Prevalence of eyeworms (Nematoda: Thelazioidea) in beef cattle grazing different range pasture zones in Alberta, Canada.

The eyes from 410 beef cattle grazing 6 range pasture zones in Alberta were dissected and examined for species of Thelazia. Thirty-eight (9.3%) contained eyeworms: 25 (6.1%) contained Thelazia skrjabini alone, 3 (0.7%) contained Thelazia gulosa alone, and 10 (2.4%) had a mixed infection of T. skrjabini and T. gulosa. Prevalence was related to the type of pasture. The prevalence was lower in cattle grazing short- and mid-grass pastures (zones 1, 2; 0% and 1.5%, respectively) than those grazing transitional or aspen parkland pastures (zones 3, 5; 7.4% and 6.2%, respectively) or rough fescue or woodland type pastures (zones 4, 6; 11.3% and 18.4%, respectively). The data suggest that prevalence is related to pasture type that limits the number of its vector, Musca autumnalis.