AbstractThe hookworm Ancylostoma caninum is the most prevalent nematode parasite of dogs. Recently, we confirmed multiple-drug resistance (MDR) in several A. caninum isolates to all anthelmintic drug classes approved for the treatment of hookworms in dogs in the United States (USA). Cases of MDR hookworms appear to be highly overrepresented in greyhounds, suggesting that the MDR worms evolved on racing greyhound farms/kennels. The aims of this study were to evaluate the range of drug-resistant phenotypes and genotypes of the A. caninum infecting greyhounds. Fecal samples from recently retired greyhounds originating from geographically diverse areas of the USA were acquired from two greyhound adoption kennels, one active greyhound racing kennel, and three veterinary practices that work with adoption kennels. Fecal egg counts (FECs) were performed on fecal samples from 219 greyhounds, and despite almost all the dogs having been treated with one or more anthelmintics in the previous two to four weeks, the mean FEC was 822.4 eggs per gram (EPG). Resistance to benzimidazoles and macrocyclic lactones were measured using the egg hatch assay (EHA) and the larval development assay (LDA) respectively. We performed 23 EHA and 22 LDA on either individual or pooled feces, representing 81 animals. Mean and median IC50 and IC95 values for the EHA were 5.3 uM, 3.6 uM, and 24.5 uM, 23.4 uM respectively. For the LDA, mean and median IC50 values were 749.8 nM, >1000 nM respectively. These values range from 62 to 68 times higher than those we measured in our susceptible laboratory isolates. Pre-treatment fecal samples could not be obtained, however, post-treatment samples representing 219 greyhounds were collected. For samples collected <10 days post-treatment with albendazole, moxidectin, or a combination of febantel-pyrantel-moxidectin, the mean FEC were 349, 333, and 835 EPG, respectively. Samples collected 10-21 days post-treatment with albendazole, moxidectin, or pyrantel, yielded mean FEC of 1874, 335, and 600 EPG, respectively. Samples collected >21 days post-treatment with albendazole or moxidectin yielded mean FEC of 1819 and 1117 EPG, respectively. We obtained DNA from hookworm eggs isolated from 70 fecal samples, comprised of 60 individual dogs and 10 pools from multiple dogs. Deep sequencing of the isotype 1 β-tubulin gene revealed the presence of the F167Y (TTC>TAC) resistance polymorphism in 99% of these samples, with 69% having ≥75% resistant allele frequency. No resistance-associated polymorphisms were seen at any of the other β-tubulin codons previously reported as associated with benzimidazole resistance in Strongylid nematodes. These clinical, in vitro, and genetic data provide strong evidence that racing and recently retired greyhound dogs in the USA are infected with MDR A. caninum at very high levels in terms of both prevalence and infection intensity.
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