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Whatever became of Isospora bigemina?
Parasitology Today, voL 3, no. 4, 1987
The
I 01
Isospora- Toxoplasma-
Sarcocystis Confusion
Once upon a time, the taxonomy of coccidian parasites was a relatively simple affair- at least for specialists - when all could be assigned to the genus Coccidium. But as information has accumulated about structures, life cycles and host range, the picture has become systematically confused. Eimeriid oocysts, probably first seen in rabbit bile by Antony van Leeuwenhoek / .e S t , in 1674, were original~ ascribed to existing genera such as Monocystis. Schizogony stages were then assigned to ELmeria and., when the corm~tion between schizonts in the viscera and oocysts m the faeces was realized, the genus Coccidium was used for these parasites of n~ammals, with avian forms usually assigned to Isospora or Diplospora. C.A. Hoare" s "periodic table', published in 1933, classified the coccidia according the number of sporozmtes per sporocyst, and sporocysts per oocyst, but this assumed that these parasites were homoxenous - all development occurring in one host. Not so, for studies of Toxoplasma and Sarcocystis revealed that they too have a coccidian life cycle but are heteroxenous. Today, not only the names, but their life cycles and interrelationships continue to give confusion, and in this debate, Norman Levine and John Baker attempt some clarification. .
m
~ .~
/ ~o
)
"By the way, what's your name?'
Whatever became of
bigemina?
Isospora
N.D. Levine In the good old days, all the coccidian protozoa whose sporulated oocysts contained two sporocysts, each with four sporozoites, were assigned to the genuslsospora. In 1891 Stiles I gave the name Coccidium bigeminum to an oocyst that he found in the intestine of a dog. The oocysts were already sporulated in the intestine, and the nanle bigeminum (two twin) refers to the fact that they contain two similar sporocysts (Fig. 1). Laterthegeneric name Coccidium (which had been used for all coccidia regardless of the numbers of sporocysts and sporozoites in each oocyst) was dropped, and the name Isospora (introduced by Schneider in 1881; Ref. 3) was used for all species whose sporulated oocysts contained two sporocysts, each with four sporozoites. It was soon discovered that dogs had two sizes of'I. bigemina' oocysts, and these were called the large and small forms. It was also discovered that similar oocysts occurred in a variety of other carnivores, and these were called varieties of I. bigemina. Thus we had I. bigemina var. cati, I. bigemina var. zorillae etc.
Over 55 genera and some 2000 species of what might be called the true coccidia (suborders Adeleorina and Eimeriorina) have new been named 3,4,5,19.They are all parasitic ~)1987, Elsevier Pubhcations,Cambridge 0169-4758/87/$02.00
protozoa of the phylum Apicomplexa Levine, 1970 (Ref. 6). Their life cycles include both asexual and sexual phases. The sporulated oocysts are generally ingested by a host; sporozoites emerge from the oocysts and enter a host cell, where they develop. They round up and grow, becoming meronts, which divide
Department of Veterinary Pathobiology University of Illinois 2001 S. Lincoln Avenue, Urbana, IL 6180 I, USA
Parasitology Today, voL 3, no, 4, 1987
102
Fig. I. A sporuluted oocyst of Sarcocystis miescheriana (-Isospora bigemina = Sarcocystis bigemina) from the dog as passed in the faeces (from Ref72),
Fig. 2. The structures of a sporulated oocyst of Eimeria as seen under the light microscope, showing the sporocysts and sporozoites. These structures occur in the oocysts of almost all coccidia (from Ref 7).
by a process of multiple fission into a number of merozoites - the number depending upon the species and genus. This process is asexual. The merozoites enter new host cells, become meronts, and repeat the process, usually more than once. The next phase is sexual. Merozoites enter new host cells. Some of them turn into macrogamonts and others into microgamonts (i.e. female and male gamonts, respectively). The macrogamonts turn into macrogametes (i.e. female gametes) and the microgamonts form a number of microgametes (i.e. male gametes). FertiliTation takes place; the resultant zygote develops a wall around itself and (in most genera) breaks out of the host cell. It is now an oocyst and contains a single cell. Sporulation then occurs and
sporocysts may form within the oocysts (Fig. 2) or naked sporozoites may be formed. The sporozoites and merozoites have an apical complex (Fig. 3) consisting of a polar ring, conoid, micronemes, rhoptries and subpellicular microtubules, all of which can be seen only with the electron microscope. The zygotes are diploid; all other stages are haploid. Until recently, all coccidia were thought to be homoxenous - both asexual and sexual development taking place in the same host animal. Then it was found, almost at the same time, that Toxoplasma and Sarcocystis, whose taxonomic positions had been uncertain and in which only asexual reproduction was known, both produce oocysts and are actually coccidia. This was done independently for Toxoplasma by Hutchison et al. 9 in Scotland and Denmark, Frenkel et al. 10 in Kansas, Sheffield and Melton n in Maryland, Gordon Wallace (unpublished) in Hawaii, Wetland and Kuhn 12 in Germany and Overdulve 13in The Netherlands. It was done independently for Sarcocystis by Fayer 14 in Maryland and Rommel et al. 15 and Heydorn and Romnle116,17in Germany. Both findings have been amply confLrmed. Both these genera are heteroxenous, with asexual multiplication taking place in one host, and sexual reproduction in another. Oocysts are produced in the intestinal cells of a predator, and are passed out in the faeces. A prey animal then ingests them, the sporozoites emerge, and the asexual phase of the life cycle proceeds. It was found that the so-called Isospora bigemina cannot be transmitted from one dog to another, but that its oocysts are actually those of Sarcocystis and can be transmitted only to a prey animal. Commonly, most of the sporulated oocysts of Sarcocystis release their sporocysts in the intestine, so that sporulated sporocysts are passed in the faeces. A further complication is the status of the genus Atoxoplasma. This genus was named by GarnhalT115 for parasites resembling Toxoplasma in the blood cells of birds. Then Box 19 found that the species in canaries underwent asexual multiplication in blood cells, but formed oocysts (which she called Isospora serini) in the intestinal cells. I therefore resurrected Garnham's genus and listed 19 species, of which Atoxoplasma serini was one2°. All four genera have oocysts containing two sporocysts, each with four sporozoites. Isospora and Atoxoplasma are homoxenous, with both sexual and asexual reproduction in the same host animal. Sarcocystis and Toxoplasma are heteroxenous, with asexual
Parasitology Today, voL 3, no. 4, 1987
103
reproduction in a prey animal and sexual reproduction in a predator (except that Toxoplasma can have both in cats). In Iso-
spora, Toxoplasma and Atoxoplasma, the oocysts are undeveloped and contain only a single cell when shed in the faeces. In these genera, sporulation occuls outside the host. In Sarcocystis however, sporulation occurs within the host, and sponflated oocysts containing sporocysts (Fig. 1) as well as sporulated sporocysts (Fig. 4) are shed in the faeces. Isospora bigemina of dogs is now a synonym of Sarcocystis miescheriana; I. bigemina var. cati of cats is a synonym of Sarcocystis muris; and some other of the organisms formerly known as I. bigemina are actually S. bertrami, S. cruzi, S. hirsuta, S. levinei, S. tenella, etc. depending on the definitive hostS, 21,22. Other former varieties of I. bigemina are probably also Sarcocystis. The species that were formerly assigned to Isospora have now been placed in seven genera - Isospora, Toxoplasma, Atoxoplasma, Sarcocystis, Arthrocystis (which resembles Sarcocystis but has spherical merozoites and meronts jointed like b a m boo in the muscles), Frenkelia (which resembles Sarcocystis but has relatively small meronts in the central nervous system) and Besnoitia (whic)h resembles Toxop/asma but has host-cell nuclei within the
meront wall). We now recognize 349 species of Isospora, 120 of Sarcocystis, 19 of Atoxop/asma, 9 of Toxoplasma, 7 ofBesnoitia, 2 of Frenkelia, and 1 of Arthrocystis 5. The whole situation deserves further study. Taxonomy, which some people consider old-fashioned, is flourishing and advancing.
The ToxoplasmaTangle J.R. Baker The parasitic protozoan Toxoplasma gondii has been known uneqttivocally as a parasite and potential pathogen of man since 1937 (Ref. 23). For many years the taxonomic status of the genus remained unclear and it was possible to assign it to a separate class (Toxoplasmea) within the phylum Sporozoa (now called Apicomplexa)24.. But in 1965, evidence was published that T. gondii could be transmitted via the faeces of infected cats25 within, it was thought, the ova of nematodes. Subsequently, it was found that nematode eggs played no essential role; the infective stage in feline faeces was a caccidian oocyst of typical isosporan structure, and the parasite underwent a characteristic coccidian life cycle involving merogony (= schizogony), gametogony, fertilization and sporogony within the intestinal cells and (finally) lumen of cats 26. Thus was the mystery of the correct classification and full life cycle of Ibxoplasma finally solved. The discovery revolutionized a (~ 1987, Elsevier Publications,Cambridge 0169 4758/87/$0200
branch of parasitology, illuminating hitherto obscure aspects of the epidemiology and transmission of toxoplasmosis as well as most satisfactorily fitting the recalcitrant organism into the protozoologists' taxonomic schemez7 and opening the door to the solution of similar problems relating to several other generazS. However, the peaceful interlude resulting from the resolution of earlier arguments was short. Very soon a controversy arose concerning the correct name for the parasite 13,29,30, which has not yet been resolved. It has been argued by Overdulve 13,31and others that toxoplasms and isosporans differ only in the extent to which they can develop extra-intestinally in animals other than their definitive hosts (in which sexual reproduction occurs). Therefore, they would deserve taxonomic distinction at the sub-generic level, at the most, whereupon the law of priority decrees that the generic name Toxoplasma NicoUe and Manceaux, 1909,
4 Belvoir Road Cambridge CB4 lJJ, UK
The
I 01
Isospora- Toxoplasma-
Sarcocystis Confusion
Once upon a time, the taxonomy of coccidian parasites was a relatively simple affair- at least for specialists - when all could be assigned to the genus Coccidium. But as information has accumulated about structures, life cycles and host range, the picture has become systematically confused. Eimeriid oocysts, probably first seen in rabbit bile by Antony van Leeuwenhoek / .e S t , in 1674, were original~ ascribed to existing genera such as Monocystis. Schizogony stages were then assigned to ELmeria and., when the corm~tion between schizonts in the viscera and oocysts m the faeces was realized, the genus Coccidium was used for these parasites of n~ammals, with avian forms usually assigned to Isospora or Diplospora. C.A. Hoare" s "periodic table', published in 1933, classified the coccidia according the number of sporozmtes per sporocyst, and sporocysts per oocyst, but this assumed that these parasites were homoxenous - all development occurring in one host. Not so, for studies of Toxoplasma and Sarcocystis revealed that they too have a coccidian life cycle but are heteroxenous. Today, not only the names, but their life cycles and interrelationships continue to give confusion, and in this debate, Norman Levine and John Baker attempt some clarification. .
m
~ .~
/ ~o
)
"By the way, what's your name?'
Whatever became of
bigemina?
Isospora
N.D. Levine In the good old days, all the coccidian protozoa whose sporulated oocysts contained two sporocysts, each with four sporozoites, were assigned to the genuslsospora. In 1891 Stiles I gave the name Coccidium bigeminum to an oocyst that he found in the intestine of a dog. The oocysts were already sporulated in the intestine, and the nanle bigeminum (two twin) refers to the fact that they contain two similar sporocysts (Fig. 1). Laterthegeneric name Coccidium (which had been used for all coccidia regardless of the numbers of sporocysts and sporozoites in each oocyst) was dropped, and the name Isospora (introduced by Schneider in 1881; Ref. 3) was used for all species whose sporulated oocysts contained two sporocysts, each with four sporozoites. It was soon discovered that dogs had two sizes of'I. bigemina' oocysts, and these were called the large and small forms. It was also discovered that similar oocysts occurred in a variety of other carnivores, and these were called varieties of I. bigemina. Thus we had I. bigemina var. cati, I. bigemina var. zorillae etc.
Over 55 genera and some 2000 species of what might be called the true coccidia (suborders Adeleorina and Eimeriorina) have new been named 3,4,5,19.They are all parasitic ~)1987, Elsevier Pubhcations,Cambridge 0169-4758/87/$02.00
protozoa of the phylum Apicomplexa Levine, 1970 (Ref. 6). Their life cycles include both asexual and sexual phases. The sporulated oocysts are generally ingested by a host; sporozoites emerge from the oocysts and enter a host cell, where they develop. They round up and grow, becoming meronts, which divide
Department of Veterinary Pathobiology University of Illinois 2001 S. Lincoln Avenue, Urbana, IL 6180 I, USA
Parasitology Today, voL 3, no, 4, 1987
102
Fig. I. A sporuluted oocyst of Sarcocystis miescheriana (-Isospora bigemina = Sarcocystis bigemina) from the dog as passed in the faeces (from Ref72),
Fig. 2. The structures of a sporulated oocyst of Eimeria as seen under the light microscope, showing the sporocysts and sporozoites. These structures occur in the oocysts of almost all coccidia (from Ref 7).
by a process of multiple fission into a number of merozoites - the number depending upon the species and genus. This process is asexual. The merozoites enter new host cells, become meronts, and repeat the process, usually more than once. The next phase is sexual. Merozoites enter new host cells. Some of them turn into macrogamonts and others into microgamonts (i.e. female and male gamonts, respectively). The macrogamonts turn into macrogametes (i.e. female gametes) and the microgamonts form a number of microgametes (i.e. male gametes). FertiliTation takes place; the resultant zygote develops a wall around itself and (in most genera) breaks out of the host cell. It is now an oocyst and contains a single cell. Sporulation then occurs and
sporocysts may form within the oocysts (Fig. 2) or naked sporozoites may be formed. The sporozoites and merozoites have an apical complex (Fig. 3) consisting of a polar ring, conoid, micronemes, rhoptries and subpellicular microtubules, all of which can be seen only with the electron microscope. The zygotes are diploid; all other stages are haploid. Until recently, all coccidia were thought to be homoxenous - both asexual and sexual development taking place in the same host animal. Then it was found, almost at the same time, that Toxoplasma and Sarcocystis, whose taxonomic positions had been uncertain and in which only asexual reproduction was known, both produce oocysts and are actually coccidia. This was done independently for Toxoplasma by Hutchison et al. 9 in Scotland and Denmark, Frenkel et al. 10 in Kansas, Sheffield and Melton n in Maryland, Gordon Wallace (unpublished) in Hawaii, Wetland and Kuhn 12 in Germany and Overdulve 13in The Netherlands. It was done independently for Sarcocystis by Fayer 14 in Maryland and Rommel et al. 15 and Heydorn and Romnle116,17in Germany. Both findings have been amply confLrmed. Both these genera are heteroxenous, with asexual multiplication taking place in one host, and sexual reproduction in another. Oocysts are produced in the intestinal cells of a predator, and are passed out in the faeces. A prey animal then ingests them, the sporozoites emerge, and the asexual phase of the life cycle proceeds. It was found that the so-called Isospora bigemina cannot be transmitted from one dog to another, but that its oocysts are actually those of Sarcocystis and can be transmitted only to a prey animal. Commonly, most of the sporulated oocysts of Sarcocystis release their sporocysts in the intestine, so that sporulated sporocysts are passed in the faeces. A further complication is the status of the genus Atoxoplasma. This genus was named by GarnhalT115 for parasites resembling Toxoplasma in the blood cells of birds. Then Box 19 found that the species in canaries underwent asexual multiplication in blood cells, but formed oocysts (which she called Isospora serini) in the intestinal cells. I therefore resurrected Garnham's genus and listed 19 species, of which Atoxoplasma serini was one2°. All four genera have oocysts containing two sporocysts, each with four sporozoites. Isospora and Atoxoplasma are homoxenous, with both sexual and asexual reproduction in the same host animal. Sarcocystis and Toxoplasma are heteroxenous, with asexual
Parasitology Today, voL 3, no. 4, 1987
103
reproduction in a prey animal and sexual reproduction in a predator (except that Toxoplasma can have both in cats). In Iso-
spora, Toxoplasma and Atoxoplasma, the oocysts are undeveloped and contain only a single cell when shed in the faeces. In these genera, sporulation occuls outside the host. In Sarcocystis however, sporulation occurs within the host, and sponflated oocysts containing sporocysts (Fig. 1) as well as sporulated sporocysts (Fig. 4) are shed in the faeces. Isospora bigemina of dogs is now a synonym of Sarcocystis miescheriana; I. bigemina var. cati of cats is a synonym of Sarcocystis muris; and some other of the organisms formerly known as I. bigemina are actually S. bertrami, S. cruzi, S. hirsuta, S. levinei, S. tenella, etc. depending on the definitive hostS, 21,22. Other former varieties of I. bigemina are probably also Sarcocystis. The species that were formerly assigned to Isospora have now been placed in seven genera - Isospora, Toxoplasma, Atoxoplasma, Sarcocystis, Arthrocystis (which resembles Sarcocystis but has spherical merozoites and meronts jointed like b a m boo in the muscles), Frenkelia (which resembles Sarcocystis but has relatively small meronts in the central nervous system) and Besnoitia (whic)h resembles Toxop/asma but has host-cell nuclei within the
meront wall). We now recognize 349 species of Isospora, 120 of Sarcocystis, 19 of Atoxop/asma, 9 of Toxoplasma, 7 ofBesnoitia, 2 of Frenkelia, and 1 of Arthrocystis 5. The whole situation deserves further study. Taxonomy, which some people consider old-fashioned, is flourishing and advancing.
The ToxoplasmaTangle J.R. Baker The parasitic protozoan Toxoplasma gondii has been known uneqttivocally as a parasite and potential pathogen of man since 1937 (Ref. 23). For many years the taxonomic status of the genus remained unclear and it was possible to assign it to a separate class (Toxoplasmea) within the phylum Sporozoa (now called Apicomplexa)24.. But in 1965, evidence was published that T. gondii could be transmitted via the faeces of infected cats25 within, it was thought, the ova of nematodes. Subsequently, it was found that nematode eggs played no essential role; the infective stage in feline faeces was a caccidian oocyst of typical isosporan structure, and the parasite underwent a characteristic coccidian life cycle involving merogony (= schizogony), gametogony, fertilization and sporogony within the intestinal cells and (finally) lumen of cats 26. Thus was the mystery of the correct classification and full life cycle of Ibxoplasma finally solved. The discovery revolutionized a (~ 1987, Elsevier Publications,Cambridge 0169 4758/87/$0200
branch of parasitology, illuminating hitherto obscure aspects of the epidemiology and transmission of toxoplasmosis as well as most satisfactorily fitting the recalcitrant organism into the protozoologists' taxonomic schemez7 and opening the door to the solution of similar problems relating to several other generazS. However, the peaceful interlude resulting from the resolution of earlier arguments was short. Very soon a controversy arose concerning the correct name for the parasite 13,29,30, which has not yet been resolved. It has been argued by Overdulve 13,31and others that toxoplasms and isosporans differ only in the extent to which they can develop extra-intestinally in animals other than their definitive hosts (in which sexual reproduction occurs). Therefore, they would deserve taxonomic distinction at the sub-generic level, at the most, whereupon the law of priority decrees that the generic name Toxoplasma NicoUe and Manceaux, 1909,
4 Belvoir Road Cambridge CB4 lJJ, UK