If you believe the headlines, sixty percent of us may be infected with a parasite that changes behaviour. Toxoplasma, a natural parasite of cats, is said to cause rage, impulsiveness and even mental illness. Most people only know Toxoplasma as the reason pregnant ladies aren’t supposed to clean kitty litter. In recent times, though, this parasite has garnered quite a reputation as a mind-controlling zombie parasite. A reputation that may in fact be slightly exaggerated.
Rest assured the mind control phenomenon does exist in nature; I myself have been studying a different mind controlling parasite, Dicrocoelium dendriticum over the course of my Ph.D. What most people don't realize though is that us scientists have little idea how exactly these parasites hijack their hosts. In most cases researchers must determine if the parasite is in fact willfully controlling the host by assessing the evolutionary value of the trait.
The most common instances of manipulation occur when a parasite must be eaten by the next host in the life cycle. In the case of the organism I study this involves making the leap from an ant into a grazing mammal. Because deer don’t tend to search out and eat ants the parasite hijacks an ant and forces it to climb and cling to plants; which grazing mammals do eat. There they wait until something comes along and gobbles them up along with the plant they are on. Even more spectacular is the fact the parasite turns the behaviour on and off. When temperatures rise and the clinging ants are in danger of overheating the parasite relinquishes control but as soon as the temperature cools they come straight back, sometimes to the exact flower they left.
It seems like a no brainer that the parasite must be controlling the ants but a careful examination of the behaviour in question needs to be undertaken in lieu of finding the chemical or mechanical means it may be using. First the parasite must have a reason to manipulate the host. In the above example this would be the need to move from an ant into a grazer. Next, any behaviour being changed by a parasite should act to achieve the desired outcome and only occur in infected individuals. Again looking at the Dicrocoelium example we can see that the clinging behaviour shown only by infected ants will in fact increase their exposure to grazing mammals. The story of Toxoplasmas alleged behaviour manipulation doesn’t quite meet these criteria.
The story begins with the unique relationship between Toxoplasma and cats. Toxoplasma infects all not just cats but everything from lizards to humans and they all become infected in one of three ways: 1) coming into contact with the parasite offspring in cat feces 2) eating infected meat or 3) from ones mother while in the womb. Due to a quirk of biology, though, it is only within cats that Toxoplasma completes sexual reproduction. This is why cats are the only ones to shed Toxoplasma in their poop and why pregnant ladies shouldn’t go near said poop. It has long been theorized that Toxoplasma should prefer being in cats due to the benefits of sexual reproduction. Sex mixes the gene pool which benefits a population by creating novel gene combinations that drive evolution. This in theory is the reason Toxoplasma would manipulate a host to enter cats. Many believed they confirmed this theory when infected rodents started to display some odd behaviours. Since then a slew of studies, have claimed that Toxoplasma influences behaviour, including studies famously investigating humans. It wasn’t until a lab from Australia started digging through these studies that anyone questioned the claims.
The original work on rodents stated infected hosts showed reduced movement and anxiety plus a remarkable attraction to the smell of cat urine. This definitely seems like something the parasite might induce to increase the chances it gets eaten. Upon closer inspection however the criteria necessary to conclude Toxoplasma has evolved the trait of manipulation are not fully met. Many studies report either no effects on behaviour or changes to behaviours that have no bearing on the chances of being eaten by a cat. Behaviours like social interactions or dominance. This casts doubt onto the idea that Toxoplasma evolved the adaptive trait of mind control. Rather it looks like some rodents with parasites sometimes do weird things. Plus there is another species of parasite, Eimeria veriformis, that also causes an attracting to cat urine in mice but this parasite dies if eaten by a cat offering no advantage to manipulation.
What then of the presumed benefit of cat hosts? Is it really enough that selection would favour the mind control trait? Again, upon closer inspection maybe not. The genetic profile of Toxoplasma populations globally appears to be a predominantly asexual one. There is little genetic mixing and only three main lineages dominate in Europe and North America. Meaning the parasite seems to be doing fine without having sex in cats and is being transmitted via the food chain or from mother to offspring. Toxoplasma certainly hasn’t had a problem invading many environments including those with a distinct lack of cats. Beluga whales in the arctic are infected and now pose a health risk to Inuit people who eat them. And foxes too on cat free islands off the coast of Norway now carry Toxoplasma.
Finally we must now question the bold claims that Toxoplasma causes aberrant behaviours in humans. In the largest study to date looking for the top reported behaviour changes in humans, things like impulsiveness, anxiety, depression and mental illness, no correlation between infection and behaviour was found.
The ability of parasites to manipulate hosts is one of the most fascinating areas of biological research. As the Toxoplasma story illustrates though it is very easy to attribute a potential quirk in behaviour to a highly specialized parasite gaining control over its host. Not all hope is lost though for Toxoplasmas master manipulator reputation. New research into the potential mechanisms used to influence behaviour may discover the smoking gun that definitively implicates Toxoplasma. All though this research isn’t yet producing headlines about zombie parasites this is exactly the research the field needs. And it may ultimately yield the best headlines of all. “Key to mind control found” or “Zombie gene discovered by parasite researchers.” Now that should move some magazines.