CROSS-TRANSMISSION EXPERIMENTS WITH Dictyocaulus viviparus ISOLATES FROM ROCKY MOUNTAIN ELK AND CATTLE

¹ Department of Veterinary Medicine, Oregon State University, Corvallis, Oregon 97331

Infectivity of Dictyocaulus viviparus larvae isolated from elk was tested experimentally in bovine calves. Patent infections or clinical symptoms did not develop in four Holstein calves. They were inoculated per os with single doses of 6,000 to 10,000 elk strain D. viviparus infective larvae, (L₃).

Degree of resistance induced by this exposure was evaluated by challenge with homologous D. viviparus L₃, from cattle. Two of the calves were refractory to challenge with 45,000 cattle strain D. viviparus L₃ 19 months after inoculation with elk strain D. viviparus. The other two Holstein calves developed light patent infections after challenge with 21,000 homologous D. viviparus L₃ 5 months after initial inoculation. Infections in these calves were characterized by a longer prepatent period (27 vs 23 days), a shorter patent period (30 vs 43 days), reduced output of larvae in feces, and only slight respiratory changes when compared to the response of a challenge control calf.

Eosinophilia, lymphoid hyperplasia, and interstitial emphysema were the prominent histopathologic changes in the lungs of the four calves 3 months after challenge. These lesions are characteristic of resistant animals.

A patent infection was established in a captive elk calf after oral inoculation with 24,000 cattle strain D. viviparus L₃.