Heartworm Disease – What Is It And What Causes It?
Heartworm disease is a serious disease that results in severe lung disease, heart failure, other organ damage, and death in pets, mainly dogs, cats, and ferrets. It is caused by a parasitic worm called Dirofilaria immitis.The worms are spread through the bite of a mosquito. The dog is the definitive host, meaning that the worms mature into adults, mate, and produce offspring while living inside a dog. The mosquito is the intermediate host, meaning that the worms live inside a mosquito for a short transition period in order to become infective (able to cause heartworm disease). The worms are called “heartworms” because the adults live in the heart, lungs, and associated blood vessels of an infected animal.
Photos courtesy of Matt W. Miller, DVM, MS, Diplomate ACVIM (Cardiology), College of Veterinary Medicine, Texas A&M University
The Heartworm Lifecycle In Dogs
In an infected dog, adult female heartworms release their offspring, called microfilariae, into the dog’s bloodstream. When a mosquito bites the infected dog, the mosquito becomes infected with the microfilariae. Over the next 10 to 14 days and under the right environmental conditions, the microfilariae become infective larvae while living inside the mosquito. Microfilariae cannot become infective larvae without first passing through a mosquito. When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite wound. In the newly infected dog, it takes between six and seven months for the infective larvae to mature into adult heartworms. The adult heartworms mate and the females release their offspring into the dog’s bloodstream, completing the lifecycle.
Heartworm disease is not contagious, meaning that a dog cannot catch the disease from being near an infected dog. Heartworm disease is only spread through the bite of a mosquito.
Inside a dog, a heartworm’s lifespan is five to seven years. Adult heartworms look like strands of cooked spaghetti, with males reaching about 4 to 6 inches in length and females reaching about 10 to 12 inches in length. The number of worms living inside an infected dog is called the worm burden. The average worm burden in dogs is 15 worms, but that number can range from 1 to 250 worms.
How Is A Dog Tested For Heartworms?
The most common test that a veterinarian uses to check a dog for heartworms is called an antigen test. This blood test detects specific proteins, called antigens, which are released by adult female heartworms into the dog’s bloodstream. In most cases, antigen tests can accurately detect infections with one or more adult female heartworms that are at least seven or eight months old, but the tests generally do not detect infections that are less than five months old.
There are also tests that detect microfilariae in a dog’s bloodstream. Microfilariae in the bloodstream indicate that the dog is infected with adult heartworms (because only adult heartworms can mate and produce microfilariae). Microfilariae can be detected in a dog’s bloodstream about six to seven months after it is bitten by an infected mosquito (because six to seven months is the time it takes the heartworms to develop from infective larvae into adults that mate and produce microfilariae).
When Should A Dog Be Tested For Heartworms?
The timing and frequency of heartworm tests depend on many factors. Some of these factors include:
- The dog’s age when heartworm prevention is started;
- If the owner forgot to give heartworm prevention and for how long;
- If the dog is switched from one type of heartworm prevention to another;
- If the dog recently traveled to an area where heartworm disease is more common; and
- The length of the heartworm season in the region where the dog lives.
Dogs older than six to seven months of age should be tested for heartworms before starting heartworm prevention. A dog may appear healthy on the outside, but on the inside, heartworms may be living and thriving. If a heartworm-positive dog is not tested before starting a preventive, the dog will remain infected with adult heartworms until it gets sick enough to show symptoms. Heartworm preventives do not kill adult heartworms. Also, giving a heartworm preventive to a dog that has an adult heartworm infection may be harmful or deadly. If microfilariae are in the dog’s bloodstream, the preventive may cause the microfilariae to suddenly die, triggering a shock-like reaction and possibly death.
Annual testing of all dogs on heartworm prevention is recommended. Talk to your dog’s veterinarian about the best time for your dog’s annual heartworm test.
What Are The Symptoms Of Heartworm Disease In A Dog?
The severity of heartworm disease is related to how many worms are living inside the dog (the worm burden), how long the dog has been infected, and how the dog’s body is responding to the presence of the heartworms. The dog’s activity level also plays a role in the severity of the disease and in when symptoms are first seen. Symptoms of heartworm disease may not be obvious in dogs that have low worm burdens, have been recently infected, or are not very active. Dogs that have heavy worm burdens, have been infected for a long time, or are very active often show obvious symptoms of heartworm disease.
There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms.
- Class 1: No symptoms or mild symptoms such as an occasional cough.
- Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
- Class 3: General loss of body condition, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
- Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die.
Not all dogs with heartworm disease develop caval syndrome. However, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.
Is There A Treatment For Heartworm Disease In Dogs?
The drug Immiticide (melarsomine hydrocholoride) is FDA-approved to kill adult heartworms in dogs. Immiticide contains arsenic and is given by deep injection into the back muscles to treat dogs with stabilized class 1, 2, and 3 heartworm disease. Another drug, Advantage Multi for Dogs (imidacloprid and moxidectin), is FDA-approved to get rid of microfilariae in the dog’s bloodstream. Advantage Multi for Dogs is a topical solution applied to the dog’s skin.
The treatment for heartworm disease is not easy on the dog or on the owner’s pocket book. Treatment can be potentially toxic to the dog’s body and can cause serious complications, such as life-threatening blood clots to the dog’s lungs. Treatment is expensive because it requires multiple visits to the veterinarian, bloodwork, x-rays, hospitalization, and a series of injections with Immiticide.
The Best Treatment Is Prevention!
Many products are FDA-approved to prevent heartworms in dogs. All require a veterinarian’s prescription. Most products are given monthly, either as a topical liquid applied on the skin or as an oral tablet. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites).
Year-round prevention is best! Talk to your dog’s veterinarian to decide which preventive is best for your dog.