The Parasite That Makes a Rat Love a Cat

Toxoplasma gondii alters activity in a rat’s brain

Toxoplasma gondii requires the cat digestive system for reproduction, so it hitches a ride in a rat
Toxoplasma gondii requires the cat digestive system for reproduction, so it hitches a ride in a rat Courtesy of flickr user cobalt123

The life cycle of the parasite Toxoplasma gondii goes like this: Toxoplasma reproduces inside the intestine of a cat, which sheds the parasite in its feces. Rats then ingest the parasite when they consume food or water contaminated with cat feces. The parasite takes up residence in the rat’s brain and, once the rat gets eaten by a cat, it starts the cycle all over again.

Researchers have known for a few years that a rat infected with Toxoplasma loses its natural response to cat urine and no longer fears the smell. And they know that the parasite settles in the rat’s amygdala, the part of the brain that processes fear and emotions. Now a new study in the journal PLoS ONE adds another bizarre piece to the tale: When male rats infected with Toxoplasma smell cat urine, they have altered activity in the fear part of the brain as well as increased activity in the part of the brain that is responsible for sexual behavior and normally activates after exposure to a female rat.

The double messages of “you smell a cat but he’s not dangerous” and “that cat is a potential mate” lure the rat into the kitty’s deadly territory, just what the parasite needs to reproduce. Scientists still don’t know how the parasite works to alter the brain, though there apparently is a link to production of dopamine, an important neurotransmitter in the systems for decision-making and reward.

How the parasite makes the rat brain do what it needs is a particularly interesting question because rats and cats aren’t the only animals that can become infected with Toxoplasma. There is concern, for example, about the parasite’s effect on sea otters. And grazing livestock can become infected after eating contaminated vegetation. More worryingly, though, is that one-third of humans test positive for exposure to Toxoplasma (the most common ways for humans to come into contact with the parasite is through kitty litter and by consuming undercooked meat). Not only can pregnant women pass on the parasite to an unborn child (putting the child at risk of blindness or mental disability) but recent studies have also found an association between the parasitic infection and increased risk of schizophrenia and obsessive compulsive disorder.

If you’re worried about Toxoplasma, there’s no need to give up your beloved cat, but there are some precautions you can take (and definitely should take if you’re pregnant), as the CDC states:

  • Avoid changing cat litter if possible. If no one else can perform the task, wear disposable gloves and wash your hands with soap and warm water afterwards.
  • Ensure that the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat’s feces.
  • Feed your cat commercial dry or canned food, not raw or undercooked meats.
  • Keep cats indoors.
  • Avoid stray cats, especially kittens. Do not get a new cat while you are pregnant.
  • Keep outdoor sandboxes covered.
  • Wear gloves when gardening and during contact with soil or sand because it might be contaminated with cat feces that contain Toxoplasma. Wash hands with soap and warm water after gardening or contact with soil or sand.

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