Vaccination still best option for controlling ovine enzootic abortion despite possible link between vaccine and disease
17 May 2010
Scientists from Moredun Research Institute in Edinburgh, funded under the BBSRC Combating Endemic Diseases of Farmed Animals for Sustainability (CEDFAS) initiative, are encouraging sheep farmers to continue to vaccinate against Ovine Enzootic Abortion (OEA) on the day that they publish research that shows the vaccines used to control OEA in UK sheep could themselves cause abortion. Vaccination is still the most effective way to control OEA and can reduce incidence of abortion to 2% or less in flocks and the research has been welcomed by companies producing such vaccines. This development will enable further detailed investigation into the causes of OEA in vaccinated flocks in the future.
Enzootic (chlamydial) abortion is a major cause of lamb mortality across the world and accounts for 44% of diagnosed infectious abortions in UK sheep (source: VIDA submissions 1995-2008). The most effective way to control Enzootic abortion in sheep flocks is through the use of vaccines. While uncontrolled OAE in a flock can lead to abortion levels between 5-10%, with "abortion storms" of up to 25%, vaccination can bring abortion down to 2% or less.
Moredun Research Institute is internationally recognised for its work on Chlamydophila and in fact it was scientists at Moredun in the 1950s who were the first to identify C.abortus as the cause of abortion in sheep. Moredun requires a regular supply of freshly isolated C. abortus to support their chlamydial research programmes, and in Autumn 2009 SACVS passed tissue samples to Moredun that had been submitted to SAC Veterinary Investigation (VI) centres across Scotland during the lambing period earlier in the year. These samples had already been screened by SAC VI staff and, on the basis of gross pathology and serology; VI staff suspected that the cause of abortion was due to chlamydial infection.
On further analysis Moredun scientists confirmed that 35 of the 40 placental samples received contained C. abortus organisms and from the information supplied by the farmer when the samples were first submitted, found that 14 of the 35 positive C. abortus samples were from flocks that had been vaccinated against the disease.
Moredun scientists then conducted further tests on the isolates recovered, including gene sequencing and showed that 5 of the 14 samples contained DNA only present in the vaccinal strain and that the bacteria was present in numbers large enough to be the probable cause of abortion. The results of this study were published today (Monday 17 May) in 'Vaccine' the international scientific journal for those interested in vaccines and vaccination.
Dr David Longbottom, Head of the Chlamydia research group at Moredun commented, "It is only due to a comparison of the recently published gene sequence (DNA code) of the vaccine strain with wild-type strains, combined with the application of gene sequencing technology that we have been able to differentiate between these wild-type and vaccine like strains of C. abortus." He added, "It is important to stress that we do not know the true incidence of potential abortions caused by the vaccinal strain of C. abortus in the UK, as this data was only collected from a very small number of samples submitted across Scotland, and was not part of an organised or scheduled experiment. This is, however, an important development that will in the future enable a more detailed investigation of the causes of chlamydial abortion in vaccinated flocks."
There are two vaccines available to control enzootic abortion in the UK - EnzovaxTM produced by Intervet / Schering-Plough Animal Health and CEVAC ChlamydiaTM produced by CEVA Animal Health, both of which are based on the same C. abortus 1B strain. This 'vaccinal 1B' strain differs slightly from 'wild-type' strains found in the environment but is sufficiently similar to evoke an excellent immune response in sheep, enough to give a strong lasting protection against the wild-type strains and any abortions they may cause. The 1B strain in both vaccines is a live attenuated (weaker) strain of C. abortus that grows less well than wild-type strains at 39oC (the body temperature of sheep), therefore the risk of this 'vaccinal 1B' strain causing disease was thought to be negligible.
No vaccine is ever 100% effective and, although uncommon, sheep that have been vaccinated with a chlamydial vaccine can still abort at the next lambing. This may be because the ewe was already infected before she was vaccinated, or because of incorrect storage or administration of the vaccine. Moredun scientists can now confirm that the 'vaccinal 1B' strain of C. abortus may also play a part in these abortions.
Dr Nick Wheelhouse, the research scientist who identified the vaccinal 1B strain in the samples and demonstrated that they were the probable cause of abortion said, "Before now, it was assumed that positive C. abortus samples from vaccinated flocks were due to either latent infection in ewes or incorrect use or storage of the vaccine, not that the vaccine itself may have been the possible cause of the abortion." He added, "Moredun is very keen to look into issues relating to vaccine breakdown and this study highlights the need for further research to be funded."
Professor Julie Fitzpatrick, Scientific Director of Moredun is keen that farmers continue using the chlamydial vaccines available. She commented "Moredun has a strong commitment to communicate our latest research findings to farmers so that they can make informed choices about disease control on their farms. Although our findings are important, our message to farmers about controlling enzootic abortion on their farms remains the same. We would encourage farmers who have a vaccination strategy for enzootic abortion on their farms to continue vaccinating, as this is the most effective way to safeguard their sheep from disease."
She added, "We strongly recommend that farmers work with their vets and local VI centres and continue to investigate the cause of any abortions they may have on their farms. If farmers receive a positive C. abortus result from a vaccinated ewe which has aborted, we would encourage them to work with their vet and contact the vaccine provider who will investigate the situation."
These new findings were made with the collaborative support of the farm animal disease surveillance activities carried out by SAC Consulting: Veterinary Services. Group Manager Brian Hosie urges farmers to continue vaccinating their flocks. He commented, "Despite these latest findings the risk is thought to be small whereas the benefits of vaccination are immense. The use of vaccines against Enzootic Abortion has eliminated the scourge of major 'EAE abortion storms' that once devastated sheep flocks and lost many lambs. C. abortus is also an infection that can cause very serious human health problems". He added, "At present the reasons why the vaccine strain of C. abortus can cause occasional abortions are unclear. They may involve the sheep's failure to generate an immune response to the vaccine. The development of immunity is highly complex and many factors can interfere with it. We would therefore advise farmers and their vets to submit abortion material to SAC VI Centres for investigation."
Alasdair King, veterinary manager at Intervet / Schering-Plough Animal Health comments, "We have never seen this type of result before in the extensively researched vaccine's fifteen year history but we are investigating further collaborative research in this area.
"We are pleased that the Moredun Research Institute is continuing to recommend that flocks are vaccinated with our chlamydial vaccine because, as with any vaccine, the risk benefit analysis is vital in deciding whether to vaccinate animals. In this case, the benefits to the national flock in using the vaccine by far outweigh the risk to the individual sheep.
"Enzootic abortion is the single most common infectious cause of sheep abortion in the UK and can affect up to 25% of ewes in a flock. Since its initial use in 1993, approximately 5 million ewes have been vaccinated with our chlamydial vaccine, helping reduce the impact of the problem."
A spokesman from CEVA Animal Health commented "CEVA Animal Health welcomes credible scientific research into veterinary medicines and responsible reporting thereof. Vets and farmers should be reassured by the Moredun Research Institute's own statement that it recognises the continued benefit of vaccination against Ovine Enzootic Abortion. The risk/benefit of vaccination remains very positive as evidenced by the ongoing approval of these vaccines by the Veterinary Medicines Directorate. We will work closely with the researchers to discuss their findings, and in particular their conclusions, regarding any possible causal link between finding the vaccine strain in vaccinated ewes and abortion.
Notes to editors
'Evidence of Chlamydophila abortus vaccine strain 1B as a possible cause of Ovine Enzootic Abortion' by Wheelhouse et alhas been published in Vaccine,the international scientific journal for those interested in vaccines and vaccination.
The farm animal disease surveillance activities carried out by SAC Consulting: Veterinary Services are part-funded by the Scottish Government as part of its Public Good Veterinary and Advisory Services.
About Moredun Research Institute
Moredun Research Institute conducts internationally recognised research on the infectious diseases of livestock, caused by important viruses, bacteria and parasites. It employs 170 scientists and vets that work to improve animal welfare, ensure food safety, reduce adverse impacts on the environment, and to contribute to sustainable communities through economic development across Scotland and beyond. Moredun's research focuses on understanding the interaction of the disease pathogen with the host species, in identifying new targets for improved diagnostic tests and in development of novel vaccines for many diseases. Moredun Research Institute is part funded by Scottish Government Rural and Environment Research and Analysis Directorate.
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