Cyst burden in the brains of Wistar rats fed Toxoplasma oocysts

Six strains of Toxoplasma oocysts were used to infect groups of 4–24Wistar rats, with each rat being fed 101–104 oocysts from a single strain. After 2 months, the rats were killed, their brains screened for Toxoplasma cysts and then bioassayed in mice if negative. Toxoplasma was either observed in the form of brain cysts or was recovered using the bioassay, from 113 out of 138 (82%) rat brains. As few as ten oocysts were capable of initiating a brain infection that lasted for at least 2 months in eight of the nine rats inoculated. However, judging from bioassay 102–104 oocysts did not give rise to progressively higher rates of infection. Brain cysts were seen in only 68 of 138 rats (49%). The number of Toxoplasma cysts formed in the brains of rats was generally in the order of tens to hundreds. The frequency of infection in the brains with Toxoplasma and the number of brain cysts formed appeared to be influenced by the individual resistance of the rats as well as by the doses of oocysts and the Toxoplasma strains used. The information gathered is considered to be a basis for a rat model of immunity against acquired toxoplasmosis.1–6/1,000 births. Although most infected newborns are asymptomatic at birth, adverse sequelae may develop later in life in a large proportion of the affected patients (Remington and Desmonts 1990). In addition, reactivation of a latent Toxoplasma gondii infection from brain cysts is often fatal in patients who are immunosuppressed in the course of acquired immunodeficiency syndrome (AIDS), during cancer therapy or after organ transplantation (Luft and Hafner 1990). Of the several approaches that can be used to prevent toxoplasmosis in animals and humans, considerable experimental effort has been put into developing protection for animals by vaccination (Nielsen et al 2000). However, to date no efficient vaccination strategy exists to prevent cyst formation in either animals or in humans. Brain cyst formation in rats was reviewed by Dubey and Frenkel (1998) and the burden of brain cysts in rats fed on cysts was investigated by Freyre et al. (2001b). The purpose of this present study was to add to the existing information by investigating the influence of the strain and dose of Toxoplasma cysts on the numbers of cysts formed in the brains of Wistar rats, as a preliminary step towards developing a rat model of immunity against acquired toxoplasmosis.

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