During 1978-79 there was an outbreak of abortion in a large sheep flock during which approximately 10 per cent of the breeding ewes aborted. Both Toxoplasma gondii and Chlamydia ovis (the agent of enzootic abortion of ewes) were considered to be involved. In the year following this outbreak (1979-80), 156 ewes (11.4 per cent) aborted and the majority of cases were diagnosed as enzootic abortion: only one case showed gross pathology typical of toxoplasmosis. Serology carried out on sera collected from ewes at the time of abortion and at two post abortion samplings demonstrated that large numbers of animals had high titres against enzootic abortion of ewes while the prevalence of sheep with titres against toxoplasmosis was relatively low. Following the introduction of control measures to reduce the spread of enzootic abortion of ewes, the abortion rate in 1980-81 fell to 2.2 per cent. A small-scale trial was carried out to investigate the prophylactic effect of long acting oxytetracycline against enzootic abortion of ewes when given to pregnant sheep three weeks before lambing. Results indicated that treatment reduced the number of abortions in comparison with untreated controls.
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