Nematospiroides dubius: response of the late fourth-stage larva to anthelmintics in vitro.

Approximately 60% of fourth-stage larvae of Nematospiroides dubius recovered from mice 6 days after infection, developed to the young adult stage when cultured over a 7-day period in a complex medium in vitro. Larvae at the late fourth stage of development were found to be highly susceptible to most broad spectrum anthelmintics under in vitro conditions, the benzimidazole, imidazothiazole, pyrimidine, isothiocyanate and macrocyclic lactone compounds all being active at very low concentrations. Narrow spectrum anthelmintics active only against ascarids, pinworms, filariae, cestodes or trematodes had little or no effect on these larvae. Ineffective also were those chlorinated hydrocarbon, substituted phenol and salicylanilide compounds known to affect Haemonchus but not trichostrongylid worms in general. It is concluded that in vitro assays employing larvae of N. dubius are useful for the stringent screening of compounds for broad spectrum antitrichostrongyle activity. Used in conjunction with in vivo screens employing N. dubius in mice, they also afford means for detecting intrinsic activity against the parasite in a system free from any complicating host pharmacokinetics.