Anthelmintic resistance in companion animal parasites

Dr. John Gilleard, Professor Department of Comparative Biology and Experimental Medicine, Faculty of Veterinary Medicine, University of Calgary, Calgary, AB

The regular use of anthelmintics has led to widespread drug resistance in many gastro-intestinal nematode species of grazing livestock and horses (Kaplan and Vidyashankar, 2012). Anthelmintic resistance has been shown to occur against multiple anthelmintic classes in multiple phylogenetic groups of nematodes including trichostrongyle species of ruminants, small strongyles of horses and ascarids of horses and poultry (Kaplan and Vidyashankar, 2012)

To date, anthelmintic resistance has been less of a problem in parasites of dogs and cats and there has been considerable debate as to the risk of its emergence. However, in the last few years macrocyclic lactone resistance has been confirmed in the canine heartworm Dirofilaria immitis (Wolstenholme  et al., 2015) and, very recently, resistance to multiple  anthelmintic classes in the canine hookworm Ancylostoma caninum in the USA (Dr Ray Kaplan, University of Georgia, personal communication). These are discussed in more detail below but it is important to emphasize that the tests available to detect anthelmintic resistance in companion animal parasites are not well established or widely applied. Consequently, our ability to detect resistance, particularly in its early stages, is very limited and its true prevalence is unknown. It is also noteworthy that the pattern of resistance emergence in livestock parasites has generally been of a relatively rapid increase in prevalence following the initial confirmed cases. Consequently, the recent reports of anthelmintic resistance in canine heartworm and hookworm are a cause for concern.

Prevention of canine heartworm is dependent on the routine use of macrocyclic lactone drugs. Concerns about potential resistance to this drug class were first raised in the USA by the observation of an increase in “lack of efficacy” reports to the FDA for heartworm control products between 1998 and 2003 (Hampshire, 2005). This led to a protracted debate as to whether this increase was due to the emergence of macrocyclic lactone resistance or just due to increased reporting of control failures due to owner non-compliance. The first peer-review report providing direct evidence of macrocyclic lactone resistance in a D. immitis isolate was a case in Canada of a dog that had been relocated from Louisiana following hurricane Katrina (Bourguinat et al., 2011). Following adulticide treatment, this dog remained microfilaremic for 18 months in spite of being adult antigen negative and having received repeated high dose milbemycin oxime treatments. Subsequently, two additional independent D. immitis isolates from Louisiana, derived from dogs with persistent circulating microfilaria following repeated macrocyclic lactone treatments, have been characterised in detail (Geary et al., 2011; Pulaski et al., 2014). Both of these have been established as experimental infections and shown to be phenotypically resistant to ivermectin by demonstrating the establishment of patent infections in dogs following challenge with mosquito derived L3s in the face of carefully controlled monthly interval treatments with sub-cutaneous ivermectin. This, and further characterisation of these isolates, has unequivocally confirmed the emergence of macrocyclic lactone resistant D. immitis in the Mississippi delta region of the USA (Wolstenholme et al., 2015). The relocation of large numbers of dogs from the region in the wake of hurricane Katrina has raised concerns about potential spread of resistant parasites beyond the region but the current extent and geographic range of the problem is unclear. However, there is currently a lack of sensitive diagnostic tools appropriate for large scale surveys for resistance. 

Pyrantel resistance was confirmed in the dog hookworm Ancylostoma caninum in Australia as long ago as 2007 demonstrating the capacity of this nematode species to develop resistance (Kopp et al., 2007). Pools of L3 larvae derived from 8 shelter dogs were used in a placebo- controlled trial that determined pyrantel efficacy to be just 25.7%. There has been no new confirmed reports of anthelmintic resistance in A. caninum in the peer review literature since that time. However, a lack of anthelmintic treatment effectiveness against A. caninum, particularly in greyhounds, is not uncommon but this has generally been assumed to be due to “larval leak” in which somatic larvae reactivate and develop to adult worms in the small intestine following anthelmintic treatment. However, three independent cases of multiple anthelmintic resistance in A. caninum from the southern US have been confirmed in 2017 by both in  vivo and in vitro testing following their establishment as experimental strains (Dr. Ray Kaplan, University of Georgia, personal communication). These cases suggest that resistance to pyrantel, macrocyclic lactones and benzimidazoles is an emerging problem in the southern US.

In summary, cases of anthelmintic resistance in two highly pathogenic canine nematode parasites have been confirmed in the USA in the last few years. As yet, there are no confirmed reports of anthelmintic resistance in Toxocara species, or in other dog or cat internal parasite species. However, this should be interpreted in the context of a general lack of appropriate diagnostic tools, a lack of routine testing and the tendency of resistance to emerge rapidly following the first cases detected. Consequently, there is a need for much greater vigilance in the assessment of anthelmintic efficacy in companion animal parasites and greater consideration given to the risk of the emergence of resistance when planning preventive anthelmintic treatment protocols. 

Hookworm infections in dogs that are non responsive to treatment (Dr. Andrew Peregrine)

In recent years, there have been increasing reports of dogs in Canada with hookworm infections that are not eliminated by standard treatment protocols (e.g. two treatments, 10-14 days apart). These dogs have usually originated from the southern USA and the infection is typically Ancylostoma caninum. Historically, these cases were thought to be due to the “larval leak” phenomenon. Unlike most dog dewormers, Advantage Multi (moxidectin + imidacloprid) is approved with activity against immature stages of A. caninum. Thus, if the problem is detected at a time of the year when heartworm risk is a concern in Canada, it makes most sense to use Advantage Multi on a monthly basis as a heartworm preventive until November. A fecal sample should then be collected 4-8 weeks after the last treatment to see if there is repopulation of the gut after the product has been discontinued.

If the problem is detected when heartworm infection is not a risk, dogs should be treated twice with Advantage Multi, one month apart. A fecal sample should then be examined 4-8 weeks after the second dose to evaluate efficacy.

If the fecal sample is still positive for hookworm eggs, particularly in dogs originating from the southern USA, it should be recognized that this could be due to both drug resistance and the larval leak phenomenon. Thus, treatment with multiple drug classes at monthly intervals may be required to eliminate an infection. Since Lopatol (nitroscanate) is not available in the USA, but is available in Canada, it makes sense to use this dewormer if resistance to the other three drug classes is suspected.


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Geary, T.G., Bourguinat, C., Prichard, R.K., 2011. Evidence for macrocyclic lactone anthelmintic resistance in dirofilaria immitis. Top. Companion Anim. Med. 26, 186–192.

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Pulaski, C.N., Malone, J.B., Bourguinat, C., Prichard, R., Geary, T., Ward, D., Klei, T.R., Guidry, T., Smith, G.B., Delcambre, B., Bova, J., Pepping, J., Carmichael, J., Schenker, R., Pariaut, R., 2014. Establishment of macrocyclic lactone resistant Dirofilaria immitis isolates in experimentally infected laboratory dogs. Parasites and Vectors 7.

Wolstenholme, A.J., Evans, C.C., Jimenez, P.D., Moorhead, A.R., 2015. The emergence of macrocyclic lactone resistance in the canine heartworm, Dirofilaria immitis. Parasitology 142, 1249–1259.

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