17 Feb 2021
Kate Hovers runs through details of infection, diagnosis, treatment and prevention for a signifi-cant cause of abortion in UK sheep.
Kate Hovers
Job Title
Image © dybcio / Adobe Stock
Enzootic abortion of ewes (EAE) is the most commonly diagnosed infectious cause of abortion in UK sheep, as shown by Veterinary Investgation Diagnosis Analysis data1. This is despite the availability of effective vaccines.
EAE is a significant cause of abortion worldwide, and is classed as a zoonosis.
The economic impact to the UK industry was calculated at £6 million to £20 million in 20032,3. The current cost per abortion is estimated at £85; the costs to consider are shown in Panel 1.
Vets in farm animal practice are in a unique position to help reduce these losses.
EAE is caused by the bacteria Chlamydophila abortus, which is spread in aborted material and postabortion vaginal discharges from the ewe. The organism survives a few days in warm conditions and longer in cold conditions4.
Ewes are infected by the oronasal route and we only have to watch a few ewes checking out where another ewe has lambed to understand how easily this spreads. Lambs fostered on to an aborted ewe can be infected from vaginal discharges on the udder and wool.
Some work has shown that venereal transmission is possible5,6, but this is unusual and not an important part of the epidemiology.
Ewes usually become infected during one lambing season, have a normal lambing, but are latently infected and abort the subsequent pregnancy. The organisms become active after day 90, causing a placentitis and near-term abortions, stillbirths or birth of live, but sickly, lambs. Ewes are rarely unwell. Aborted lambs appear grossly normal for the stage of gestation, but a placentitis will be grossly visible.
Ewes do not abort a second time due to EAE (they may abort from other infectious causes), but they can excrete infection in smaller amounts from vaginal fluids at subsequent “normal” lambing. Aborting ewes in the flock is the main cause of infection spread. Other sources include lambing fluids from ewes that aborted the previous year, infected placenta and/or fetuses moved by wildlife (such as foxes or crows) and aborting ewes from other flocks on common grazing.
Initially, a few ewes abort. Each abortion infects several other ewes and in subsequent years a “storm” of 20% to 30% of the flock may be affected.
C abortus is a zoonosis that causes flu-like symptoms, but it can also cause severe symptoms with abortion in women7. Pregnant women should not have contact with sheep flocks during lambing, or with dirty overalls and other equipment that may be contaminated. This should be emphasised to farm clients.
A placentitis with grossly unaffected fetus is suggestive of EAE, but not conclusive and a laboratory diagnosis is needed. Ideally, freshly aborted fetuses and placenta are submitted in a clean bag. If this is not feasible, the samples described in Table 1 should be taken and submitted for a full abortion investigation.
Approximately 50% of ovine abortion submissions do not reach a diagnosis; repeated submissions from the same outbreak make a diagnosis likely. It is important to submit fresh samples that are not grossly contaminated. Submissions without placenta make a diagnosis of EAE unlikely.
If a diagnosis is not reached during lambing, blood samples from affected ewes can be submitted to a range of laboratories for an ELISA test. Pharmaceutical companies producing vaccines have schemes to subsidise testing.
Unfortunately, a reliable test for latent infection is not currently available.
It should be assumed that any abortion is infectious and standard control measures of cleaning and disinfection, isolation and investigation should be followed (Panel 2).
All aborted material and contaminated bedding should be removed and the area disinfected. Gloves should be worn for this, and hands well washed afterwards.
Affected ewes should be identified and moved to a pen, building or paddock separate from the unaffected animals for at least three weeks. Ewe lambs that are to be retained for breeding should not be fostered on to ewes that have aborted.
Treatment of in-lamb ewes with long-acting oxytetracycline may reduce the incidence in the face of an outbreak, but this is only effective when the organism is active after approximately day 90, so in prolonged lambing periods it may need repeating. While this will reduce abortions8, it does not prevent excretion and infection spread.
The biggest risk is the purchase of infected replacement ewes.
The premium sheep and goat health scheme run by Scotland’s Rural College has an EAE accreditation element9. However, most replacements are bought through markets and from non-accredited sources.
Purchased ewes may already be vaccinated against EAE (and Toxoplasma, the second major cause of abortion in UK sheep), and it has become more common for vendors to advertise the vaccine status of breeding stock. This is not a guarantee against disease, as the ewes could be latently infected before vaccination.
Ideally, bought-in ewes should be managed separately from the flock until after lambing. A more realistic compromise is to identify and separate the ewes from the main flock at scanning or housing (or moving them to lambing fields in outdoor flocks) so they can lamb separately.
Biosecurity to prevent mixing with other flocks is always important, plus a timely reminder that EAE can be included on the long list of diseases that can be introduced.
As Chlamydophila can survive for at least a few days it could be transmitted by fomites. Visitors should not be allowed access to the flock, particularly during farm-to-farm sales of “pet” lambs. Facilities for boot cleaning and disinfection should always be provided, and any farm workers that may have their own flocks or work elsewhere should also wear clean, separate overalls for different flocks.
While injection of antimicrobials may be used in the face of an outbreak, and judiciously in the first year of vaccination, it is not an ongoing control measure for several reasons:
The Sheep Veterinary Society’s antibiotics policy asserts that whole-flock antibiotic treatments for enzootic abortion is one of three areas where “veterinary surgeons may be able to make the most impact to reduce the use of antibiotics on sheep farms”12.
Vaccination should play an important part in prevention, except in those flocks maintaining accreditation free from EAE.
Currently two live vaccines are available (Cevac Chlamydia, Ceva Animal Health; and Enzovax, MSD Animal Health) and one dead vaccine (Inmeva, Hipra UK and Ireland). The live vaccines are administered between four months and four weeks before tupping, and they can also be given to ewe lambs from five months of age.
Sheep should not be treated with antibiotics at the same time. Both vaccines are licensed to be administered at the same time (different sites) as the live Toxoplasma vaccine.
The exact duration of immunity is unknown and it is suggested ewes may benefit from a further vaccine after four years. However, in the author’s experience this is rarely necessary.
In year one, all breeding females should be vaccinated four months to four weeks before tupping. In subsequent years, all bought-in female replacements should be vaccinated four months to four weeks before tupping. All home-bred ewe lambs should also receive a vaccination from five months of age.
Women of childbearing age, and particularly pregnant women, should not handle or administer the live vaccines.
These vaccines will reduce excretion of infectious C abortus bacteria in infected animals and reduce the number of abortions. It must be emphasised vaccines will not prevent all abortions in already infected animals. Managing expectations in the first year of vaccination is important for confidence and future compliance.
The inactivated vaccine also protects against the rare and reportable Salmonella abortus ovis, which is rarely seen in the UK (World Organisation for Animal Health reports one case since 2005). Two injections are required three weeks apart and an annual booster before tupping.
Best practice and farmer engagement
Removal of infected material, disinfection of the area, and identification and isolation of affected ewes – all while observing good hygiene – are the most important first steps.
It is useful to have a crib sheet for reception staff and vets available during the lambing period. This can also be sent by text or email to farm clients.
Investigation and diagnosis requires suitable sampling at a busy time, but it may offer an opportunity for better engagement with sheep farmers, if a visit to collect samples and see sheep is advised and agreed. This could be combined with sampling ewes for metabolic profiles, or neonatal lambs for colostral absorption, as well as assessing ewes’ body condition scores, and the sheds or lambing fields for hygiene and feed space allowances.
Vaccination is a cornerstone of EAE control. However, unfortunately many farms only vaccinate after an abortion outbreak.
Facilitating discussion at practice farmer meetings helps enforce messages. The cost of a single abortion can be estimated using the information in Panel 1.
The vaccines are all POM-V, so it should be straightforward to send vaccination reminders from practice records, and include those who have had a recent diagnosis of EAE. A list of sheep farmers who do not vaccinate is useful for future health planning conversations and for specific targeting of information.
The “For Flock’s Sake – Vaccinate!” campaign from Ceva Animal Health has eye-catching farmer information and a useful cost benefit flyer. The campaign includes a survey for sheep farmers on experiences and attitudes to EAE (available at https://bit.ly/3gaaJ6T).
Several other causes of infectious abortion are described in other texts13-15, and clients should understand this and be advised to investigate abortion outbreaks in vaccinated ewes.
