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Surveillance Focus
Enzootic abortion of ewes

Abstract

This focus article has been prepared by Amanda Carson and Rudolf Reichel of the APHA Small Ruminant Species Expert Group and Maggie He, data analyst at the APHA Surveillance Intelligence Unit.

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Sheep abortion can have a significant impact on the profitability of a flock and some causes have zoonotic implications.

Fig A shows the four most common causes of abortion – enzootic abortion of ewes (EAE), Toxoplasma, Campylobacter and Listeria – as a percentage of diagnosable submissions in Great Britain 2009–2019. In 2019, abortion due to Campylobacter ranked second, ahead of Toxoplasma, for the first time. This was described in the March 2019 APHA disease surveillance report for England and Wales (VR, 6 April 2019, vol 184, pp 431-436).

Fig A: Incidents of sheep abortion as a percentage of diagnosable submissions in Great Britain 2009–2019 (VIDA)

Both EAE and toxoplasmosis are preventable by vaccination and pose a zoonotic risk. Private veterinary surgeons who had submitted samples to the APHA in England and Wales were contacted in July when EAE and toxoplasma had been diagnosed in their clients’ flocks. This was to help inform flock health planning and discussion of vaccination and biosecurity to prevent these diseases in future lambing seasons. In Scotland, vets were also offered a summary report on request.

Antibiotics can be used during an EAE abortion storm to reduce further abortions but the blanket use of antibiotics to manage EAE is contrary to the guidelines of the Responsible Use of Medicines in Agriculture Alliance (RUMA). The RUMA Targets Task Force report of 2017 commented that an unpublished survey undertaken in 2015 indicated that prophylactic use of antibiotics to manage EAE was routine for 10 per cent of sheep farmers.1

The APHA Surveillance Intelligence Unit investigated how many farms had repeat occurrences of EAE and carried out an analysis of the APHA’s Farmfile and Veterinary Investigation and Diagnosis Analysis (VIDA) databases.

Considering 2009–2019 we found that 2819 farms in Great Britain had had a diagnosis of EAE. These occurred in all sheep farming regions as shown in Fig B. It should be noted that Fig B does not give an indication of the regional prevalence of EAE; it reflects the number of submissions received, which can be influenced by sheep farm density and farmers’ willingness to investigate disease.

Fig B: Regions of Great Britain having diagnoses of enzootic abortion of ewes 2009–2019 (darker shading indicates higher submission counts)

We also looked at the number of farms where there had been repeat diagnoses on the same farm between 2009 and 2019 reached by submissions to APHA Veterinary Investigation Centres, SRUC Disease Surveillance Centres and partner postmortem examination providers (Table 1).

Table 1: Repeat diagnoses of enzootic abortion of ewes, 2009–2019

Reviewing the case histories for a selection of farms having EAE incidents in four or more years with the most recent diagnosis in the 2018/19 lambing season revealed that the absence of a vaccination programme was a consistent finding.

It was apparent that some farmers do not always appreciate the real cost-benefit of vaccination. However, in one case, the impact of EAE had pushed a farmer towards deciding to cease breeding sheep and move to keeping store lambs only. In another case, monitoring abortions identified a breakdown in the vaccination programme following a change in staff, and this allowed a timely review of management practices.

The impact of EAE had pushed a farmer towards deciding to cease breeding sheep and move to keeping store lambs only

The APHA sample submission form records the number of losses at the time of the submission, but does not reveal the total losses which may continue afterwards. Nevertheless, in one flock, losses were as high as 26 per cent (43 lambs dead in a flock of 160) and in another 68 lambs had been lost at the start of the investigation.

It has been calculated that the cost of a lamb lost in the neonatal period is £25,2 which does not include the potential earnings lost from lambs not going on to be raised and sold. In an abortion outbreak this can accumulate to a significant cost to the producer: the two cases described above represent a cost due to neonatal losses of at least £1075 and £1700, respectively. Compared to the average cost of a vaccine of £2.40 per ewe, which is given once, the potential savings are significant.

In an ideal situation all breeding females in the flock should be vaccinated once per year, including all replacement females at any time up to four weeks before mating. The manufacturer of one inactivated vaccine reports that a reduction in abortion and infection was evident in pregnant ewes challenged following vaccination and in pregnant ewes vaccinated after challenge.3 On the basis of this evidence, vaccination could be used in the control of Chlamydia abortus infection, even in the face of an outbreak. As with any vaccine the datasheet should be read carefully for possible contraindications.

Control of EAE

Enzootic abortion is usually introduced with purchased sheep. Replacement breeding females and orphan lambs are the most common sources. It is also possible that vectors such as wildlife can carry aborted material between farms.

Preventing EAE entering a flock depends on:

  • Maintaining a closed flock;

  • Avoiding introduction via orphaned lambs;

  • Sourcing replacement ewes from Premium Sheep and Goat Health Scheme EAE accredited free flocks (however, there are a limited number of accredited flocks);

  • Purchasing replacements from as few flocks as possible and not mixing newly purchased sheep with the main flock (alternatively vaccinate all replacement ewes);

  • Investigating abortions if more than 1 per cent of the flock is affected or multiple abortions occur over a short period.

Elimination of EAE from an infected flock is difficult. Infected ewes shed huge numbers of chlamydiae in placental material, vaginal discharges and aborted fetuses, and infection spreads to other ewes via inhalation or ingestion. Vaginal discharges can remain infective for up to 3 weeks and the organism can survive in the environment for 6 weeks.

Ewes that have previously aborted due to EAE become immune to further abortion but may continue to shed C abortus in subsequent lambings.

Continual monitoring of abortion is important to determine the cause as other agents may be involved. If EAE is identified as the cause, the threat to naive ewes in that flock is significant and can lead to abortions occurring in the following year as C abortus cannot be detected in newly infected ewes until the organism invades the placenta around the 90th day of pregnancy.

Control in infected flocks requires:

  • Isolation of aborted ewes and any surviving lambs as they are sources of infection;

  • Avoiding fostering female lambs onto ewes that have aborted;

  • Carefully disposing of aborted material and contaminated bedding followed by cleansing and disinfection.

Antibiotics should only be considered in a severe outbreak as an emergency measure but should not be used as routine prophylaxis, as this may contribute to antimicrobial resistance on the farm.

Vaccination campaign

The ‘Plan, Prevent, Protect’ campaign to promote vaccination to reduce abortion due to EAE and reduce reliance on antibiotics to control EAE was launched in June 2019 by the Sheep Antibiotic Guardian Group (a subgroup of the Sheep Health and Welfare Group).

Details are available at www.farmantibiotics.org/tool_links/target-enzootic-abortion-in-ewes-this-year-and-cut-your-losses/

This link provides access to a webinar, Powerpoint slides and infographics that can be used to promote best practice and support practitioners in flock health planning conversations.

Vaccination

Vaccination may not completely eliminate abortion in a flock in the following year but will reduce the number of abortions. Inactivated as well as live vaccines are available and vaccination of all replacement females coming into the flock is recommended.

The RUMA Targets Task Force has as one of its aims an increase in the use of vaccines for EAE by 5 per cent over the next 5 years. Recent RUMA reports indicated that sales of enzootic abortion vaccines in England and Wales increased significantly in 2017; however, this was not sustained through 2018. The latest report reveals that doses sufficient to vaccinate 1 million sheep were purchased in 2018 against an estimated 3.5 million replacement ewes in Great Britain.4

References

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