A month or so ago, I spoke about ovine brucellosis at the Australian Poll Dorset Association conference at Orange.
Like most rural veterinarians who work in the sheep industry, I have had quite a bit of experience eradicating ovine brucellosis (OB) from sheep flocks.
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But how important are ewes as reservoirs for Brucella ovis and does this have implications for ovine brucellosis eradication?
We know that ewes mated to B. ovis infected rams can develop a short-term reproductive tract infection, and that ewes served by infected rams can transmit the infection to uninfected rams, not only during the current oestrus, but also at the next oestrus and possibly later.
We also know that in rare cases B. ovis can infect the placenta causing foetal undernutrition, death and abortion of the foetus or the birth of a stillborn or underweight lamb.
While Brucella ovis abortions are rare, they pose a risk of infecting rams either from inhaling bacteria from the abortus or from serving ewes post abortion. B. ovis has also been recovered from the mammary gland of lactating ewes on several occasions.
A small proportion of blood test positive ewes can carry B. ovis in the udder for long periods of time. Ram lambs could become infected and after sexual maturity, pass this infection on to other rams in the usual manner.
While this all sounds disconcerting, is it important in practice? Kym Abbott, author of new text 'The Practice of Sheep Veterinary Medicine', sums up the opinion of most of my colleagues.
"Experimental infection of ewes at mating and field evidence indicate that infection can persist in the ewe, leading to returns to service, abortion, birth of weak lambs and perinatal mortality. The incidence of infection in ewe flocks is, however, low and the role of persistently infected ewes in the maintenance of infection in the ram flock is insignificant."
Therefore, in flocks that follow usual management practices (including the recommendations below), B. ovis carryover in ewes is most unlikely to infect previously uninfected rams.
As the infection is usually cleared after the next cycle in ewes joined to infected rams but fail to conceive, leave a gap of at least a month before rejoining these ewes to new rams; Keep rams away from infected lambing ewes for four months; Maintain integrity of boundary fences to stop straying ewes or rams; Buy replacement rams only from studs that belong to the NSW Ovine Brucellosis Accreditation Scheme.