A mathematical study of environmental effects and genetic drug resistance on the life cycle of the nematode Teladorsagia circumcincta

The parasitic nematode, Teladorsagia (Ostertagia) circumcincta is the primary cause of Parasitic Gastro-Enteritis (PGE) in lambs in Britain. Control of this parasite has largely depended on the use of broad spectrum anthelmintic drugs since their inception three decades ago. Widespread and unconstrained use of anthelmintics has resulted in selection for resistant strains of nematode, particularly within the T. circumcincta species. Control of PGE now involves optimizing parasite control whilst preserving the susceptibility of the parasites to the anti-parasitic drugs. Two aspects of the epidemiology of T. circumcincta are investigated in this thesis. First, the effect of temperature on the development and survival of the free-living stages is investigated. The conventional nematode development models are replaced by more sophisticated and biologically meaningful methods of describing temperature-dependent development rate phenomena in nematodes. The effect of geographical, temporal and developmental variation on the population dynamics of T. circumcincta are explored to determine possible sources of observed variability in infection levels in the field. Next, a suite of models generic to most direct life cycle parasites undergoing intensive drug therapy, is constructed and analysed. Provision is made within these models to explore the impact of important life history events such as refugia and immigration on the evolution of resistance. A novel technique in resistance control involving overwhelming a resistant strain of nematode with a susceptible strain is modelled and suggestions made for the practical implementation of such a method.The parasitic nematode, Teladorsagia (Ostertagia) circumcincta is the primary cause of Parasitic Gastro-Enteritis (PGE) in lambs in Britain. Control of this parasite has largely depended on the use of broad spectrum anthelmintic drugs since their inception three decades ago. Widespread and unconstrained use of anthelmintics has resulted in selection for resistant strains of nematode, particularly within the T. circumcincta species. Control of PGE now involves optimizing parasite control whilst preserving the susceptibility of the parasites to the anti-parasitic drugs. Two aspects of the epidemiology of T. circumcincta are investigated in this thesis. First, the effect of temperature on the development and survival of the free-living stages is investigated. The conventional nematode development models are replaced by more sophisticated and biologically meaningful methods of describing temperature-dependent development rate phenomena in nematodes. The effect of geographical, temporal and developmental variation on the population dynamics of T. circumcincta are explored to determine possible sources of observed variability in infection levels in the field. Next, a suite of models generic to most direct life cycle parasites undergoing intensive drug therapy, is constructed and analysed. Provision is made within these models to explore the impact of important life history events such as refugia and immigration on the evolution of resistance. A novel technique in resistance control involving overwhelming a resistant strain of nematode with a susceptible strain is modelled and suggestions made for the practical implementation of such a method