Epidemiology and Control of Gastrointestinal Nematodes of Cattle in Northern Climates

Gastrointestinal nematodes populations infecting cattle throughout Canada and northern United States are generally lower than in southern states because of shorter grazing seasons and harsh winters. In northern climates, gastrointestinal nematodes diminish cattle weight gains by roughly 0.05-0.15 kg/day during the grazing season. Persistent anthelmintics have been used during the spring to recover some of these losses; however, increasing levels of anthelmintic resistance are decreasing the effectiveness of this control strategy. This is particularly true for species such as Haemonchus placer whose distribution is expanding into northern states even though its larvae are not able to overwinter in pastures.