Heartworm disease can result 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.
In the US, heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states. Each year, the number of cases in the West has risen.
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. See the graphic below of the heartworm lifecycle in dogs.
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.
How is a Dog Tested for Heartworms?
A veterinarian uses blood tests to check a dog for heartworms. An antigen test detects specific heartworm 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. The earliest that the heartworm proteins can be detected in a dog’s bloodstream is about 5 months after it is bitten by an infected mosquito.
Another test detects 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). The earliest that microfilariae can be detected in a dog’s bloodstream is about 6 months after it is bitten by an infected mosquito, because it takes about that long for 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 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
- 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.
We will explain the symptoms of heartworm and treatment in our next GoldenGram. Stay tuned!
