The parasite Toxoplasma gondii causes the condition toxoplasmosis. T. gondii infects many mammals, including sheep, but its definitive host is the cat. Within the intestine of a cat, the parasite produces thousands of oocysts, which pass out of the cat in its feces. Ingestion of these oocysts in contaminated water or feed causes infection in sheep.
Infected sheep exhibit no signs of infection. It is, however, a major cause of fetal loss and stillbirths in sheep. Ewes catching a primary infection during pregnancy pass the infection onto the fetus this causes abortion and stillbirths of the lambs.
Developing a toxoplasma infection in early pregnancy causes abortion and resorption of the fetus. The infection in such sheep is only recognized when they prove barren at lambing time. Infection late in pregnancy causes stillbirths and the birth of weak lambs not all infections late in pregnancy will cause this and some healthy lambs are born to infected ewes. A primary Toxoplasma infection mid way through the ewe’s pregnancy leads to fetal death followed by mummification. Eventually the ewe delivers a brown colored mummified fetus and placenta. Sometimes fetal retardation occurs owing to placental dysfunction in addition to fetal infection.
Ewes losing their lambs owing to a toxoplasma infection will not suffer this in second or subsequent pregnancies, as they are immune to the infection.
The drugs Decoquinate (Deccox™) or Monesin sodium (Rumensin™) given with supplementary feeding aids in controlling the infection. In practice, ewes are given the drugs in the last eight weeks of pregnancy as the cost of the supplementary feed required makes their use throughout pregnancy prohibitive.
In farms where no previous cases of toxoplasmosis have been seen, the first indication of a problem is often a series of late abortions. In such cases oral sulphonamide antibiotics given to the rest of the flock can decrease the number of stillborn lambs.
In the UK and the rest of Europe a vaccine, Toxo-Vax, is licensed for the use in sheep. Toxo-Vax™ is a live vaccine containing an incomplete strain of T. gondii called S48. As of August 2009, this vaccine has not been granted licensed for use in the USA.
The prevention of ingestion of the oocysts prevents toxoplasmosis. Kittens are the most common source of such oocysts so maintaining a healthy, neutered, adult population of cats on a farm decreases the incidence of toxoplasmosis.
Reference sources:
Vaccination of sheep with a live incomplete strain (S48) of Toxoplasma gondii and their immunity to challenge when pregnant. D Buxton, K Thomson, S Maley, S Wright and HJ Bos. The Veterinary Record, Vol 129, Issue 5, 89-93
Decoquinate and the control of experimental ovine toxoplasmosis, D. Buxton, S. Wright, S. W. Maley, K. M. Thomson, J. Brebner and K. Millard. The Veterinary Record, Vol 138, Issue 18, 434-436
Livestock Management – Toxoplasmosis in Sheep. Robert E. Pitts University of Virginia Extension Service.
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