Sequential histological changes in Biomphalaria glabrata during the course of Schistosoma mansoni infection

Abstract

The evidences of susceptibility or resistance to Schistosoma mansoni infection presented by different species of Biomphalaria snails may vary considerably (Newton 1952, Pan 1965, Souza et al. 1995. Such variability can even be observed in snails belonging to a same species, from different geographic areas (Correa et al. 1979). The capacity of an infected snail to shed cercariae, is well correlated with histopathology. In highly susceptible B. glabrata, for instance, sporocysts and cercariae are observed in great number, simply displacing the structures, in the absence of any host tissue reaction. On the other hand, resistant snails, eliminating few cercariae, exhibit marked focal and diffuse hemocyte proliferation in several organs and tissues, usually resulting in encapsulation of disintegrating sporocysts and cercariae (Lie et al. 1980, Lewis et al. 1993.We observed that such histological features indeed reflect resistance. However, it is not a fixed characteristic of a given snail strain, but may gradually develop along the course of infection in snails previously presenting a susceptible pattern of cercarial shedding.A group of highly susceptible B. glabrata from Feira de Santana, BA, Brazil, originated from a single egg, were used. A total of 30 snails were submitted to infection with 15 miracidia, during 50 min with a Feira de Santana strain of S. mansoni (Andrade & Sadigursky 1985). Later, each of them was individually exposed to bright light during 40 min, for cercarial shedding and counting, starting 30 days post-infection and continuing thereafter at every 15th day, up to the 142nd after miracidium exposure. At each experimental point, two infected snails were selected for histological examination, the one representing the highest number of eliminated cercariae and the other, the lowest number in a particular day. These snails were anesthetized with menthol crystals before being taken out of the shell, and immediately fixed in Bouin' s fluid for 4 h. They were then washed in 70% alcohol, dehydrated in absolute alcohol, cleared in xylol and embedded in paraffin. Sections were stained with hematoxylin and eosin. The curve representing cercarial shedding (Fig. 1)

Changes induced in Biomphalaria glabrata (Say, 1818) following trials for artificial stimulation of its internal defense system

“…Of course there is close association between a histological picture of hemocyte proliferation, with formation of encapsulating granulomatous-like structures around degrading parasitic forms, and the degree of resistance measured by cercarial elimination (Godoy et al 1977, Lemos & Andrade 2001.…”

“…Sullivan et al (1982) subjected B. glabrata to irradiated Ribeiroia marini miracidia and observed the development of resistance to Echinostoma paraensei. Lemos and Andrade (2001) investigated the reason for a finding already known by others, that cercarial elimination presents a progressively decreasing curve with passing time. Their data disclosed that histological changes similarly and progressively varied from no-reaction at the beginning of cercarial elimination, to diffuse hemocyte proliferation, with formation of encapsulating reactions around sporocysts and developing cercariae, at the final period of observation.…”

Prevalence of Infection of Biomphalaria glabrata by Schistosoma mansoni and the risk of urban Schistosomiasis mansoni in Salvador, Bahia, Brazil

“…Focal and diffuse proliferation of hemocytes accompanied by an expansion of the extracellular matrix in a manner similar to that seen in granulomas, was observed in B. glabrata 16 . Lemos and Andrade 17 proposed that these tissue changes may develop gradually in infected snails that had previously eliminated cercariae. However, these tissue changes do not guarantee complete eradication of the infection, since some sporocysts that remain may be able to complete the development cycle of the parasite, whereby cercariae may be released at any time within 9 months following infection 16 .…”

Molecular Genetic Variations Among Adjacent Populations of Freshwater Snails That Are Target and Non-Target to Schistosoma Infection

“…Such variability can even be observed in snails belonging to the same species , "from different geographical areas 32 Abdel -Hamid Z. Abdel -Hamid et al ( Richards, 1984). Thus, resistance or susceptibility is genetically dependent rather than acquired immunity ( Richards 1976 ) which may ultimately be related to genetic background (Lemos and Andrade, 2001).…”