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Toxoplasma gondii is an economically important parasite that can cause spontaneous abortion in sheep and humans. The importance of the different transmission routes of the parasite is unclear. However, three routes are recognised: (1) shedding of infective oocysts by the definitive host, the cat (2) consumption of undercooked, infected meat and (3) congenital transmission (mother to offspring). Previous work, based on a single farm (Valley Farm, Worcestershire) revealed the controversial finding that congenital transmission may be highly important. In this thesis, I have set out to investigate this in detail and establish more conclusive evidence. A study carried out on a different sheep farm (Heighley Castle Farm, Cheshire) demonstrated congenital transmission was occurring in 44% of pregnancies, thus confirming the importance of the congenital route. A prediction arising from this finding is that different families on a single farm would have different prevalences of T. gondii infection. This hypothesis was tested by investigating infection rates between families of Charollais sheep. A significant difference was found between families (n=27,) both in terms of frequency of abortion (P<0.01) and T. gondii infection (P<0.01). In addition, a significant correlation was found between abortion and infection (correlation co-efficient = 0.89, d.f= 27, (P<0.01)). Furthermore, a study of a large Charollais sheep farm (Foulrice Farm, Yorkshire) showed the same pattern - significantly different frequencies of abortion among different families. This study has shown that one of the principle dogmas of T. gondii research may need revising, namely that sequential abortion and infection does not occur due to acquired immunity. A study of sheep families at Valley Farm demonstrated numerous occurrences of sequential abortion and infection. Taken together, the data presented in this thesis suggests that there are some deficiencies in our understanding of T. gondii epidemiology, and that current sheep husbandry practices may need to be amended.