Hookworm cases are on the rise—and not just in the South, where warm and humid conditions provide a perfect environment for them. In 2018, the prevalence of hookworms (Ancylostoma caninum) was just four per cent in dogs across the United States. Three years prior, it was only two per cent, meaning, it had risen by almost 50 per cent in that period. In fact, according to a DOGPARCS study,1 prevalence in dogs visiting dog parks is almost 70 per cent.
The reason may be the emergence of drug resistance in the blood-sucking nematodes. A peer-reviewed study published in Clinician’s Brief in August 2020 found half of dogs diagnosed with hookworms in the United States have some level of drug-resistant hookworms.2 That is resistance to all three major classes of dewormers.
That was a shocking finding, says Ray Kaplan, DVM, PhD, DEVPC, DACVM (Parasitology), who was one of the authors. “No one expected that, but the data is absolutely clear.”
What started the resistance
Prior to this, there had never been a single report of drug resistance in hookworms in dogs in the U.S. In fact, while resistance to drugs is common around the world in many species of nematode parasites found in sheep, goats, cattle, and horses, it was not a problem in dogs.
However, the farming of greyhounds for racing—Kansas being the epicentre of the greyhound industry—laid the groundwork for hookworms to become endemic in the breed. Greyhounds are raised on long sand runs to protect their feet, says Michael Dryden, DVM, PhD, professor of veterinary parasitology at Kansas State University (K-State). Sandy soil contaminated by the feces of often hundreds of heavily parasitized dogs offers fertile ground for hookworms to survive and reproduce.
With a large number of dogs being treated for parasites monthly or even more frequently, over a span of decades, the situation was ripe for development of multiple-drug-resistant (MDR) hookworms.
“There’s a huge parasite population that’s under drug selection for resistance,” Dr. Kaplan says. “It requires large, large numbers for this to happen because what makes worms drug-resistant are genetic changes. These mutations are very rare, so you need to have a whole lot of parasites to have a mathematical probability that they are going to occur. And once they occur, then you have to have a situation where they can be amplified and transmitted further.”
The transfer of the dogs from farms to racetracks, and with a percentage eventually adopted into pet homes, enabled just such a situation.
Fighting the resistance
Combined with the popularity of dog parks, where dogs of many different breeds gather, it is no surprise that MDR hookworms began to be found more widely in the general pet dog population, most likely related to the emergence of drug resistance.
“We can’t really prove any of this, but the evidence is very strong,” Kaplan says. “We have genetic evidence, as well as epidemiological evidence supporting this. Can we say it with 100 per cent certainty? No, but we can be very, very confident.”
That means routine diagnostic surveillance of dogs for hookworms is more important than ever. Veterinarians and owners can no longer assume deworming products are successfully ridding dogs of hookworms as a fecal flotation is only a qualitative test. It identifies the presence of hookworms, but not whether they are drug resistant.
Fecal egg count reduction tests (FECRT), detailed in the Clinician’s Brief paper, are necessary to count eggs and measure response to treatment. It is familiar to large-animal veterinarians, but not so much a routine in small-animal practices. To perform an FECRT, the number of eggs per gram of feces must be quantified pre- and post-treatment, with the pre-treatment fecal sample collected the day before or the day of treatment and refrigerated until testing to prevent development and hatching of eggs beforehand.
In their paper, Kaplan and lead author Pablo David Jimenez Castro, DVM, recommend performing two separate fecal egg counts (FECs) on the pretreatment sample and two on the post-treatment sample.
Calculate the FEC reduction by comparing the average eggs per gram (EPG) for both pretreatment and post-treatment FECs. By repeating the FEC, the variability of each FEC measurement will be reduced by half, improving the accuracy of the measured FEC reduction, they write.
An FECRT is sensitive for detecting resistance because dead worms do not shed eggs. Effective treatment produces a high reduction in the number of eggs shed, while ineffective treatment yields a low reduction in eggs shed. Egg-shedding levels are highly variable, making it easy to overinterpret results. For example, 25 per cent and 70 per cent reduction both indicate resistance, but the results should not be interpreted as being greatly different. FEC reduction between 75 per cent and 95 per cent is inconclusive, and the FECRT should be repeated at the next treatment.
“If you have a dog that comes in positive for hookworms and they’ve been on any type of routine treatments, like monthly heartworm treatments that cover intestinal worms, you need to be very suspicious that you have resistance,” Kaplan says. “If you do a flotation that’s positive, you should follow it up with a fecal egg count, so you know how many eggs per gram of feces there are. Then you treat the dog and check again 10 to 14 days later to determine what the efficacy was. If you don’t do the egg count, you really can’t get a good feel for whether there’s resistance or not.”
Performing additional diagnostics
The additional diagnostics require an inexpensive fecal counting chamber. Both types—the McMaster and the mini FLOTAC—work well and are reusable many times. The technique is not difficult to learn, and online videos are available to help hone skills. The FECRTs are the only way to get objective information to properly diagnose and manage cases of hookworms, Kaplan says.
Faced with a case of resistant hookworms, how should veterinarians respond? Some have turned to emodepside, a product formulated for cats. (The drug is approved for use in dogs in other countries, but not approved for use in dogs in the U.S.) To ensure hookworms do not become resistant to emodepside as well, treatment with it should be avoided until a proper diagnostic workup has proven the presence of MDR hookworms and treatment confirms that no other drugs will work.
“Just assuming you have drug-resistant hookworms and starting to use emodepside is not recommended at all and should not be done,” Kaplan says. “Before deciding emodepside is a potential option, you should first confirm that using a triple combination of all three groups of the dewormers is ineffective.”
That is because for now it is the last defense against MDR hookworms. Hookworms can kill dogs—puppies and small dogs are especially susceptible—so if hookworms start developing resistance to it, Dr. Dryden says, “We’re done.” Some kennels already routinely give emodepside to dogs, he adds. “That’s disastrous, in all honesty, and could leave us in grave peril. I know it sounds like hyperbole, but it’s not.”
Taking a step further
A number of combination drugs are available to try before moving on to emodepside. They include praziquantel, pyrantel pamoate, and febantel; imidacloprid and moxidectin; and fenbendazole, moxidectin, and pyrantel, Dryden says. It is important to document they have failed before using emodepside off-label.
What if a dog has a very low level of hookworm infection, with no clinical signs? There are two options. One is to consider using emodepside to clear the hookworms. The other is to keep the dog on the other drugs and advise the owner to maintain strict fecal hygiene to prevent eggs still being passed from contaminating the environment and reinfecting the dog, creating a worse problem.
Hookworms are not only deadly to dogs, but they also have zoonotic potential, so control is essential. A prevention task force is working to develop a set of recommendations to follow in terms of diagnosing and managing hookworms as well as getting the word out about drug resistance and ways to manage those cases.
“It’s not something to panic about, but it’s something to be vigilant about,” Kaplan says.
Kim Campbell Thornton is a frequent and longtime contributor to Veterinary Practice News Canada. She is a Southern California-based freelance writer who specializes in pet-related topics.
1 Detection of gastrointestinal parasitism at recreational canine sites in the USA: the DOGPARCS study. Stafford, K, Kollasch T.M., Duncan, K.T., et al. Parasites & Vectors. 2020. June 1. https://parasitesandvectors.biomedcentral.com/articles/10.1186/s13071-020-04147-6
2 Jimenez Castro PD, Kaplan RM. Persistent or suspected-resistant hookworm infections. Clinician’s Brief. August 2020. Accessed September 2022. https://www.cliniciansbrief.com/article/persistent-or-suspected-resistant-hookworm-infections