History and diagnosis of multi-anthelmintic drug resistant hookworms

Emily Singler, VMD - 4/10/2024

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Have you managed a case where your patient keeps coming up positive for hookworms despite appropriate prevention and treatment?

It could be from reinfection or lack of compliance with the treatment plan. But it also could be multi-anthelmintic drug resistance (MADR).

Cassan N. Pulaski, DVM, MPH, PhD, clinical assistant professor and director of the Diagnostic Parasitology Lab at the University of Georgia College of Veterinary Medicine and small animal relief veterinarian, has been studying trends in parasite resistance. She reports that cases of hookworm MADR are more concerning to her than heartworm resistance ever was.

MADR hookworm resistance: Beyond greyhounds

Twenty years ago, hookworms that were resistant to pyrantel pamoate were being reported in racing greyhound populations. These dogs were being treated as a herd: They were routinely dewormed every two weeks without being weighed, meaning some dogs likely received suboptimal doses of dewormer, Pulaski says.

Greyhounds were housed in kennels with sand bedding, which Pulaski describes as “paradise for hookworms.” They were being transported around the country for races, where they mingled with or were housed in close proximity to other dogs.

All of these practices helped to select for mutations coding for resistance against multiple commonly used parasiticides and to propagate these resistant worms around the country.

While resistance to parasiticides in hookworms is no longer just a greyhound problem, the breed is still overrepresented in Pulaski’s consulting work. Pulaski reports that when she sees outbreaks or hot spots of hookworm infestations in other breeds of dog, they are most often tied to a dog park, daycare, kennel, or breeding facility.

Diagnosing MADR hookworms

When Pulaski sees a hookworm positive dog where resistance is suspected, these are the questions she asks:

1. Was the correct dose of dewormer used to treat the patient?

If the patient was underdosed or not treated at an appropriate frequency, they may not have been able to clear their infection.

2. Is this a case of reinfection?

In this case, the treatment may have cleared the infection, only for the dog to be reinfected by hookworm exposure either in their own yard or another site such as a dog park or boarding facility.

3. Is this a case of larval leak syndrome?

In this syndrome, worms are not resistant, but rather encysted in tissue outside the intestinal tract and therefore not affected by oral anthelmintic treatment.

4. What is the dog’s fecal egg count per gram of feces?

Counts can reach as high as 800-1,200 hookworm eggs per gram.

Answering all these questions can help determine if resistance is likely or if another cause for the persistent infection is to blame.

Both history and diagnostic testing results are necessary to rule out these other differentials. Determining whether a dog has been reinfected, for example, requires knowing not only the exposure history and whether the owner is adequately preventing hookworm egg shedding in the dog’s home environment, but also whether a fecal sample was at some point negative after treatment.

Always recheck fecal samples

Pulaski stresses that we must always recheck a fecal sample after treating for intestinal parasites. Performing a fecal egg count reduction test (FECRT), which is different from a standard fecal flotation test, allows for measuring of any reduction in fecal egg counts.

In cases where it appears that dogs are continually infested after appropriate anthelmintic treatment, it is ideal to perform a FECRT before treatment and 10 to 14 days after treatment, depending on the drug used for treatment.

Combining medications to increase efficacy

To help identify cases that are not truly multi-anthelmintic drug resistant (MADR), Pulaski will use certain anthelmintics, either alone or in combination, and then monitor fecal egg counts post-treatment. Advantage Multi has a high enough dose of moxidectin to be effective against most cases of larval leak syndrome, she explains.

For cases that do not respond to Advantage Multi alone, she will use a combination treatment such as Advantage Multi with Drontal Plus or Advantage Multi with three days of Panacur.

This is a strategy that is used a lot in large animal medicine that Pulaski advocates for more use in small animal medicine—combination of medications not so much to improve their spectrum, but rather their efficacy.

Some of these dogs may respond to repeated monthly combination therapy if their fecal egg counts were somewhat reduced after the first round of combination treatment. For these dogs, it is important to look for continued response while also ensuring that the dog is not contaminating their own environment to prevent reinfection.

For dogs with no reduction in their fecal egg count, they are likely infested with MADR hookworms.

“Drug resistant worms tend to produce very high numbers of eggs, and that doesn’t change” even after treatment, Pulaski says.

Dogs with MADR hookworms are unlikely to respond to any of the anthelmintics typically recommended for treating intestinal parasites in dogs.

In the next article in this series, we will examine the extra-label use of a drug for treatment of MADR hookworms and important recommendations for practitioners who elect this treatment option.

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History and diagnosis of multi-anthelmintic drug resistant hookworms

MADR hookworm resistance: Beyond greyhounds

Diagnosing MADR hookworms

Always recheck fecal samples

Combining medications to increase efficacy

Further reading