When Persistence Pays Off for Parasite Prevention

With parasite prevalence increasing, veterinarians must keep up the good fight of educating hesitant pet owners on the importance of prevention.

Dr. Chris Adolph’s interest in parasitology originated from the sheer volume of patients who would visit his veterinary clinic in Oklahoma with some form of parasitic condition. From the late 1990s to 2015, he diagnosed countless numbers of pets with heartworm, intestinal parasites, flea infestations and tick-transmitted diseases.

“It was so prevalent, but I hadn’t paid enough attention to parasitology in veterinary school,” said Adolph, DVM, MS, Diplomate ACVM (Parasitology), senior veterinary specialist, CAD Veterinary Specialty Operations for Zoetis. “So, I started to develop that aspect of my practice by focusing on parasitology.”

He went to as many CE events as he could, educated himself on the science and best practices, then trained his team on those learnings. In 2015, Dr. Adolph passed the American College of Veterinary Microbiologists Parasitology exam.

As Dr. Adolph and his team focused on parasitology, compliance grew. “Compliance grew because we didn’t take the first ‘no,’ for an answer,” he said. If an owner declined their recommendations for heartworm or flea and tick control, Dr. Adolph and his team were ready with brochures, pamphlets, and educational resources. They adopted the attitude that those owners just needed more information so they could make the best decision possible for their family. “We did that with competence because we knew our stuff.”

Clinical and consumer science

Today, Dr. Adolph has an industry-level view of the fight against parasites in pets. In addition to his full-time role as a senior veterinary specialist with Zoetis, Dr. Adolph serves as the vice president of the American Heartworm Society, the Zoetis industry representative for the Companion Animal Parasite Council (CAPC), the ACVM Board of Governors and exam committee, and the Oklahoma Veterinary Medical Association District 1 director. Dr. Adolph also works a few days a month as an associate veterinarian in Owasso, Oklahoma.

Dr. Adolph believes there are several factors that come into play regarding whether a client will purchase parasite control and actually keep up with it. First, veterinarians still have the financial hurdle to overcome. Preventives are often below the budget line of pet owners, and veterinary teams are trying to convince them it’s important enough to get above that line.

“We’re trying to make it so important that it displaces something else they spend that money on,” Dr. Adolph said. “Maybe it’s giving up a cappuccino once a week, or a dinner out. Whatever it is, we’re still competing for those discretionary dollars like we always have.”

Owners today are more knowledgeable about parasite control, which can be both good and bad, depending on the source of their information. Sometimes veterinarians must have myth-busting conversations with pet owners who’ve read something inaccurate on the internet but taken it as fact.

“It’s easier for me to talk to a client who knows nothing than a client who knows the wrong thing,” Dr. Adolph said. For example, last summer, Dr. Adolph pulled three ticks off a dog while discussing the importance of parasite control with the pet owner, who was in the exam room. Despite the evidence right in front of him, the pet owner declined preventives, citing an upcoming trip to the groomer where he would purchase a flea dip. “So, I wasn’t even starting from ground zero,” Dr. Adolph said. “I had to start from a negative number and work my way up to zero on the education of the product.”

Early on in his consulting work, Dr. Adolph watched a video from a focus group of veterinary clients that changed the way he thought of the parasite prevention conversation. There were no veterinarians in the room for the focus group, just pet owners. The interviewer asked for their thoughts on what circumstances would lead them to seek out a veterinarian. Their responses were that the veterinarian was for when their dog or cat was sick, and to be the one to give shots that kept the pet from getting sick. But when it came to the topic of parasite, flea and tick control, the focus group did not feel the veterinarian needed to be in the loop. They said it was their job as pet owners. They would go to the internet for research and find the best over-the-counter medication to use on their pet.

“That was a real eye opener in knowing how clients thought,” said Dr. Adolph. “It wasn’t just one or two pet owners in the focus group – it was all of them. They all really felt it was their job to protect their pet. They essentially said, ‘I go to the vet when my pet is sick or needs shots, but when it comes to flea and tick control, I can go to the pet store or the grocery store. I can go wherever I get milk, bread and eggs. I’ll pick something up there.’ This thinking is something we have to overcome as veterinarians.”

Drug-resistant worms

The veterinary community also must overcome a more threatening parasite population. In 2005, a publication pointed toward the possibility of drug-resistant heartworms, something unheard of for years.

Indeed, since the first once-a-month preventive came out 1987 and more followed in the 1990s, preventives were considered bulletproof. If you gave them on time, every month, they would be 100% effective, the thinking went. “That’s what was taught in veterinary schools, and that’s what veterinarians came to believe,” Dr. Adolph said. “If you use this, it’s 100%, 100% of the time.”

Their effectiveness was due in part to the fact that the parasite population had never seen this class of drugs prior to 1987. But 18 years after the first monthly product was launched, there was worry in the scientific community that some parasites were showing resistance. “There were enough reports to say there’s some smoke here, and we should investigate.” The 2005 publication served as a call to action.

For the next several years, the scientific community gathered samples from dogs that were positive that should not have been. To study those cases, samples were collected, and genetic testing was done. A 2011 study concluded that with the particular strain of worms used in the research, many of the dogs tested were still heartworm positive even after receiving one drug dose from each class.

Soon, a growing body of evidence released concluded that, yes, resistance did exist in the parasite population. There were strains out in the wild that if they got into a dog, not all of the immature worms would die from one dose of parasiticide. Some of the adult worms would grow up to be adults. That population of worms’ offspring would carry a resistant genetic code. And once they got into mosquitoes, they would then get into the ecosystem, and most likely infect non-medicalized dogs. “This is how this phenomenon can grow,” Dr. Adolph said.

New thresholds

Around the same time these conclusions were released, the agency that approves the medications changed the approval process. Previously, criteria for approval included a lab study where you took a strain of heartworms that had been collected from a naturally infected dog. After testing, if the dogs treated had no worms, and the dogs that were not treated did have worms, the product passed. “If one dog had one worm, you failed,” Dr. Adolph said. “It was a 100% threshold.”

After drug-resistance in certain heartworms was confirmed, the rules changed. Now, there needed to be two studies instead of one. Plus, manufacturers could not use old strains in their tests. They needed to go into the field and collect fresh samples. Regulators also put geographic restrictions and requirements for case studies. One of the studies needed to be done in the area of the United States where the majority of the resistance reports originated, basically the area from the Lower Mississippi River Valley to the Atlantic Ocean, east and west borders, from the Mason Dixon Line down to the Gulf of Mexico. In short, the bar to get a drug approved became higher. Because of that, there were changes to label language on the product’s effectiveness with more than one dose of treatment. “There are products on the market that have been approved, that say this product was 100% effective in worm reduction when given in doses for three consecutive months, or six consecutive months,” Dr. Adolph said. “So that’s a little bit different, and something all veterinarians should be aware of.”

New products are still being developed, but the research needed is extensive, costly, and time consuming – even more so with the added approval process. “The particular challenge with heartworms is, from the time a dog is infected until the time they’re fully grown adults reproducing, it’s around six months,” Dr. Adolph said. “Because they’ve got an incredibly long life cycle, and you are evaluating so many unknowns, the process is laborious.”

Feline heartworm

Particularly taxing to the veterinary field is what to do with feline heartworm disease. Dr. Adolph did his master’s thesis on it, and still calls it “a mystery, wrapped around a riddle, dropped into a quagmire. It’s the exact same parasite that infects dogs, but in a cat, different things occur.”

Basically, though the same worm infects the cat, and goes through the same maturation process migrating through muscle before finally finding its way to the vascular system, the cat’s immune system goes haywire when they’re infected with heartworm. “They mount what I would call a robust immune response,” Dr. Adolph said.

Cats have types of inflammatory cells that dogs don’t. When they become infected, there is a massive fight happening in the cat’s chest that produces a volatile inflammatory process. Yet some worms may survive and grow up to be adult worms that live in the vessels around the cat’s heart.

Not every infected cat has adult worms, but they have all the damage that goes along with the battle, Dr. Adolph said. “So, if you’re relying on an antigen test that picks up particles of adult worms, well, guess what? One of those may be negative on a cat that was infected, fought the battle, and there’s no adults there to find.”

Where dogs can have dozens of worms, cats could have just one or two. If they have adult worms, and if those adults are putting all those antigens into the bloodstream, then the cat’s immune system comes into play. The cat’s natural antibodies kick in and complete a new structure. Even today’s tests could give a lot of false positives, Dr. Adolph said. “So, you have to go through special testing techniques where you keep the samples, destroy the antibodies, free the antigen and rerun the test,” he said. “Not everybody wants to put in that much effort and spend that much money to find heartworm in a cat.”

The most accurate way to screen cats is to run an antigen test plus and antibody test, but that’s costly and not as widely accepted in the profession. “So, it is my opinion that there’s a lot more positive cats out there than we think,” he said. “It’s just there are more hurdles to finding them with routine screening. I think veterinarians should understand the limitations of the routine test. They’re not as accurate as they are with dogs. But I also understand that if we let perfect be the enemy of good, then we’re never going to get where we need to get with testing cats.”

Spread of parasites

Still, veterinarians should not get complacent when advocating for year-round prevention. “The one thing that stays the same through the course of your career is that everything changes,” Dr. Adolph said.

Judging by the yearly results CAPC releases, parasites have been on the move for years. For example, when Dr. Adolph graduated veterinary school in the mid-90s, the western states did not experience parasite problems as much as the eastern states. Over the years, through multiple factors, that has changed. People travel with their pets, and pets are being rehomed from areas of high prevalence to areas of low prevalence. Not all the dogs that are rehomed are tested before they travel, so whatever they have, they’re bringing with them.

“My mentor, Dr. Susan Liddell, always said, ‘When you adopt a pet, you adopt its parasites as a package deal,’” Dr. Adolph noted. “With all of these pets moving around, different climate changes, different ecology changes, you now have previously low prevalence areas seeing more and more cases.”

For example, a recent study by Jason Drake and Rudolph S. Parrish examined how many dogs were brought into Colorado through other states from 2013 to 2017. Over 100,000 dogs were accounted for moving from other states to Colorado. Almost 50% of those dogs came from Texas and Oklahoma, two hotbed states for heartworm disease. While the rest of the United States saw about a 21% increase in heartworm disease from 2013-2017, Colorado saw a 67% increase in heartworm disease. “This is what happens when you take dogs from a high prevalence area and put them in a low prevalence area,” Dr. Adolph said.

Indeed, low prevalence is not the same as no prevalence. Every dog should be tested for heartworm and tick transmitted disease, regardless of geography. “It’s our job to find out who’s positive and make them better,” Dr. Adolph said. “With so many pets on the move and the myriad of pet ownership shifts in the U.S., it’s more important than ever that veterinarians stay vigilant and advocate strongly for checking all of these pets.”

Parasite Forecast

In its 2024 Pet Parasite Forecast, CAPC said veterinarians in states with historically lower prevalence are again cautioned about the increasing risk of heartworm infection and are encouraged to have a discussion with their clients about the changing prevalence. This is particularly important in southern Indiana, central and southern Illinois, southern Iowa, Kansas, and Lower Michigan and Ohio in the Great Lakes region. capcvet.org/articles/2024-annual-pet-parasite-forecasts/

Chris Adolph, DVM, MS Diplomate ACVM (Parasitology) Senior veterinary specialist, CAD Veterinary Specialty Operations, Zoetis

Source: Companion Animal Parasite Council

Photo credit: istockphoto.com/Nadzeya Haroshka