Farmers should continue to vaccinate against Ovine Enzootic Abortion (OEA) despite the fact chlamydial vaccines may cause abortions in sheep.


This advice comes after the Moredun Research Institute tested placentas from aborted sheep and identified the vaccine strain of Chlamydophila abortus bacteria in large enough numbers to cause abortion.

Using a new testing method, 14 out of 35 positive C abortus samples were identified as being from flocks that had been vaccinated against the disease, with 5 out of 14 containing DNA only present in vaccines.

However, scientists stress vaccination is still the most effective way to control enzootic abortion in sheep flocks.

Despite these findings, the risk is thought to be small where as the benefits of vaccination are immense, says SAC‘s Brian Hosie.

Although our findings our important, our message to farmers about controlling enzootic abortion on their farms remains the same, agrees Moredun’s scientific director Julie Fitzpatrick.

“We encourage farmers who have a vaccination strategy for enzootic abortion to continue vaccinating as it is the most effective way to safeguard their sheep from disease.”

With uncontrolled Ovine Enzootic Abortion leading to abortion levels of 5-10% and “abortion storms” of up to 25%, vaccinating could bring abortion levels down to less than 2%.

“We strongly recommend farmers work with their vets and local VI centres and continue to investigate the cause of any abortions.”

And when receiving a positive result for C abortus in a vaccinated ewe, farmers should work with their vet and contact the vaccine provider who will investigate further.

The two vaccines available in the UK are Enzovax from Intervet Schering-Plough and CEVAC Chlamydia produced by CEVA Animal Health – both are based on the same C abortus 1B strain.

No vaccine is ever 100% and although uncommon, vaccinated sheep can still abort at the next lambing, says David Longbottom, head of Chlamydia research at Moredun.

“This may be because the ewe was already infected before vaccination or because of incorrect storage or administration of vaccine.

“However, Moredun scientists can now confirm vaccinal 1B strain of C abortus may also play a part in these abortions.”

Mr Longbottom stressed further research was needed to understand the cause of abortion in vaccinated flocks.

“It is important to realise this data has only been collected from a small number of samples – we do not know the true incidence of potential abortions caused by the vaccinal strain of C abortus.”

Mordeun is keen to look into issues relating to vaccine breakdown and this study highlights the need for further research to be funded, agrees research scientist Nick Wheelhouse.

Intervet’s Alasdair King says the company are investigating further collaborative research into this area, but stresses the benefits of vaccination far outweigh the risk to individual sheep.

“We must remember this is new knowledge, rather than a new situation – the new testing regime from Moredun highlighted the problem, but nothing has changed with the vaccine.”

This means any effects will have always been going on, suggesting any abortion caused by the vaccine is small.

A spokesman from CEVA Animal Health commented that vets and farmers should be reassured by Moredun’s own statement that it recognises the continued benefit of vaccination against OEA.

“We will work closely with researchers to discuss findings and in particular their conclusions regarding any causal link between finding the vaccine strain in vaccinated ewes and abortion.”