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What Are Heartworms and What Can I Do About Them?

Picture of a heartworm.

Picture of a heartworm.

What Is Heartworm?

Like many pet owners, you probably have heard of fleas, ticks, mites, and other sundry nasty intestinal parasites that can afflict your fuzzy (and yes, even scaley or feathered) friends. But, one parasite in particular is absolutely terrifying. It's as deadly as rabies, but with a very quiet means of infiltration. What could this terrible species be?

They're called heartworms, or Dirofilaria immitis, a species of roundworm that infects the cardiovascular system of its primary host. Infection is spread by mosquitoes—its secondary host—which means it's far more prevalent in places that are warm, wet, and otherwise tropical. However, that doesn't mean that living in a desert will protect your pet!

Heartworms cause (creatively) "heartworm disease," which is difficult to detect during the fist six months of infection. Bitten by a carrier mosquito, the unlucky host will carry the microscopic larval form of the adult worm in its bloodstream until they reach the lungs and heart. (Rarely, a heartworm will become "lost" and end up somewhere outside of the usual destinations, such as the eye or a different major artery elsewhere, causing unusual and complex symptoms such as blindness.)

As heartworms mature in the heart, they cause more obvious problems, including (but not limited to) coughing, fatigue, exercise intolerance, and other symptoms generally associated with heart complications. As cases advance without treatment, the tissue of the lungs may begin to break down and cause the afflicted to cough up blood. The patient may lose weight and become weak, or may develop fainting spells. Eventually, a significant infection will cause congestive heart failure, and the patient will die.

How Do You Know If Your Pet Is Infected?

Easy! There's a simple test for both dogs and cats that only requires three drops of the patient's blood and about ten minutes. It's a simple "snap" test, which means it's a small plastic indicator on which there are small dots that will react with various proteins, antigens, and other markers for various infections. (The one we use in my clinic for dogs also tests for Ehrlichia, Anaplasma, and Lyme diseases, all tick-borne diseases.) Based on what dots turn blue at the end of the test, your veterinarian or veterinary technicial will know whether or not your pet is positive for heartworm infection.

Three drops of blood are mixed with four drops of conjugate, then poured into the "well" of the plastic unit. The mixture will run up the "window", covering the indicator dots. Once it covers the window completely, the technician will "snap" the test downward, breaking the fluid-filled compartment housed on the underside of the plastic unit, which will cause the fluid to push the blood mixture back towards the well. Any blood containing the appropriate antigens will slowly turn blue on the corresponding dot.

Don't panic if you see one blue dot! Most tests have a "control" indicator to prove that the test is working correctly.

A simple "snap" test in action.

A simple "snap" test in action.

What If My Pet Is Infected?

Treatment is a much harder option than prevention. If your pet turns up positive for heartworm, be prepared for a long and difficult journey.

For dogs, there is a treatment available that takes nearly a year to complete. First, your veterinarian will likely prescribe an antibiotic and steroids, as a particular species of bacteria often carried by heartworms has shown some association with the inflammation brought about by heartworm disease. After that, there are at least three injections into the dogs' lower back muscles (which is about as unpleasantly painful as you'd imagine it would be), each at no more or less than 30 days apart. After the injections are administered, the dog must be kept under close hospital monitoring to ensure no reaction the injections.

After that ordeal, the patient must then be kept on strict kennel rest for no less than six months to ensure that they don't develop complications from the dead worms potentially lodging in important blood vessels. Then, they will need to be retested for heartworm disease. Which, if they still have larvae in their system, they very well could be. And thus will require more injections and more kennel rest.

All of this treatment costs thousands of dollars. And that's assuming it works.

Sometimes the infection is so severe it cannot be treated any other way than surgically going in and manually extracting the adult heartworms, which costs even more and has a fairly high risk of complications. It is, however, a better alternative than not treating the animal at all.

For cats?

Cats are a different story. Heartworm disease affects cats differently, as they are not the target species. Cats have a remarkable inflammatory response that actually tends to kill off most heartworm larvae... which means instead of 25-40 adult heartworms, they are more likely to develop two or three. Cats are more likely to have the larvae become "lost" and end up in non-target organs and areas of the cat's body. Due to the nature of the inflammatory response, symptoms vary from coughing, difficulty breathing, and fatigue to vomiting, anorexia, blindness, and convulsions. Cats are far more likely than dogs to suddenly die from heartworm disease without warning.

Given all this and given that cats are more apt to throw false-negatives on anything other than an antigen-test, the American Heartworm Society recommends trying to wait out the 2 to 3 year lifespan of the heartworms. This, of course, is provided the cat does not appear sick, in which case you would have had to diagnose the problem asymptomatically. If the cat appears to be suffering from effects of the inflammatory response their body has mounted against the heartworms, your veterinarian may suggest steroids to help alleviate distress. Otherwise, the only other real option to treat an active heartworm infection in cats is to administer the same medication used for dogs . . . which is notoriously toxic to cats and can cause lethal damage on its own.

Note the difference between the hosts.

Note the difference between the hosts.

So, What Can You Do?

Heartworm is easily preventable, thankfully. For dogs, there are once-a-month tablets they can consume that will release medication into the dog's bloodstream and interfere with the developing larvae's nervous systems. For those of you with picky pups, there is an injectable option available which lasts for six months. This is especially useful if you're anything like me and have trouble remembering when the last time it was that you gave your pet their heartworm prevention pill.

For cats, there are several forms of chewable tablets, pills, and topical applications that you administer monthly, but as of yet there are no injection-based preventions available. Just because your cat is indoor-only does not eliminate risk! Put even indoor kitties on a careful regimen of heartworm preventative. Mosquitoes can get inside with one swing of your door, and all it takes is one carrying the disease to ruin your cat's health.

Prevention Is Key

Prevention! Heartworm disease is nasty, expensive, painful, and a serious risk to your pet's health and welfare. If you have ferrets, they can also be infected, so be sure to talk to your veterinarian about prevention options for them.

. . . and in case you're wondering, no, this disease does not affect humans. Our bodies are inhospitable to the larvae!

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

Questions & Answers

Question: Are rats susceptible to heartworms?

Answer: Thankfully, there is no evidence that rodents, lagomorphs, or mustelids are at risk for contracting heartworm disease.


BirdPerson on June 15, 2020:

Are Birds prone to get heartworms? pls tell me as I have never read about this before