I should admit straight up that I’m no fan of cats. Like any zoologist I treasure the rare glimpses I have had of lions, leopards, serval and even ocelot.
Which is one of the many reasons I relish each new scientific finding regarding the protozoan parasite Toxoplasma gondii and its intriguing effects on humans.
Cats are essential to the life-cycle of this insidious parasite. As the evidence mounts of the havoc Toxoplasma wreaks when it infects humans, I naively imagine people will grow less enthusiastic about keeping cats.
As with its close relative, the malaria-causing parasite Plasmodium, Toxoplasma undergoes the sexual part of its life cycle in one host and a period of asexual reproduction in another.Meet Toxoplasma gondii
The all-important sexual stage happens inside the gut of a cat, whereas the asexual stage can happen in any mammal or bird.
Most often this intermediate host is a rodent that has eaten food contaminated by cat feces. But humans commonly become infected, usually due to poor hygiene and food preparation.
Infection can be catastrophic for patients with compromised immune function and for foetuses whose mothers suffer a new Toxoplasma infection during pregnancy. But most infections involve a brief, mild flu-like illness. Around one in three people worldwide, and as many as 80% of people in some countries have antibodies to T. gondii, indicating they have previously been infected.
After infection, the parasite doesn’t go away. It forms cysts inside muscle cells and neurons, where it avoids the host’s immune system. These cysts don’t seem to cause any immediate trouble and, until recently, nobody paid them much attention.
Rat, meet cat
The interesting thing about so many parasites is the way they manipulate a host’s behaviour to their own advantage. People with colds, for example, emit impressive volumes of snot, often sharing it in aerosol form. That virus-sodden liquid isn’t merely an unhappy side-effect of the infection. It is an adaptation – on the part of the virus rather than the human. Viruses that cause runny noses and sneezing make the leap from host to host, whereas more considerate viruses might never propagate themselves. As a result, natural selection leaves us with the inconvenient kind of virus.