Most sheep flocks experience some abortions each year but some can experience an abortion storm with up to 25% of the flock aborting. The two most common causes of abortion are chlamydial (enzootic) and toxoplasma abortion. Some flocks may be at an increased risk this year due to the unavailability of vaccines for these abortions last year.
Chlamydial abortion is caused by a bacterium, Chlamydophila abortus, that spreads to the womb and afterbirth of an unprotected sheep and kills the developing lambs. Aborted lambs, afterbirth and discharges from aborted ewes are heavily contaminated and can infect other pregnant sheep. Non-pregnant female sheep, including newborn lambs, can pick up the infection from an aborting ewe and the organism will remain latent until the animal is 90 days pregnant and then become active, causing the animal to abort. The disease can be brought into a clean flock through bringing in infected sheep that picked up the infection at lambing time.
Where an outbreak occurs, it can be controlled by treating the flock with long-acting oxytetracycline at 95-105 days of pregnancy. Toxoplasmosis is caused by sheep becoming infected from ingesting oocysts shed by cats on pasture or in contaminated feed, bedding or water. Infection of sheep early in pregnancy may result in unnoticed abortions or barrenness whereas infection in later pregnancy may cause stillbirth, mummified foetus or birth of a weak lamb. Following infection, sheep develop immunity which will protect them against the disease in subsequent pregnancies.
There are no drug treatments that can control or cure sheep infected with toxoplasmosis. Deccox (Decoquinate) has been shown to give some control of the disease when fed as a feed additive from mid pregnancy. However, it is only effective if fed to ewes before they encounter the disease. Deccox can only be mixed in rations under veterinary prescription.
Control measures for aborted sheep
Isolate aborted ewes immediately as they can spread disease through vaginal discharges for some time after aborting. Chlamydia (enzootic) may be present in discharges for up to two weeks after abortion and survive in bedding for up to six weeks. Mark or identify the ewes for future blood sampling or culling. Collect aborted lambs together with all afterbirth material and submit through your local vet to the regional veterinary laboratory for diagnosis. A number of fresh samples may need to be sent for a definitive diagnosis. The lambing area should be disinfected and freshly bedded.
Pregnant women are a major risk group for both diseases and should avoid all contact with ewes at lambing time.