What's the story with Toxoplasma abortions after Toxovax?
Sara Sutherland, BVSc, MSc, BSc(Agr) Vet Services Wairarapa
This year we have had a number of farmers bring up the idea that Toxovax isn’t working, or that there is a “new strain” of toxoplasmosis that the vaccine doesn’t protect against.
Before I address these myths, here is a brief review of how this disease works. Cats eat an animal (mouse or lamb or afterbirth) infected with Toxoplasma gondii. Within about 10 days, millions of eggs (oocysts) start to be shed in the faeces, and continue to be shed for several days. These oocysts survive for a long time (months to years) in the environment and are infective after a couple of days. When a mouse, or a sheep that is not pregnant, accidentally eats one of these microscopic eggs, the organism spreads throughout the body and makes little cysts. The Toxoplasma organisms sit quietly in the cysts and wait to be eaten by a cat. If a pregnant, unvaccinated sheep eats an oocyst, the organism spreads through the body and into the foetus. Depending on the stage of pregnancy, the sheep will either detect that the foetus is infected and spit it out (abortion), or the infected foetus will die (abortion), or the lamb will be born infected and either have brain damage and die soon after birth or survive but be slower and smaller than an uninfected lamb, or be born normal. This is why there is a variation of presentations of toxoplasmosis in a flock – from dries at scanning to lambs that are born small and weak.
Vaccination with Toxovax infects the non-pregnant ewe with a strain of Toxoplasma which has been modified so that it cannot spread from the ewe to infect cats and continue the cycle. The ewe mounts an immune response against the organism. If the ewe then encounters Toxoplasma while she is pregnant, her immune system detects and closes down the organism so that it cannot form cysts or spread through the body, or get into the foetus. The immunity from the vaccine is not as great as the immunity from natural infection, but it is normally good enough to prevent the clinical signs – abortions and dead lambs.
Worldwide, there are hundreds of strains of Toxoplasma gondii, and there are probably strains of Toxoplasma that the vaccine doesn’t protect against. However, none of these strains are present in New Zealand. Where there have been apparent failures of the vaccine to protect against abortion, this has never been due to the vaccine failing to cause an immune response in the ewe. Nor is it due to a “new strain” that the vaccine doesn’t protect against. What likely happens in that situation is that there are so many Toxoplasma oocysts around that the immune system of the ewe can’t close them all down. Then odd hogget may still abort despite being vaccinated. If you did not vaccinate in that case, you would get many more abortions – having these failures is not a reason to avoid vaccination. This is supported by the observations of the original trial data, which demonstrated that ewes vaccinated with Toxovax may abort if the field challenge is extremely high.
This year we had two or three farmers that decided not to vaccinate with Toxovax despite having done it for many years previously. When they started seeing abortions they got us to take samples from the foetus and work up the cause. In ALL cases where the farmer had not vaccinated with Toxovax, toxoplasmosis was the cause of abortion. Where they worked out the cost/benefit of vaccination compared with all those dry ewes and dead lambs, vaccination came out ahead.
We did see a lot of abortions this year, particularly in hoggets. In the cases of abortion that we investigated this year, there were NO cases due to toxoplasmosis where the farmer had vaccinated with Toxovax.
If you see abortions, particularly if you have vaccinated against Toxoplasma and Campylobacter, we always recommend bringing in fresh foetuses for post mortem examinations to find out the cause. Without knowing the cause, you don’t know if there is anything you can do about it. We find a cause for the abortion about 80% of the time. Sometimes we don’t, despite a full workup, and this is very frustrating for the farmers and for us. In these situations it could be that there was a non-infectious cause, such as a toxin or fungus, or that the organism causing the abortions was not at a detectable level in the foetus. In general, the more foetuses you bring in the better the chance of finding something.
Can you get rid of Toxoplasma by shooting cats and not vaccinating? There is an old joke about a farmer who lived so far out in the wops that he had to import his own tomcat to get his farm cats pregnant. We don’t have any farms that remote in the Wairarapa! There are a lot of cats out there, and you don’t see them. The cats that shed the very high levels of oocysts on pasture are generally young cats, so don’t necessarily line up your pet moggy in the gun sights.
If you have questions about abortions, or about Toxovax, please contact one of our vets to discuss.
Published Wednesday 15th of August 2018