Reducing abortion rates in your flock 

Many Irish flocks will experience a few abortions each year, but how many abortions is too many? Targets for abortion and overall empty rate are set at <2 per cent and <5 per cent, respectively, although many flock owners accept rates higher than this. Approximately one third of lamb losses occur between tupping and scanning, another third is lost to abortion or stillbirth, and the remaining third die in the neonatal period. Abortion in ewes can happen due to non-infectious causes, such as injury or twin-lamb disease. But when abortion rates rise above 2 per cent an infectious agent should be suspected. 

Common causes

The two most common causes of ovine abortion in Ireland are Chlamydophila abortus – also known as enzootic abortion of ewes (EAE) and Toxoplasma gondii (toxoplasmosis). In the most recent All Island Disease Surveillance Report, 28 per cent of foetuses submitted to regional veterinary laboratories were positive for EAE, while 20 per cent were positive for toxoplasmosis. Other infectious causes of ewe abortion include listeriosis, campylobacteriosis, leptospirosis, salmonellosis, border disease virus, Schmallenberg virus and Escherichia coli.
Without diagnosis, ewe abortion outbreaks are impossible to address. It is advised to submit aborted foetuses plus accompanying placentas to your nearest veterinary laboratory. Samples should be submitted as soon as possible, as aborted materials begin to degrade rapidly or may be scavenged by wildlife, making them less suitable for diagnostic testing. Ewes can also be blood tested three weeks post abortion, although these results may be less reliable in some instances. Both EAE and toxoplasmosis are readily identifiable in fresh foetuses and placentas if present. 
It is worth noting that both EAE and toxoplasmosis are transmissible to humans (zoonotic). In a 1990 study carried out in Northern Ireland, 11.1per cent of farmers' blood tested positive for Chlamydia psittaci (now called Chlamydia abortus) antibodies and 73.5per cent tested positive for T. gondii antibodies. Exposure to these pathogens can pose harm to immunocompromised or pregnant people. Pregnant women should avoid contact with sheep for this reason, particularly around lambing time. EAE can also cause flu-like symptoms in children and adults. Care should always be taken when handling aborted materials and aborted ewes, and when caring for weak lambs. 

Enzootic abortion of ewes 

EAE caused by the bacterium C. abortus, is usually introduced to a flock by purchasing infected replacement ewes. Naïve ewes contract this bacterium during the previous year’s lambing season and carry it until the following lambing season, when they will usually abort. Infection occurs when they are exposed to the foetal fluids or aborted materials of infected ewes and they, in turn, newly infect other naïve ewes at the following lambing season, and so the cycle continues.
Abortions usually occur in the last three weeks of pregnancy, when ‘abortion storms’ can commonly occur. Aborted ewes can remain infectious to naïve ewes for approximately three weeks after aborting. Lambs born to infected ewes can be infected with C. abortus at an early age and, in the case of ewe lambs, go on to abort at first lambing. Following EAE abortion, ewes may subsequently have normal lambings but continue to harbour and shed the bacterium.
EAE control measures include quarantining aborted ewes, maintaining a closed flock, purchasing replacements from EAE-free flocks, prophylactic antibiotic treatment and vaccination. Aborted ewes should be quarantined for a minimum of four weeks and should be kept well away from replacements, in particular. The most recent advice is to keep aborted ewes in the flock once the quarantine period has passed, as these ewes will have acquired immunity to the bacterium and will heighten flock immunity overall. The antibiotic oxytetracycline can also be used to reduce abortions in the face of an outbreak.
Vaccinating with a product such as Enzovax provides protection against EAE and is an effective method of reducing EAE abortions in an infected flock. Vaccination of ewes already infected with C. abortus will also reduce the incidence of abortion in this cohort, though it is important to realise that some may abort due to latent infection that was present silently prior to vaccination. Usually only one Enzovax vaccination is required over the lifetime of the ewe. However, in some instances, ewes over four years of age may require a booster. Ewes should be vaccinated with Enzovax at least four weeks pre-lambing. 

Toxoplasmosis 

T. gondii is a protozoal parasite found in the ewe’s environment. It can infect all warm-blooded mammals, including humans. Cat faeces, lambing ewes, and undercooked meat have historically been sources of toxoplasmosis infection in humans. The cat is the definitive host of the parasite. Young cats often get infected when they go out hunting for the first time. Rodents and birds are reservoirs for the parasite, which infects the cat on ingestion of these prey. The young cat is infected for approximately a week before it becomes immune and no longer sheds the parasite in its faeces. Fifty grams of cat faeces can contain 5,000,000 T. gondii oocysts (eggs). While most studies in the past have shown that thousands of oocysts are required to produce abortion in ewes, a recent study found abortions in sheep fed only 10 oocysts in mid-pregnancy. Toxoplasma oocysts can survive in the environment for up to two years. 
Sheep are infected with T. gondii on ingestion of oocysts in feed or soil. Clinical signs of toxoplasmosis in ewes depend on when the ewe is initially infected. A non-pregnant ewe infected with T. gondii may show no signs and becomes immune to the parasite. A ewe infected in early pregnancy may appear barren at scanning or suffer early embryonic death. Later infections result in abortions, mummified foetuses and the birth of weak lambs that may die shortly after birth.
Control measures for toxoplasmosis differ from those of EAE, as toxoplasmosis is not a ewe-to-ewe disease. As mentioned previously, only young cats shed T. gondii oocysts and for a short period of time. It is recommended to keep neutered adult cats on farm as vital rodent control. The presence of neutered adult cats will deter young strays from entering farm grounds. Cats should not be able to access feed stores. Ewes confirmed to have aborted due to toxoplasmosis should be immune for life and can be kept in the flock. Control of toxoplasmosis can prove difficult due to the parasite being ubiquitous in the environment.
Vaccination against T. gondii is regarded as the only way to control the effects of the parasite in an infected flock. Toxovax vaccination reduces the effects of infection with the parasite, namely early embryonic death, barrenness and abortion. Generally, only one vaccination is required during the lifetime of the ewe. Natural exposure to the parasite boosts immunity. Rarely, where natural exposure may not have occurred, a second Toxovax vaccination is given after two lambing seasons. Toxovax should be administered at least three weeks pre-tupping.
Vaccination, in combination with disease control protocols, is your flock’s best bet against EAEcand toxoplasmosis. If these conditions have been diagnosed in your flock, speak to your vet about implementing a vaccination programme.