Intraspecific variation in Schistosoma mansoni infection and modulation of its expression by vertebrate host genetics was studied by evaluation of some biological parameters of the infection in BALB/c and C57BL/6 mice infected with one Brazilian (BH) and two Venezuelan (YT and SM) laboratory strains of the parasite. Mice infected with 60 cercariae of each parasite strain were euthanized at 5, 6, 8, and 12 weeks. Parameters recorded included the number of adult worms recovered by portal perfusion (infectivity); the number of eggs in the feces, the intestine, and the liver; and the ability of the eggs to cross the intestine, expressed as a quotient of the number of eggs in the intestine versus the feces. Results showed that the parasite appeared to determine the infectivity, the sex ratio, the onset and timing of oviposition, the number of eggs produced, initial egg laying toward the liver, and the ability to cross the intestinal wall. In this sense the BH strain appeared to be the most efficient and the SM strain, the most delayed; the YT strain was intermediate, although closer to the SM strain. On the other hand, the host appeared to influence the susceptibility to infection, the fecundity, and the percentage of eggs distributed in the liver and in the intestine during the chronic stage. In this sense, although they have been shown to be less susceptible to infection than BALB/c mice, C57BL/6 mice permit more eggs to be produced and exhibit similar numbers of eggs in the intestine and the liver at certain time points. It appears from these results that parasite genetics is essential for the outcome of infection with S. mansoni, but some characteristics may be quantitatively modulated by host genetics.

En el presente estudio se demostró que Teladorsagia circumcincta (Stadelmann, 1894) y T. trifurcata (Ransom, 1907), nemátodes Trichostrongylidae de la sub-família Ostertagiinae, parasitan un espectro de hospedadores similares. La ausencia de barreras reproductivas entre T. trifurcata y T. circumcincta, así como la estabilización rápida de las proporciones de las dos entidades en el seno de la población, evidencia que T. trifurcata es un morfo de T. circumcincta.
It is shown in this study that two species of nematodes: Teladorsagia circumcincta (Stadelmann, 1894) and T. trifurcata (Ransom, 1907), (Nemátoda, Trichostrongylidae, subfamily Ostertagiinae), have a similar range of host species. Furthermore, the abscence of reproductive barriers, as well as the fact that both forms soon reach stable proportions in the host population, indicate that T. trifurcata is a morph of T. circumcincta

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INFECTION DE LYMNAEA CUBENSIS PAR FASCIOLA HEPATICA dans une region d'altitude, au Venezuela. G. MORALES et L. A. PINO* RÉSUMÉ. Dans le foyer de distomatose étudié, situé dans l'État de Trujillo à plus de 1000 m d'altitude, où presque 100 % des Bovins sont parasités, le vecteur est Lymnaea cubensis ; le quart des mollusques est infecté et héberge en moyenne 20 rédies, capables d'émettre chacune 16 cercaires. La fréquence du parasitisme des mollusques augmente avec leur taille et est maximale chez les limnées de 4-5 mm de long, mais les limnées de 3 mm produisent déjà des cercaires et les traitements par mollusquicides doivent être effectués dès qu'apparaissent des formes de cette taille.

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