Epidemiology and control of bovine ostertagiasis in South America.

Gastrointestinal parasitism has been recognized by practitioners as the most common disease in beef cattle, mainly in weaning calves and fattening steers. Among the different genera, Ostertagia ostertagi is the predominant parasite in the temperate climate, in which the major beef and dairy cattle area of South America is situated. Outbreaks of Type I ostertagiasis are usually seen after weaning time (autumn-winter) when larvae counts are high and food availability is low. The development of the disease is rapidly established and 15-30 kg are lost in 30-50 days. Epidemiological studies have demonstrated a fast evolution of parasite eggs to larvae (L3) in summer (1 week or less), evolution being 30-45 days during winter. Inhibition O. ostertagi occurs during spring (September-December) and development resumes in late summer and early autumn. The production effect is seen as a significant reduction in body weight gain and occasionally clinical Type II ostertagiasis appears. A similar epidemiological pattern of inhibition of Ostertagia sp. has been recorded in Uruguay and temperate areas in southern Brazil.

Metabolism and growth in housed calves given a morantel sustained release bolus and exposed to natural trichostrongyle infection.

Three groups of calves, previously grazed on permanent pastures contaminated with bovine trichostrongyle (Ostertagia and Cooperia species) infective larvae, were housed from October to May. During the grazing season one group had received fenbendazole at fortnightly intervals to suppress trichostrongyle infections, one received a morantel sustained release bolus before grazing to limit trichostrongyle contamination of the pasture, and the control group was only medicated when heavy infections caused clinical type 1 ostertagiasis. Digestibility of the whole diet was poorer in the control cattle, particularly for dry matter, crude protein and energy fractions. Balance studies conducted throughout the winter housed period showed that both increased faecal and urinary nitrogen outputs in the control animals contributed to a significantly reduced overall nitrogen retention. The effects were most apparent during clinical type 2 ostertagiasis, which occurred in March in the control group.

Production, parasitological and carcase evaluation studies in steers exposed to trichostrongyle infection and treated with a morantel bolus or fenbendazole in two consecutive grazing seasons.

Friesian steers were exposed to natural trichostrongyle infection and left untreated (controls) or treated with either a morantel sustained release bolus or fenbendazole at two week intervals (clean) in each of two consecutive grazing seasons. In the second grazing season the mean liveweight gains of the clean, morantel treated and control groups of cattle over 152 days were 105, 131 and 109 kg respectively (morantel treated greater than clean, control P less than 0.02). The cattle were slaughtered following an indoor fattening period. The dressed carcase weight, killing out percentage and related carcase measurements were superior in the treated groups. Eighty per cent of the clean and morantel treated cattle graded satisfactorily and qualified for subsidy payment whereas less than 30 per cent of the control cattle were successful. Rib joint analyses showed significantly lower total weight, eye muscle weight and area together with a higher bone content in control cattle.