Population effects of parasite survival of host death: experimental studies of the interaction of Ichthyophthirius multifiliis and its fish host

Abstract

Current models of the regulation of host populations by parasite infection assume that death of a host results in death of all the parasites contained within or on the host. This assumption acts as a density-dependent constraint on parasite population growth and contributes to the stability of the interaction between host and parasite populations. The protozoan Ichthyophthirius multifiliis, a parasite of freshwater fish, is one of a number of pathogens in which the reproductive capacity of an individual is not entirely destroyed by the death of its host. The effect of host death on the reproductive potential of I. multifiliis was investigated with a series of short-term experiments examining (i) the relationship between parasite burden and host mortality, (ii) parasite survival on the host and (iii) the relationship between parasite age and the number of infective stages released/parasite. Results of a series of experimental epidemics in which host numbers were maintained constant showed large oscillations in parasite numbers. Deterministic models ignoring the age structure of the parasite population were unable to reproduce the behaviour observed in the experimental epidemics. Using the results of the short-term experiments, a difference equation model was developed. This model was able to qualitatively reproduce the observed epidemic behaviour.