Controlled From Within - Influences On Behavior
We like to think that we’re the masters of our destiny, not puppets to invisible intruders in our bodies. But when you consider the complex interplay involved in something as simple as malaria, it’s hard to escape the feeling we’re being driven by outside sources. Of course there are many factors that control human behavior such as: social influences, environmental conditions, genetics and even music, but these factors are only the tip of the iceberg.
According to the World Health Organization, there are approximately 216 million cases of malaria a year. This parasite-transmitted disease causes shaking, fever, chills and convulsions and can sometimes end in death. Transmitted by mosquitos, the infectious agent transmits in the mosquitos’ saliva as she feeds on human hosts. This agent, called plasmodium, has an effect on both the infected human… and its mosquito transport system.
Plasmodium-infected mosquitoes initially behave more cautiously, resting instead of feeding again and giving the
plasmodium time to replicate. Once the parasite is ready, though, the mosquitos become more likely than usual to bite multiple hosts and spend more time drinking the blood. This insures the infection is passed. Once a human is infected, his or her pheromones make him more attractive to mosquitoes. He is bitten by an uninfected mosquito, who contracts the parasite- and the life cycle continues.
Humans, however, are not helpless under the skin. Sickle cell disease is a condition where an affected person’s red blood cells take on a sickle shape. A recessive disease, one must receive two copies of the gene to be affected. Those who carry only one copy of the gene have, instead, a natural defense against malaria. The trait originated and is still largely found in the parts of the world where malaria is endemic. Children with one copy of the gene are much more likely to avoid malaria or live through it if contracted.
Picture the plasmodium parasite, invisible to the naked eye, pitched against a recessive gene in a thousand year battle. And this is hardly the only subsurface battle going on. The rabies virus is perhaps the most famous, compelling those infected to bite and avoid water to better spread the disease. Flatworms infect ants and cause them to climb to the top of a blade of grass and clamp down there, the better to be eaten by a grazing sheep and propagated in the sheep’s digestive system. Parasitic wasps lay their eggs inside spiders and the larvae force the spiders to spin cocoons for them. Hairworms cause grasshoppers to jump into water and drown, spreading the worms through the water supply. On the opposite end of the spectrum, it has been noted that hookworm infestation in humans relieves seasonal allergies, possibly by giving the immune system something else to fight.
Recently, an evolutionary biologist has speculated that a parasite excreted by cats can infect humans and cause them to manifest self-destructive behavior and even mental illness. So next time you have a self-destructive urge, consider: is it myself, or my parasites?