By Lead Vet Annie Kerr
Enzootic abortion continues to be the leading cause of sheep abortion in Cumbria this year. It can cause massive abortion storms with 20 to 50 per cent of ewes aborting at late term gestation, with very little indication beforehand.
This is caused by an organism called Chlamydophila abortus. The main source of infection is infected sheep shedding at lambing time in birth fluids and membranes. This can happen from newly infected sheep or recovered carrier sheep that aborted during a previous pregnancy. Newly infected sheep can look healthy, but the organism can remain dormant in them and cause abortions the following year.
Transmission
Enzootic abortion is mainly spread at lambing time but may also be spread in the mating season. Sheep can become infected by coming into contact with any of the following:
• Aborted lambs and placentas
• Discharges from infected animals contaminating the environment (during the spring the bacteria can survive on pasture for several days)
• Contaminated hands, boots and clothing
• Abortion material carried by wildlife and birds
• Infected ewe replacements added to the flock
Economic Impacts of enozootic abortion
• Financial loss to the industry is estimated to be £15 million per year
• A 5% abortion rate can reduce potential gross margin by 10%
• Each ewe that aborts can represent a reduction in gross margin of £95
If you encounter a sheep abortion episode it is important to establish a definitive diagnosis as there are a number of things that could cause abortion.
This involves testing aborted foetuses (stomach contents, placenta and serum sample from ewe) to get a definitive diagnosis. The cost of abortion is variably quoted as £95 per aborted ewe and vaccination costs roughly £3.50 per ewe so it is economically sensible to vaccinate in order to prevent a catastrophic outbreak.
Control of enzootic abortion requires careful consideration.
Retaining sheep in the flock after an abortion episode: Sheep that have aborted with enzootic abortion are immune, probably for life, although they may shed organisms in subsequent lambings.
The old approach to disease was to cull these sheep. However, thinking on this has changed as economic evaluation has rendered culling of affected cases hard to justify. It is therefore now recommended to retain as many sheep in the flock that were exposed to disease as possible after the outbreak as these sheep will maintain a relatively high level of flock immunity, meaning that only replacements will have to be vaccinated.
It must be understood that cases of latent infection will continue to arise for a few years after a vaccination programme has started. Therefore, vaccination will not result in an immediate prevention of abortion in every flock.
Biosecurity
Buying in latently infected sheep is the principal initial source of infection. Maintaining a closed flock policy or purchasing sheep from known enzootic free flocks will aid in control and prevention. Ensuring hygiene around lambing time with disposal of foetal membranes and isolation of affected cases will limit spread within the flock. Avoiding fostering replacement ewe lambs onto an aborted ewe will reduce the risk of that lamb becoming infected and aborting the following year.
Vaccination
The vaccine is administered ahead of tupping. Further information is available from your local vets, so please speak to them. There are sometimes issues with the supply of this vaccine so have a plan and speak to your vets as soon as you know how many replacements you want to vaccinate in order to secure the required number of doses.
It is vitally important to have a plan for prevention of abortion in your flock and the vaccine is really effective, so it is surprising how prevalent enzootic abortion still is.