The urine scent of rats infected with Toxoplasma gondii—a protozoan parasite that infects wild and domestic animals and birds—drives away mice, their prey, shows a new study by researchers from Nanyang Technological University, Singapore.
This notorious parasite is not confined to animals. Notably, a third of the world’s population is infected and approximately 60 million people in the US alone are infected, often without showing any symptoms. It is mainly spread by eating undercooked meat or unwashed fruits and vegetables laced with soil containing infected cat feces. While it is not a concern in people with healthy immune systems, it can be serious in pregnant women and immunocompromised individuals.
Previous studies have shown that Toxoplasma deviously manipulates rats to display unusual behaviors toward cats, their predators. They lose their innate fear of cat odors; instead, they are attracted to cat urine odors making them more likely to be preyed upon, thereby allowing the parasite access to cats, where it can sexually reproduce.
Normally, females of a species would shun males infected with parasites for obvious reasons. But interestingly, the team earlier found that female rats were more attracted to urine odors of Toxoplasma-infected male rats compared with urine from uninfected rats and that Toxoplasma is sexually transmitted in rats. These odors contain pheromones—chemical substances emitted to communicate with other members of the same species, which usually respond by altering their behavior—that are often involved in attracting members of the opposite sex for mating. Parasites can hijack and exploit a hosts’ pheromone production. Many pheromones are volatile and are thus transmitted through the air as specific odors, which are detected by faraway individuals.
Parasitism usually harms the host but in this case attracting females is favorable for both the infected rats and Toxoplasma: rats acquire more mates and prolific opportunities for reproduction, and this in turn helps to transmit the parasite. However, other species can take advantage of the pheromonal cues, which may ultimately harm the host. Since pheromones are an openly broadcasted communication system, they can also affect the behavior of other unintended species, such as prey or predators, which can espy these signals for their own benefit. They may seize and decode these chemical cues to glean information about their surroundings and act accordingly. For example, predators may use them to find prey whereas prey may become vigilant of predators.
Mice are preyed upon by rats; they exhibit an innate fear of rats, even when rats are not physically present. The researchers suspected that mice may seize pheromonal signals from infected rat urine, possibly using them to their advantage. To find out, they placed twelve mice into a cage divided into two sections: one containing urine scent marks from Toxoplasma-infected rats and the other from non-infected ones. They then recorded the time spent by mice in each section during twenty-minute trials.
Urine consists of volatile substances that fade away with time. To test if a volatile pheromone was causing this response, the researchers repeated the experiment using urine that was three-days old.
They found that nine out of twelve mice preferentially spent more time in the section containing fresh, uninfected rat urine. In other words, they avoided the reeky Toxoplasma-infected rat urine. Even old urine had a similar effect on mice: Most mice still avoided the parasite-infected urine compared with uninfected rat urine, indicating that the substance responsible for this aversion is non-volatile.
Although it is a small study, the results are interesting. But what does this mean for the infected rats? Parasites and hosts are constantly evolving and trade-offs are involved when parasites manipulate host behavior. Although boosting pheromone production benefits infected rats—and parasites—by providing ample reproductive opportunities, it also imposes a cost on the host by warding off their prey. Infected rats may suffer as a result of lower prey availability. In addition, they have a greater risk of being eaten by cats. Scientists believe that parasites evolve a fine balance when imposing behavioral changes on their host to ensure they derive maximum benefit.
Vasudevan A and Vyas A (2014) Toxoplasma gondii infection enhances the kairomonal valence of rat urine [v1; ref status: indexed, http://f1000r.es/37q%5D F1000Research 2014, 3:92 (doi: 10.12688/f1000research.4166) – See more at: http://f1000research.com/articles/3-92/v1#ref-20