Anthelmintic resistance in the canine hookworm, Ancylostoma caninum

The canine hookworm, Ancylostoma caninum (Nematoda: Strongylida: Ancylosmastoidea) (Figure 1), is the most prevalent and important intestinal nematode parasite of dogs in the USA, as well as in other parts of the world. A survey published in 1996 evaluating 6458 faecal samples from animal shelters across the USA yielded a prevalence for A. caninum of 19.19% (Blagburn et al, 1996). This was followed by a prevalence of only 2.5% from 1 199 293 faecal samples of pet dogs evaluated in 2006, with the prevalence depending on age, level of care and geographic location of the dog (Little et al, 2009). A study evaluating archived data of over 39 million canine faecal samples from 2012–2018 showed a stable prevalence of hookworm infections from 2012–2014 ranging between 1.5% and 2.3%. From 2015, the prevalence began to steadily rise each year, with an overall increase from 2015–2018 of 47% (Drake and Carey, 2019). This decrease in prevalence of more than 75% from 1996 to 2012–2014, even though samples from different populations of dogs were examined, could be largely because the awareness of anthelmintics for companion animals was very different than at the beginning of the 1990s. Additionally, a more frequent treatment to cover gastrointestinal nematodes was pushed by both veterinarians and parasitologists. However, in one large scale study evaluating the prevalence of intestinal parasites in dog parks throughout the USA, 7.1% of the samples were positive for hook-worms and of those, 98.2% were positive for A. caninum (Stafford et al, 2020). Interestingly, this prevalence is more than twice as high as that reported for 2018 by (Drake and Carey, 2019), and is more than 70% higher than the mean prevalence for 2017–2019 reported by Sweet et al (2021). There could have been some degree of underdiagnosing, thus underestimating, these prevalence rates because of technical factors (eg not all the eggs being detected at the faecal floatation because a centrifugal step was not used), or biological factors (eg different maturity of infections, as some could have been too early or too old to detect, and the females could be dying off as a result of senescence). However, taken together, these data suggest that hookworm prevalence is rapidly increasing, and that dogs that visit dog parks are at a higher risk of infection. Also, this shows that anthelmintics labelled for hookworms are showing not to be efficacious, as these pets are most likely to be on a monthly preventive for the canine heartworm, Dirofilaria immitis, and all these products have a drug with a label efficacy against hookworms. The high prevalence of hookworm found in the face of presumed anthelmintic use for heartworm could also be because of poor owner compliance, as demonstrated in European countries, where heartworm prevention should also be in place (McNamara et al, 2018).