Studies on Polyparasitism: Helminth Infections in Scottish Sheep and Laboratory Rodents

A study of the nature and abundance of parasitic helminth communities of sheep in Scotland (Part 1), and complementary experimental investigations of concurrent helminth infections in rats (Part 2) are described in this dissertation. In Part 1, a brief discussion on the life histories of common parasites of sheep in temperate climates is presented followed by a review of publications on various aspects of helminth infections of Scottish sheep. The effects of gastrointestinal helminth infections on sheep productivity are also considered, and the rough cost of such infections to the meat industry in Scotland is estimated. The parasite fauna of Scottish sheep was investigated in an 18 month survey based at the Glasgow abattoir. Most sheep (N =511) were of Lowland origin, and parasite status was assessed by identifying and counting helminth eggs and protozoan oocysts in the faeces. The diversity and abundance of the helminth community of sheep in Scotland was found to have changed little over the last 70 years. Strongylid nematodes were found to be most common, and Trichuris ovis (Nematoda), Strongyloides papillosus (Nematoda) and Capillaria longipes (Nematoda) were also found to occur frequently. Eggs of Fasciola hepatica (Digenea) and of Moniezia expansa (Cestoda) were detected in slightly less than 10% of samples. Oocysts of the protozoon, Eimeria sp. were discovered in about a fifth of all samples. Host sex was not found to have any effect on prevalence or egg output of any of the parasite species investigated but, with the exception of sheep infected with F. hepatica, young sheep were found to harbour the majority of the infections. The prevalence of most parasite species, and the egg counts of strongylid nematodes and of T. ovis, were shown to conform to a distinct seasonal pattern, being high in July and October and low in January and April. Increased exposure to the infective stages of the various helminth species in late spring and summer may have been a consequence of the elevated helminth egg output of the peri-parturient ewe in spring. Additionally, climatic conditions for the development and transmission of the infective stages of these parasite species may be optimal in spring and summer. Associations between high egg counts of the strongylids, T. ovis and S. papillosus. and between the presence of strongylids and high T. ovis and S. papillosus egg counts, were detected by stepwise multiple regression analysis and by logistic regression analysis, respectively. Seasonal effects were, again, thought to be partly responsible for these observations since temperature and rainfall were found to explain a considerable proportion of the variation in helminth egg counts. Despite this, when season, source and age variables were removed, a positive, though weak, relationship between strongylid and T. ovis egg count intensities was discovered in sheep from certain batches. It is suggested that behavioural, physiological and immunological processes may also determine this particular relationship. In Part 2, a review of the literature on experimental investigations of concurrent parasitic infections in mammalian hosts concentrated specifically on antagonistic and synergistic interactions between parasite species and the consequences of effects on the host. In the experimental studies described, the relationship between Hymenolepis diminuta (Cestoda) and F. hepatica in rats was examined. The possibility of interactions between the helminth species was investigated. Both mature and immature F. hepatica infections in rats were found to confer no resistance to oral challenge with H. diminuta cysticercoids. In fact, H. diminuta survival was shown to be slightly enhanced by prior patent infection with F. hepatica, which may have resulted from some degree of immunosuppression associated with the concurrent F. hepatica infection, or from the increase in bile glucose symptomatic of patent F. hepatica infection in rats. The extensive intestinal mast cell response of F. hepatica-infected rats around day 35 p.i., may have been responsible for the slightly poorer growth of H. diminuta when administered at this time. Rats with mature H. diminuta infections offered no resistance to oral challenge with F. hepatica metacercariae. Density-dependent effects on F. hepatica growth and on H. diminuta fecundity were demonstrated. Intraspecific competition for a limiting resource, or increased stimulation of the immune system, both as a consequence of the increased worm burden, may explain these observations. (Abstract shortened by ProQuest.)