Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

First Advisor

James C. Williams


This experiment was conducted to further define epidemiological events involved in the induction, duration and maturation phases of inhibited development of Ostertagia ostertagi in naturally infected calves and to characterize some aspects of the immune response associated with the inhibition phenomenon. During March, April and May 1988, 3 groups of 18 calves were grazed on the same experimental pasture for 3 weeks. At the end of each grazing period, 2 calves of each group were slaughtered for analysis of worm population; the remaining calves were divided into 2 subgroups of 8 calves. One subgroup was returned to pasture while the other was placed in confinement. At 2-week intervals herbage samples were collected for larval recovery. Large numbers of O. ostertagi early 4th-stage larvae (EL4) were observed in calves placed on pasture in March and April, while calves introduced in May had smaller numbers of parasites and lower percentages of EL4. Ostertagia ostertagi larval inhibition was attributed to seasonal factors. Similar level of EL4 in grazed and confined calves, suggested that inhibition was not induced by a density-dependent phenomenon. Maturation occurred in September, i.e., as spontaneous and marked qualitative changes in the worm population. This supported the theory of a similarity of nematode larval inhibition and diapause in insects as a means of survival during periods of environmental adversity. No differences were observed in plasma pepsinogen values among the 3 groups at the time of maturation. Numbers of O. ostertagi and the total number of worms in grazed calves and those in confinement were not different. Lengths of adult worms and numbers of eggs in utero were significantly smaller (P $<$ 0.05) in grazed calves through September. However, differences in vulval flap, lengths of EL4 and lengths of spicules were not significant. Grazed calves exhibited higher IgG titers. No response of lymphocytes stimulated with O. ostertagi L3 antigen was observed until the end of July, but a significant lymphocyte proliferation occurred in August. Abomasal lymph node lymphocytes displayed higher levels of proliferation than those from internal iliac lymph nodes and peripheral blood.