Bighorn sheep pneumonia: transmission dynamics of pathogens

Bindu Raghavan

Pneumonia in bighorn sheep (BHS; Ovis canadensis) transforms from an epizootic, with all-age die-offs, to an enzootic form with recurrent lamb deaths in subsequent years. This suggests the presence of carriers of pathogens within the herd that transmit pneumonia to other naïve animals. We commingled three naïve, pregnant bighorn ewes with two rams from a wild, previously pneumonic, herd and known to be carriers of the main pathogens, Mycoplasma ovipneumoniae and leukotoxin-producing (lkt+) members of Pasteurellaceae. All ewes, and lambs born to them, developed mild to severe pneumonia and lambs died of acute bronchopneumonia at 4-6 weeks of age. Lambs born to the surviving ewes in two successive years also died of acute bronchopneumonia, indicating the ewes had become carriers. To test whether abrogation of ewe carrier state would result in lamb survival, we treated four (treatment group) of eight pregnant bighorn ewes with two doses of a macrolide antibiotic, Zuprevo®, known to be highly effective against Pasteurellaceae in cattle, prior to lambing. However, while there was a suppression in carriage for 2-3 weeks, the treatment did not clear Pasteurellaceae. All lambs were exposed to the pathogens and succumbed to acute bronchopneumonia. These results are summarized in Chapter One. To test whether inadequate maternally-derived antibody titers from carrier ewes render bighorn lambs more susceptible to fatal pneumonia, we purified IgG from domestic sheep and administered it intravenously to day-old bighorn lambs (treatment group) born to four carrier ewes positive for lkt+ Pasteurellaceae. Two control lambs received 1X phosphate buffered saline (PBS). All lambs were free of apparent disease until 12 weeks of age. The lambs and ewes were then commingled with two ewes that were carriers for M. ovipneumoniae also. Soon after commingling lambs developed acute bronchopneumonia and succumbed to it. We repeated this experiment with ewes that were carriers for both lkt+ Pasteurellaceae and M. ovipneumoniae and administered higher concentrations of IgG to lambs. However, lambs still succumbed to acute bronchopneumonia by 2-6 weeks of age. Thus, passively derived antibodies seem unable to provide prolonged protection from pneumonia in bighorn lambs. These results are summarized in Chapter two.

Bighorn sheep pneumonia: transmission dynamics of pathogens

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