Laboratory infection of zebrafish (Danio rerio) and channel catfish (Ictalurus punctatus) with the protozoan parasite Ichthyophthirius multifiliis: A model for parasite persistence

The protozoan parasite Ichthyophthirius multifiliis (Ich) is believed capable of infecting nearly all species of freshwater fish. Ich typically causes high mortality in fish populations leading to catastrophic host mortality and parasite extinction. It seems clear that the parasite is not maintained on any of the host species in which we normally encounter it. The purpose of this thesis was to examine and describe the quantitative and qualitative properties of the kind of host-pathogen relationship that might allow Ich to become established as an endemic infection in a host species population. The model host was the laboratory-reared TLF strain of zebrafish (Danio rerio). Zebrafish, individually and in cohorts, were infected with Ich and the burden of parasitism, distribution of parasites, and progress of infection over time were examined. As a control and comparison species, fingerling channel catfish (Ictalurus punctatus) were concurrently infected. A mathematical model of Ich infection dynamics was developed to account for the observed outcomes of infection. TLF zebrafish were found to suffer much less severe infection, as measured by total number of parasites per fish, than catfish exposed to similar numbers of infective stage parasites. Zebrafish experienced much lighter gill infection than catfish, and parasites collected from zebrafish demonstrated significantly lower reproductive potential (measured as number of new infective stages produced) than parasites collected from catfish. Zebrafish cohorts infected with Ich experienced lower cumulative host mortality than catfish cohorts infected with Ich. From the point of view of population dynamics, the differences between the two systems turn out to be quantitative, rather than qualitative, in the sense that a single mathematical model can be used to describe both host-pathogen systems. The systems differ only in the numerical values assigned to the model parameters. While the TLF zebrafish might not be the natural reservoir host of Ich, it does represent a laboratory model of one suite of mechanisms for the persistence of this parasite in nature.