Researchers from the Centre d’Écologie Fonctionnelle et Évolutive (Centre for Functional and Evolutionary Ecology) have shown that chimpanzees infected with toxoplasmosis are attracted by the urine of their natural predators, leopards, but not by urine from other large felines. The study suggests that parasite manipulation by Toxoplasma gondii is specific to each host. It fuels an ongoing debate on the origin of behavioural modifications observed in humans infected with toxoplasmosis: they probably go back to a time when our ancestors were still preyed upon by large felines. The study findings are published on 8 February in the scientific journal Current Biology.
Parasites such as those that cause toxoplasmosis take various pathways, some of them complex, in order to develop into their adult form and reproduce in a so-called definitive host. These pathways may include stages in which intermediate hosts are infected. In order to pass from one such host to another, some parasites are able to induce behavioural changes in their hosts. However, this process, known as parasite manipulation, is rarely observed in mammals.
The parasite that causes toxoplasmosis, Toxoplasma gondii, is an exception. This protozoan, which infects a wide range of species including humans, can only reproduce in felines, that become infected by ingesting a parasitized prey. Studies on mice have shown that Toxoplasma gondii induces olfactory modifications in parasitized rodents. Unlike healthy individuals, parasitized mice appear to be attracted by the odour of cat urine, thus making it more likely that these intermediate hosts – mice infected by the parasite – are eaten by cats, a definitive feline host. So, increasing the chance for the parasite to successfully reproduce, an evolutionary survival mechanism.
In humans, other studies have shown changes in behaviour in parasitized individuals, such as personality changes, prolonged reaction times and reduced long-term concentration. However, no beneficial effects for the parasite have been observed, since modern humans are no longer hunted by felines.
In order to understand the origin of such behavioural change in humans, the researchers performed behavioural tests based on olfactory cues on chimpanzees, humans’ closest relatives, which are still preyed upon in their natural environment by a feline: the leopard. The tests showed that, whereas uninfected individuals avoided leopard urine, parasitized individuals lost this aversion. More surprisingly, this behavioural modification is not observed when parasitized chimpanzees are exposed to the urine of felines that are not their natural predators (lions and tigers), thus suggesting that parasite manipulation induced by Toxoplasma gondii is highly specific.
These findings fuel an ongoing debate on the origin of behavioural and olfactory modifications observed in humans: rather than being simple secondary effects of toxoplasmosis, such modifications probably go back to a time when our ancestors were still preyed upon by large felines. In addition to chimpanzees, the researchers now hope to focus on a wider range of species undergoing different predation pressures, so as to shed light on the evolutionary history of Toxoplasma gondii and unravel the circumstances under which the parasite manipulates its hosts.
And yes it appears that some more research has to be conducted:
(Source: Else-Vet YouTube channel)
(Source: CNRS press release, 02.02.2016)