Influence of latentToxoplasmainfection on human personality, physiology and morphology: pros and cons of theToxoplasma–human model in studying the manipulation hypothesis

Abstract

Toxoplasma as the model organism for studying the parasite manipulation hypothesisToxoplasma gondii is a parasitic protozoan whose effects on human behaviour, personality and other phenotypic traits have been studied most thoroughly, often in the context of the manipulation theory, the theory suggesting that many parasites change the phenotype of their host to increase their chances of transmission to a new host by, for example, predation. There are various reasons why this particular protozoon has become a favoured model for evolutionary parasitologists, biologists and also psychiatrists. First of all, Toxoplasma is a very common parasite both in developed and developing countries, and some forms of diseases caused by Toxoplasma infection have very serious impacts on human health; taken together, all forms of toxoplasmosis are a serious socio-economic burdens throughout the world (Pappas et al., 2009;Torgerson and Macpherson, 2011). It is also important to note that the study of the influence of toxoplasmosis on the behaviour of laboratory animals has a very long tradition; this includes a series of about 20 studies that started in the laboratory of William M. Hudtchison in the early 1980s, followed by studies by Joanne P. Webster and Manuel Berdoy in the 1990s, which were succeeded by several other teams (for reviews, see Skallová et al., 2006;Webster, 2007;Webster and McConkey, 2010).Toxoplasma is an excellent model for studying the manipulation hypothesis as it is trophically transmitted from an intermediate to a definitive host by predation. In contrast to behavioural patterns induced by directly or, more commonly, sexually transmitted parasites, the behavioural patterns induced by a trophically transmitted parasite are relatively easy to recognize (Parker et al., 2009). For example, in a sexually transmitted parasite, the parasite's and the host's genes have similar interests: they both 'try' to program the host to increase the probability of host reproduction. In contrast, the interests of the host and its trophically transmitted parasite radically differ. The intermediate host, e.g. the mouse, needs to survive (and reproduce) for as long as possible while the parasite, e.g. Toxoplasma, 'wants' the definitive host (here, a cat) to kill and SUMMARY The parasitic protozoan Toxoplasma gondii infects about one-third of the population of developed countries. The life-long presence of dormant stages of this parasite in the brain and muscular tissues of infected humans is usually considered asymptomatic from the clinical point of view. In the past 20years, research performed mostly on military personnel, university students, pregnant women and blood donors has shown that this ʻasymptomaticʼ disease has a large influence on various aspects of human life. Toxoplasma-infected subjects differ from uninfected controls in the personality profile estimated with two versions of Cattellʼs 16PF, Cloningerʼs TCI and Big Five questionnaires. Most of these differences increase with the length of time since the onset of infe...

Host Manipulation by Parasites: Cases, Patterns, and Remaining Doubts

“…The parasite cannot achieve sexual reproduction in rodents and the described alterations increase the chances of transmission to its final, feline host. T. gondii can also infect other mammals (Poirotte et al, 2016) and is considered to be one of the most frequent infections in humans (Flegr, 2013b;Worth et al, 2013). Behavioral alterations that have been associated with latent toxoplasmosis include multiple forms of schizophrenia, personality disorder, Parkinson disease and Alzheimer disease, as well as increased aggressiveness, decreased willingness to follow social rules, and slower reaction times in men (Flegr, 2013a).…”

“…Therefore, the absence of sickness behavior or a lack of any discrimination against parasitized mates (Hari Dass et al, 2011;Adamo, 2014;Adamo et al, 2014) have been suggested as examples of successful host manipulation (Table 2). Second, there is clear evidence that men and women, or male and female rodents, behave differently when suffering from latent toxoplasmosis (Flegr, 2013b). Sexspecific or ontogenetic differences in the responses of hosts to certain parasites are likely to be common, and host manipulation is likely to be routinely overlooked in studies that compare parasitized and non-parasitized individuals at the population level without considering individual parameters such as sex, ontogenetic stage or nutritional status.…”

“…In the case of STDs "the parasite's and the host's genes have similar interests" (Flegr, 2013b) and "there is no threshold density for disease spread" (Lockhart et al, 1996). Therefore, sexually transmitted parasites might generally be under a low selective pressure to evolve manipulation, at least in terms of enhancing sexual activity over normal levels.…”

Toxoplasmosis, Pancreatitis, Obesity and Drug Discovery

“…However, no association was reported to link anti-Toxoplasma IgG antibody and obesity in 985 individuals examined from Iceland, Sweden and Estonia [16]. In contrast, excessive gestational weight gain was reported during pregnancy in infected women with Toxoplasma compared with uninfected pregnant women [17,18]. In addition, excess weight gain was detected in a murine pregnancy model that mimics toxoplasmosis complications in humans [19].…”

Anti-Toxoplasma IgM and IgG Seropositivity Among Individuals Referred to a Clinical Laboratory of Isfahan, Central Iran

“…Infection is mostly self-limiting with no or subtle symptoms, yet can cause severe disease and disorders in infants, when infected during pregnancy, and also in immunocompromised individuals (3,4). The parasite stays dormant for a long time in form of tissue cysts and causes no symptoms (5), yet some authors believe that it causes some personality changes at this stage (6,7).…”