title: Field-derived Schistosoma mansoni and Biomphalaria pfeifferi in Kenya: a compatible association characterized by lack of strong local adaptation, and presence of some snails able to persistently produce cercariae for over a year.
abstract: BACKGROUND
  Schistosoma mansoni is widely distributed in sub-Saharan Africa with Biomphalaria pfeifferi being its most widespread and important snail intermediate host. Few studies have examined the compatibility of field-derived B. pfeifferi snails with S. mansoni miracidia derived from human hosts. We investigated compatibility (as defined by shedding of cercariae following exposure to miracidia) of two isolates of S. mansoni from school children from Asao (western Kenya) and Mwea (central Kenya) with B. pfeifferi collected directly from Asao stream or the Mwea rice fields.
  
  
  METHODS
  We exposed snails from both regions to four different doses of miracidia (1, 5, 10 and 25) from sympatric or allopatric S. mansoni, and maintained them in a shaded, screened out-of-doors rearing facility in Kisian, in western Kenya. Both snail survival and the number of snails that became infected were monitored weekly. This was done for 25 weeks post-exposure (PE). Those infected snails which survived beyond this period were monitored until they all died.
  
  
  RESULTS
  Although overall survival of Mwea snails maintained in western Kenya was generally low, both sympatric and allopatric combinations of parasites and snails exhibited high compatibility (approximately 50% at a dose of one miracidium per snail), with an increase in infection rates as the miracidial dose was increased (P < 0.002). Schistosomes were no more compatible with sympatric than allopatric snails, nor were snails less compatible with sympatric than allopatric schistosomes. Snail mortality increased significantly with dose of miracidia (P < 0.05). Approximately 3% of Asao snails exposed to a low dose of sympatric miracidia (1 or 5) continued to shed cercariae for as long as 58 weeks post exposure.
  
  
  CONCLUSIONS
  There were no significant local adaptation effects for either schistosomes or snails. Also, the existence of "super-survivor" snails is noteworthy for its implications for current control initiatives that mostly rely on mass drug administration (MDA). Long-term shedders could provide an ongoing source of cercariae to initiate human infections for many months, suggesting care is required in considering how human MDA treatments are timed. Future control programs should incorporate means to eliminate infected snails to complement chemotherapy interventions in controlling schistosomiasis.
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. mmutuku@kemri.org.
authors.forename: Martin W
authors.initials: MW
authors.lastname: Mutuku
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. slestynn@gmail.com.
authors.forename: Celestine K
authors.initials: CK
authors.lastname: Dweni
authors.affiliation: Centre for Public Health Research, KEMRI, P.O Box 20752-00200, Nairobi, Kenya. rewelmwangi@yahoo.com.
authors.forename: Moses
authors.initials: M
authors.lastname: Mwangi
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. jkinuthia@kemri.org.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Kinuthia
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. indungu@kemri.org.
authors.forename: Ibrahim N
authors.initials: IN
authors.lastname: Mwangi
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. gmaina@kemri.org.
authors.forename: Geoffrey M
authors.initials: GM
authors.lastname: Maina
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. elelo@kemri.org.
authors.forename: Lelo E
authors.initials: LE
authors.lastname: Agola
authors.affiliation: Center for Evolutionary and Theoretical Immunology, Department of Biology, and Parasitology Division, Museum of Southwestern Biology, University of New Mexico, Albuquerque, 87131, U.S.A. zhangsm@unm.edu.
authors.forename: Si-Ming
authors.initials: SM
authors.lastname: Zhang
authors.affiliation: Department of Zoology, Jomo Kenyatta University of Agriculture and Technology, P.O Box 62000-00200, Nairobi, Kenya. bellamaranga2010@yahoo.com.
authors.forename: Rosebella
authors.initials: R
authors.lastname: Maranga
authors.affiliation: Center for Evolutionary and Theoretical Immunology, Department of Biology, and Parasitology Division, Museum of Southwestern Biology, University of New Mexico, Albuquerque, 87131, U.S.A. esloker@unm.edu.
authors.forename: Eric S
authors.initials: ES
authors.lastname: Loker
authors.affiliation: Centre for Biotechnology Research and Development, Kenya Medical Research Institute (KEMRI), P.O Box 54840-00200, Nairobi, Kenya. gmkoji@kemri.org.
authors.forename: Gerald M
authors.initials: GM
authors.lastname: Mkoji
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001700:Biomphalaria / Q000469:parasitology*; D058487:Cercaria / Q000502:physiology*; D002648:Child; D005243:Feces / Q000469:parasitology; D006801:Humans; D007630:Kenya / Q000453:epidemiology; D016017:Odds Ratio; D012550:Schistosoma mansoni / Q000502:physiology*; D012555:Schistosomiasis mansoni / Q000453:epidemiology / Q000469:parasitology