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.

Sources of T. gondii infec­tion in humans. The var­i­ous sources of food-​borne and envi­ron­men­tal con­t­a­m­i­na­tion of humans are rep­re­sented. (Source: Clin. Micro­biol. Rev. April 2012 vol. 25 no. 2 264 – 296)

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: CNRS press release, 02.02.2016)