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
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.