Infectivity and pathogenicity of three isolates of Ostertagia ostertagi in cattle.

Two laboratory-maintained isolates of Ostertagia ostertagi and 1 recently isolated from grazing cattle, were compared for their infectivity and pathogenicity in Holstein-Friesian calves. Calves were inoculated with 200 000 and 300 000 infective larvae in 2 experiments. Calves in both experiments developed anorexia and diarrhea; generally calves were more severely affected by the larger inoculum. Patent infections and clinical signs developed later in the calves inoculated with the field isolate. Despite decades of laboratory passage of 2 of the isolates and the geographic diversity of all 3 isolates studied, there were no significant differences among them in their infectivity as measured by numbers of nematodes and percentage of arrested development at necropsy and by pathogenicity in calves.