Is there a new heartworm in town?

Dirofilaria immitis isn’t the only heartworm that can infect dogs in North America. Angiostrongylus vasorum, commonly called French heartworm, has historically been confined to Europe and a handful of other regions, with only scattered reports from North America, including Canada and a single fox in West Virginia. That changed when 2 University of Tennessee graduate students, working independently on a black bear and a coyote, each found the parasite in their respective study animals within 2 weeks of each other—marking the first documented case in a bear anywhere and the first coyote host in Tennessee.

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology), associate professor of parasitology at the University of Tennessee College of Veterinary Medicine, joins Adam Christman, DVM, MBA, to unpack the discovery, published in Veterinary Parasitology: Regional Studies and Reports. The conversation covers how French heartworm differs from Dirofilaria in appearance, transmission, and diagnosis; what it means for wildlife and domestic dogs; and a broader look at heartworm resistance and prevention adherence in general practice.

Below is a partial transcript, edited lightly for clarity.

Adam Christman, DVM, MBA: Let’s talk about the paper you just published. What’s the biggest takeaway for veterinarians?

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology): The very first thing I’d take away is that just because a parasite hasn’t been documented in your area doesn’t mean it’s not there. We found this by happenstance. I had 2 individual graduate students—one working on coyotes and one working on black bears—doing really good diagnostics, and we got lucky. It just reiterates that there are a lot of emerging and new parasites out there, and that veterinarians absolutely have to keep going to [continuing education] meetings and stay current. Things change so fast that without that lifelong learning, you’re going to be left behind.

Adam Christman, DVM, MBA: This paper, published in Veterinary Parasitology: Regional Studies and Reports, is called “Two cases of Angiostrongylus vasorum, a cardiopulmonary nematode, in a wild black bear and coyote of Tennessee.” Tell us how you found it.

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology): It’s kind of a funny story. Katie, a master’s student working on black bears, came to me one day and said, “I think I found Angiostrongylus vasorum in the black bear.” I said, “You’re kidding. Tell me what you found and why you think it’s that.” She said the bear had severe pneumonia, and under the microscope, she saw severe bronchointerstitial pneumonia with intralesional dorsal-spine larvae, which points to a metastrongylid lungworm, though none of the usual candidates are known to affect canids or ursids. So we ran [polymerase chain reaction], the sequence came back, and it was Angiostrongylus vasorum. I couldn’t believe it. Then about 2 weeks later, Eliza, a PhD student who’d just finished her DVM here, walked up and said, “Hey, we found Angiostrongylus vasorum.” I said, “I know, Katie found it in the bear.” She said, “No, I found it in my coyote.” The bear had come from the Smoky Mountains, and the coyote was from just south of the Kentucky border in northeastern Tennessee—nowhere near each other, but they both turned up the same parasite within a 2-week window. Until that point, that parasite had only been reported in 1 other place in the US: a fox in West Virginia, reported by one of my colleagues at Georgia. Since then, it’s also turned up in a domestic dog in Washington that had been born in Oregon. So, we really don’t have a good handle yet on where this is or how far it’s spread.

Adam Christman, DVM, MBA: We all know Dirofilaria immitis, heartworm. Is there another species of heartworm that infects dogs in North America?

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology): Yes—it’s the one we’re talking about, Angiostrongylus vasorum. It’s often called French heartworm. The adults look very different from Dirofilaria but infect a similar location—usually the vessels, whether the aorta or the pulmonary artery. Dirofilaria worms can be up to almost 15-cm long, white, thin worms. The adult Angiostrongylus are only about 14 to 20 mm, so much smaller, with a bit of a pinkish hue. And they’ll sometimes sit inside the vessel wall, so you may have to dissect the vessel wall to find them. It’s considered the most pathogenic lungworm of dogs. Historically, it’s been found in Europe, Africa, and a few other locations, and more recently—prior to our findings and the case in West Virginia—in Nova Scotia and Newfoundland, Canada.

Adam Christman, DVM, MBA: Geographically, it’s spreading. What host species have been infected?

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology): Until this bear was diagnosed, it was thought to be strictly a canid parasite. This is the first report of a bear getting it, and it killed the bear. There’s no question about that when you look at the histology of the lungs. Foxes, coyotes, domestic dogs, and wolves can get it too. The natural host is considered to be wild canids like foxes and coyotes, but as we’re seeing, it can get into other species.

Adam Christman, DVM, MBA: Are the clinical signs similar to heartworm—coughing, for instance?

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology): There’s not much difference as far as clinical signs: coughing, respiratory distress, dyspnea. You won’t see the classic reverse-D heart shape on radiographs, because you don’t get the same impact from worm number or size. But clinically, you really couldn’t distinguish it from Dirofilaria.

Adam Christman, DVM, MBA: What about diagnostics? Would this show up on our heartworm antigen test?

Richard Gerhold, DVM, MS, PhD, DACVM (Parasitology): There’s some thought that it can cross-react with Dirofilaria antigen tests; there are a couple of papers that go back and forth on that. The interesting diagnostic difference is that, unlike Dirofilaria, where microfilariae circulate in the blood because of the mosquito vector, Angiostrongylus is a metastrongylid, so it requires a gastropod—a snail or slug—as an intermediate host. Affected dogs shed dorsal-spined L1 [first-stage] larvae in their feces, which eventually get into a gastropod. Dogs get infected either by eating a snail or slug carrying the L3 stage, and frogs have also been shown to be a paratenic host, so there are potentially different routes of infection.

Want to hear more of this conversation, including what can be done to minimize transmission? Check out the full episode here or wherever you listen to podcasts!