today, voL 6, no. 9, 1990 but living in similar habitats may lead to sympatric speciation among parasites. T.. DeMee0s and his colleagues (University of Montpellier, France) suspect that this may have occurred in the copepod parasites of three species of Mediterranean fish. R. Colwell (University of Connecticut, CT, USA) pointed out that host specificity may have little to do with the nutritional or physiological characteristics of the host. He reported that the specificity of species of flower mites is selectively driven by the,. difficulties of finding a mate and does not relate to food. To improve their odds, different mite species therefore have become specialized at disembarking from their hummingbird transport hosts at specific plant species. Even well-known parasites harbour some surprises that may shed light on questions that have arisen in other disciplines. This is vividly demonstrated in work on the genetics of Trypanosoma brucei. L Jenni (Swiss Tropical Institute, Basel, Switzerland) reported that in T. brucei a fair amount of genetic exchange occurs. Recombination may explain why some phenotypic traits, such as the variable surface glycoprotein coat, drug resistance or pathogenicity, show variation and instability. Genetic exchange in parasites is of intrinsic interest to evolutionary biologists because of the increased potential of the parasite to undergo evolutionary change and because of its consequently increased ecological impact.
Models for Public Health The sessions on 'parasites and man' not only focused on the understanding of this host-parasite relationship, but also provided an outlook for the control of parasitic infections at the community level. A framework was provided by R.M. Anderson (Imperial College, UK) who discussed the various levels of genetic, social, behavioural and spatial heterogeneities found within and between host and parasite populations interactions that are non-linear in nature. Mathematical models have demonstrated their scope for predicting epidemiological scenarios and provide the basis for decision making in public health policy, eg. the necessary vaccination coverage for a certain level of protection, or the age group that can be immunized to the best effect in population-based programmes. At the level of an individual host, the dynamics of a parasite population is driven by rates of multiplication and
279 development. Similar factors can be incorporated in an 'ecological' approach for the analysis of the interactions between parasite populations and host immune cells at a molecular level. The concept of 'immuno-ecology' or 'ecoimmunology', as advocated by Anderson, should provide a new stimulus for immunologists. This could become a vital part of the interplay between the bench and the field and aid the translation of research findings into applications. The ubiquity of parasitic diseases and their significance to humans was illustrated by the reports of various field studies in Europe and the tropics. A case-control study of paediatric cryptosporidiosis has revealed the importance of this parasite in Europe and indicated that person-to-person rather than animal-to-human transmission is the main route for maintaining the disease (D. M~usezahl et al., Swiss Tropical Institute, Basel, Switzerland). A series of studies on Lyme borreliosis and on tickborne encephalitis in Switzerland was presented by A. Aeschlimann and his colleagues (University of Neuchatel, Switzerland). Their field studies have helped to form an understanding of how this disease spreads, but less encouragingly, laboratory experiments show that the protein profiles of different Borrelia burgdorferi strains can be altered after passage through its vector. Such variation may have epidemiological implications (C.M. Hu, University of Neuchatel, Switzerland). The pressing problem of malaria was highlighted by an intervention study that compared the effects of intradomicilliary spraying against mosquitoes on the incidence of malaria in children under ten years of age (Mnzava et al., Swiss Tropical Institute, Basel, Switzerland). Residual spraying was found not only to be effective at controlling populations of the endophilic Anopheles gambiae (sensu stricto) but also those of the exophilic A. arabiensis. Transmission of a parasite is not always straightforward and amenable to control measures. Complex transmission mechanics have been implicated as a problem in a study assessing the importance of intestinal parasitoses in a squatter area of Manila, Philippines (C. Auer, Swiss Tropical Institute, Basel, Switzerland): a high degree of multiparasitism seemed to stimulate the host's immune system, but inappropriately so. This study was carried out in the broader context of urban health, an area that has been neglected for a Iongtime despite its growing impact in the developing world. It will 15e a major challenge for health
planners and biomedical scientists in the years to come to solve the problem of urban health, and both parasitologists and epidemiologists will have to be involved in this task. Health intervention unfolds within the dynamic world of host-parasite relationships. To help unravel this complexity J. Koella (Swiss Tropical Institute, Basel, Switzerland) has developed mathematical models that aim to predict the effects of intervention, particularly in relation to age-dependent chloroquine resistance in Plasmodium falciparum. These ideas have been tested during the long-term monitoring of children living in an endemic area of Tanzania. It appears that immunity develops against the most frequent and persistent strains of parasite. If protective epitopes vary between P. falciparum strains, and the degree of immunity varies, then children under five will harbour more drug-resistant isolates than schoolchildren, a pattern that is actually observed. The evolution of drug resistance in helminths was discussed by S. Gupta (Imperial College, London, UK) who presented a model of genetic heterogeneity, formulated in terms of resistance and susceptibility of parasite alleles using simple Mendelian genetics. The model shows that although mean worm burden will recover rapidly from a single drug dose, the susceptible parasite population may become sufficiently suppressed to allow the fixation of a rare, recessive, resistant allele. When the frequency of drug application decreases, the relative advantage of the resistant over the susceptible population also decreases but the declining rates of heterozygote (susceptible) mortality may exert a positive effect on population growth. Such models suggest that controlling the intensity of disease may be more effective than eradication. Ultimately, field studies and field experiments are essential for understanding disease processes, particularly for vector-borne agents. C. Dye (London School of Hygiene and Tropical Medicine, UK) demonstrated the many subtleties of the interactions between selective host attraction for male sandflies and the subsequent female response. This will have obvious implications for sandfly control strategies involving pheromone traps.
Prospects Concluding the meeting, T. Freyvogel (Swiss Tropical Institute, Basel, Switzerland) emphasized the need to connect
280
Parasitology Today, vol. 6, no. 9, 1990
benchwork and fieldwork both within and between disciplines to design rational as well as cost-effective approaches and strategies to control parasitic infections. Although it has not been possible to describe all the work presented, the diversity of topics covered at this meeting has shown the potential for progress when researchers of diverse disciplines
join efforts, but at the same time, many gaps and misunderstandings have also become apparent. The prize at stake is learning about the role of some of the most important evolutionary and ecological agents in nature. References I Rau, M.E. and Caron, F.R. (1979)Can.J. Zool. 57, 2466-2468
2 Moore, J. (1984) Sci.Am. 250, 82-89 3 Hamilton, W.D. and Zuk, M. (1982) Science 218,384-387 4 Park, T. (1948) Ecol. Monogr. 18,265-307
Paul Schmid-Hempel is at the Zoologisches Institut der Universitdt, Rheinsprung 9, CH4051 Basel, Switzerland, and Marcel Tanner is at the Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland.
Helminths and Hydatid Disease Soils, Uruguay: December 1989 Following the very successful first workshop in 1987 (Ref. I), the International Workshop on Helminth Basic Research and Second Regional Workshop on Basic Research on Hydatid Disease set out to expand current research programmes between South American countries and promote new collaborating investigations with European laboratories. The International Scientific Cooperation Programme of the EC was the primary agency behind the meeting, and was represented by Alejandro Herrero (Belgium). More than 100 scientists from 13 countries in the Americas, Europe, Japan and Australia participated and over 50 papers on hydatid disease and other helminth infections were presented. In his opening address, Luis Yarzabal (University of Uruguay) stressed the impact of parasitic disease in the Third World, and the importance of collaboration between developing and developed countries in different areas of research.
Epidemiology The prevalence of asymptomatic hydatidosis in humans in Uruguay is in the range 1.37-4.05% (Carmelo Alvarez, University of Uruguay; Rosario Bonifacino, Instituto de Higiene, University of Uruguay; and Elbio Paolillo, Fundacion San Antonio de Padua, Durazno, Uruguay). These studies have revealed a considerable lack of awareness of the methods of transmission and control of hydatidosis by the population at risk, and have also highlighted the usefulness of ecography (ultrasound) as a diagnostic procedure. Furthermore, Perla Cabrera (University of Uruguay) drew attention to the low efficacy of current methods of examination at slaughter houses for the
detection of hydatid infections; it is estimated that 20% of offal passed fit for human consumption may be infected. Histopathological studies on hydatidosis in cattle and wild animals in Australia (Tsukasa Sakamoto, Iwate University, Japan) have shown that a sylvatic cycle involving the dingo ( Canis familiaris dingo) and macropods exists with cattle as the accidental host. Hector Alcaino (University of Chile) reported that in Chile Fasciola hepatica is the principal parasitic infection of veterinary importance, with up to 31% of cattle being infected (the human infection rate was found to be 0.64%). He also stressed the potential role of other domestic stock and wild animals in transmission. Elena Duran (University of Uruguay) found a high prevalence of precipitating antibodies to Toxocara canis in 246 children with eosinophilia (mean age 3.65 years). Reviews on the importance of socioeconomic development on the epidemiology of parasitic infections were provided by Ivo de Carneri (Universita degli Studi di Pavia, Italy) and Michael Gemmell (University of Cambridge, UK). Naftale Katz (Centro de Pequisas 'Rene Rachou', Belho Horizonte, Brazil) discussed similar aspects with regard to the distribution of schistosomiasis in the Americas. Over 95% of infected individuals live in Brazil, but the wider distribution of potential intermediate hosts raises questions about the future spread of the disease.
Biochemistry and Molecular Biology The biochemistry and molecular biology of hydatid disease was reviewed by Ricardo Ehrlich (University of Uruguay) and discussed by a number of participants. Ignacio Reisin (Centro de
Investigaciones M~dicas Albert Einstein, Buenos Aires, Argentina) described work on the ion transport mechanisms of Echinococcus granulosus in experiments using membranes reconstituted as planar lipid bilayers for the identification and characterization of specific K+ channels. An analysisof proteases and protease inhibitors in E. granulosus was reported by Marta Marco (University of Uruguay) who has identified a protease of 100 kDa in hydatid fluid. Anne-Fran~oise Petavy (Universite Claude Bernard, Lyon, France) reported on the lipid components of E. multilocularis and on enzymes involved in parasite glycogen metabolism, noting changes in the corresponding enzymes in the infected host. No doubt studies in vitro will be greatly assisted by the work of Gustavo Salinas (University of Uruguay), who described methods of isolatingviable cells from the protoscoleces of E. granulosus following disruption by sheared glass, collagenase digestion and purification over a percoll gradient', after six days in culture, 40% of the isolated cells remained viable. Guillermo Denegri (National University of La Plata, Argentina) reported on the establishment of the complete cycle of Spirometra erinacei in the laboratory, the cycle involving Paracyclops, an anuran intermediate host, with cats and dogs as final hosts. The impact of molecular biology on the study of hydatid disease was reviewed by Don McManus (Queensland Institute of Medical Research, Australia), and examples of cloned sequences described include genes encoding actin (Helio Ferreira and Arnaldo Zaha, Centro de Biotecnologia do Estado de Rio Grande do Sul, Brazil) and thymidylate synthetase (Liane Luz Livi and Arnaldo Zaha, Centro de Biotecnologia do Estado de Rio Grande do Sul, Brazil), repetitive sequences ~) 1990,ElsevierSciencePublishersLtd,(UK)0169~4707/90/$02.00