Heartworm disease is a potentially deadly parasitic infection in dogs. Without treatment, a dog with heartworm disease will eventually die. Although the disease can often be treated, prevention is the best way to keep dogs safe.
What is Heartworm Disease?
Heartworm disease is caused by an infection with a parasite called Dirofilaria immitis. This parasitic nematode is known to affect many animal species, but its ideal host is the dog. Dirofilaria immitis invades the heart, lungs, and nearby blood vessels of the dog, which can eventually lead to death.
Heartworms are possibly the most dangerous parasites that affect dogs. While heartworm disease is quite common in dogs, it can also be easily prevented with the help of your veterinarian. Heartworm disease can also occur in cats, but this is less common. People can be infected with Dirofilaria immitis, but this rarely causes complications because the parasite is not viable in the human body.
Signs of Heartworm Disease in Dogs
Clinical signs of heartworms do not typically appear until the disease is somewhat advanced. No signs are exhibited in the early stages while larvae are still maturing. Dogs may develop coughing and exercise intolerance once adult heartworms are present in the lungs and heart. As the heartworms reproduce and more develop into adults, dogs experience difficulty breathing, abdominal swelling, collapse, and even sudden death.
Contact your vet right away if your dog is coughing or showing other signs of illness. A veterinarian can perform a heartworm test, listen for abnormal heart sounds, and perform other diagnostic tests to help determine the severity of heartworm disease.
Cause of Heartworm Disease in Dogs
Heartworm disease is transmitted between animals by way of the mosquito. First, a mosquito bites a dog or other animal that is infected with heartworm microfilariae (immature heartworm larvae) in the bloodstream. When the mosquito ingests these microfilariae, within two to four weeks they develop into infective larvae within the body of the mosquito.
When the mosquito bites another dog, the microfilariae enter the dog's skin and continue to develop for three to twelve days. The mature larvae then migrate through the dog's body (abdomen, thorax, and skin) for 50-70 days until they enter the dog's bloodstream and become young adult worms. These young adult heartworms travel towards the heart and lungs as they mature to reproductive age. At this stage, they are about one to two inches long.
Within seven months of the first transmission by the mosquito bite, Dirofilaria immitis will reach maturity. Mature male heartworms are about 15-18 cm long. Females are 25-30 cm and resemble angel hair pasta. Adult heartworms mate in the blood vessels of the lung. Their offspring, the microfilariae, then make their way through the bloodstream until they are ingested by a mosquito, and the life cycle is repeated.
A single adult heartworm can survive in a dog for five to seven years. Adult heartworms generally live in the dog's heart and pulmonary blood vessels, causing damage and inflammation to the lining of the vessels and surrounding tissue. The more worms present, the greater the complications. The worms can obstruct blood flow through the arteries and the valves of the heart. Cardiac output may be decreased, leading to possible heart enlargement and pulmonary hypertension. All of these issues can also lead to the malfunction of other organs in the body, including the liver and kidneys.
Dogs positive for heartworm disease first undergo diagnostic testing to help determine the severity of the disease. This typically includes lab work and radiographs but may vary depending on the clinical signs, if any. Dogs with severe disease may not survive treatment and are often not considered eligible for treatment.
Adult heartworms are killed with the use of an adulticide. The protocol most vets follow is based upon the guidelines set by the American Heartworm Society, which involves injections of an adulticide called melarsomine (Immiticide) and adjunct treatments, to help manage the complications associated with heartworm disease and treatment.
Many vets will use a pre-treatment of antihistamines, antibiotics, and anti-inflammatory drugs to prevent a reaction as the immature larvae die, in response to the administration of a preventive. The dog may need to be monitored in the hospital after the initial dose of heartworm preventive is given. Year-round heartworm prevention will often be necessary for the remainder of the dog's life.
The heartworm-positive dog is generally started on oral doxycycline or minocycline for four weeks. These antibiotics are given to fight bacteria released by the dying heartworms. They are also thought to weaken the worms.
The veterinarian would determine the best protocol for the dog. In some cases, a two-dose protocol (24 hours between the two doses) would be used, and in others, a three-dose protocol would be selected. Melarsomine is injected into a muscle along the lumbar spine and the dog is observed for a day in case of any reaction.
With the three-dose protocol, after the first dose of melarsomine is administered, the dog will need to return to the hospital for the second dose about 30 days later. The same preliminary treatments mentioned before are usually given to prevent a reaction. The dog is typically hospitalized overnight and is given a third melarsomine injection the next day.
Heartworm treatment is risky, mainly because of the blood clots that can occur as the worms die. Restriction of the dog's activity is essential throughout treatment and should be strictest during and after adulticide injections. Exercise, excitement, and overheating will all increase the likelihood of complications. Vets typically recommend activity restriction for one or two months following heartworm treatment.
After a dog has been treated for heartworm disease, reinfection can occur if a heartworm preventive is not used. Discuss with your vet whether your dog should be on year-round heartworm prevention.
How to Prevent Heartworm Disease in Dogs
You can spare your dog (and your bank account) from the risky treatment process by taking actions to prevent heartworm disease from occurring in the first place. Talk to your vet about the best heartworm prevention plan for your dog.
Heartworm Prevention Drugs
Heartworm prevention drugs are parasiticides that kill any microfilariae present in the bloodstream, preventing them from maturing into adult worms. Heartworm preventives are often given in the form of a monthly chewable tablet or topical application. An injectable form of heartworm prevention is available at some veterinary clinics. Heartworm preventives are about 99% effective and are tolerated well by most dogs.
The American Heartworm Society recommends year-round heartworm prevention for all dogs in the United States, regardless of region. It's important to follow your vet's recommendations regarding heartworm prevention. Never stop or skip your dog's regular heartworm prevention unless instructed to do so by your veterinarian. Notify your veterinarian if your dog misses one or more doses of heartworm prevention.
Veterinarians have a quick and simple blood test to check for the presence of antigens released by adult female heartworms. All dogs should be tested for heartworms at least once a year, even if they have been on a heartworm preventive consistently. If heartworm preventives are missed or delayed, dogs should be retested in about six months to ensure no microfilaria have developed into adult heartworms. Heartworm testing is typically part of a dog's annual wellness visit.
Talk with your vet about the best plan of action for preventing heartworm in your dog. Following proper prevention methods is the best way you can keep your dog healthy and heartworm free.
“Heartworm Disease in Dogs.” Merckvetmanual.Com, https://www.merckvetmanual.com/dog-owners/heart-and-blood-vessel-disorders-of-dogs/heartworm-disease-in-dogs.
CDC-Centers for Disease Control, and Prevention. CDC - Dirofliariasis - Frequently Asked Questions (FAQs). 2010, https://www.cdc.gov/parasites/dirofilariasis/faqs.html.
American Heartworm Society. Prevention, Diagnosis, and Management of Heartworm Infection in Dogs. 2020, https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines.