Mixed strain schistosome infections of snails and the evolution of parasite virulence

C. M.  DAVIES  a1 c1 , E.  FAIRBROTHER  a1 and J. P.  WEBSTER  a1
a1 Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3FY

Article author query
davies c   [PubMed][Google Scholar] 
fairbrother e   [PubMed][Google Scholar] 
webster j   [PubMed][Google Scholar] 


Mathematical models often propose that within-host competition between parasites can be a major factor in the evolution of increased parasite virulence. Kin selection predicts that as the coefficient of relatedness between infecting parasites decreases, the benefits of competition to individual genotypes increases. Thus where parasites can adjust their behaviour in response to current conditions, higher virulence is predicted in multiple genotype infections. There is limited experimental data, however, regarding the effects of mixed strain infections on host and parasite fitness. We investigated, for a snail–schistosome system, whether a conditional increase in replication rates occurred in mixed genotype infections and resulted in increased virulence. Four groups of Biomphalaria glabrata snails were exposed to 1 or 2 laboratory strains of Schistosoma mansoni. Mixed genotype infections were observed to be more virulent than single genotype infections, in terms of reductions in host reproductive success and survival. Parasite reproductive rate was also increased in mixed strain groups. Reduced host reproductive success was suggested to be directly due to the genetic heterogeneity of the parasitic infections resulting in increased host defence costs. Reduced host survival was consistent with an adaptive conditional parasite response.

(Received March 24 2001)
(Revised July 19 2001)
(Accepted July 21 2001)

Key Words: virulence; facultative response; mixed infection; Schistosoma mansoni; Biomphalaria glabrata.

c1 Corresponding author. Tel: +01865 281356. Fax: +01865 310447. E-mail: