Enzootic Abortion of Ewes

Also known as: EAE — Ovine Enzootic Abortion — Chlamydophila abortus

Introduction

The gram negative intracellular bacterium Chlamydophila abortus, previously named Chlamydia psittaci is the infectious agent of EAE which is the most frequently diagnosed cause of abortion in sheep in the UK. It occurs most commonly in flocks that are intensively managed over the parturient period.

Australia, New Zealand, and some areas of the Scottish Highlands are free from the disease.

It is important due to its zoonotic potential and is a serious infection in pregnant women.

Epidemiology and transmission

Infection occurs by the oral route and is not apparently until the next pregnancy, when the organism invades the placenta at 90 days' gestation causing a suppurative necrotising placentitis at the time of rapid foetal growth.

In some flocks ewes may become infected and abort in the same pregnancy if infection occurs at least 6 weeks before they are due to lamb.

Ewes which abort or drop stillborn or weak lambs shed vast numbers of the organism in the diseased placentas and uterine discharges. The Chlamydiae organisms remain viable for several days and allow spread of infection. Survival is longer if temperatures drop or freeze.

In rams, infection can be seen as orchitis and the organism is excreted in the semen, though venereal spread is thought to be uncommon.

Experimentally, ewes have been observed to have persistent infections of the reproductive tract, which are reactivated at subsequent breeding seasons with transient excretion of the organism at each oestrus.

Lambs born from infected ewes and lambs fostered to ewes that have aborted are very likely to be infected as a result of close contact with the mothers, although no clinical infection may be apparent.

Neither colostrum or milk are direct vehicles for transmission of the organism, but infected vaginal excretions may contaminate the udder and milk.

If bred in their first year, up to one third of lambs born from infected ewes may develop a placental infection and abort in their first pregnancy.

Clean flocks usually become infected through the introduction of replacement breeding females which contaminate the flock during parturition. The following year can bring a serious outbreak with up to 30% of ewes aborting. Ewes have solid immunity post-abortion and thereafter, only younger females will pick up the infection and an annual incidence of 10-20% can be expected if no control measures are put in place.

Clinical signs

Abortion storms are a feature of EAE, especially in housed, intensively managed flocks. Aborting ewes may have vague malaise for up to 24 hours but are not noticeably sick.

Abortions typically occur in the last three weeks of pregnancy and weak, live lambs may be born in the same litter as dead lambs.

Most aborted lambs appear fresh and well-developed although the placenta presents a variable extent of necrotic change with a thickened intercotyledonary area.

For several days after abortion, ewes pass some discoloured uterine discharge but are clinically normal. Occasionally, the placenta is retained and metritis can develop, leading to loss of condition and potentially death.

Diagnosis

The diagnostic method of choice is examination of fresh placentas. The organism is demonstrated in smears from intercotyledonary areas of the placenta using a modified Ziehl-Neelsen stain.

Vaginal swabs or swabs from the coat of the aborted foetus can be used if placenta is not avaiable.

It is important to note that Yersinia and Coxiella burnetti also stain red with the Ziehl-Neelsen stain and may complicate the diagnosis.

Histopathology reveals an inflammatory infiltrate and intracytoplasmic inclusions in chorionic epithelial cells. Immuno-histo chemistry can be used to demonstrate antigen in the tissues if necessary.

Serological testing forms the basis of most flock health schemes available in the UK. It is most useful to find ewes which have never encountered the organism, and interpretation can be difficult in vaccinated animals.

PCR testing has also been developed for use on vaginal swabs and placental tissue from sheep.

Prevention and control

There are three vaccines available in the UK that provide protection against EAE. The two live-attenuated vaccines can only be used in non-pregnant females but the inactivated vaccine can be used during pregnancy.

The inactivated vaccine has been shown to reduce the number of abortions in a flock of already-infected sheep and it can be used during an outbreak to control the number of abortions.

Whole-flock treatment with long-acting oxytetracycline can be used 3-6 weeks before lambing to increase the number of viable lambs born to ewes known to be infected with EAE. Repeat injections might be necessary and ewes still excrete the organism if the lambs are born alive.

In the face of an EAE outbreak, it is best to mark and isolate the aborting ewes, dispose of the bedding and aborted products and disinfect the pen thoroughly.

Ewes that have aborted should not be used as foster ewes.

The purchase of accredited, disease-free stock and vaccination offer the best way of avoiding the disease.

If the status of bought-in replacements is not know, the option of vaccination should always be considered.

References

Mearns, R. (2007) Abortion in sheep, 1:Investigation and principal causes In Practice 29:40-46

Aitken, I. (2007) Diseases of sheep Wiley-Blackwell