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InFocus

Enzootic abortion in ewes

While sporadic abortion in ewes is usually attributed to handling or movement stresses, when the numbers seem to suddenly increase, there is a case to investigate infectious causes

Royal College of Veterinary Surgeons (RCVS) Knowledge logo

Imagine this clinical scenario: a 3,000-ewe lowland mule flock suffered from sporadic abortions last year but has suffered from an abortion storm this year, with 120 ewes aborting in the fortnight before lambing was due to start. Enzootic abortion of ewes (EAE) is suspected, based on the clinical presentation.

You want to confirm this assumed diagnosis, and utilise the “Plan, Prevent, Protect” principles to implement effective infection control measures to support the flock and reduce reliance on antimicrobials.

What is the plan to prevent further abortions? What can be done to protect the flock in the future?

Enzootic abortion of ewes (EAE) is the most common infectious cause of sheep abortion in the UK, accounting for 37 percent of all ovine abortions between 2012 and 2021 (APHA and SRUC, 2021), and is caused by Chlamydia abortus.

When enzootic abortion of ewes is first introduced to a naïve flock, the bacteria may circulate unnoticed, as there can often be a small number of abortions. This is often followed by an abortion storm the following year, which can affect up to 30 percent of the flock. There is a need to confirm the diagnosis of Chlamydia abortus with placental samples or serology of aborted or empty ewes. Confirmation of diagnosis assures that farmers are invested in the correct management strategies to protect the flock from further abortions (Silk, 2016).

With this scenario, there are two infection control periods to consider: the immediate actions that are required to support the flock, and the future actions to plan, prevent and protect the flock against enzootic abortion of ewes in the longer term.

Short term

All affected ewes should be isolated and all abortus and infected materials should be disposed of correctly

Immediately, there is a need to prevent further infection spread within the flock. All affected ewes should be isolated and all abortus and infected materials should be disposed of correctly. All abortion-infected bedding should be removed and destroyed. The housing should be thoroughly cleaned and disinfected. Aborted ewes should not be allowed to foster lambs.

Immediately and dependent on the timings, the administration of long-acting oxytetracycline may reduce the risk of further abortion in the flock. There has been a tendency historically to take a precautionary approach and inject all ewes with antimicrobials on an annual basis to prevent further abortion storms. This indiscriminate use of routine antimicrobials is totally inappropriate and there is a better way to control enzootic abortion of ewes in the flock.

Th[e] indiscriminate use of routine antimicrobials is totally inappropriate and there is a better way to control enzootic abortion of ewes in the flock

Long term

There are several measures that can be taken to protect naïve flocks as well as infected flocks in subsequent years. Vaccination is a key tool. The flock is vaccinated four weeks prior to tupping and usually this only needs to be administered once in the ewe’s lifetime. While vaccination will not necessarily prevent all abortions in ewes that were infected in the previous year, without doubt it is the most cost-effective route to controlling this disease (NADIS, 2021).

The cost of vaccinating a flock is offset by the protection offered, which for most flocks is a single cost per ewe. The price of vaccination at approximately £3 per ewe compares favourably to the cost of an aborted ewe which has been estimated at between £123 and £177 (Robertson et al., 2018). It is recommended that the whole flock is vaccinated in the first year and that all replacement ewes are vaccinated in subsequent years.

If pregnant ewes are purchased, it is essential that these are managed as a separate flock, not mixed until after the first lambing and then vaccinated four weeks prior to next tupping

A flock that is currently free from enzootic abortion but that buys in replacements is at high risk of introducing the disease as enzootic abortion of ewes is most commonly introduced to a flock through the purchase of infected replacement ewes. A replacement policy that seeks ewes from as few sources as possible or from EAE-accredited flocks can minimise the risks. If pregnant ewes are purchased, it is essential that these are managed as a separate flock, not mixed until after the first lambing and then vaccinated four weeks prior to next tupping.

In summary

Do not plan to use routine antimicrobial treatment as a control measure for abortion – this is only acceptable in the face of an outbreak or following a confirmed laboratory diagnosis in the immediately preceding year

Abortion in ewes can carry both a large economic and psychological burden for those involved in the care of the flock, but various steps can be implemented to ensure both the short-term and longer-term safety of the flock. Do not plan to use routine antimicrobial treatment as a control measure for abortion – this is only acceptable in the face of an outbreak or following a confirmed laboratory diagnosis in the immediately preceding year. While antimicrobial treatment may be an effective short-term disease control measure, the routine misuse of antimicrobials contributes to the development of antimicrobial resistance, which will ultimately be detrimental to flock health as it becomes harder to treat diseases.

To learn more about antimicrobial stewardship and infection control in farm animals, become a Farm Vet Champion. Farm Vet Champions is a major collaborative project, run by RCVS Knowledge and funded by the Veterinary Medicines Directorate, to unite and empower UK farm animal veterinary practitioners as they establish good antimicrobial stewardship in practices and on farms. Access the 20 hours of free on-demand CPD now on the RCVS Knowledge website.

For further information on infection control best practice, watch RCVS Knowledge’s five-part webinar series.

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