Experimental Toxoplasmosis in Rats Induced Orally with Eleven Strains of Toxoplasma gondii of Seven Genotypes: Tissue Tropism, Tissue Cyst Size, Neural Lesions, Tissue Cyst Rupture without Reactivation, and Ocular Lesions

Abstract

BackgroundThe protozoan parasite Toxoplasma gondii is one of the most widely distributed and successful parasites. Toxoplasma gondii alters rodent behavior such that infected rodents reverse their fear of cat odor, and indeed are attracted rather than repelled by feline urine. The location of the parasite encysted in the brain may influence this behavior. However, most studies are based on the highly susceptible rodent, the mouse.Methodology/Principal FindingsLatent toxoplasmosis was induced in rats (10 rats per T. gondii strains) of the same age, strain, and sex, after oral inoculation with oocysts (natural route and natural stage of infection) of 11 T. gondii strains of seven genotypes. Rats were euthanized at two months post inoculation (p.i.) to investigate whether the parasite genotype affects the distribution, location, tissue cyst size, or lesions. Tissue cysts were enumerated in different regions of the brains, both in histological sections as well in saline homogenates. Tissue cysts were found in all regions of the brain. The tissue cyst density in different brain regions varied extensively between rats with many regions highly infected in some animals. Overall, the colliculus was most highly infected although there was a large amount of variability. The cerebral cortex, thalamus, and cerebellum had higher tissue cyst densities and two strains exhibited tropism for the colliculus and olfactory bulb. Histologically, lesions were confined to the brain and eyes. Tissue cyst rupture was frequent with no clear evidence for reactivation of tachyzoites. Ocular lesions were found in 23 (25%) of 92 rat eyes at two months p.i. The predominant lesion was focal inflammation in the retina. Tissue cysts were seen in the sclera of one and in the optic nerve of two rats. The choroid was not affected. Only tissue cysts, not active tachyzoite infections, were detected. Tissue cysts were seen in histological sections of tongue of 20 rats but not in myocardium and leg muscle.Conclusion/SignificanceThis study reevaluated in depth the rat model of toxoplasmosis visualizing cyst rupture and clarified many aspects of the biology of the parasite useful for future investigations.

Reexamining Chronic Toxoplasma gondii Infection: Surprising Activity for a “Dormant” Parasite

“…On a more practical note, the brain is easily recovered intact for histological studies and easily homogenized while maintaining the integrity of tissue cysts which can be quantified microscopically or purified using Percoll gradients as originally developed [49] and subsequently refined [21••]. Studies, to establish the tissue cyst burden in rodents, both mice and rats, have been reviewed in detail [50, 51••]. Of note, tissue cyst burdens can vary significantly, with these differences impacted by the vertebrate host, host strain, as well as the toxoplasma strain being used [24, 51••].…”

“…Studies, to establish the tissue cyst burden in rodents, both mice and rats, have been reviewed in detail [50, 51••]. Of note, tissue cyst burdens can vary significantly, with these differences impacted by the vertebrate host, host strain, as well as the toxoplasma strain being used [24, 51••]. This variability appears to be intrinsic as seen with our experience using the Type II ME49 strain, serially passaged by i.p.…”

“…Cyst numbers, particularly at later time points, can be the result of reactivation mediated re-seeding in addition to the cysts established at the onset of the chronic infection. This does not appear to be the case in rats, where cyst rupture without reactivation occurred frequently [51••] or in CBA/J mice where the loss of a cyst was essentially balanced by its apparent replacement, implying that vast majority of bradyzoites within the cyst were cleared [21••]. This equilibrium appears to be responsible for the average cyst burden remaining relatively stable across the 5 time points tested (weeks 3, 4, 5, 6, and 8 post-infection) [21••].…”

“…The size of tissue cysts, measured in situ [11, 51••, 52], in homogenized brain tissues [51••, 53, 54] and following purification in Percoll gradients [21••] has been shown to cover a large range from 20 to over 100 microns in diameter. Of note, in rats, tissue cysts rarely reach a diameter of 70 microns, and despite variability between toxoplasma strains, no clear pattern is observed [51••].…”

“…Of note, in rats, tissue cysts rarely reach a diameter of 70 microns, and despite variability between toxoplasma strains, no clear pattern is observed [51••]. These distributions varied somewhat based on the specimens being measured with measurements from histological and electron microscopy tending to be smaller than that seen for specimens measured in brain homogenates and following tissue cyst purification.…”