Parasites are playing in America’s dog parks

dvm360dvm360 March 2021
Volume 53

Research reveals there may be worse things lurking in your local dog park than inattentive pet owners: namely, intestinal parasites and lots of them. Learn what this means for your veterinary practice and neighborhood.

Marina /

Marina /

Dogs and their owners aren’t the only living things thriving in dog parks throughout the United States. Recent research from Elanco, Idexx, and a team of investigators at Oklahoma State University found intestinal parasites in 85% of the 288 parks included in a recent study.1 During a Fetch dvm360® virtual conference session sponsored by Elanco, Michael W. Dryden, DVM, PhD, DACVIM (parasitology), emeritus professor at Kansas State University College of Veterinary Medicine, discussed the details of these findings and their implications for people, their best friends, and veterinary professionals.

DOGPARCS methodology

The DOGPARCS (Detection Of Gastrointestinal Parasitism At Recreational Canine Sites) study collected data between July and August 2019. Researchers visited 288 designated dog parks in 30 metropolitan areas across the United States and collected 10 freshly deposited canine samples from each. One dog was sampled per household, and dogs with professional dog walkers were excluded. Each dog owner participant verbally responded to a short questionnaire.

All samples were evaluated at a single Idexx laboratory using zinc sulfate centrifugal flotation (specific gravity 1.24), and coproantigen immunoassays (Fecal Dx Giardia Test). For Dryden, the inclusion of these methods is key.

“Most of us recognize now that a centrifugation fecal is much better than a passive flotation fecal, which can miss almost two-thirds of positive whipworm numbers,” he explained. “And fecal antigens are really remarkable. These new fecal antigen tests that Idexx has for rounds, hooks, and whips—they’re not an egg antigen, and that’s important. They’re a worm antigen…[which] can often be detected prior to eggs being shed. So, you could have a dog with a negative fecal and a positive antigen test because it can pick up prepatent infections.”

Of the more than 3000 dogs tested, 1 in 5 had parasites. And, as noted, parasites were found in 85% of the parks included in the study. The overall regional breakdown doesn’t show much variability. Of the dog parks sampled, parasites were found in 90% of those in the South, 87% in the Midwest, 80% in the Northeast, and 79% in the West.

Dryden addressed what the results might mean for veterinary professionals and the public at large (and offered some expert diagnostic advice as well).

Hookworms: between a rock and a hard place

Hookworms were the most common nematode parasite detected in the study (Table 1).1 The numbers are sobering for several reasons, not least of which is the rise of multiple-drug resistant (MDR) hookworms.2 “Over the past few years, multidrug resistant Ancylostoma caninum has been documented,” Dryden said. “We’re talking about hookworms that are resistant to fenbendazole, pyrantel, milbemycin oxime, and even high levels of bovine ivermectin. So, this is really scary.”

Table 1

Table 1

According to Dryden, these resistant organisms likely originated from greyhound farms, where dogs were dewormed repeatedly in heavily contaminated environments before heading to the tracks. Although retired greyhounds have long been adopted into homes, he noted that with more and more states outlawing dog racing, the number of greyhounds transitioning from the track to the dog park is increasing. “Recently it seems [as though] more cases [of MDR Ancylostoma] are now seen in breeds [that are not greyhounds] than in greyhounds themselves,” Dryden said. “This is highly problematic because this is a hookworm that we can’t control with Heartgard Plus (Boehringer Ingelheim) or Interceptor (Elanco). We’re struggling.”

Although a combination of Drontal Plus (Elanco) and Advantage Multi (Elanco) given on a monthly basis has shown promise in such cases, “sadly, we’ve recently seen cases here where [that combination] isn’t working anymore,” Dryden explained. “A recent publication showed that the drug emodepside may be effective in these cases. The problem is that it’s only approved as a topical for cats in this country, so we’re between a rock and a hard place right now.”

Moreover, Dryden noted that hookworms are highly pathogenic. “Hookworms can kill dogs,” he said. “I’ve seen young dogs exsanguinated by hookworms.” The zoonotic potential also is worth mentioning. The infective larvae of both A caninum and A braziliense can penetrate human skin and migrate in subcutaneous tissues; in rare cases they can travel to the lungs and produce pneumonitis. A caninum also can end up in the small intestine of humans and cause eosinophilic enteritis, although this is rare also. At the very least, the study’s results make a compelling case for dog owners to keep their shoes on when visiting dog parks.

Whipworms: historically underdiagnosed

Whipworms were the second most common nematode found in study samples (Table 1).1

What about Toxocara canis?

Michael W. Dryden, DVM, PhD, DACVIM (parasitology), explained that dogs aged 1 to 3 years accounted for 46% of all dogs in the study. “Dogs that might have roundworms, [such as] dogs under 6 months of age, only accounted for 2.4%,” he said. “There weren’t a lot of puppies going to dog parks. That’s probably why the numbers were so low.”

“The pathogenesis of Trichuris vulpis is vastly underappreciated by most practitioners,” Dryden said. “In fact, I would argue that worm for worm, whipworm is more pathogenic than hookworm.” He explained that adults “thread” their filamentous anterior end in the superficial glandular mucosa and that larvae and adults both have a mouth spear that cuts and shreds tissue and blood vessels. “Clinical signs depend on worm numbers, extent of gut penetration, and the reaction of the host,” Dryden said. “They can cause significant damage and even necrosis of the cecum and areas of the colon as well. These cases can be severe.”

Because because whipworm eggs are so heavy, the parasite often goes undetected on passive fecal flotation. Dryden shared data he presented at the 2019 World Association for the Advancement of Veterinary Parasitology (recently published in the International Journal of Veterinary Research3) that compared centrifugation with passive fecal flotation (Table 2).

Table 2

Table 2

“Notice that with a passive flotation system, you miss almost two-thirds of all whipworm cases,” Dryden saud. “You [have] to centrifuge it in a 1.24 to 1.27 [specific gravity] solution, whether [it’s] sugar or zinc sulfate, to get that job done.”As a result, he said that the parasite is “markedly historically underdiagnosed in many veterinary practices.”3

Giardia: Overplayed zoonotic potential

The most popular parasite on pup playgrounds? Giardia. It was found in more than 70% of dog parks (Table 1).1

Considering how ubiquitous it is in nature, how worried should veterinarians be about Giardia from a zoonotic standpoint? According to Dryden, not very. He explained that the classification and zoonotic potential of Giardia have been undergoing constant revision and that the various genotypes of Giardia species are now called genetic assemblages, which go from A to G. Assemblage A has been subdivided (AI to AIV), and so has assemblage B (BI to BIV). “We know that most dogs have assemblages C and D, and most cats have assemblage F,” Dryden said. “These are almost never found in humans anywhere in the world and haven’t ever been found in humans in North America.”

Here’s where things have been a little muddled in the past. “We used to say, ‘Dogs and cats can have A and B, and A and B are human assemblages,’” Dryden explained. “But now that these assemblages have subtypes, that’s all changed. AI and AII are human subassemblages that have been rarely found in dogs and cats. Most animals, if they have A, have AIII and AIV. So, you can’t just say that if a dog has assemblage A that it’s zoonotic.”

And it’s similar with B, Dryden said. BIII and BIV are human subassemblages, whereas BI and BII are animal subassemblages and rarely found in humans. “There are older studies that said 30% to 40% of dogs have A or B,” he continued, “but that’s irrelevant. It’s the subassemblage that’s relevant.”

Dryden noted that as understanding of Giardia has increased, it’s zoonotic potential has decreased. He provided quotes from a few organizations as evidence:

A 2016 retrospective analysis by the CDC that looked at giardiasis outbreaks in the United States between 1971 and 2011 adds further proof.7 Of 242 outbreaks (ie, 2 or more cases of giardiasis that were epidemiologically linked by time or location of exposure to a common source), 181 were waterborne, 38 were foodborne, 14 had no known source, 6 were person-to-person, and 3 were from animal contact (rabbit, reptile, cattle). Not a single outbreak during that 40-year period was linked to a dog or cat. “We’re not saying people can’t get Giardia from a dog or cat,” he explained. “We’re not even saying a person hasn’t had it. What we’re saying is, in North America, no one has ever proven a human has [had] Giardia [transmitted] from a dog or cat. I think we’ve markedly overplayed the zoonotic potential.”

However, that doesn’t mean Giardia isn’t still a real problem. It can be particularly difficult to diagnose. Dryden noted that yeast, pollens, and other debris are often misidentified as Giardia cysts and that microscopes in private practices aren’t equipped with micrometers that allow for measurement of cysts. Moreover, passive fecal flotation tests don’t do well with Giardia, and veterinarians have incorrectly relied on response to treatment (with metronidazole) as a diagnostic tool. Further complicating matters is the fact that cyst shedding is intermittent. “From a clinical standpoint, due to intermittent cyst shedding, the number of cysts in a fecal flotation or the intensity of the spot on a SNAP Giardia Test (Idexx) is irrelevant,” Dryden explained. “They have no relationship to the severity of clinical disease. Whether I see 1 Giardia cyst or 5000, it’s the same. Tomorrow, they could be flipped.”

As for treatment and retesting, Dryden said it’s a common myth that fecals remain positive for an extended period following treatment. They don’t. Fecals (cysts or cyst antigen) are usually negative within 1 to 5 days of successful treatment. “Giardia has a prepatent period as short as 6 days,” he explained. “Therefore, if I treat an animal for Giardia and I come back 2 weeks later and retest that dog and it’s positive, that doesn’t mean my treatment failed me. The dog could have [become] reinfected….That’s why it’s recommended with a SNAP Giardia Test that following treatment that you retest on day 5.”

“Why isn’t my preventive working?”

As part of the study, 1 of the questions owners at the dog park were asked was whether their dog was currently on a heartworm or intestinal worm preventive or medication. Unfortunately, those owners who said they had their dogs on a preventive still had significant levels of parasitism, Dryden said. Why is that? “First of all, what’s the preventive they were on?” he asked. “Are they giving it correctly and on a regular basis? We know they’re not. How many of these owners bought a year’s supply last year and still have 6 months left, or bought a 6-pack last year and still have 2 doses left?”

Dryden stressed the importance of annual fecals (specifically fecal flotation with centrifugation plus antigen testing) and argued that, based on the study findings, dogs that visit dog parks should be tested twice a year. Obviously, veterinarians are then at risk of hearing this dreaded question: “How can my dog have a positive fecal when he’s on a preventive?” Regardless of the cause (eg, pet owner error, prepatent infection detection), Dryden said veterinarians can turn this into a positive interaction that reinforces compliance. Try saying, “What this is telling us is that your dog is under heavy exposure. Thank goodness we had him on a preventive or he might have clinical disease now. We’re doing the right thing by keeping him on a monthly preventive.”

Sarah Mouton Dowdy is a freelance writer and editor in Kansas City, Missouri.


1. Stafford K, Kollasch TM, Duncan KT, et al. Detection of gastrointestinal parasitism at recreational canine sites in the USA: the DOGPARCS study. Parasit Vectors. 2020;13(1):275. doi:10.1186/s13071-020-04147-6

2. Jimenez Castro PD, Howell SB, Schaefer JJ, et al. Multiple drug resistance in the canine hookworm Ancylostoma caninum: an emerging threat? Parasit Vectors. 2019;12(1):576. doi:10.1186/s13071-019-3828-6

3. Dryden MW, Herrin BH, Payne PA, Smith V, Ritchie D, Hegarty E. Further comparison of centrifugation versus passive fecal flotation for the recovery of Toxocara canis, Trichuris vulpis and Ancylostoma caninum eggs. Int J Appl Res Vet Med. 18(1), 69-77, 2020.

4. Giardia: Dog. Companion Animal Parasite Council. Accessed February 6, 2021.

5. Giardia and pets. Centers for Disease Control and Prevention. July 21, 2015. Accessed February 6, 2021.

6.Spickler AR. Giardiasis. Center for Food Security and Public Health. December 2012. Accessed February 6, 2021.

7. Adam EA, Yoder JS, Gould LH, Hlavsa MC, Gargano JW. Giardiasis outbreaks in the United States, 1971–2011. Epidemiol Infect. 2016;144(13):2790-2801. doi:10.1017/S0950268815003040

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