Effect of Trypanosoma lewisi ( Kinetoplastida : Trypanosomatidae ) on the infection of white rats with Toxoplasma gondii ( Eucoccidia : Sarcocystidae ) oocysts

White rats were inoculated with 1 ()6 trypomastigotes of TT}panosoma lewisi, simultaneously or two days before and after inoculation with lOS oocysts of T. gondií. A greater number of cysts was found in the brain of the animals having concomitant inoculations, as compared with rats inoculated with either one of the two par­ asites. An apparent immunosuppressive effec! is Iikely. Since both orgartisms can be found in rats, ít is possible that infections with T. lewisi, could make this rodent another intermediate host for Toxoplasma infections.

The rat, natural host of Trypanosoma lewisi as well as• a satisfactory model of Toxoplasma gondii infection of human origin, is naturally resistant to this parasite (Ruchmand & Fowler 1951, Lainson 1955, Chinchilla el al. 1981).We have studied the interactions of both parasites in concurrent infections (Guerrero el al. 1997).In this paper we described an apparent 1. lewisi-induced immunosuppression which enhanced T. gondii multiplication using tachyzoites to infect the animals.Since ingestion of oocysts appears to be the natural infection way in this coccidian (Frenkel et al. 1970), several experiments were designed to explore if T. lewisi had any similar effect to that found for tachyzoites, on oocysts infections in the white rat.For the experiments, 35 Wistar white rats (1 OOg body weight) were inoculated intraperitoneally with 1 ()6 T. lewisi trypomastigotes and 1 ()6 T. gondii oocysts obtained according to the Dubey et al. procedure (1970).Five groups of seven rats each were separated and infected acoording to the next scheme: Group l. T. gondii oocysts irioculated two days before T. lewisi infection.Group 2. Simultaneous infection with oocysts of T. gondii and trypomastigotes of T. lewisi.
Group 3. T. gondii oocysts inoculated two days after T: lewisi infection.The Tukey HSD test was used for the sta tistical analysis.
Apparently there are les s brain cysts in animals inoculated with T. gondii after try panosome infection, but this is not statistically significartt.However, we believe that this is a biological phenomenom: animals are.used as the experimental unit and the cyst distribution is not homogeneous, therefore by increasing the number of rats, differences between groups 1,2 and 3 might become significant.
Our results suggest that T. lewisi infec tions induce an increase of Toxoplasma multi plication in the rats after oocyst ingestion.This finding is interesting from the immunological and epidemiological point of view.
Immunosuppression in trypanosome models has been found.Darji et al (1992) explained the mechanisms for that effect in African tri panosomes.Later, the role of gai:nma interfer on in this phenomenon was considered (Darji et al. 1996), and Chanon and Kasper (1996) have found that T. gondií induces immunosup pressive factor such as gamma interferon.
Something similar could happen for our model with T. lewisi.On the other hand, although mice are the first source of T.gondii infection for cats (FrenkeI1973), rats may become.nat urally infected in lhe wild with this parasite (Chinchilla 1978) in spite of their innate resis tance (Chinchilla et al. 1981).Since T. lewisi is a natural parasite for this rodent, it could increase the number of brain cysts, making the rat an important Toxoplasma infectíon source for cats.
Group 4. T. gondii oocysts infection on1y.Group 5. T. lewisi trypomastigotes infection only , REVISTA DE BIOLOGIA TROPICAL Survival time of all animals was measured in 30 d and animal s were killed to search an count Toxoplasma cysts in brains.Parietal and frontal lobes portions (total three) were weighted and the number of cysts were deter mined, to calculate the number per gram(Holst and Chinchilla 1990).