The reaction of calves to helminth infection under natural grazing conditions

Abstract

The reaction to helminth infection was studied in calves grazing natural pasture in an area of south-eastern Queensland. Calves raised worm-free were exposed each month to natural infections with Cooperia punctata, C. pectinata, Trichostrongylus axei, Haemonchus placei, and Oesophagostomum radiatum. The observations extended over 2 years. Calves first exposed during the months of higher rainfall (December–April) suffered a mortality of 93%, whereas the mortality was 63% in those placed on the pasture during the drier months. Weight loss and death of calves during the first 8 weeks' grazing was associated with a pasture larval contamination of more than 50 larvae of the above species per sq yd. Evidence was obtained that O. radiatum was the main pathogen during the experimental period, and egg counts of this species greater than 380 eggs per gram of faeces were associated with clinical signs of infection. Some calves, which were initially exposed to infection shortly after seasonal rains had fallen, acquired the highest worm burdens and survived for less than 10 weeks. Calves which survived for 11–22 weeks were on a better than average pasture, and were first exposed to infection at least 4 weeks after suitable rains when the larval population had commenced to decline. These calves acquired approximately half the worm burden recorded for the calves which did not survive beyond 10 weeks. Calves surviving for longer than 22 weeks were first exposed to infection 2–3 months after seasonal rains, and showed a mean maximum O. radiatum egg count less than two-thirds of that recorded for the animals which survived for 11–22 weeks. Body weights in the group surviving for more than 22 weeks did not increase until after 20 weeks, when the egg counts had decreased to consistently low levels. Infective larvae of O. radiatum ingested after the first week did not contribute significantly to the adult population found at necropsy. Despite the inability of these larvae to complete their development, pathogenic effects of the continuous intake continued until about the 20th week. No evidence was found that the presence of haemagglutinating and complement fixing antibodies was correlated with the survival of calves.