Seasonal infection pattern of gastrointestinal nematodes of beef cattle in Minas Gerais State--Brazil.

Tracer calves were used to assess the seasonality of infections of gastrointestinal parasites in beef cattle extensively raised at a farm in the State of Minas Gerais, Brazil. Tracer calves acquired infections during all months of the year, however, highest worm burdens were observed in the rainy season. The following nematode species were recovered from tracer calves: Cooperia punctata, C. spatulata, C. pectinata, Haemonchus similis, H. placei, Oesophagostomum radiatum, Trichostrongylus colubriformis, T. axei, Bunostomum phlebotomum and Trichuris discolor. Cooperia was most prevalent, representing 74.4% of the total of all nematodes recovered. This was followed by Haemonchus 19.2%, Oesophagostomum 4.5%, Trichostrongylus, Trichuris and Bunostomum, represented less than 1% of the total. Thirty Nellore cows with calves were used to evaluate the dynamics of the infections. The cows were the principal source of contamination to pasture and to infection of suckling calves during the pre-weaning period, and probably, also contributed to increased infection of weaned calves at the beginning of the rainy season. Strongyloides was the first infection observed in the calves. The peak egg count was observed in the 2nd month, however, all Strongyloides egg counts became negative in the 7th month. Strongylate nematode egg counts were low in calves from the 3rd month of age onward, and increased gradually until calves were 1 year old. A gradual reduction then occurred until the end of the trial. From the 3rd month onward Cooperia and Haemonchus infective larvae (L3) were most prevalent in fecal cultures. Fecal cultures after the 4th month also resulted in recoveries of Oesophagostomum, Trichostrongylus, and Bunostomum L3. Cooperia was the predominant genus in fecal cultures until the calves were 9 months old. After the calves were 9 and 12 months of age, L3 of Cooperia and Oesophagostomum, respectively decreased in prevalence. Trichostrongylus and Bunostomum L3 were only minimally present in the fecal cultures at this time.