Is there a disease that makes us love cats? – Jaap de Roode
Is there a disease that makes us love cats,and do you have it?Maybe,and it’s more likely than you’d think. We’re talking about toxoplasmosis,a disease caused by toxoplasma gondii. Like all parasites, toxoplasma livesat the expense of its host,and needs its host to produce offspring. To do that, toxo orchestrates a brainmanipulation schemeinvolving cats,their rodent prey,and virtually all other birds and mammals,including humans. Documented human infections go as far backas ancient Egypt. We found samples in mummies. Today, about a third of the world’spopulation is infected,and most of them never even know it. In healthy people, symptoms oftendon’t show up at all. When they do, they’re mild and flu-like. But those are just the physical symptoms. Toxoplasma also nestles into our brainsand meddles with our behaviorbehind the scenes. To understand why, let’s take a lookat the parasite’s life cycle. While the parasite can multiplyin practically any host,it can only reproduce sexuallyin the intestines of cats. The offspring, called oocysts,are shed in the cat’s feces. A single cat can shed up to a hundred million oocysts. If another animal, like a mouse,accidentally ingests them,they’ll invade the mouse’s tissuesand mature to form tissue cysts. If the mouse gets eaten by a cat,the tissue cysts become activeand release offspringthat mate to form new oocysts,completing the cycle. But there’s a problem. A mouse’s natural desire to avoida cat makes it tough to close this loop. Toxoplasma has a solution for that. The parasites invade white blood cellsto hitch a ride to the brainwhere they seem to override the innatefear of predators. Infected rodents are more recklessand have slower reaction times. Strangest of all, they’re actuallyattracted to feline urine,which probably makes them more likelyto cross paths with a catand help the parasite complete its life cycle. How does the parasite pull this off?Although the exact mechanism isn’t known,toxo appears to increase dopamine,a brain neurotransmitter that is involvedin novelty-seeking behavior. Thus, one idea is that toxo tinkerswith neurotransmitters,the chemical signals that modulate emotions. The result?Fatal attraction. But mice aren’t the only animalsthat end up with these parasites,and that’s where humans,and all of toxo’s other hosts, come in. We can accidentally ingest oocystsin contaminated water,or unwashed produce,or from playing in sandboxes,or cleaning out litter boxes. This is behind the common recommendationthat pregnant women not change cat litter. Toxo can cause serious birth defects. We can also get toxo from eating undercooked meatfrom other animals that picked upsome oocysts. And it turns out that toxo can mess withour brains, too. Studies have found connections betweentoxo and schizophrenia,biopolar disorder,obsessive compulsive disorder,and aggression. It also slows reactions and decreases concentration,which may be why one study foundthat people involved in traffic accidentswere almost three times more likelyto have toxoplasma. So is toxo manipulating our brainsas an evolutionary strategyto get predatory cats to eat us?Or are our brains just similar enoughto a rodent’sthat the same neurological tricks thatlure them in catch us in the net, too?And is toxo the reason so many peoplelove cats and keep them as pets?Well, the jury’s still out on that one. Some recent studies even contradict the idea. Regardless, toxoplasma has definitelybenefited from humansto become one of the world’s most successful parasites. It’s not just our willingness to letcats on our dining room tablesor in our beds. Raising livestock and building cities which attract rodentshas provided billions of new hosts,and you and your cat may be two of them.