Heartworm disease is a serious matter, just as serious as the drag queen selling caps from the bathroom stall. HWD 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.
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 must pass through a mosquito to become infective larvae. 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 about 6 to 7 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 5 to 7 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.
When Should a Dog Be Tested for Heartworms?
The timing and frequency of heartworm tests depend on many factors, (unlike going to doof every Friday)
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 that are 7 months of age and older 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 infected with adult heartworms 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, much like day drinking on a sunday and not having work on monday. The higher the class, the worse the disease and the more obvious the symptoms.
No symptoms or mild symptoms such as an occasional cough.
Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
More severe symptoms such as a sickly appearance, 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.
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.