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.
So how would a protozoan find a cat to infect and thus complete it’s life-cycle? How about by turning the normally cat-averse rodents that act as the intermediate hosts into cat-seeking soon-to-be-ex rodents?
That, it seems, is exactly what T. gondii does to rats. Normally, rats are furtive, neophobic (afraid of new things) and desperately keen to avoid places that smell of cat urine. But infected rats become more active, more interested in novel objects, and they lose their aversion to cat urine.
In fact, rats and mice tend to be attracted (rather than merely indifferent) to urine from bobcats and domestic cats.
This is your brain on Toxo
People who have been infected with Toxoplasma gondii show a number of behavioural changes, including a tendency to take risks, trust strangers and be more outgoing than people who don’t have the antibodies.
The idea that human behaviour can be manipulated by a mere protozoan parasite might sound implausible, but the rate at which new findings accumulate suggests the strangest findings are yet to come. For now Kathleen McAuliffe’s wonderful article in February’s The Atlantic gives a well-rounded introduction to these effects and an entertaining profile of Jaroslav Flegr, the remarkable Czech evolutionary parasitologist who has done more than anybody to expose the many consequences of Toxoplasma infection. Curiously, many of the consequences of Toxoplasma infection seem to be sex-specific. Men are more likely to become introverted and suspicious, and to care less what others think of them. Infected women, however, tend to become more trusting, outgoing, and concerned with their appearances. Interestingly, Flegr later showed Toxomplasma infection raises testosterone levels in men but lowers them in women. According to McAuliffe’s article, Flegr suspects that these sex-specific responses might be due to sex differences in how women and men respond to heightened anxiety and emotional stress.
Women are more likely than men to respond to emotional strain by becoming more pro-social, whereas men are more likely to become withdrawn, individualistic and anti-social (not to dismiss the many nuances in sex-specific responses to stress). The links between Toxoplasma, testosterone, stress and behaviour will take some delicate teasing out.
Nobody is suggesting the effects on humans are a scaled-up equivalent of the cat-rat relationships, making humans more prone to being eaten by lions or tigers. Instead, the idea is that due to similarities shared by all mammalian brains, the program of manipulation that evolved to make Toxoplasma-riddled rats more susceptible to cat predation also incidentally targets similar pathways in the brain of humans unlucky enough to become infected. And Flegr’s research suggests these effects can mean bad news for infected people. Infected subjects tend to be less vigilant and have slower reactions. That makes them worse drivers, prone to lapses in concentration and to mistakes. In one large study of military drivers, Toxoplasma infection was associated with up to six times the accident rate of Toxoplasma-free subjects. Infected people are also two to three times as likely to suffer from mental illness, notably schizophrenia. While schizophrenia is known to run in families, genomic searches for the genes involved haven’t been particularly fruitful.
But the field is now progressing by discarding the anachronistic nature-nurture dichotomy and looking at interactions between infections such as Toxoplasma, immune system genes and genes involved in the maintenance of brain tissues. Evidence is also emerging that Toxoplasma infection can almost double the risk of several brain cancers. Perhaps we shouldn’t be surprised that an organism that encysts in brain cells might trigger tumour growth. In fact, a cross-national study in Biology Letters shows that about one fifth of the variation among 37 countries in brain cancer incidence can be explained by national Toxoplasma infection rates.
Time for kitty to stay inside?
The Biology Letters study set the cat among the pigeons, with global media coverage suggesting the beloved family feline was as likely to bring brain cancer into the home as a dead bird. What better excuse could exist for world-wide domestic feline house arrest? Unfortunately, it seems that cat ownership isn’t a useful predictor of brain cancer incidence. Studies suggest it is either a weak or non-significant predictor of Toxoplasma infection. While the link between cats and Toxoplasma infection is beyond dispute, it seems cat owners are not much more likely to become infected, especially if they follow good hygiene practices in the kitchen and when cleaning their cats’ litterboxes. It seems under-cooking meat or failing to wash vegetables presents the biggest risk of infection.
Cats aren’t off the hook, though. While they might not elevate the chances of their owners contracting Toxoplasma and all its unfortunate symptoms, those costs might well be spread among cat owners and non-owners alike. Toxoplasma infection might be an externality of cat ownership, and societies that choose to forego cat ownership may yet find themselves much better off.