The distribution of Toxoplasma gondii cysts in the brain of a mouse with latent toxoplasmosis: implications for the behavioral manipulation hypothesis

Abstract

Background: The highly prevalent parasite Toxoplasma gondii reportedly manipulates rodent behavior to enhance the likelihood of transmission to its definitive cat host. The proximate mechanisms underlying this adaptive manipulation remain largely unclear, though a growing body of evidence suggests that the parasite-entrained dysregulation of dopamine metabolism plays a central role. Paradoxically, the distribution of the parasite in the brain has received only scant attention.

Methodology/principal findings: The distributions of T. gondii cysts and histopathological lesions in the brains of CD1 mice with latent toxoplasmosis were analyzed using standard histological techniques. Mice were infected per orally with 10 tissue cysts of the avirulent HIF strain of T. gondii at six months of age and examined 18 weeks later. The cysts were distributed throughout the brain and selective tropism of the parasite toward a particular functional system was not observed. Importantly, the cysts were not preferentially associated with the dopaminergic system and absent from the hypothalamic defensive system. The striking interindividual differences in the total parasite load and cyst distribution indicate a probabilistic nature of brain infestation. Still, some brain regions were consistently more infected than others. These included the olfactory bulb, the entorhinal, somatosensory, motor and orbital, frontal association and visual cortices, and, importantly, the hippocampus and the amygdala. By contrast, a consistently low incidence of tissue cysts was recorded in the cerebellum, the pontine nuclei, the caudate putamen and virtually all compact masses of myelinated axons. Numerous perivascular and leptomeningeal infiltrations of inflammatory cells were observed, but they were not associated with intracellular cysts.

Conclusion/significance: The observed pattern of T. gondii distribution stems from uneven brain colonization during acute infection and explains numerous behavioral abnormalities observed in the chronically infected rodents. Thus, the parasite can effectively change behavioral phenotype of infected hosts despite the absence of well targeted tropism.

Figure 1. Transient reduction of body weight in the infected mice.

The abscissa shows body weight, the ordinate shows time (in days) after inoculation. Error bars denote the 95% confidence interval.

Figure 2. Numbers of brain tissue cysts and T. gondii antibody titers are not correlated.

The abscissa shows total numbers of brain cysts, the ordinate shows antibody titers determined by the complement fixation test.

Figure 3. Toxoplasma gondii cysts in the brains of chronically infected CD1 mice.

(A) A pair of cysts (arrows) in the molecular layer of the cerebellar cortex. Arrowheads point to the Purkinje cells. GrL, granular layer; fis, cerebellar fissure; MoL, molecular layer. (B–C) A single cyst (B) and a cyst triad (C) in the isocortex. Scales, 100 µm (in C for B, C).

Figure 4. The distribution of T. gondii cysts in the forebrain of a mouse with latent toxoplasmosis.

Coronal diagrams compile results from five infected CD1 mice. Tissue cyst locations are indicated by color circles (each circle represents a single cyst); colors refer to the individual animals investigated. The cyst density within brain regions is classified into five classes – very high, high, medium, low, and very low, which are colored red, pink, white, blue, and dark blue, respectively. Grey indicates the dopaminergic system. The antero-posterior positions of the sections are determined by Bregma coordinates. See Abbreviations S1 for abbreviations.

Figure 5. The distribution of T. gondii cysts in the caudal forebrain, the cerebellum and the brainstem of a mouse with latent toxoplasmosis.

See caption to Fig. 4 for explanation.

Figure 6. Histopathological lesions associated with latent toxoplasmosis.

(A–E) Perivascular and leptomeningeal infiltrations of inflammatory cells. (A, B) Detail of perivascular cuffing. (C–E) Leptomeningeal and perivascular cuffing on the surface of cerebral cortex (C), in the interhemispheric region (D) and in the cerebellum (E). (F) Extensive necrosis in the hypothalamus (observed in one mouse). (G) Vacuolisation of brain parenchyma in the thalamus. Note that vessels were dilated post mortem by pressure of the perfusion liquids. Scales, 100 µm (A), 200 µm (B, C, G, in G for D–G).