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

In this paper we describe a procedure that enables the identification of species of infective third stage (L 3) Trichostrongylus larvae. Lambs were infected with putatively monospecific infections of three species of Trichostrongylus commonly found in New Zealand ( T. axei, T. colubriformis and T. vitrinus) and Teladorsagia ( Ostertagia) circumcincta. After recovering L 3 from faecal cultures, the lambs were slaughtered and adult male worms recovered and examined for spicule morphology to verify identification. L 3 were examined for morphological features and measurement of their length. Further L 3 were exsheathed and examined under high power optics to observe posterior morphological features (tubercles). The posterior of T. colubriformis has 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. The range of lengths of L 3 of Trichostrongylus spp. (600–858 μm) and T. circumcincta (700–914 μm) 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. circumcincta in a culture. A combination of tubercle number, with overall length and anterior morphology of L 3, can be used to differentiate nematode populations of T. axei, T. colubriformis, T. vitrinus and T. circumcincta.