Annual variation in serum antibody concentrations against gastrointestinal nematodes in beef calves from semi-arid rangelands of western Canada.

Sera from calves originating on a 17,000 ha ranch in southeastern Alberta, were collected over three consecutive years (2008-2010) at weaning and analysed for the presence of anti-Ostertagia antibodies using the SVANOVIR(®)Ostertagia ostertagi-Ab ELISA kit. Faecal samples taken at the same time were analysed for presence of nematode eggs using the modified Wisconsin faecal flotation technique. Mean optical density ratio (ODR) values for all calves were similar in 2008 and 2010 while the 2009 values were significantly higher. Similar cumulative frequency distribution profiles were found for 2008 and 2010 while the 2009 profile had an increased number of values in the higher bin categories. This pattern indicates higher nematode transmission on pasture in 2009. There was no significant difference in ODR values between egg positive and egg-negative calves and no significant correlation was detected between individual ODR values and faecal egg output. Significantly higher ODR values in 2009 appear contradictory to general assumptions about the ecological requirements of larval development, as 2009 was a relatively dry year. However, examination of weather data identified 5-6 rainfall events that may have resulted in episodic enhanced survival of nematode larvae. Crown