You’d think that years of research, the availability of easy-to-administer preventives and better, safer treatment would help put a damper on the spread of heartworm disease in dogs.

They haven’t.

Today danger is in virtually every corner of the United States (to see maps of incidence from 2001, 2004 and 2007, click here) and more mosquito species can pass the disease from animal to animal – including dogs.


Dogs cannot give heartworm to other dogs. It takes the bite of a mosquito, like this Aedes triseriatus to pass the tiny larvae that grow into worms. About 200 species of mosquito live in the United States. Photo courtesy Centers for Disease Control/Robert S. Craig.

It’s something akin to our problem with antibiotics. They’ve been over-prescribed by some physicians and abused by people who do and don’t need them because they don’t take the full prescription when they start to feel better. And, thus, bugs evolve or develop that are stronger, more resilient and can withstand the infiltration of the pharmaceutical industry’s best drugs.

We have heartworm preventives available for our dogs, as chewable tablets, injectables and spot-ons, but most of us don’t use them.

The New York Daily News  reported in mid-March that Floridians should prepare for an invasion of Gallinipper mosquitoes, or Psorophora ciliate, calling the half-inch long insects “notoriously aggressive.” While “invasion” isn’t quite the right word because the mosquitoes have lived in Florida for a long time, the warning is that there likely will be many more this summer than in previous years.

What do mosquitoes have to do with heartworm in our dogs, you might ask? Everything. Absolutely everything.


Even mosquito species that don’t carry diseases, such as West Nile virus and malaria, can transmit heartworm to dogs. Photo courtesy Centers for Disease Control/James Stewart.

A dog with circulating heartworm larvae, or even with fully developed adult heartworms, cannot pass the heart-destroying parasites directly to another dog or other mammal. It takes a mosquito to do that. So, the more mosquitoes that live around our dogs and the longer they can be active throughout the year, the better the chance that a heartworm-carrying mosquito will bite your dog. And if your dog – or any dog – is infected, but isn’t diagnosed, and a heartworm-free mosquito bites it, that mosquito will become a carrier of heartworms too.

In wetter years, we have more mosquitoes. In dry years, fewer. But that’s not the only variable. In fact, Wallace Graham, D.V.M., president of the American Heartworm Society,  says, “I think what affects the spread of heartworm more than just about anything is the mobility of our society and the mobility of our dogs. For example, heartworm-infected dogs from New Orleans were transported all over North America after Hurricane Katrina.” Although Graham is not an internist, nor a parasitologist, he has a special interest in heartworm disease. His veterinary practice is in southern Texas on the coast, and that’s long bed a hotbed of heartworm activity, along with the rest of the Gulf Coast and the Atlantic Coast. “We have lots of mosquitoes and lots of heartworm,” he says.

Mosquitoes must feed on the blood of an animal, including us, to survive. And we have no shortage of the pesky –and sometimes dangerous – bugs in the U.S. The American Mosquito Control Association says about 200 species call the U.S. home. While not all can transmit the diseases we fear for ourselves, such as West Nile virus and malaria, even the Gallinipper, which transmits none of them, can still serve as what is called a “vector” for heartworms, carrying around the microfilariae, kind of like eggs, that become long, spaghetti-like worms – over time – inside your dog.

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Summer evenings, when you may enjoy summertime evening walks with your dogs by your side, are prime feeding time for mosquitoes. Photo © Can Stock Photo.

Prevention Is Easy, Yet Neglected
It’s easy to prevent heartworm from invading one of your dogs’ most precious organs – yes, the heart, as the parasite’s name implies. Start it on a preventive as a puppy, and give it each and every month for the rest of its life. For most of us, however, that’s more easily said than done. We miss a month or two, maybe in the summertime, and we can’t be absolutely sure that Duke or Daisy hasn’t been bitten by a mosquito in the interim. So now we need to have the dog tested before we can put it back on the preventive. And heartworm tests aren’t inexpensive. Depending on where you live, one can cost from about $50 to more than $100. When you find out your dog is negative for heartworm – hopefully – you can start her on a preventive. Everything’s hunky dory, until you forget again.

Then, another test. “Testing after a lapse in preventive administration usually occurs as soon as the lapse is discovered and again about six months later, since it takes about six months from the time the mosquito infects the dog until the test can detect the infection,” Graham explains.

The reason testing is important is that a female heartworm can continue to lay larvae, Graham says, even after a dog receives the preventive. Different chemicals are needed to prevent and treat heartworms. “It’s incredibly important that we don’t give heartworm-positive dogs preventive without following through with the treatment,” he says.

The American Heartworm Society recommends annual testing of dogs even if they are on a preventive.

Why aren’t we consistent in administering the preventive?

A number of things get in the way. For one, human nature. We forget. We run out. We have a work or kennel or family crisis that comes before everything else. Or, we just dislike the idea of putting something potentially toxic into our dogs when we don’t see any evidence among our dog-owing friends and colleagues that it’s necessary.

Graham understands the hesitance, but says, despite the fact that he likes to practice “without drugs as much as I can, but I use pharmaceuticals in my practice every day. From a philosophic point of view, I’m sympathetic with that opinion. However, with as mobile as dogs are and as tenacious as mosquitoes are, I am just strongly of the opinion that you just can’t take a chance with a disease as serious as that.

“Millions and millions and millions of doses of this stuff have been administered, and the incidence of reactions we don’t like is so tiny. It’s hard to justify not giving it. The drugs have proven to be so incredibly safe. No drug is 100 percent safe, but these things have had such an incredible safety record for such a long time. The disease is awful. In my opinion, it’s really hard to justify not doing it.”

In addition, the need for heartworm prevention is not as ingrained in dog owners’ minds as flea and tick prevention is. (See a map  showing that 64 percent of dogs in the U.S. leave their veterinarians’ offices without a heartworm preventive.) Fleas and ticks are obvious on a dog, at least after a certain point. A dog infected with heartworm can go for months, even longer, yet have no disease symptoms, or clinical signs as veterinarians call them. Coughing, the inability to exercise normally and abnormal lung sounds can be signs of moderate heartworm disease.

Diagnosis and Treatment Make Strides
One positive development in the diagnosis of heartworm infection is that many veterinary clinics now have digital X-ray machines. “It makes the characteristic changes to the heart and lungs easier to see,” Graham says. When a heartworm test comes back positive, the American Heartworm Society recommends a second test – of a different kind – to ensure the first wasn’t a false positive. If the second test is also positive, the private practice veterinarian can take a chest X-ray, then analyze it for “evidence of change in the shape of the heart and the arteries that serve the lungs.”

Treating a dog for heartworm is “not something you want to do unless you’re certain you need to do it,” Graham says.

However, when a dog does need treatment, it’s not nearly as likely to result in death as it was a decade ago.

“The drug that we use [an organic arsenic compound, Melarsomine dihydrochloride] is a better drug than we had 10 or 15 years ago. The addition of doxycycline has made a big change in the number of post-treatment complications that we have. I see far, far fewer significant problems with the dogs that I treat now.” The major complication from treatment is the lodging of dead worms in the dog’s pulmonary arteries. “Doxycycline kills a bacterium that benefits the heartworm. So heartworms lose weight, and there’s not so much mass when they die,” Graham explains. “The smaller the biomass, the fewer complications.” Research done in the last four or five years has allowed the American Heartworm Society to strongly recommend the use of doxycycline. “We don’t make any recommendation that we can’t back up with good, published research,” he says.

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All standing bodies of water, whether lakes or simple low areas in a backyard, can be mosquito breeding grounds. Photo © Can Stock Photo.

In addition, the timing of treatment has changed as researchers have learned more about the life cycle of heartworms. “Once the mosquito infects the dog, we can kill those larvae for a month or so,” Graham says. An anthelmintic is used – some of the same chemicals used in preventives. The drugs used to treat heartworm, however, cannot kill “middle-aged” worms. So, the initial part of the treatment is to kill larvae, then the next part of the treatment kills fully developed adult worms.

Diagnostics and treatment may soon take more leaps forward.

“Since our current tests detect only adult female heartworms, there’s a great deal of interest in developing tests that detect male heartworms,” Graham says. “Hopefully someday we’ll be able to detect those too.”

Although no new molecules that might allow development of a new preventive have been discovered, “I would just bet that someone is looking for a new molecule. It would be good if someone could discover one that would perform as well or better than what we have.”

Numerous papers detailing research into this pervasive parasite will be presented at the triennial American Heartworm Society Symposium in September in New Orleans. “We’ll have people from South America, Europe and Japan,” Graham says. “There’s going to be some cutting-edge research presented, some of it centered around how certain conditions in dogs may affect the tests” for heartworm.

Best In Show Daily will bring you a full report on the symposium in the Fall.