Researchers at the Dutch Wildlife Health Centre (DWHC) working together with members of the Zoogdiervereniging (Dutch organisation for the research and conservation of native mammals) and the RIVM (National Institute for Public Health and the Environment), have confirmed that more than half of the squirrels that underwent post-mortem exam in autumn of 2014 died from toxoplasmosis. The majority of the remaining squirrels had injuries, often consistent with roadkill. There was no evidence for viral infection or poisoning.
Toxoplasmosis is caused by infection with the Toxoplasma gondii parasite. The life cycle of this parasite involves definitive (felids that have not previously been infected) and intermediate (all warm-blooded animals, including humans) hosts. Only in the definitive host are eggs (oocysts) produced and these are excreted by the million in the feces and can survive for up to 1.5 years in the environment. These oocysts are the source of infection of intermediate hosts in which motile forms of the parasite leave the oocyst and migrate through the body forming cysts in muscle and nervous tissue (tissue cysts). Cats or other predatory animals can become infected upon consumption of tissue cysts in their prey.
The death of large numbers of squirrels in the Dutch province of Twente in autumn 2014 prompted the DWHC and the Zoogdiervereniging to call on members of the public to submit any dead squirrels for post-mortem investigation. In response, the DWHC received more than 350 reports of dead squirrels of which 35 underwent post-mortem investigation; in half of those cause of death was identified as toxoplasmosis. The research project was continued in 2015, with another 32 squirrels undergoing post-mortem exam between January and October. Whilst a proprtion of these animals also tested positive for toxoplasmosis the number of animals examined so far is too small to determine any temporal or geographic trends.
Humans cannot be infected with Toxoplasma by contact with dead squirrels. People can become infected in two ways: 1) ingestion of eggs via contaminated water or foods such as foraged fruit and nuts or unwashed fruit and vegetables, working in contaminated soil, or cleaning out cat litter trays;
2) ingestion of tissue cysts in undercooked or not pre-frozen contaminated meat.
Dogs and cats can become infected via consumption of uncooked meat containing tissue cysts e.g. prey. In general the infection remains sub-clinical, however, on the website of the RIVM is a single report of a cat showing clinical signs associated with toxoplasma infection.
Many questions regarding toxoplasmosis in squirrels remain unanswered: Was 2014 an unusual year or is this disease a common cause of death in Dutch squirrels? Are temporal or geographic differences in the occurrence of toxoplasmosis? How important are factors such as biotope and weather (both directly and indirectly i.e. mast years, mushroom growth)?
In order to address these questions it will be necessary to investigate a large number of squirrels over a long period. The DWHC and the Zoogdiervereniging hope to be able to continue this research over the coming years.
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