The promise of heartworm prevention: An expert Q&A

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Stephen Jones, DVM, knows a thing or two about heartworms. A general practitioner and partner at Lakeside Animal Hospital in Moncks Corner, South Carolina, Dr. Jones has treated thousands of heartworm infections in pets over the years and served on the board of the American Heartworm Society for more than a decade.

During a lecture at the 2020 Western Veterinary Conference, Dr. Jones noted that despite the fact that Dirofilaria immitis is increasingly found in both endemic and non-endemic areas across the U.S., questions remain about the need for heartworm disease prevention. He set out to answer those questions.

What pets need prevention?

Regardless of where you live, if you have mosquitos, there are mosquitoes that carry heartworms, Dr. Jones said. But if the average dog doesn't have exercise intolerance, is not coughing and is alert, you won’t be able to tell it has a problem. “You see a dog that has heartworms, but ‘not the clinical disease.’ Annual testing, prevention and treatment are not stressed [as much as they should be],” he said.

Can a dog or cat be heartworm positive and not have heartworm disease?

I have heard experts say, “This dog has a heartworm infection but does not have heartworm disease.” The fact is that a lack of clinical evidence does not mean the patient does not have heartworm disease.

Why don’t pet owners worry more aboutheartworm disease?

There’s been a “heartworm conspiracy” going around on the internet. Some people think heartworms aren’t real. “I’m not sure how you could fake it, though,” Dr. Jones said.

“Believing in testing, treatment and prevention starts from the ‘top down,’” he noted. “Veterinarians must believe heartworm disease is real, and once they do veterinary technicians will then believe it is a valid concern. This pulls through to clients." Even in low endemic areas, heartworm should be taken seriously, he said.

Why is heartworm more common in some places than others?

There are still parts of the country where no data exist because there is no population to treat and no veterinarians because there is no population to treat, Dr. Jones said.

Heartworm has been present longer in the Gulf Coast states and in Florida and South Carolina than in other areas of the country. Long summers and a growing population of mosquitos in the low country like the Mississippi River Delta and Gulf Coast saw the population rise.

“Historically, America’s early colonizers who settled in the Gulf Coast, Florida and South Carolina brought heartworms here in their dogs, infected our neighborhood mosquitos, and that is where the initial problem began,” said Dr. Jones.

Why is heartworm disease more prevalent today?

Part of the reason is that the tremendous amount of geographic movement among people and pets that’s happening today. “People didn’t move around as much even a few decades ago,” Dr. Jones said. So, for a long time, heartworm disease remained a Southeast Coast problem. Now it’s everywhere.

“That’s not to say heartworm won’t be a bigger problem in the future in low-endemic areas,” Dr. Jones warned. “The increasing prevalence we see today resulted from a problem that once we had no way to test for or prevent. Every state has mosquitos that are capable of transmitting heartworm disease.”

How much disease do heartworms cause?

Heartworm causes tremendous obstructive disease in the arteries of the lungs. A lot of heartworms can cause a lot of disease, but so can two worms that inhabit a host for a short period of time. A lot of worms can also do a lot of damage in as few as three months, and just a few worms can cause disease once infective larvae grow. Heartworm disease diagnosed in young dogs and cats can cause health issues if not treated, Dr. Jones noted. “Early disease causes early problems,” he said.

What about cats?

In cats, it is the same thing, but the disease and testing are somewhat confusing. “In my practice, I saw an anorexic cat with fever of unknown origin. One heartworm caused that damage,” Dr. Jones said. “The cat was normal on Friday and passed away on Monday.”

Compared with dogs, the number of cats harboring mature worm infections is not as high, but the infection rate between species is similar.

“Most adult worms don’t reach maturity in cats because their immune system eliminates them and that poses problems; they can still have long-term pulmonary disease, even after adult worms are eliminated. It causes other problems that can carry on for years,” said Dr. Jones.

Can heartworm disease be cured?

We can eliminate heartworms, but the disease can be permanent. The bottom line is that heartworm positive equals heartworm disease. Our job as veterinarians is to prevent, not just treat, the disease.