Guide to preventing enzootic abortion in ewes

Enzootic abortion, also known as chlamydial abortion, is caused by infection with the bacteria Chlamydophila abortus.

The bacteria damages the placenta and reduces the transfer of nutrients to the growing lamb and the production of the hormones that regulate pregnancy. The bacteria can also cross the placenta and damage the lamb directly.  

James Hipperson, of Belmont Farm and Equine Vets, answers some of the key questions about EAE and how to manage and prevent it.

See also: A guide to toxoplasmosis in ewes

What are the symptoms of EAE?

The first sign that there might be EAE in a flock is usually late-term abortions, stillbirths or births of weak, ill-thrift lambs. Abortions usually occur in the last three weeks of gestation.

Aborting ewes usually display a red-brown discharge, which may only be seen in the fleece around the perineum.

Ewes typically present as asymptomatic despite being infected.

How contagious is the disease?

EAE is a contagious disease that is amplified when flocks are housed for lambing. The close proximity of animals at the time of lambing can result in naïve animals coming into contact with infected or aborted materials. 

In unvaccinated flocks, the infection can quickly spread through the flock and lead to “abortion storms” and large-scale losses.

For these reasons, it is essential for aborted materials (dead lambs and placental material or discharge) to be removed as soon as identified. Pens must be disinfected thoroughly between ewes, and bedding changed.

Aborting ewes should be isolated until all discharge has dried up and these ewes should be marked for identification purposes.  

Enzootic abortion is also a zoonotic disease, which means it can pass from animals to humans. The main at-risk group is pregnant women and immune-compromised people. Both of these groups should not come into contact with ewes during lambing time. 

How is it diagnosed? 

A history of abortion within the flock and an abortion rate greater than 2% in a given season would raise the suspicion of an infectious cause of abortion. 

A presumptive diagnosis can be based on visual inspection of the aborted lambs and placenta.

To confirm a diagnosis, fresh samples of the aborted placenta can be sent to a laboratory, where they are stained to confirm the presence of chlamydial elementary bodies. During an abortion investigation, a vet will also sample foetal stomach contents, foetal fluids and brain material, as there can be other causes of abortion besides EAE.

Should bought-in sheep be blood tested? If so, when?

The major issue with EAE is the latency of infection, because animals exposed to infectious materials at lambing may not show signs of infection until the subsequent lambing. There is no test to identify EAE during this latent period while the organism is dormant in their bodies. 

Ewes that have EAE and have aborted or lambed within the past three months will produce an immune response to the infection. Therefore, blood samples should be taken from aborted or barren ewes up to three months after lambing in situations where no foetal material was present or there was insufficient time to investigate.

Bought-in sheep can only be tested if they have lambed in the previous three months.

How much does the blood testing cost?

Costs will vary between different labs, but it is about £7-£9 for a Chlamydophila abortus Elisa test, depending on the number of samples submitted. There is also the cost of the vet visit, clinical time and consumables to consider in addition to the lab fees. 

Once infected, will ewes continue to abort in future years?

After a ewe is infected, it will not typically abort or show other signs of EAE until the following pregnancy and, once it has aborted, it is less likely to abort in subsequent years.

Aborting ewes should ideally be culled from the flock, as they can act as carriers of the disease and pass it on to other animals.

How should surviving ewe lambs from aborted sheep be managed?

It may be possible for viable lambs born from a ewe with EAE to carry EAE, abort in their first pregnancy and pass EAE on to their lambs or other naive animals in the flock.

Ideally, ewe lambs from infected ewes should be recorded, fattened and sold to slaughter. It would be unscrupulous to sell these animals for breeding purposes. 

If home-bred animals are kept as replacements for the flock, the cohort should be vaccinated to prevent the spread of EAE should a ewe lamb from an infected dam be retained.

Can antibiotics be used when an outbreak occurs?

Oxytetracyclines may be used as a last resort in abortion outbreaks to try to reduce overall losses from EAE. However, they will not prevent infection and abortions in subsequent pregnancies. With responsible use of medicines in mind, the use of antibiotics in an abortion outbreak is less than ideal, especially when the disease can be managed and prevented through vaccination. 

What quarantine procedure should be followed when buying in sheep?

To prevent the introduction of new diseases, it is best to maintain a closed flock. However, if you are sourcing replacement ewes, try to source from an accredited, or vaccinated flock, to minimise the risk of disease introduction. 

If replacement animals are coming from an accredited flock, these animals should have been blood tested. After a standard three-week quarantine period, where suitable consideration is given to other problems such as foot-rot, contagious ovine digital dermatitis, liver fluke, resistant worms and scab, they can be introduced to the flock. 

When introducing ewes of unknown health status to the flock, the gold standard is to manage them as a separate group from your original flock, avoiding any crossover of rams, and lambing each group separately. 

Lambing is the major risk period, so it is the most important time to instigate strict quarantine and biocontainment policies. 

When should animals be vaccinated?

Timing of vaccination depends on the product used. All vaccines are required to be given before breeding, so farmers should ensure they act promptly and don’t leave it until it’s too late.

Farmers should discuss vaccination options with their vets and weigh up the cost of lamb losses against the cost of vaccination. A vaccination policy is a cheap insurance policy compared with the potential losses. 

The vaccines are licensed for all breeding females. For complete vaccine coverage, all breeding females should be vaccinated.

Because of cost considerations some farmers will just implement the vaccination of incoming or replacement ewes. This isn’t such a robust vaccination protocol, but it will ensure the most naïve animals are protected and, with annual repetition, the flock should become fully covered after four to five years, depending on the replacement rate.