Experimental Transmission of Protostrongylus stilesi to Bighorn-Mouflon Sheep Hybrids

Abstract

The life cycle of the lungworm, Protostrongylus stilesi Dikmans, 1931, was completed by experimental transmission of infective-stage larvae in the terrestrial snail, Vallonia pulchella, to bighorn X mouflon hybrid (Ovis canadensis canadensis X 0. musimon) sheep. Infection attempts were successful in 3 of 4 cases. Proof of infection was substantiated through identification of male and female adult P. stilesi from lung tissue of the animals. The activity of infective-stage P. stilesi in these hybrid sheep is strongly indicative that similar results do occur in the bighorn sheep. Other variations of the life cycle, i.e., prenatal transmission, may also exist in bighorn sheep. However, the present research has shown that bighorn sheep-terrestrial mollusc-bighorn sheep is no doubt an actual mode of transmission of P. stilesi. To that extent, the life cycle is now known. The life cycle of Protostrongylus stilesi in bighorn sheep (Ovis canadensis) is incompletely known. The adult nematode lives in the lung parenchyma of the definitive host. The eggs are laid in the lung, hatch into firststage larvae, and eventually make their way into the intestinal tract, leaving the body of the host with the feces. The first-stage larva must leave the sheep feces and enter an intermediate host where it develops to the infective stage. This stage must then enter the definitive host. This part of the cycle has been the subject of intensive experimentation (Pillmore, 1955, 1959 a-c, 1961; Post and Winter, 1957; Post, 1958). All species of Protostrongylus with known life cycles require terrestrial molluscs as intermediate hosts. Therefore, studies on the life cycle of P. stilesi have been focused on finding a terrestrial mollusc which will transfer the infection back to the definitive host. Past research has shown several species of molluscs, including Vallonia pulchella, will accept firststage larvae of P. stilesi. Metamorphosis from first to third stage has been observed on several occasions (Honess, 1955; Pillmore, 1955, 1956; Post and Winter, 1957; Post, 1958). Attempts to complete the life cycle of the worm have met with inconclusive results or failure beReceived for publication 13 July 1971. * This research was financed by Rachelwood Wildlife Research Preserve, New Florence, Pennsylvania (Publication No. 11) and administered through the Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado. cause lungworm-free bighorn sheep have not been available for experimentation and domestic sheep appeared to be resistant. The purpose of the present research was to complete the life cycle of P. stilesi by transmission of the infective larvae from the intermediate host to the definitive host. Vallonia pulchella was used as the experimental intermediate host. Definitive hosts were mouflonbighorn sheep hybrids (Ovis musimon X O. canadensis canadensis). MATERIALS AND METHODS First-stage larvae of P. stilesi were obtained from the feces of wild-ranging bighorn sheep in Rocky Mountain National Park, Colorado. Fecal droppings were collected in bighorn sheep bedding areas and air-dried for maximum preservation of larva . A Baermann apparatus was used to separate Ist-stage lungworm larvae from fecal samples. Land snails of the species V. pulchella were obtained from Laramie, Wyoming (Post and Winter, 1957). These snails were collected from a greenhouse-eliminating the possibility of prior exposure to P. stilesi. Hybrid sheep from crosses of bighorn and mouflon were provided by Rachelwood Wildlife Research Preserve, New Florence, Pennsylvania. Two animals were 4 bighorn and 3/ mouflon. The other 2 animals were 1/2 bighorn and 1/2 mouflon. These animals harbored infections of Muellerius sp. and Pneumostrongylus sp. These natural infections were determined by means of preliminary fecal sampling. The sheep were treated with Tramisol (American Cyanamid Co., Princeton, New Jersey) for removal of the Muellerius infection. The animals were considered free of this lungworm when larval lungworm production dropped to zero and remained there through 6 weeks of daily fecal sampling. Pneumostrongylus first-stage larvae remaining in fecal samples were