PREVALENCE AND EPIDEMIOLOGY OF TRICHOSTRONGYLIDS IN WYOMING CATTLE WITH CONSIDERATION OF THE INHIBITED DEVELOPMENT OF OSTERTAGIA-OSTERTAGI

During 1988, monthly collections of abomasa and the cranial portion of the small intestine of 208 native Wyoming cattle were examined for ad ult and larval helminth parasites. Egg counts were performed on coloni c faecal specimens. The animals ranged from 7 months to 8 years of age and were sampled at slaughter houses or diagnostic laboratories from five different counties in the State. Most of the cattle had ranged fo r a long time on non-irrigated, high altitude grass plains from about 1250 to 2500 m above sea level, All animals examined were found infect ed, Nematode genera (and number of species) found included Ostertagia( 4), Trichostrongylus (2), Haemonchus (1), Cooperia (4) and Nematodirus (2). One trematode, Fasciola hepatica, and a cestode, Moniezia benede ni, also were found. Trichostrongylus axei, Trichostrongylus longispic ularis and Cooperia lyrata were found for the first time in Wyoming, O stertagia circumcincta was found for the first time in cattle in Wyomi ng, and Haemonchus placei, tentatively identified in a previous survey , was definitively confirmed. Predominant species (and prevalence) wer e Ostertagia ostertagi (98%), Cooperia oncophora (60.6%), Ostertagia b isonis (41.8%), Cooperia bisonis (33.6%) and T. axei (27.9%). One or m ore of the four species of Ostertagia were found in all animals. Inhib ited O. ostertagi and O. bisonis larvae predominated from November to April and reached a peak in January to comprise 88% of the total nemat ode burden for that month. The adult population of abomasal nematodes peaked in April and predominated thereafter until October. Haemonchus placei also spent the autumn-winter period in an inhibited state of de velopment, Faecal egg counts were lowest during winter months, when th e highest numbers of worms were present as L(4) larvae, as determined by luminal and mucosal counts. Nematode populations in the small intes tine peaked in June and persisted until November. The results of this study showed the magnitude and species composition of nematode parasit ism in cattle raised on high altitude grass plains, underscored the in accuracy of faecal egg counts as an indicator of worm burden during fa ll and winter seasons, and provided information on which anthelmintic treatment timing would be most efficacious.