There are many potential causes of abortion in late pregnancy. Incidents caused by physical factors such as a ewe getting hurt or injured are generally not of major concern. The same could be said with a case linked to twin lamb disease, for example, with the bigger concern here relating to why the ewe contracted twin lamb disease and whether nutrition and flock management are adequate.
Cases caused by infectious agents, on the other hand, are a different matter, as it can be the first indication of a potential outbreak. Therefore, every case of abortion should be handled with care and assessed on the potential cause. Alarm bells should sound once the incidence rate in a flock hits 3%, or three ewes in a 100-ewe flock.
The two most common causes of abortion remain toxoplasmosis and chlamydial, or enzootic abortion. The warning signs for toxoplasmosis can often be a high barren rate in ewes. It is best characterised by the birth of weak or mummified lambs, while it is common in multiple litters for ewes to give birth to one mummified or weak lamb and one seemingly healthy one. Another tell-tale sign includes white spots on the placenta. There is unfortunately nothing that can be done in the face of an outbreak.
Enzootic abortion generally manifests itself in the final two weeks of pregnancy. The disease has similar symptoms to toxoplasmosis, with the birth of lambs that are mummified, dead for a period and decomposing, or lambs which have gone full-term but are born weak. In contrast to toxoplasmosis, where the placenta appears with white dots, it can appear thickened with enzootic abortion.
Enzootic abortion generally manifests itself in the final two weeks of pregnancy
Administering a long-acting antibiotic such as oxytetracycline to remaining ewes in the flock - in consultation with your vet - can help in reducing the number of subsequent abortions. It is critical that samples are submitted for post-mortem examination to get an accurate diagnosis. In exceptional circumstances, treatment can be administered on day 110 of gestation and every two weeks for two or three treatments. This should not be a routine treatment, as it can lead to resistance developing over time.
The main route of spreading for enzootic abortion is through lambing fluids, aborted materials and infected lambs that remain living. Ewes that pick up the infection during this lambing will generally abort in the subsequent lambing. Those that abort will develop a level of immunity and are unlikely to abort at subsequent lambings. They will, however, remain carriers and spread the disease at lambing. This highlights the importance of segregating and isolating sheep that abort and also removing any lambing materials from the lambing site.