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Toxoplasma gondii - a major
Toxoplasma gondii is a major cause of infectious abortion in sheep and is also a significant human pathogen. Scientists at Moredun are conducting research to further our understanding of the biology, transmission and host-pathogen interactions with the parasite to help develop effective prevention and control strategies.
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Toxoplasma parasites are tiny single celled organisms and are among the most successful parasites worldwide. Toxoplasma can infect all warm-blooded animals, including humans. Domestic cats and members of the cat family are the definitive hosts of the parasite, i.e., the parasite can only complete its life cycle in a member of the cat family. Cats become infected for the first time when they start hunting and eat infected wild rodents and birds. Following a primary infection, cats can shed millions of tough shelled oocysts (parasite eggs) into the environment, where they can survive and remain infective for several years, preferring temperate, moist conditions. Following infection, most cats develop immunity to the parasite and tend not to re-shed oocysts. As young cats are the main shedders of the parasite, it is good practice to have resident, neutered and healthy farm cats.
Sheep may become infected with Toxoplasma through the ingestion of infective oocysts on pasture or in contaminated feed or water. A primary infection during pregnancy may cause disease and abortion of the foetus. Sheep develop immunity following a primary infection that will protect against disease in a subsequent pregnancy. If they become infected outwith pregnancy, they show very few clinical signs but may have a transient fever. Research at Moredun has shown that as few as 200 infectious oocysts may cause Toxoplasma abortion in a susceptible ewe.
Following infection, the parasite persists in the animal within tissue cysts and consumption of undercooked meat from infected food animals is a common route of transmission to people. Contamination of water with the oocyst stage of the parasite is also a possible route of infection for humans and may be more important than previously thought.
Sheep with their lambs.
Diagnosis
As with all disease situations, early and accurate diagnosis of the problem is key to minimising losses and applying effective prevention and control strategies.
Always isolate the aborting ewe immediately, clean up the aborted material and bedding and get a diagnosis as soon as possible. This is very important in case the diagnosis informs that an infectious agent is responsible which could spread to other animals in the flock. For this reason, it is advisable not to use ewes that have had late abortions, mummified lambs or suspiciously weak lambs as foster mothers until diagnostic tests have confirmed this is due to toxoplasmosis and not enzootic abortion, which is transmissible to lambs.
Sensitive and specific tests are available for Toxoplasma diagnosis: • Dead lambs and their placentas (if available) should be submitted to the veterinary investigation laboratory, where tissue samples will be examined for the presence of
Toxoplasma and for lesions associated with
Toxoplasma infection.
• Molecular diagnostic tests (PCR) can detect
Toxoplasma specific DNA from infected tissue samples. • In addition, your vet can take blood samples and submit these to the lab to be examined for the presence of Toxoplasma antibodies, which will indicate exposure to the parasite.
Prevention and control
There is a live vaccine available, Toxovax™, which is licenced for use in the UK and should be administered to ewes at least 3 weeks prior to tupping. As toxoplasma is transmitted to sheep through consumption of oocysts on the pasture or in feed and water, it is difficult to prevent the disease spreading within a flock through the maintenance of a closed flock. Feed bins should be covered to prevent cats from gaining access and cats should be kept away from hay used for feeding sheep. Although there are no drugs that can cure sheep infected with Toxoplasma some research done at Moredun showed that feeding the coccidiostat decoquinate during pregnancy can reduce lamb losses due to Toxoplasma infection.
Risk to people
Toxoplasma can also infect people and particular risk groups include pregnant women and immune-compromised individuals. The main route of transmission is through consumption of the parasite in contaminated food and water and eating undercooked meat containing Toxoplasma cysts.
Current Toxoplasma research at Moredun
Moredun’s aim is to advance our understanding of how sheep can be protected against toxoplasmosis and subsequently use this knowledge to develop next generation vaccines that will be safer, offer better protection and cost less. Scientists are also in the process of developing new molecular diagnostics to identify Toxoplasma strains in livestock, wildlife, humans, and the environment and understand transmission routes. Finally, Moredun are investigating new in vitro systems to study the virulence of Toxoplasma parasites and help develop new prevention and control strategies.
As part of their popular animation series, Moredun recently released “Tackling Toxoplasmosis” in conjunction with scientists in Brazil and the UK’s Biotechnology and Biological Sciences Research Council. View it on the website at: https://moredun.org.uk/ resources/videos/tackling_toxoplasmosis. A more detailed fact sheet is also available to members of the Moredun Foundation, “Control of Toxoplasma Abortion in Sheep” (Vol. 7, No. 2).
