Evaluation of the resistance to Schistosoma mansoni infection in Nectomys squamipes (Rodentia: Cricetidae), a natural host of infection in Brazil.

The development of resistance in three stages throughout an active infection (pre-ovular, acute and initial chronic stages) was studied, comparing the total number of adult worms recovered from the reinfected group and the control groups. It was shown that Nectomys squamipes was unable to develop resistance in the tested conditions and, on the other hand, reinfection in the pre-ovular period of the parasite led the rodent to present the phenomenonacilitation, with reduction of natural resistance and an increase in the parasite load. These results suggest the existence of other forms of immunity diverse from the concomitant immunity in the host-parasite relationship, according to the employed model.

Comparative study on the localization of adult Schistosoma mansoni worms in albino mice anesthetized with pentobarbital sodium, ether or chloroform.

The effect of anesthetic drugs on the localization of adult worms in albino mice was compared. The animals with 56 days of infection were anesthetized with pentobarbital sodium, ether or chloroform. Perfusion was carried out immediately after, recovering the worms and classifying them in relation to their localization on the liver or portal vein and the mesenteric veins. Our results showed that pentobarbital sodium produced a greater displacement of the worms to the liver (89%) than ether (76%) and chloroform (34%) did, when compared to the control group (22%). The difference between pentobarbital sodium and ether was significant (p < 0.05). We suggest that anesthetic drugs may not be used in studies on the distribution of adult worms in several hosts.

Conditions for labeling of Schistosoma mansoni cercaria with technetium-99m.

Study of the labeling of Schistosoma mansoni cercaria with technetium-99m (99mTc) at room temperature (25 degrees C) and 37 degrees C shows that the incorporation of radioactivity in this cercaria increases with the increase in stannous chloride concentration, reaching a constant value threshold at 130.00 microM. Strong binding of the 99mTc was obtained since the radioactivity was not washed out. The characteristic motion of the cercaria, labeled at 25 degrees C and 37 degrees C, was only modified at the concentration of 1300 microM for both temperatures, showing that the methodology here described can be applied to living structures. With this technique it is possible to obtain a cercariae suspension labeled with a radionuclide that is very inexpensive and readily available. Furthermore, it is a gamma emitter with a photon energy of 140 keV that would permit one to make scannings of the infected animals and causes less of an environmental impact and present fewer radioactive disposal problems than the long lived radionuclides.

Natural infection of wild rodents by Schistosoma mansoni. Parasitological aspects.

The evaluation of the role of rodents as natural hosts of Schistosoma mansoni was studied at the Pamparrão Valley, Sumidouro, RJ, with monthly captures and examination of the animals. Twenty-three Nectomys squamipes and 9 Akodon arviculoides with a schistosomal infection rate of 56.5% and 22.2% respectively eliminated a great majority of viable eggs. With a strain isolated from one of the naturally infected N. squamipes, we infected 75% of simpatric Biomphalaria glabrata and in 100% of albino Mus musculus mice. The adult worms, isolated from N. squamipes after perfusion were located mainly in the liver (91.5%) and the mesenteric veins (8.5%). The male/female proportion was of 2:1. The eggs were distributed on small intestine segments (proximal, medial and distal portions) and the large intestine without any significant differences in egg concentration of these segments. In A. arviculoides, the few eggs eliminated by the stools were viable and there was little egg retention on intestinal segments. Considering the ease to complete S. mansoni biological cycle in the Nectomys/Biomphalaria/Nectomys system under laboratory conditions, probably the same is likely to occur in natural conditions. In support to this hypothesis there are also the facts that human mansonic schistosomiasis has a very low prevalence in Sumidouro and endemicity among the rodents has not changed even after repeated treatments of the local patients. Based on our experiments, we conclude that N. squamipes has become a natural host of S. mansoni and possibly may participate in keeping the cycle of schistosomiasis transmission at Pamparrão Valley.

Nectomys squamipes (Rodentia: Cricetidae) as an experimental model for Schistosomiasis mansoni.

Twenty specimens of Nectomys squamipes born in captivity, were infected with 500 cercariae by the transcutaneous route. Coprologic examinations were carried out from the 5th to 23rd week after infection. On the 7th, 8th, 12th, 16th, and 23rd weeks the animals were sacrificed and perfused. The oogram was performed in segments of the small intestine (proximal, medial and distal portions) and the large intestine. The average pre-patent period was of 42 days. The average number of eggs varied from 350 on the 6th week, to 800 on the 13th. From the 14th week on, the average number of eggs eliminated was lower than 50 per gram of feces. The recovery of worms kept steady on the 7th, 8th and 12th week (16.85%; 15.45% and 11.95%), decreasing to 7.70% on the 16th week and 8.45% on the 23rd week. The proportion of male/female worms was about the same on the first two weeks, but from the 12th week on, the proportion was: 1.4/1 on the 12th week; 2.5/1 on the 16th week and 1.8/1 on the 23rd week. These observations suggest that N. squamipes may be used as an experimental model for schistosomiasis mansoni, to which it develops resistance mechanism, useful for immunity studies.

Roedores silvestres como modelos experimentais da esquistossomose mansonica: Akodon arviculoides (Rodentia: Cricetidae) / Wild rodents as experimental models of schistosomiasis mansoni: Akodon arviculoides (Rodentia: Cricetidae)

A infeccao experimental do A. arviculoides atraves de diferentes vias de penetracao das cercarias de Schistosoma mansoni (transcutanea e subcutanea) foi estudada pela cinetica de eliminacao de ovos nas fezes, pela recuperacao dos vermes adultos (no sistema porta e nos vasos mesentericos) e pelo oograma quantitativo. Demonstrou-se que a infeccao do A. arviculoides assemelha-se a dos camundongos albinos que serviram de controle, quanto a eficacia das vias de penetracao, ao habitat dos vermes adultos e ao oograma. Esses resultados sugerem que se avaliem outros aspectos da relacao parasito-hospedeiro para que o A. arviculoides possa ser indicado como modelo experimental alternativo, nos estudos sobre a esquistossomose.

[Wild rodents as experimental models of schistosomiasis mansoni: Akodon arviculoides (Rodentia: Cricetidae)]. / Roedores silvestres como modelos experimentais da esquistossomose mansônica: Akodon arviculoides (Rodentia: Cricetidae).

The experimental infection of A. arviculoides through different routes of penetration of Schistosoma mansoni cercariae (Transcutaneous and subcutaneous) was studied by the kinetics of egg elimination in stools, by the recovery and localization of adult worms (in the portal system and the mesenteric veins) and through the quantitative egg count. It was shown that A. arviculoides infection is similar to the albino mice which served as control in relation to the efficiency of penetration routes, to the adult worms habitat and the egg count. These results suggest that other aspects of the host-parasite relationship should be evaluate so that A. arviculoides may be indicated as an alternative experimental model in schistosomiasis studies.