The Role of the Soil as a Potential Reservoir for Infective Larvae of Ostertagia ostertagi

Abstract

During investigations into the epidemiology and control of bovine parasitic bronchitis and ostertagiasis we have observed outbreaks of these diseases when young cattle were introduced onto silage or hay aftermath grazing in later summer. In one outbreak, helminth-free calves were grazed on a hay aftermath in August and developed clinical signs of parasitic bronchitis within 3 weeks of grazing (Duncan et al., 1979). In another, calves grazed on a pasture with low number of Ostertagia ostertagi infected larvae (L₃) were treated with the anthelnintic, levamisole (ICI Alderley Edge, Macclesfield, England) and transferred to a silage aftermath in late July. A marked increase in O. ostertagi L₃ numbers on the aftermath occurred within 7 days of the calves being introduced and clinical ostertagiasis occurred 4 weeks later (Bairden et al., 1979). In both these outbreaks the aftermath pastures had not been grazed by livestock since the previous autumn and the interval between entry to the aftermath and clinical or other evidence of infection precluded the possibility of the calves themselves being responsible for cycling of the infection. The source of infection in these outbreaks was obscure but one possibility considered by us and previously suggested by Nelson (1977) in relation to Dictyocaulus viviparus larvae, was the existence of a reservoir of infective larvae in the soil which had persisted from previous grazing seasons.