Parasites can influence the behavior of the organisms they inhabit.
For example, mice infected with the protozoan Toxoplasma gondii, the organism that causes toxoplasmosis, become lethargic and lose their fear of cats, the primary host of the parasite
Clearly, in a cat-infested environment, such mice don't last very long. And the cats that eat the infected mice become infected themselves and then spread the eggs (oocysts) through their feces.
The behavior modification caused by other parasites in other organisms are even more bizarre.
So, could human behavior also be influenced by some of the parasites we all carry? Some people think yes.
Our guts are filled with bacteria. Many of these bacteria are beneficial. For example, gut bacteria produce most of the B vitamin biotin that we need. Other bacteria can cause obvious harm, for example, gut inflammation, pain, and diarrhea. The diarrhea benefits the bacteria because it increases the probability that other people will come in contact with the abundant fluid and become infected themselves.
Effects on behavior could be more subtle. We know that animals infected with rabies virus behave differently. They become more aggressive and tend to bite. Because the virus colonizes the salivary gland, such bites pass the infection on.
But why am I babbling about all this, interesting though it might be?
It's because I'm wondering if it's gut bacteria that program some people to eat more than normal, causing obesity. Why would the bacteria do that? Well, the more you eat, the more food there will be in the gut, which means the more the bacteria could grow.
There is some evidence that gut bacteria are related to obesity: overweight people tend to have different types of bacteria than normal-weight people. And some animal studies showed that transferring the gut bacteria from mice prone to metabolic syndrome into normal mice caused the normal ones to develop metabolic syndrome too.
So this idea that gut bacteria are associated with obesity is not new. Whether the bacterial population causes the obesity or the obesity provides a gut environment friendly to certain types of bacteria, or perhaps both in a vicious circle, has not yet been definitively proved.
A recent study reported at the Stockholm meeting of the European Association for the Study of Diabetes showed that transplanting fecal matter from thin people into obese people with prediabetes did not result in any weight loss. However, the recipients did see their insulin resistance decrease.
Clearly, obesity, type 2 diabetes, and gut populations are related somehow. One possibility is that certain bacteria are especially efficient converters of food and fiber into compounds that can easily be taken up in the gut, essentially adding calories to whatever we eat.
But I'm wondering if there's more than a metabolic effect. I wonder if the gut bacteria, like the parasites that change behavior in mice and spiders, are subtly changing the behavior of their hosts.
If the bacteria made the hosts feel sluggish, they wouldn't want to move around a lot and burn off calories. If the bacteria made the hosts hungry all the time, they would eat more than they needed to maintain their weight.
The bacteria could then happily munch on the extra calories, rapidly multiply, and infect other people.
Is this really true? No one knows. But the idea intrigues me.