Comparative Morphology of Infective Nematode Larvae of Kansas Beef Cattle and Its Use in Estimating Incidence of Nematodiasis in Cattle

Abstract

The transmission of parasitic intestinal roundworms of cattle, with the exception of the genera Neoascaris and Trichuris, is accomplished by ingestion of hatched larvae which have developed to the infective stage. Third stage (infective) larvae of the genera Bunostomum, Strongyloides, and Oesophagostomum can also invade the definitive host per cutaneously. The definitive diagnosis of roundworm infections is based on either the identification of the parasites at post mortem examination or the identification of their eggs by fecal examination. The latter method of diagnosis of helminthic infection is not always reliable because of similarity in the size and shape of roundworm eggs. Only eggs of the genera Neoascaris, Strongyloides, Nematodirus, Trichuris, and Bunostomum can be recognized with certainty in fecal examinations. The eggs of other intestinal roundworms of cattle can be placed into the following groups on the basis of similarities in morphology; the Haemonchus-Nodular (Oesophagostomum) group and the Cooperia-Ostertagia-Trichoslrongylus group (Porter et al, 1941). Dikmans and Andrews (1933) separated the latter five genera parasitizing sheep on the basis of the morphology of their infective larvae. Keith (1953) published on the morphology of infective larvae of some common nematode parasites of beef cattle in Australia. No work has been published summarizing the distinguishing morphological characters of infective larvae of beef cattle in the United States. The first objective of the present study was to investigate the morphology of infective nematode larvae of beef cattle in Kansas and to determine whether these larvae could be differentiated at the generic and species level. When this objective was completed, it was possible to determine the applicability of differential larval counts as a method for estimating seasonal incidence of nematodiasis in range cattle. The third objective was to evaluate the accuracy of the total and differential counts of eggs obtained by the egg-count method with that of the larval-count method. MATERIALS AND METHODS The following procedures were used to obtain infective larvae for the morphological and incidence studies. Eggs of the various genera of nematodes parasitic in beef cattle were obtained from female worms collected from beef cattle slaughtered at a local packing plant. The only genera not collected were Trichostrongylus and Strongyloides. Larvae of these genera were obtained from mixed cultures of fresh calf manure. The female worms collected from cattle intestines were stimulated to oviposit by incubating them at 25?-28? C for 6-8 hours in Petri dishes