FECAL EGG OUTPUT, CONTAMINATION OF PASTURES AND SERUM PEPSINOGEN CONCENTRATIONS IN HEIFERS WITH NATURAL GASTROINTESTINAL NEMATODE INFECTIONS IN NORTH-WEST SPAIN

In 1988, 1989 and 1990 second year grazing heifers, naturally infected with gastrointestinal nematodes, were separated into two groups, one of which was treated orally with albendazole. In 1988 and 1989 treatme nt was administered immediately after parturition February), and in 19 90 during the last term of pregnancy (December). Both treated and cont rol animals were grazed on separate plots in a rotational system. Maxi mum faecal egg counts were observed around parturition, except in 1990 , when treatment was given at the end of gestation. The main genera id entified were Cooperia, Trichostrongylus, Ostertagia and Oesophagostom um. The number of Ostertagia larvae in the treated groups increased fr om 1989 to 1990, while the others decreased. Pasture contamination wit h third stage larvae (L3) was lower on the plots grazed by treated hei fers. Maximum numbers of L3 were found in autumn, at the end of winter , and at the beginning of spring. Mean serum pepsinogen concentrations were significantly higher in the untreated groups. This concurs with the pattern for L3 on pasture. The trial shows that if a single treatm ent against gastrointestinal nematodes is carried out, and the animals remain on contaminated pastures, the parasitic load tends to level ou t after 4-5 months under favourable climatic conditions. However, the percentages of nematode genera occurring in the new populations may di ffer from those in the original infection.