Cat’s intestine the only place it can multiply
Puppet-masters at work
The body-snatchers of the animal world, parasites are known for bending the minds of their hosts. A famous “zombie-ant fungus” drops toxic spores from the canopy onto a carpenter ant’s head in the Brazilian Amazon. The infection causes the ant to wander onto a leaf vine, locking its mandibles to the plant in a final act. The fungus sprouts from the ant’s head and drops more spores onto the rest of the colony below, completing its life cycle.
While toxoplasma doesn’t explode from a cat’s head when it’s ready to move on, some researchers believe T. gondii also evolved in the Amazon before making its debut on the world stage.
A cat’s intestine is the only place it can multiply, so to get back and complete its life-cycle, it hijacks the behaviour of feline prey.
Early studies found rats infected with toxoplasma had their innate aversion to cats turn into a fatal attraction. When a cat eats an infected rat, it increases the likelihood that toxoplasma will make it into the feline’s feces and spread back into the environment. Others have found toxoplasma-infected chimpanzees are drawn toward leopard urine in a morbid attraction.
Because cats rarely eat people, T. gondii usually hits a reproductive brick wall when it infects humans. But that doesn’t stop it from influencing our behaviour years after initial infection.
The parasite creates tiny scars in the brain, embedding itself in cysts where it will lie dormant. When the individual has been weakened by disease or age, they can burst open, releasing the parasite so that symptoms re-surface years after the initial infection.
For decades, scientists have been building a body of evidence that suggests those tiny scars could be contributing to a range of human diseases. Beyond mind-control, latent toxoplasmosis has been associated with mental disorders like schizophrenia and Alzheimer’s, as well as epilepsy, autism, cognitive and vision deficits, cancers and increased severity of other diseases like HIV.
Global infection rates shifting
Despite its staggering global infection rate, toxoplasmosis in humans appears to be on the decline, says Lafferty. That’s largely due to rising standards in hygiene, food production and water treatment, as well as recognition among many pet owners that cats live a healthier and longer life indoors.
But according to recent research out of the University of British Columbia, those global declines could be masking some important regional differences.
The study, published in the journal Proceedings of the Royal Society B, examined more than 45,000 cases of toxoplasmosis in more than 200 wild mammals at over 1,000 locations across the globe.
Layering positive toxoplasmosis cases onto maps showing human density, the researchers found 19 per cent of all animals tested positive for the virus. Higher rates of infection were found in wildlife near urban centres, where domestic cat populations climb and the filtering effects of wetlands, damaged and destroyed by urbanization, decline.
In one example, grizzly bears were found to have lower toxoplasma-infection rates than their black-bear cousins, who more often live near human population centres, said lead author Amy Wilson, a UBC forestry professor and practising veterinarian in Vancouver. Some of the most significant increases were found in aquatic ecosystems, where contaminated storm runoff can carry the toxoplasma eggs into the bodies of whales, fish and sea otters.
“Habitat conservation is not just a concept. It’s also a public-health intervention,” Wilson said. “People need to understand that there are services that these ecosystems provide — with climate change it’s carbon sequestration, but there’s also pathogen filtration.”
At the same time, her research suggests turning off the source of toxoplasma will require cat owners to make millions of individual decisions, all landing on “don’t let your animals wander freely.”
Does that kitten you adopted during the pandemic make you more susceptible to parasitic mind control? No, Wilson tells Glacier Media. If you have an indoor cat, the likelihood of them shedding the pathogen will be very low, especially if they are not killing wild prey and you regularly clean their litter box.
“There can be some simple interventions that we can do here that benefit wildlife and also benefit us, too. It’s a shared fate,” said Wilson.
Climate change is a mixed fate
As with any living creature, toxoplasma’s spread is under the influence of global climate patterns. Wilson’s study found that the parasite’s eggs survived as temperatures increased, leading to questions about whether climate change will shift infection levels to new parts of the world.
A separate study in Europe suggested climate change could increase toxoplasmosis infection rates in northern France, Belgium and Great Britain over the coming decades. At the same time, climate change is expected to lead to warmer, drier climes in Southern Europe, making it less hospitable for the parasite.
Could climate change offer toxoplasma a more comfortable niche in Canada? The reality is, T. gondii has already made its way from the Amazon to Inuit communities in the Canadian Arctic. Lafferty says that trend is only likely to continue as pockets of warm, moist conditions migrate across the globe in the coming decades.
“The prediction about toxoplasma, which is true for malaria, is that its ideal distribution is going to shift toward the poles.”
On the other hand, if your home is slated to transform into an arid desert scape, at least you’ll have a better shot at avoiding the world’s most prolific parasite.
Living with parasites
T. gondii was first discovered in 1908 in a Tunisian lab when two scientists isolated the parasite in a tissue sample from a gundi (hence “gondii”), a hamster-like African rodent. It would be another 30 years before it was first found in a human — an infant born by caesarean section in New York City — and not until 1970 when scientists started to understand cats’ unique role as the only host that offers the right conditions for the parasite to reproduce sexually.
But toxoplasma has likely been living alongside human populations much longer, itself a startling example of life’s ability to adapt.
Today, we know enough about the parasite to tap into some of our worst fears, fears Lafferty says need to be tempered with a sense of wonder. “It’s terrifying to have this parasite in your brain controlling your behaviour,” he said. “It gets in our brains and does these subtle things, things that are subtle enough that we can detect them in humans, and we can demonstrate them in experiments with rodents.”
A fascinating way to look at the parasite, says Lafferty, is to understand that humans and toxoplasma have been interconnected for a long time — so long, it may be hard to define where we end and where the pathogen begins. Lafferty says we’ve failed as a society to have a conversation about toxoplasma, a parasite affecting a high percentage of the population and whose latent effects are rarely talked about in public health.
“If I told you you could take a drug that would cure you of it, would you take it? Would you give up that part of your personality that you’d become accustomed to?” Lafferty said.
“You live an interesting life, and maybe part of that interesting life is the fact that you’re infected with toxoplasma.”