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Could a Parasite Make Your Dog Reckless? Maybe.

By Caroline Coile

Is it possible for a parasite to affect your dog’s behavior? Maybe. Look at the case of the gray wolf and toxoplasmosis. In a study that encompassed 26 years of data on more than 200 gray wolves from Yellowstone National Park, researchers found that wolves infected with the parasite Toxoplasma gondii developed markedly increased risk-taking behaviors.

When wolves were infected at an early age, they would leave the pack to strike out on their own at a premature age. About 50% of infected male wolves would separate from the pack at the age of only 6 months, when normally a young wolf separates on average at about 21 months. The same trend was true for females: Infected females separated from the pack at the age of 30 months instead of 48 months. Overall, an infected wolf is 11 times more likely to disperse. In addition, an infected wolf is 46 times more likely to become a pack leader, a role it gains by being reckless and aggressive.

Toxoplasmosis can only multiply in cats, but can affect many species, including dogs and humans. If acute infection occurs during pregnancy it can lead to birthing complications, spontaneous abortions, and stillbirths; and in young or immunosuppressed individuals it can cause fatal encephalitis. But more often no signs of note exist to raise the alarm. The parasites enter the body and travel to the brain and muscle tissue, where they become encysted. Once there, they change the host’s behavior–just how isn’t yet known.

Experimental studies have shown that chronic infections, even in healthy individuals, can lead to increased dopamine and testosterone production in both rodents and humans. These hormone changes can cause increased aggression and risk-taking behavior such as increased hyperactive movement and decreased fear of novel situations.

The phenomenon is most well-known in rodents: Mice infected with T. gondii seem to lose their inherent fear of cats and no longer avoid the scent of cat urine. These mice make easy prey for cats, and when the cat catches and eats them, it acts as the new primary host for the parasite.

If it happens in wolves, could it happen in dogs? As yet, no data exists. Dogs can acquire T. gondii by eating infected raw meat, such as that from rodents, or cat feces. In Portugal, a study found 38% of dogs tested positive for T. gondii antibodies. Could it be worth testing for T. gondii in a dog that suddenly exhibits risky or aggressive behavior? Probably.

Meyer, C.J., Cassidy, K.A., Stahler, E.E. et al. Parasitic infection increases risk-taking in a social, intermediate host carnivore. Commun Biol 5, 1180 (2022).

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Posted by on Apr 4 2023. Filed under Current Articles, Featured, Health & Training. You can follow any responses to this entry through the RSS 2.0. Both comments and pings are currently closed.

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