Evolution and coevolution of human blood flukes, Schistosoma, and host snails

Trematode blood flukes of the genus Schistosoma infect 200–300 million people in the tropics and subtropics causing schistosomiasis or bilharzia. The life cycle of these 1 cm long worms involves a period in a freshwater snail intermediate host. The major parasites that I have studied and their obligate intermediate host snails are:
S. mansoni/Biomphalaria Africa and the Neotropics
S. japonicum/Oncomelania China, the Philippines, Sulawesi
S. mekongi/Neotricula Mekong River, Thailand and Laos
I was originally attracted to the host snails of the genus Biomphalaria out of frustration. Cerion would never breed in the laboratory while Biomphalaria has eight generations a year and can be selfed or out-crossed. I was interested in studying coevolution and cospeciation and intrigued by the discovery in 1970 that some snails were genetically resistant to parasite infection. I spent several years trying to locate the genes for resistance and testing my idea that human infection rates could be reduced by increasing the number of resistant snails in a local population. Publication 71 presents the case for the genetic control of schistosomiasis, Publication 72 provides an example of our contributions to the mapping of genes for pigmentation, allozymes and adult resistance to S. mansoni in the snail Biomphalaria glabrata. Suffice it to say, my work on linkage mapping and genetic control did not go far enough and, to this day, it is unclear whether resistance involves fitness costs that prevent snails from evolving the ability to exclude parasites.

My work on the genetic characterization of host snail species and parasite species has been more productive and we have made a number of significant discoveries. Using allozymes as genetic markers my students and I elucidated the evolutionary history of the Asian schistosomes and discovered that parasitic worms in the Mekong valley and the Philippines are specifically distinct from the only species then recognized by the medical profession, S japonicum (Publications 56, 61, and 83). Our discovery of large genetic distances between Chinese and Philippine S. japonicum (Publication 87) alerted the medical world to the great diversity in this so-called "species". I went on to document the evolution of the host snails and discovered new species in Thailand, Laos, the Philippines and Indonesia. In Publication 97 we show that Oncomelania hupensis snails in China and the Philippines are distinct species. In Publication 106 we reported that Neotricula aperta, the intermediate snail host of the newly recognized Schistosoma mekongi, is probably a group of at least four sibling species. In Publication 70 we used population genetic markers to trace the long-distance movements of Biomphalaria straminea, a Neotropical schistosome-transmitting snail, recently introduced into China. In Publication 183, I explore the application of the genetic cluster species concept to Asian Oncomelania snails which have been traditionally regarded as subspecies but which I show should be elevated to the rank of species.

Our most significant recent contribution (Publication 164) concerns Neotropical schistosomiasis and our discovery of the African affinities of the host snail Biomphalaria glabrata. We used genetic data to test and disprove an 80-year old hypothesis concerning host-parasite coevolution in both Africa and the Americas. Human blood flukes were probably introduced recently from Africa to the New World by infected slaves and, since 1911, it has been believed that schistosomiasis spread rapidly as a suitable intermediate host snail (Biomphalaria glabrata) was already present and had lived in the Americas for tens of millions of years. We falsified this ancient vicariance hypothesis by comparing patterns of genetic variation in African and American snails; the common host snail in the Americas is apparently of recent African origin. Contrary to dogma, the whole genus probable originated in the Americas and first dispersed to Africa about 3 million years ago. Subsequently, one species returned to South America to transmit the debilitating trematode. These discoveries have since been confirmed by a British research group using DNA sequence data.

My review of the snail-transmitted diseases of medical and veterinary importance in Thailand and the Mekong valley (Publication 136) has been used in debates over the public health risks of various proposed hydropower dam projects in this region (Publications 134, 221).

Publications

47. Woodruff, D.S. Biological control of schistosomiasis by genetic manipulation of intermediate host snail populations. Proceedings of the International Conference on Schistosomiasis 2:755. (1978e). Note: This proposal led to Publication 71 and to Fletcher & Minchella’s doctoral theses and subsequent publications.

54. Minchella, D.J., P.T. LoVerde and D.S. Woodruff. Fecundity compensation during pre-patency in Biomphalaria. Proceedings of the American Society of Parasitologists Annual Meeting Program. Abstracts, p. 18. (1979d). Note: This abstract is included here only to draw attention to Minchella’s (1980) thesis: Ecological aspects of host-parasite coevolution and their implications in the genetic control of schistosomiasis. Purdue University.

56. Fletcher, M., D.S. Woodruff and P. LoVerde. Genetic differentiation between Schistosoma mekongi and S. japonicum: an electrophoretic study. In: The Mekong Schistosome. Harinasuta, C. and J.I. Bruce, eds. Malacological Review, Supplement, 2:113–122, (1980b). Note: This was the first report of multilocus genetic variation in human schistosomes. Allozymes showed that Mekong schistosomes were a distinct species related to, but distinct from S. japonicum. [No. of citations: 23]

61. Fletcher, M., P.T. LoVerde and D.S. Woodruff. Genetic variation in Schistosoma mansoni; Enzyme polymorphisms in populations from Africa, southwest Asia, South America and the Caribbean. American Journal of Tropical Medicine and Hygiene 30(2):406–421. (1981d). Note: A multi-locus allozyme survey revealed surprisingly little variation across Africa and supported the hypothesis that the parasites were introduced into the Americas in the last 400 years. Research conceived, funded and directed by Woodruff. Graduate student, Fletcher, ran the gels and postdoc LoVerde assisted with parasite acquisition and maintenance. [No. of citations: 108]

69. Woodruff, D.S., M. Mulvey and M.W. Yipp. The continued introduction of intermediate host snails of Schistosoma mansoni into Hong Kong. Bulletin of the World Health Organization 63(3):621–622. (1985a). Note: Described the public health significance of results reported in the following paper. Woodruff and May Yipp (Hong Kong Polytechnical University) collected the snails, postdoc Peg Mulvey ran the gels. [No. of citations: 26]

70. Woodruff, D.S., M. Mulvey and M.W. Yipp. Population genetics of Biomphalaria straminea in Hong Kong: A neotropical schistosome-transmitting snail recently introduced into China. Journal of Heredity 76:355–360. (1985b). Note: Allozyme surveys showed the Neotropical snail had been introduced into China at least twice, probably in association with the Brazilian tropical aquarium plant trade. Woodruff and Yipp (Hong Kong Polytechnical University) collected the snails, postdoc Peg Mulvey ran the gels. [No. of ISI citations: 37]

71. Woodruff, D.S. Genetic control of schistosomiasis: A technique based on the manipulation of intermediate host snail populations. In: Comparative Pathobiology 8, Parasitic and Related Diseases: Basic Mechanisms, Manifestations and Control. Cheng, T., ed. Plenum, New York, pp. 41–68. (1985c). Note: I proposed that host–parasite compatibility can be manipulated genetically so as to reduce transmission of the parasite through the snail population and thus reduce the incidence of the disease in humans. A companion paper by Fletcher provided a mathematical model and computer simulations to show how this might occur. This proposal was based on Charles Richards’ (NIH) discovery that compatibility was under oligogenic control. At the time, all schisto-related research efforts were focused on the development of a vaccine and my suggestion was ignored. [No. of citations: 17]

72. Mulvey, M. and D.S. Woodruff. Genetics of Biomphalaria glabrata: Linkage relations of genes for pigmentation, allozymes and adult resistance to Schistosoma mansoni. Biochemical Genetics 23(11–12):877-889. (1985d). Note: Reports on our search for genetic markers in the intermediate host snail associated with resistance to the parasite. Full collaboration between Woodruff and his postdoc, Mulvey. [No. of citations: 12]

75. Woodruff, D.S., M.P. Carpenter, E.S. Upatham and V. Viyanant. Genetic studies of medically important snails in southeast Asia. Isozyme Bulletin 19:32. (1986c). Note: Summary of the first study of genetic variation in Asian snails that transmit schistosomes and other human parasites. Results based on analyses performed by Woodruff and his laboratory assistant Pat Carpenter. Upatham and Viyanant (Mahidol University) provided tissue samples of molluscs. [No. of ISI citations: 3]

83. Woodruff, D.S., A.M. Merenlender, E.S. Upatham and V. Viyanant. Genetic variation and differentiation of three Schistosoma species from the Philippines, Laos and Peninsular Malaysia. American Journal of Tropical Medicine and Hygiene 36:345–354. (1987b). Note: Reports our discovery that important human blood parasites, previously lumped as S. japonicum, warrant recognition as separate species. Results based on analyses performed by Woodruff and his graduate student Adina Merenlender. Upatham and Viyanant (Mahidol University) provided tissue samples of parasites. [No. of citations: 28]

87. Merenlender, A.M., D.S. Woodruff, E.S. Upatham, H.C. Yuan and V. Viyanant. Large genetic distances between Chinese and Philippine Schistosoma japonicum. Journal of Parasitology 73(4):861–863. (1987f). Note: We found that Chinese and Philippine blood flukes are highly differentiated genetically and may require recognition as separate species. This discovery may account for differences in the disease in the two areas. Results based on analyses performed by Woodruff and his graduate student Adina Merenlender. Upatham (Mahidol University) and Yuan (Shanghai) provided parasites that were cultured by Upatham and Viyanant. [No. of citations: 29]

95. Mulvey, M., D.S. Woodruff and M.P. Carpenter. Linkage relationships of seven enzyme and two pigmentation loci in the snail Biomphalaria glabrata. Journal of Heredity 79:473–476. (1988e). Note: Second paper describing our attempts to map genes associated with parasite resistance in the intermediate host snail. Full collaboration between Woodruff and his postdoc Peg Mulvey. [No. of citations: 7]

96. Mulvey, M., M.C. Newman and D.S. Woodruff. Genetic differentiation among West Indian populations of the schistosome-transmitting snail Biomphalaria glabrata. Malacologia 29:309–317. (1988f). Note: Allozyme variation patterns indicated this medically important snail dispersed by island hopping from South America to Puerto Rica. Full collaboration between Woodruff and Mulvey, who was assisted in the field by her husband, Newman. [No. of citations: 36]

97. Woodruff, D.S., K.C. Staub, E.S. Upatham, V. Viyanant and H.C. Yuan. Genetic variation in Oncomelania hupensis: Schistosoma japonicum transmitting snails in China and the Philippines are distinct species. Malacologia 29(2):347–361. (1988g). Note: First suggestion that the Asian schistosome host snails are as speciose as the parasites. Results based on analyses performed by Woodruff and his graduate student Kate Staub. Upatham (Mahidol University) and Yuan (Shanghai) provided snails that were cultured by Upatham and Viyanant. [No. of citations: 32]

101. Woodruff, D.S. Coevolution of schistosomes and their intermediate host snails: Prospects for parasite control by genetic manipulation of host snail populations. [In Chinese]. Chinese Journal of Parasitology and Parasitic Diseases 7(2):133–134. (1989d). Note: Summary of the evidence for the genetic regulation of host-parasite compatibility and its relevance to disease control.

106. Staub, K.C., D.S. Woodruff, E.S. Upatham and V. Viyanant. Genetic variation in Neotricula aperta, the intermediate snail host of Schistosoma mekongi: allozyme differences reveal a group of sibling species. American Malacological Bulletin 7(2):93–103. (1990a). Note: Puzzling heterozygote deficiencies detected in a multilocus allozymic survey of samples of this host-snail collected in the Mun and Mekong rivers of Thailand led to the discovery of four sibling species. Results based on analyses performed by Woodruff and his graduate student, Staub. Upatham (Mahidol University) and Woodruff collected snails in the Mekong; others were provided by Upatham's and Viyanant.. [No. of citations: 18]

136. Woodruff, D.S. and E.S. Upatham. Snail-transmitted diseases of medical and veterinary importance in Thailand and the Mekong valley. Journal of Medical and Applied Malacology 4:1–12. (1993b). Note: A critical review of the snail-transmitted diseases of Southeast Asia. Woodruff conceived, researched and wrote this review; Upatham facilitated his research in Thailand. [No. of ISI citations: 3; reprinted and used in university courses in Thailand]

164. Woodruff, D.S. and M. Mulvey. Neotropical schistosomiasis: African affinities of the host snail Biomphalaria glabrata. Biological Journal of the Linnean Society 60:505–516. (1997c). Note: We reported the results of our allozyme-based phylogeographic survey that led us to a major discovery concerning the African origin of the South American host-snails. Our work disproved an 80-year old hypothesis (that African and American snails were Gondwanan vicars) and necessitated a re-thinking of the course of host-parasite evolution in both Africa and the Americas. Full collaboration between Woodruff and Mulvey. Our conclusions have been confirmed by two separate DNA-level investigations. [No. of citations: 27]

183. Woodruff, D.S., M.P. Carpenter, E.S. Upatham and V. Viyanant. Molecular phylogeography of Oncomelania lindoensis (Gastropoda: Pomatiopsidae) intermediate host for Schistosoma japonicum in Sulawesi. Journal of Molluscan Studies 65:21–31. (1999b). Note: We use allozymic variation to show that the Indonesian host-snails are derived from populations in the Philippines. We strengthen the genetic argument for splitting the Asian host-snail, Oncomelania hupensis, into several species and dispute Davis’ retention of his subspecific taxonomy. Results based on analyses performed by Woodruff and his laboratory assistant, Carpenter. Upatham and Viyanant (Mahidol University) collected and cultured the snails. [No. of citations: 10]

Dr. May Yipp collecting in the New Territories, Hong Kong, where she studied the ecology of a schistosome intermediate host snail, Biomphalaria straminea, introduced to China from Brazil. Using genetic variation we were able to show that this snail had been introduced at least twice, probably with the aquarium plant trade (Publications 69–70). She also contributed to our genetic study of freshwater clams in which we discovered that over 30 “species” of Asian Corbicula were all referable to a single species, C. fluminea (Publication 116, 135).

Dr. Suchart Upatham (now President of Burapha University) collecting schistosome intermediate host snail, Neotricula aperta, in the Mun River, Thailand. The construction of the Pak Mun hydropower dam flooded this stretch of the river and we raised concerns about the introduction of the disease schistosomiasis (Publications 134, 136). Suchart and his colleagues at Mahidol University facilitated my genetic studies of both the snails (Publications 97, 105, 183) and the trematodes, contributing to the discovery of Schistosoma mekongi and to the recognition that there may be several species of parasite incorrectly lumped under the name S. japonicum (Publications 56, 83, 87).