Abstract

White rats were inoculated with 10⁶ trypomastigotes of Trypanosoma lewisi, simultaneously or two days before and after inoculation with 10⁵ oocysts of T. gondii. 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 parasites. An apparent immunosuppressive effect is likely. Since both organisms can be found in rat, it is possible that infections with T. lewisi, could make this rodent another intermediate host for Toxoplasma infections.
 

Key words

Toxoplasma gondii, Trypanosoma lewisi, immunosuppression, synergism, rats.
 

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 et al. 1981). We have studied the interactions of both parasites in concurrent infections Guerrero et al. 1997). In this paper we described an apparent T. lewisi-induced immunosuppression which enhanced T. gondii multiplication using tachyzoites to infect the animals. Since ingestion of oocysts an 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 (100g body weight) were inoculated intraperitoneally with 106 T. lewisi trypomastigotes and 10⁶ 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 1. T. gondii oocysts inoculated 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.

Group 4. T. gondii oocysts infection only.

Group 5. T. lewisi trypomastigotes infection only

Survival time of all animals was measured in 30 d and animals 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 determined, to calculated the number per gram (Holst and Chinchilla 1990).

The Tukey HSD test was used for the statistical analysis.

Rats inoculated either with T. gondii (gr.4) or T. lewisi (gr.5) did not present Toxoplasma cysts; many rats infected with both parasites (groups, 1,2,3) had evolutive stages of T. gondii (averages number ranged between 221 to 895 cysts).

There was a slight difference between groups 4 and 5 as compared with groups 1, 2 and 3 (P=0.09), but no difference between groups 1, 2 and 3.

Apparently there are less brain cysts in animals inoculated with T. gondii after trypanosome infection, but this is not statistically significant. 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 infections induce an increase of Toxoplasma multiplication 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 tripanosomes. Later, the role of gamma interferon in this phenomenon was considered (Darji et al. 1996), and Chanon and Kasper (1996) have found that T. gondii induces immunosuppressive 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 (Frenkel 1973), rats may become naturally infected in the wild with this parasite (Chinchilla 1978) in spite of their innate resistance (Chinchilla et al. 1981). Since T. lewisi is a natural parasite fo this rodent, it could increase the number of brain cysts, making the rat an important Toxoplasma infection source for cats.
 

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¹Instituto de Acueductos y Alcantarillados, ²Centro de Investigación en Enfermedades Tropicales (CIET), Departamento de Parasitología, Facultad de Microbiología, Universidad de Costa Rica, 2060 San José, Costa Rica.