Factors affecting the distribution and transmission of Elaphostrongylus rangiferi (Protostrongylidae) in caribou (Rangifer tarandus caribou) of Newfoundland, Canada

Elaphostrongylus rangiferi was introduced to caribou (Rangifer tarandus car ibou) of Newfoundland by infected reindeer (R. t. tarandus) from Norway and has caused at least two epizootics of cerebrospinal elaphostrongylosis (CS E), a debilitating neurologic disease. In an attempt to understand the cond itions necessary for such outbreaks, we examined the effects of herd densit y and climatic factors on parasite abundance. The abundance of E. rangiferi was represented by counts of first-stage larvae in feces collected from yo ung caribou (calves and yearlings) in 7 distinct caribou herds in Newfoundl and. Abundance of E. rangiferi was highest in February and in the Avalon (6 32 +/- 14 (mean +/- SE)) and St. Anthony (526 +/- 145) herds, the 2 herds i n which CSE was most frequently reported. Mean abundance in February sample s from young animals correlated positively with mean annual minimum tempera ture (r(S) = 0.829, df = 6, P = 0.04) and the number of days per year above 0 degreesC (r(S) = 0.812, df = 6, P = 0.05) and negatively with mean summe r temperatures (r(S) = -0.830, df = 6, P = 0.04). Results suggest that abun dance of E. rangiferi and the likelihood of cases of CSE are increased by m oderate summer temperatures suitable for the activity and infection of gast ropod intermediate hosts and by mild winters with little snow that extend t he transmission period. Abundance of larvae was not correlated with herd de nsity. Animals in all 7 herds also had the muscle worm Parelaphostrongylus andersoni, a related nematode with similar dorsal-spined larvae. In 2 addit ional herds (Cape Shore and Bay de Verde), P. andersoni occurred alone and larvae were passed only by young caribou. In herds with dual infections, nu mbers of P. andersoni larvae were depressed, declined more quickly in young animals, and were considered to be present in only low numbers in February samples used for E. rangiferi analysis. Upon initial infection, young cari bou develop a resistance to E. rangiferi that prevents or reduces reinfecti on later in life. This was demonstrated by examining the brains of caribou for recently acquired worms, which must develop there for up to 90 days bef ore continuing their tissue migration into the skeletal muscles. Recent inf ections were detected in only calves and yearlings in all herds with E. ran giferi except the Avalon herd, where developing worms were also found on th e brains of older caribou. The infection of older animals in the Avalon her d may reflect a lower immunocompetence of a naive herd that has only recent ly been exposed to E. rangiferi.