The life history of Echinostoma paraensei sp. n. (Trematoda: Echinostomatidae).

The life history of Echinostoma paraensei, a new Brazilian species with 37 collar spines, has been completed experimentally. The planorbid snail Biomphalaria glabrata serves as first intermediate host, and intramolluscan stages also develop in Physa rivalis. Sporocysts usually develop in the heart. Rediae are of two types, distinguishable by large or small pharynges. Cercariae released 25 or more days postexposure develop into metacercariae in the pericardial sac and kidney of snails, sometimes in various tissues of snails infected with rediae. Adults develop in hamsters, mice, or rats after they are fed metacercariae, but not in pigeons, chicks, or ducklings. The natural final host is not known. In recent papers (Basch and Lie, 1966; Lie, 1966a, 1967; Lie et al., 1965) we described the destructive antagonism of larval echinostomes to trematode sporocysts developing in the same snail. Various echinostome species naturally transmitted by Biomphalaria glabrata (Say) in Brazil are being sought and evaluated to determine their efficiency in destroying Schistosoma mansoni Sambon. This paper describes the life history of a new species of echinostome encountered during our studies. MATERIALS AND METHODS Cercariae of this species appeared in collections of B. glabrata snails made in Belo Horizonte (Bairro Sao Domingos), Caratinga, and Bambui, all in the State of Minas Gerais, Brazil. Snails were shipped by air to San Francisco, California, where we studied the life histories of their trematodes. Cercariae were permitted to encyst in laboratory-bred B. glabrata, and metacercariae were fed to hamsters and albino rats and mice. Eggs from washed hamster stools produced miracidia, which were used principally to infect laboratory-raised albino B. glabrata of a strain obtained from the National Institutes of Health, Bethesda, Maryland. Infected snails were kept in clear plastic, 1-gal aquaria at 24 to 27 C and fed on red leaf lettuce (Lactuca sativa). Techniques for the study of the parasite were the same as in previous studies (Lie, 1963a; 1965; 1966b, c). All measurements are in microns. Received for publication 12 June 1967. * Supported by the University of California International Center for Medical Research and Training (UC ICMRT, Hooper Foundation, San Francisco School of Medicine) with Research Grants TW 00144 from the Office of International Research, and AI-07054-01 from the NIAID, NIH, U. S. Public Health Service; and by Rockefeller Foundation Grant GA-MNS6654. RESULTS Egg and miracidium (Figs. 1, 2) Eggs appear in stools of hamsters and mice 11 to 13 days (in rats 14 to 16 days) after infection, in uncleaved condition, yellow-brown, 104 to 122 by 74 to 86, with thickening at nonoperculated end of shell. Eggs kept in distilled water in a petri dish at 28 C begin to hatch in 11 days. Exposure to light stimulates hatching, which usually occurs in the morning. Newly hatched miracidia swim rapidly in the water for several hours and are positively phototactic. They enter the snail host through its exposed parts while shedding their epidermal plates. Complete penetration takes about half an hour. Miracidia penetrate into B. glabrata and also into Physa rivalis (Maton and Rackett) (sometimes referred to as Aplexa marmorata (Guilding), but we prefer the more commonly used name P. rivalis in the absence of an overall revision of neotropical Physidae). In both snails they develop into sporocysts which produce rediae and these in turn pro-