Abortion - Sheep
Abortion is the expulsion before full term of a foetus which is incapable of independent life. Gestation length in ewes is usually 147 days but breed differences mean that it can vary from 140 to 150 days.
During any abortion episode, some fetuses may be infected yet still carried to term and some may be carried to term and fully viable. There are many causes of ovine abortion and sometimes the episodes can be multifactorial, with infectious agents, nutritional and management factors involved.
The national annual incidence of abortion in the UK is estimated at 2-3%, and additionally, some flocks may experience abortion storms involving large numbers of ewes, especially after the first exposure of a totally naive flock to a new infectious agent.
The stage during gestation at which abortion takes place does not usually guide to a specific cause, but can sometimes guide to the timing of infection and nutritional stress.
Causes of Abortion
These can be considered if infectious causes have been eliminated.
Others cause more generalised disease with abortion being the result of septicaemia: Salmonella typhimurium, Salmonella dublin, tick-borne fever (Erlichia phagocytophila).
Viral agents include Border Disease Virus.
Other protozoa include Neospora caninum.
By far the most important causes of abortion are Chlamydophila and Toxoplasma which account for over 40% and 35% respectively of all diagnosed incidents.
Investigation of an Outbreak
A positive diagnosis of an infectious cause only results with material from about 50% of incidents submitted to veterinary diagnostic laboratories, but it does not follow that the remaining 50% are not due to infections. Materials submitted may not have been suitable, the wrong test carried out, or fragile organisms may not have survived the journey to the laboratory.
Often, little can be done to halt an outbreak once it has begun, and abortion occurs weeks or months after an infection has been acquired.
A detailed history should be taken, including details about the flock such as management, husbandry, abortion history, recent events. Also details of the abortions seen so far such as timing of abortions, clinical signs in the ewes, which ewes are affected, appearance of the aborted foetus, isolation of the affected ewes.
Submission of Samples for Investigation
All aborted material should be retained by the farmer, and samples should be taken from the freshest case. Ideally, material should be submitted from every abortion, or at least from 10% of affected ewes.
Care should be taken as some of the agents present a zoonotic risk to the farmer and workers involved.
Most laboratories offer a batch charge for the examination of fetuses and placentas from two to four ewes. This offers the best chance of reaching a diagnosis at the least cost to the farmer.
The foetus and placenta should be submitted, or if whole samples cannot be sent, part of the placenta, foetal fluid, foetal stomach contents and fresh spleen can be collected.
Serological testing of ewes does not replace the submission of foetuses and placenta, but it can be used to rule out causes of abortion. However interpretation can be difficult, especially in vaccinated animals.
Gross findings may suggest a possible aetiology for the abortion, such as the thickened placenta seen in enzootic abortion of ewes or the white pinhead lesions on the cotyledons due to Toxoplasma infection. However several other diseases may mimic these signs and diagnosis should not be made on gross pathology alone.
Several tests can be used to determine any infectious agents involved. This includes Ziehl-Neelsen staining, gram staining, cultures of placenta and foetal stomach contents and foetal serology. Virus isolation, PCR and histopathology can also be performed if no diagnosis is reached in the preliminary investigation.
Steps to Take Following an Abortion
Aborting ewes should be marked for later identification and sampling, and isolated from other sheep.
The bedding and abortion products should be removed and the area disinfected thoroughly.
Antibiotic therapy can be initiated if the ewe is unwell or has retained foetal membranes.
Lambs should not be fostered onto ewes that have aborted due to the risk of enzootic abortion.
Good hygiene and biosecurity should be maintained to avoid zoonotic disease.
|Abortion - Sheep Learning Resources|
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|Sheep Medicine Q&A 11|
Kahn, C. (2005) Merck Veterinary Manual, Merck and Co
Hindson, J., Winter, A. (2002) Manual of sheep diseases, John Wiley and Sons
Mearns, R. (2007) Abortion in sheep: Investigation and principal causes, In Practice 29: 40-46
Mearns, R. (2007) Abortion in sheep: Other common and exotic causes, In Practice 29: 83-90
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