Parasite Pimps Rats to Cats for Sex

Most species reproduce on their own, but Toxoplasma gondii requires two species besides itself to engage in successful procreation.

 Is this single-celled organism able to infect rats and force them to act against instinctual self-preservation?  Can T. gondii over-ride host rodent brains to recognize the acrid warning stink of cat urine as sexually attractive instead of as a hazard indicator?  Does this parasite achieve its carnal desires by making rats suicidal participants in the protozoan mating process so they can be more easily be eaten by infected cats?   Researchers Patrick House, Ajai Vyas and Robert Sapolsky say the answer to all the questions is yes.  Their recent study in PLoS ONE suggests Toxoplasma gondii effectively manipulates the behavior of mule hosts in order to ensure and increase its own reproductive success.

 The sexual phase of the protozoan parasite Toxoplasma’s life cycle takes place exclusively in the dark warmth of a cat’s intestines, where hardy oocytes -immature female egg cells- are created and then excreted in feces.  Once dumped, the oocytes are unable to get where they need on their own and must use others for transport to complete their reproductive cycles.  In an astounding example of behavioral manipulation, the bacteria infects rats causing them to register the odor of cat urine as a sexual turn-on rather than a fear-inducing imminent death warning.  Rats whose brains are infested with Toxoplasma gondii cysts are cognitively impaired by the parasite which seems to re-channel olfactory recognition of cat urine from ‘defensive’ to ‘reproductive’ innate behavior.  The rats process the odor as sexual attractor rather than preservation warning and offer themselves in tasty sacrifice to hungry cats. 

Toxoplasma gondii doesn’t discriminate against carriers.  Just about any mammal, including humans, can potentially be infected with the bacteria and unwittingly coerced into carrying out the parasite’s fiendish reproductive schemes.  But the most effective sex-workers Toxoplasma recruits and pimps out are rodents.  Mice and rats are cats’ naturally preferred treats and thus the perfect delivery system for getting the oocytes back into the animal’s gut.  It is a striking evolutionary adaptation made all the more amazing because a unicellular organism is calling the shots, enslaving far more complex organisms in the pursuit of its only goal, reproduction.

Toxoplasma gondii lets nothing stand in the way of its species’ success.  It is estimated that a third of Earth’s human population is seropositive for Toxoplasma.  Why would such a large percentage of  humans, who do not make up a significant quantity of cat food, be carriers of a parasite that can only reproduce in the digestive system of Felis catus?  Adaptation is an economical strategy that uses the least change necessary to maintain survival.  Although few humans these days are eaten by cats, there was a significant period of time in the joint of evolutionary history of the cat genus Felidae and of the great ape Hominidae when some cats weighed over 100 kilos and hominids made up some part of their diet.

Research into Toxoplasma gondii is illuminating aspects of animal-animal and human-animal relations in unforeseen ways.  Elements of domestication, which is a fundamental building block of human society, may have to be reexamined to see if we are confusing human agency with microbial imperative.