Anthelmintic efficacy against tissue-arrested larvae of Ancylostoma caninum in murine hosts.

Third-stage larvae of the major human and canine Ancylostoma hookworm species have the capacity to undergo developmental arrest in the somatic tissues of an infected host. Arrested larvae reactivate at opportune periods such as pregnancy, which results in the transmammary transmission of infection to the nursing neonates. Using murine paratenic hosts to focus specifically on tissue-arrested stages of Ancylostoma caninum, the present study found that neither recommended nor elevated doses of commonly used anthelmintics were effective in eliminating latent infections at the accepted standard of greater than 90% reduction in parasite burden. Of the drugs tested, i.e., pyrantel, fenbendazole, ivermectin, and milbemycin, ivermectin was the most effective and engendered an 80% reduction in the burden of tissue-arrested A. caninum larvae but only if administered repeatedly or at elevated doses. Studies in 2 inbred mouse strains, BALB/c (H-2b) and C57BL/6 (H-2d), that typically display divergent immune responses to various infections showed no significant differences in the efficacies of the drugs tested. The results of this study indicate that there is still a need for effective strategies of eradicating latent infections with tissue-arrested hookworm larvae.