Some aspects of the epidemiology of neosporosis in sheep in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University, Palmerston North, New Zealand

Abstract

Recent reports from New Zealand indicate Neospora caninum may have a role in causing reproductive problems in sheep. However, knowledge about the epidemiology of neosporosis in sheep in New Zealand is limited. Thus, the research presented in this thesis was undertaken to further understand the mode of transmission, seroprevalence, diagnosis and treatment of N. caninum in sheep in New Zealand. The initial study investigated venereal and vertical transmission. The results suggested that although N. caninum DNA can be found in the semen of experimentally infected rams (n=16), the transmission of N. caninum to ewes (n=16) via natural mating is unlikely. In a two year study, ewes inoculated prior to mating (n=25 in Year 1; n=7 in Year 2) did not have congenitally infected lambs that year (n=0/44) but did in Year 2 (n=7/11). Ewes re-inoculated on Day 120 of gestation in Year 2 (n=9) had congenitally infected lambs (n=12/12) with more severe lesions than those not re-inoculated (n=2/11) indicating that the initial inoculation did not induce protection. Ewes inoculated for the first time on Day 120 of gestation (n=12) gave birth to lambs (n=10) that were all congenitally infected. Treatment of these congenitally infected newborn lambs (n=11) with toltrazuril (20mg/kg) on Day 1, 7, 14 and 21 was not effective as determined by serology, histopathology and qPCR. An avidity ELISA assay was able to differentiate between recently and chronically infected sheep. A longitudinal study with serology on 3 farms where N. caninum infected sheep were previously identified, found an overall seroprevalence of 0.8% (n=7/880) for N. caninum antibodies. The low seroprevalence observed across selected farms did not allow a meaningful interpretation to be made about the role of neosporosis on these farms. A consistent observation was the value of using multiple diagnostic tests to detect the presence of Neospora rather than relying exclusively on any one of them. Observation of typical lesions was generally more rewarding then the detection of Neospora DNA. Overall, further work is required to fully determine if N. caninum is causing reproductive problems in sheep in New Zealand.