MONTHLY FLUCTUATIONS OF WORM BURDENS AND HYPOBIOSIS OF GASTROINTESTINAL NEMATODES OF CALVES IN EXTENSIVE MANAGEMENT-SYSTEMS IN THE PYRENEES(SPAIN)

Monthly fluctuations of worm burdens and arrested development of gastr ointestinal nematodes in cattle from a mountainous region of Spain wer e studied, Fourteen previously helminth-naive calves grazed together w ith a herd of 120 cattle from May to November following the traditiona l extensive grazing system used in mountainous regions of Spain (perma nent calves). Each month, throughout the grazing season, two helminth- naive calves (tracer calves) were added to the herd and allowed to gra ze for 4 weeks. Every 2 weeks, throughout the grazing period, faecal a nd blood samples from the permanent calves, and pasture grass samples for larval recovery were collected. Every 4 weeks, throughout the graz ing period, two tracer and two permanent calves were removed from the herd and housed on concrete for 2 weeks before being slaughtered. The nematode parasite species identified from the animals were: Ostertagia ostertagi, O. lyrata, Teladorsagia circumcincta, Trichostrongylus axe l, Cooperia oncophora, Trichostrongylus longispicularis, Capillaria bo vis, Nematodirus helvetianus, Oesophagostomum radiatum, Chabertia ovin a and Trichuris spp. O. ostertagi was the predominant species, followe d by C. oncophora and T. axei. The highest numbers of worms recovered from the tracer calves were observed in May, June, September and Novem ber with average worm burdens of 4050, 3775, 2775 and 2825, respective ly. These dates corresponded with 2 months of spring grazing in areas below 1000 m (May-June), the last month of grazing in areas higher tha n 1000 m (September), and the last month of autumn grazing in areas be low 1000 m (November), respectively. The highest total worm burden (80 00 worms per animal) was observed in the permanent calves in June afte r 2 months of grazing below 1000 m. The average total worm burden in t he permanent calves during the study was 5825 worms per animal. As in other cool temperate regions of the Northern hemisphere, the highest p ercentage of larval inhibition was observed in autumn, with maximum le vels of 63.4% for Ostertagia spp. and 89.3% for Cooperia oncophora. Si milar inhibition levels were observed in parasites from both tracer an d permanent calves, indicating that previous exposure was not the prim ary cause of larval inhibition.