Investigations into the control of neosporosis in cattle : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Clinical Science, Massey University, Palmerston North, New Zealand

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The research presented in this thesis was undertaken to further understanding of the control of neosporosis in cattle. A prospective cohort study of primiparous heifers on a farm with a history of Neospora-associated abortion found a 0.65 risk of abortion among seropositive heifers, suggesting that identification and culling of seropositive heifer replacements may be cost-effective. A clinical trial of a registered Neospora caninum vaccine utilising 2,246 cattle from five farms with endemic N. caninum infection was assessed for efficacy in preventing abortion and vertical transmission. Overall vaccine efficacy was 0.25 (p=0.12) and vaccination increased the risk of vertical transmission. Histopathological and serological results from 148 cases of abortion from this trial were compiled to establish aetiological diagnoses. Nine of 34 cases where the fetus was examined had histopathological evidence of N. caninum infection. Histopathology revealed dual infectious aetiologies in 2 cases and serology suggested that, in another 17 cases, there had been recent exposure to a second infectious agent capable of causing abortion in conjunction with N. caninum lesions in the fetus or fetal bacteraemia. As a prelude to cattle challenge trials, a challenge study conducted on pregnant sheep revealed a strong dose-response for abortion and that indirect fluorescent antibody test results did not correlate well with infection status or pregnancy outcome. A novel challenge method of applying tachyzoites to an abraded oral mucosa was undertaken in pregnant heifers to establish whether oral lesions could facilitate the direct horizontal transmission of N. caninum between cattle. One of eight heifers seroconverted, her calf and one other were seropositive when sampled within 12 hours of birth, and three other heifercalf pairs had at least one positive polymerase chain reaction result at parturition. This method of transmission between cattle may be responsible for only a small proportion of infections but is a major new finding in the epidemiology of N. caninum infection and warrants further investigation. Finally, inoculation with mouse-passaged N. caninum tachyzoites prior to mating did not prevent abortion when heifers were challenged again on Day 70 of gestation, suggesting that live inoculation may not be a suitable control option.
Parasites, Cattle diseases, Neospora caninum, Neosporosis, Abortion in cows