Epidemiology of Ostertagia ostertagi in cow-calf herds in the southeastern USA.

Ostertagia ostertagi is commonly found in the brood cow and nursing calf in the southeastern USA, this information being derived from fecal egg counts, coproculture and necropsy results; however, clinical disease and large burdens of this parasite are rarely reported. Fecal egg counts in brood cows are routinely low and are generally reported to be 10 eggs per gram of feces (EPG) or less. Nematode egg counts in spring-born calves are also generally low prior to weaning; they increase steadily during the spring and summer and peak from late summer to fall weaning. That egg counts in spring-born calves are low for several months after birth is probably a reflection of minimal grazing activity. It appears that Ostertagia ostertagi may be of equal or less importance than other nematode genera for spring-born calves in the southeastern USA. The role that Ostertagia ostertagi plays in fall-born calves or in year-round calving herds has not been adequately investigated. Also, the role that the adult cow plays, with low egg counts and small Ostertagia ostertagi burdens, in contamination of pasture is not understood during either lactation or dry periods. Treatment of beef calves prior to, or at weaning can reduce contamination and transmission of gastrointestinal parasites on pastures which may be subsequently grazed by these or other weaned calves. Data on parasite population dynamics from tracer calf studies in cow-calf herds in the southeastern USA have identified peak periods of transmission and incidence of specific genera or stages. This information, in conjunction with routinely used cattle production and management practices such as time of calving, should provide means to more accurately define optimal timing for strategic parasite treatment programs and their overall effect on beef production in the southeastern USA.