Features
Control of Chlamydial Abortion in Sheep
by Dr David Buxton
Lambing is arguably the most important event in the flockmasters calendar when all the preparation of the previous year with its myriad of decisions and tasks comes to fruition. The site of an early newborn lambs taking its first coughing breath, flapping its head and trying to stand is always special. Equally nothing is worse then than seeing the first lambs arriving two weeks early and dead and the imagined prospect of many more deaths. Fortunately as the planned start-date of lambing arrives so do the live lambs and the mood may lighten. Such abortions and stillbirths suggest an infection, the two commonest in Britain being chlamydial abortion and toxoplasmosis with Campylobacter (Vibrio), Salmonella, Listeria and border disease virus being relatively less common but still important causes. When abortion occurs, isolate the ewe, collect diagnostic samples and establish the cause.
Chlamydial Abortion
Chlamydophila abortus (old name Chlamydia psittaci) is the bacterium responsible for chlamydial abortion (enzootic abortion of ewes/EAE) in sheep. However, a chlamydial infection can also produce full-term stillborn or weakly lambs and it is not uncommon to have one dead lamb and one or more live lambs produced by the same ewe. Also some infected ewes give birth to normal live lambs which they rear successfully but which may, in turn, be carrying the infection which can eventually cause the female lambs to abort in their first pregnancy.
The ground on which the aborted lambs are dropped becomes contaminated with Chlamydophila, which can then be picked up by “clean” sheep to cause further infections. If picked up in late gestation then infection can lie dormant until a subsequent pregnancy when it may cause abortion. However if infection is established earlier in pregnancy it may cause abortion just a few weeks later.
Work at Moredun has shown that regardless of the time that infection occurs in the ewe, Chlamydophila does not initiate disease until after 90 days gestation. Infection arrives at the placenta via the mother’s bloodstream and then spreads across the membranes. The resultant damage, swelling and inflammation gives rise to the characteristic reddish-yellow thickened appearance of the placental membranes seen at the time of abortion. In 20% of ewes which abort or produce weak infected lambs Chlamydophila proceeds to lie dormant until the next pregnancy, when it becomes activated and infects the placenta and its foetus.
Control of Chlamydial Abortion
The infection-status of the flock can only be established by carrying out a blood test on a proportion of the sheep (seek veterinary advice on how to set this up). If a flock is clear of infection then it is important to buy in replacements which also test negative and are from a safe source. This is best achieved by entering the flock into the Premium Health Scheme run by the Scottish Agricultural College’s Veterinary Services. In this case replacements may only be bought from “EAE accredited” flocks. If the flock already carries a degree of infection or is in an area where there is a lot of infection then vaccination may be a wise option
Vaccination
There are three vaccines commercially available to control chlamydial abortion; Enzovax®, (Intervet UK), CEVAC Chlamydophila® (CEVA Animal Health Ltd) and Mydiavac® (Novartis Animal Health). Your veterinary surgeon will be able to advise you as to which product would be most suitable for use on your farm. Current research at Moredun is aimed at attempting to better understand how sheep can be protected against Chlamydophila and to improve diagnosis of abortion and recognition of silent infections. Moredun scientists are also working to develop the next generation of chlamydial vaccine that will offer even better protection but also cost less.
Treatment
If active chlamydial infection is present in a flock of unvaccinated pregnant ewes then treatment with long-acting oxytetracycline (20 mg per kg body weight) will reduce the severity of infection. For best effect it should not be given before 100 to 110 days of pregnancy because infection does not establish until after 90 days and so earlier treatment will be wasted. A second injection two to three weeks later will further reduce losses. However, some ewes will still abort and many may excrete infection at lambing time. Treatment late in pregnancy will have less of an effect.
The risk of human infection
Chlamydophila abortus is a particular risk to the pregnant woman and her developing foetus, as infection will result in foetal death and also cause severe illness in the mother. Pregnant women should avoid all involvement with lambing ewes and should not handle contaminated clothing from those working with lambing ewes or new-born lambs. People with a problem with their defence (immune) system (i.e. immunosuppressed or immunocompromised) must also avoid contact with potential sources of infection at lambing time.
Footnote: this article first appeared in the Sheep Farmer magazine and is reproduced with their permission