Distinguishing morphological features of the third larval stage of ovine Trichostrongylus spp.

In this paper we describe a procedure that enables the identification of sp ecies of infective third stage (L-3) Trichostrongylus larvae. Lambs were in fected with putatively monospecific infections of three species of Trichost rongylus commonly found in New Zealand (T. axei, T. colubriformis and T. vi trinus) and Teladorsagia (Ostertagia) circumcincta. After recovering L-3 fr om faecal cultures, the lambs were slaughtered and adult male worms recover ed and examined for spicule morphology to verify identification. L3 were ex amined for morphological features and measurement of their length. Further L-3 were exsheathed and examined under high power optics to observe posteri or morphological features (tubercles). The posterior of T. colubriformis ha s a three-tubercle structure whereas T. vitrinus has a single tubercle and T. axei none. However, the tails of T. circumcincta also lack tubercles and thus T. axei cannot be readily distinguished from them on this feature. Th e range of lengths of L-3 of Trichostrongylus spp. (600-858 mu m) and T. ci rcumcincta (700-914 mu m) were found to overlap considerably. The shape of the anterior end of these two species differs and this in combination with length provides an indication of the proportion of T. axei and T. circumcin cta in a culture. A combination of tubercle number, with overall length and anterior morphology of L-3, can be used to differentiate nematode populati ons of T. axei, T. colubriformis, T. vitrinus and T. circumcincta. (C) 2000 Elsevier Science B.V. All rights reserved.