Emergence of larval Protostrongylus boughtoni (Nematoda: Metastrongyloidea) from a snail intermediate host, and subsequent infection in the domestic rabbit (Oryctolagus cuniculus).

Most nematode parasites in the superfamily Metastrongyloidea have heteroxenous life cycles, with transmission to the final host dependent on ingestion of the intermediate host, which is most commonly a gastropod (Anderson et al., 1974. CIH keys to the nematode parasites of vertebrates, No. 1. Commonwealth Agricultural Bureaux, Famham Royal, Bucks, England, 17 p.). Emergence of third-stage larvae from infected gastropods has been reported for nematodes of the metastrongyloid families Angiostrongylidae and Protostrongylidae. Ubelaker et al. (1980, J. Parasit. 66: 856-857) review such cases for species in the genus Angiostrongylus, and several Soviet authors report emergence of infective Protostrongylus larvae from snails (see summaries in Boev, 1975. Studies in nematology. Vol. XXV. Protostrongylidae. Izdatelstvo "Nauka", Moscow, 264 p.) In addition, Monson and Post (1972; J. Parasit. 58: 29-33) mention third-stage larvae of Protostrongylus stilesi emerging from the foot of Vallonia pulchella, and Rose (1957, J. Helminthol. 31: 1-16) and Zmoray et al. (1969, Biologia, Bratislava 24: 113-128) noted Muellerius capillaris larvae leaving decomposing gastropods. The potential exists for transmission of these metastrongyloid species to the definitive host via ingestion of isolated larvae on vegetation and in water. This potential was illustrated by Heyneman and Lim (1967, Science 158: 1057-1058) who determined that Angiostrongylus cantonensis third-stage larvae collected from lettuce were infective to rats. The high prevalence and intensities of Protostrongylus tauricus in Lepus europaeus in the Crimea have been attributed to numerous infective larvae in the environment (Kadenatsii, A. N., 1969. In Contributions to helminthology, S. N. Boev (ed.). Kazakh. Gosudar. Izdatel. Alma Ata. Israel Program for Scientific Translations, Jerusalem, pp. 190-195). However, there is no convincing evidence that ingestion of free-living, infective larvae is important in the transmission of metastrongyloid species. As part of a study on the life cycle and transmission of the snowshoe hare (Lepus americanus) lungworm, Protostrongylus boughtoni Goble and Dougherty, 1943, an investigation was initiated to determine if third-stage larvae would emerge from experimentally infected snails maintained in laboratory culture, and to determine their infectivity. Vallonia pulchella (Miiller), a relatively small (approx. 2 mm diameter) stylommatophoran snail, was exposed to first-stage larvae of P. boughtoni and maintained as detailed elsewhere (Kralka and Samuel, 1984, Can. J. Zool. 62: 473479). At 34 days post-exposure, the number of third-stage larvae in the foot of each of 12 snails was counted. Snails in a Petri dish were misted with water, and when they began crawling, the dish was inverted and examined under a dissecting microscope at 40 x using transmitted light. Larvae were visible as dark brown "C"'s in the translucent foot tissue. Snails were put in individual 32 ml glass vials containing lettuce, and maintained at room temperature. Larvae in the foot of each snail were counted at weekly intervals for 10 wk, and the surfaces of the vial and lettuce examined at 16 x for emerged P. boughtoni. One 3-month-old domestic rabbit, Oryctolagus cuniculus Dutch Belted strain, was exposed to 23 third-stage larvae recovered from the vials and suspended in saline, using a stomach tube (Kralka and Samuel, op. cit.). Feces from the rabbit were examined daily using zinc sulfate flotation until patency, when the rabbit was killed and examined. Isolated P. boughtoni larvae were first found in vials during the second week, and a total of 30 larvae sporadically emerged from 6 of 12 infected V. pulchella during the experiment (Table I). Seven snails died after 5 to 7 wk of the experiment, and all third-stage larvae remained in