Toxoplasmosis is the main cause of abortions in sheep in Ireland annually. Toxoplasmosis infection in the sheep is caused by a parasite which is known as toxoplasma gondii.

Cause

The pathogen produces a great number of oocysts in the gut of a cat. A few million oocysts can be produced. Oocysts are the eggs of the parasite, released in order to reproduce. The cat can then pass this disease into the surrounding external environment in its faeces (dung). The defecation acts as a source of infection which can be transferred to new hosts such as sheep and other ruminants. Sheep feed can become contaminated by the cat faeces containing oocysts. These ocoysts can then be ingested by the sheep when they consume the feed. This is a disease which seems to be particularly severe among hoggets, as they are less likely to have been exposed to the parasite in the past. Immunity is effectively lifelong, so if a ewe has been affected, she will be immune for life and not likely to abort again.

Symptoms

Sheep usually show no signs or symptoms despite being infected with the disease because it is a dormant parasite. Some ewes may even give birth to healthy lambs. The main result in ewes following infection is abortion. Sometimes, the aborted lambs may be mummified. The disease can pass on to the foetus inside of pregnant ewes which can lead to stillbirths. During early pregnancy, ingestion of oocysts is known to cause reabsorption in ewes. Some ewes may be barren after infection. Most often the abortions tend to occur prior to onset of the lambing season, or in the early stages. More often than not, it will be among the hoggets, it can be quite worrying for farmers until things settle down.

Treatment

A normal abortion rate in a flock is 1% to 2% and alarm bells should ring above this level. Just like Enzootic abortion, placentas and foetuses should be brought to a regional veterinary laboratory for analysis.

Prevention and control

Ewes that abort from toxoplasmosis are considered immune in the following year and can be retained for breeding. There is also no risk of disease being spread from sheep to sheep, as the disease is spread by cats shedding oocysts on animal feed. Where toxoplasmosis has been confirmed or there is a history of it on the farm and naïve animals are being introduced, veterinary advice is to vaccinate these animals.

Toxovax is the only vaccine available on the Irish market. It is a live vaccine, meaning it should be used within a short duration of opening (recommended maximum of two hours) and stored carefully as per manufacturer’s guidelines. Timing of administration is important and the vaccine must be given in a single 2ml dose at least three weeks before the start of the breeding season. Animals can receive the vaccine once they reach three months of age.

Finally it is worth remembering this is a zoonosis. Pregnant or potentially pregnant women should be particularly careful handling ewes lambing. Immunocompromised people like those on chemotherapy likewise should also take precautions.