Study on the inductive factors of hypobiosis of Ostertagia ostertagi in cattle

Two experiments were carried out to determine the causes producing the Oste rtagia ostertagi hypobiosis phenomenon in cattle. In the first experiment, the effect of time on third-stage larvae in the environment was studied dur ing a 2-year period. Three experimental paddocks contaminated with O. oster tagi eggs at different times of the year were used, and the levels of hypob iosis were recorded by using 'indicator' and 'tracer' calves. The results s uggest that time as such is not a hypobiosis-inductive factor. The second e xperiment was conducted under laboratory conditions, where the effects of t emperature and light on infective larvae were studied. Infective larvae wer e subjected to different conditions of temperature and light during 6 weeks , and then inoculated to parasite-naive calves, which were slaughtered afte r 4 weeks. Percentages of hypobiotic larvae in these calves varied from 3.5 to 94.8%, depending on the different storage conditions the larvae underwe nt before inoculation. Results suggest that increasing temperature and incr easing time of light exposure simulating spring conditions would be the fac tors which act upon third-stage larvae inducing them to a later hypobiotic stage in the host. (C) 1999 Elsevier Science B.V. All rights reserved.