**6. Studies on behavioral disorders in animals and humans**

Research on this issue has included experimental animal studies, mainly in rats and mice. Parasitized animals have shown various behavioral changes, becoming more active, express‐ ing less fear when examining new stimuli, reducing their natural aversion to cat odor or even becoming attracted to it, and demonstrating reduced learning ability and attention or mem‐ ory deficits [78–83]. According to the "behavioral manipulation hypothesis," these disorders in their intermediate hosts (rodents) represent an evolutionary adaptation of the parasite, facilitating their capture by their definitive host (felines) and completing their life cycle [84, 85]. Although the mechanism by which *T. gondii* induces these behavioral changes is poorly understood, various possibilities have been proposed. It may be due to a direct effect on tis‐ sue cysts in specific brain areas such as the amygdala or hippocampus, given that the host response to predator odors was changed by the parasite in male rats infected with *T. gondii* by inducing hypomethylation of the neuropeptide arginine vasopressin in the posterodorsal part of the medial amygdala, an important node of the extrahypothalamic vasopressin system that contains a large number of arginine vasopressin neurons. This epigenetic manipulation pro‐ duced a greater activation of vasopressinergic neurons after exposure to cat odor, leading to the reversion of fear into attraction [86]. It may also result from the effect of a more diffuse and wider involvement of brain tissues, with no apparent changes, that nevertheless give rise to a series of neurophysiological disorders. Changes may also result from inflammation (enceph‐ alitis) caused by the immune activation induced by parasitization, which would increase inflammatory cytokines in the rodent brain, such as tumor necrosis factor alpha (TNF‐α), interleukin‐1β (IL‐1β), IL‐10, interferon gamma (IFNγ), C‐reactive protein, tissue inhibitor of metalloproteinases 1 (TIMP‐1), or vascular cell adhesion molecule 1 (VCAM‐1), similar to observations in *postmortem* biopsies of schizophrenic patients. Finally, the behavioral changes have also been related to neurochemical mechanisms, with an increase in dopamine and homovanillic acid and a decrease in norepinephrine levels [73, 84, 85, 87–90].

Any of the above mechanisms in rodents could also produce behavioral changes in the brains of other intermediate hosts, including humans. Thus, research in humans also sug‐ gests that toxoplasmosis may alter behavior, psychomotor abilities, or personality, with the corresponding clinical consequences [84]. These disorders would be more related to latent rather than acute toxoplasmosis, given that its emergence, frequently several years after pri‐ mary infection and not during the acute phase, would indicate that it results from slow and possibly accumulative changes induced by parasite activity [91–93]. The study by Horacek et al. [76] demonstrates that, in seropositive schizophrenic patients, latent parasitization is associated with a significant reduction in gray matter volume in specific brain areas (cor‐ tical regions, hippocampus, and caudate nucleus), which is not observed in seronegative patients.

Reinforcing the relationship between the parasite and the psychiatric disease, it has been demonstrated that haloperidol, an antipsychotic drug that blocks D2 dopaminergic recep‐ tors in the mesolimbic system and often used in the symptomatic treatment of schizophrenia, inhibits the replication of tachyzoites in cell cultures *in vitro*. This effect may at least partly be due to the capacity of this drug to inhibit calcium transport, blocking cell ion channels [94]. The interaction between tachyzoites and host cells is calcium‐dependent; hence, cell inva‐ sion capacity can be inhibited by the presence of drugs that block calcium channels, such as haloperidol [95]. Experimental studies with rodents have also demonstrated that some behavioral changes caused by the infection are reverted by using the antipsychotic, and that there are fewer parasitized neurons and glial cells after the treatment; this is observed using immunohistochemical techniques [96]. It is therefore possible that its therapeutic effect can be explained in patients with schizophrenia by various mechanisms, given that on the one hand, it blocks dopamine, whose levels are often elevated in schizophrenia patients parasit‐ ized with *T. gondii* [89, 97], and on the other hand, it can inhibit parasite replication in brain cells [96]. Other antipsychotic drugs such as fluphenazine, thioridazine, trifluoperazine, or zuclopenthixol, and mood stabilizers, e.g., valproic acid, were also found to inhibit *T. gondii* proliferation in cell cultures [94, 98, 99].

Antipsychotics are especially indicated in patients with a predominance of positive symp‐ toms and agitation (as in the acute phase of schizophrenia), which are significantly more frequent in those parasitized with *T. gondii*, as noted above. The greater effectiveness of these drugs in these situations may be due not only to their dopamine blocking effect but also to their anti‐*Toxoplasma* activity. Thus, these treatments were found to reduce anti‐*Toxoplasma* antibody levels in seropositive schizophrenic patients, indicating their antiparasitic effect [44]. These findings suggest that these drugs may possibly have a beneficial effect on schizo‐ phrenic patients parasitized with *T. gondii*.

Studies to date on the possible effect in these patients of drugs with anti‐*Toxoplasma* activity (e.g., pyrimethamine, sulfadiazine, azithromycin, or trimethoprim‐sulfamethoxazole) have not demonstrated significant improvements in psychotic symptoms [100, 101]. In fact, drugs used to treat toxoplasmosis are largely active during the tachyzoite replication phase, and their effectiveness against bradyzoites in tissue cysts is drastically reduced once chronic infec‐ tion by *T. gondii* is established [102].

The etiological relationship between parasitization and schizophrenia has not yet been established, despite the above data on behavioral changes in animals or humans and on the effects of antipsychotic drugs on symptoms. In addition, differences in behavioral disorders between humans and rodents may mean that results in animal models cannot be extrapolated to humans. It should also be borne in mind that the mild behavioral modifications associated with the infection cannot necessarily be considered symptoms of a psychotic disease.
