STABILITY OF NEMATODE PARASITE COMMUNITIES OF SHEEP IN A POLISH FLOCKIN RELATION TO YEARS, SEASONS AND RESISTANCE STATUS OF HOSTS

The infection of sheep with gastrointestinal nematodes was studied dur ing 4 consecutive years in a flock from southern Poland in which ewes were grazed from May to October and lambs from June to October. Each m onth during the grazing season, 1 ewe and 2 lambs were necropsied and their worm burdens established at the species level. At the end of the grazing season in 1993 and 1994, 12 lambs selected from the progeny o f 2 resistant sires (as established by ranking the faecal egg counts o f all their progeny) and 12 from 2 susceptible sires (plus 12 from int ermediate sires in 1994), were sampled for individual egg counts and f aecal culture. Subsequently they were necropsied to verify if resistan ce was specific. Teladorsagia circumcincta and Haemonchus contortus we re the predominant species present; Trichostrongylus vitrinus, Trichos trongylus axei, Nematodirus filicollis and Cooperia curticei were also present but less abundant. Trichostrongylus colubriformis, Nematodiru s helvetianus, Strongyloides papillosus, Chabertia ovina, and Trichuri s ovis were present in low numbers except in the last year when S. pap illosus was more common. Nematodirus spp, was more common in worm burd ens of the lambs than in ewes (25% oi <2%). Male lambs were more commo nly infected with T. circumcincta and S. papillosus than females but l ess commonly infected with N. filicollis and C. curticei. Lambs of res istant sires had around half as many worms as those from susceptible s ires; although the species composition was essentially similar, except for a somewhat lower proportion of I: circumcincta in 1994 in lambs o f resistant sires. The diversity of nematodes present was assessed in terms of species richness, the Shannon index and the equitability inde x of Pielou. Except for higher richness values in 1992, these indices were not affected by the month of the grazing season, the year, the ty pe of host (ewe or lamb), the sex or the resistance status of the lamb s, despite minor differences in species composition. This indicated a high stability of the species diversity in this nock through time as w ell as between host characteristics.