Gastropods as intermediate hosts of Pneumostrongylus tenuis Dougherty of white-tailed deer

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Abstract
From May to October, 1966, gastropods from three different habitats on Navy Island, Ontario, were examined for larvae of Pneumostrongylus tenuis. Of 9940 examined individually, 4.2% contained larvae. A mean of 2.9 larvae was recovered from each infected gastropod. Deroceras laeve and Zonitoides nitidas were the most abundant and commonly infected species. The incidence of infective larvae in adult D. laeve from a wet forested area rose to 25% by late June but dropped to 1.5% during July, coinciding with the disappearance of adult slugs. The level of infection in Z. nitidus (4.3%) remained relatively constant. The wet forested habitat where gastropods were abundant, commonly infected, and active from May to October was probably most important for transmission. Experiments showed that snails already containing larvae can be reinfected and there is some evidence that they are repeatedly infected in nature. Larval development was retarded in aestivating snails. Numbers of larvae recovered and their rate of development differed in various species of experimentally infected gastropods. Snails of the family Polygyridae were the most suitable intermediate hosts studied. Larvae passed by deer were situated in the film of mucus covering faecal pellets. In this location, larvae may be dispersed by high spring water or heavy rainfall. Snails became infected when exposed to larvae in wet and dried soil. Larvae on pellets survived freezing at −15 to −20 °C for up to 306 days.