Heartworm Disease in Dogs and Cats

by Bruce W. Little, DVM


Heartworm Disease in Dogs and Cats

Heartworm disease is a serious and sometimes fatal disease in dogs, cats and ferrets that can cause severe damage to the heart, lungs and blood vessels. Damage to other organs in the pet’s body can occur, but the greatest damage occurs in the cardiopulmonary organs. There is a significant difference in the number of heartworms found in dogs and cats with a corresponding relevance to the level of damage to the pet’s body. Dogs are a natural host for heartworms, and typically harbor about 12 to 18 adult worms. There are cases where as many as 250 adult worms have been documented. Each adult heartworm can measure six to twelve inches in length, and are located in the heart and blood vessels. Conversely, infected cats and ferrets usually contain 1-3 adult heartworms at any given time. Unfortunately, once the inflammation has been created in the heart, lungs and major blood vessels then damage to the pet’s health is frequently permanent at various levels of risk.

The Spread of Heartworm Disease

Heartworm disease is found in all 50 states and many parts of the world. Fifty years ago veterinarians thought the disease was concentrated in more humid areas of the country, such as the southeast region and gulf coast area. This also includes areas along major rivers and streams that serve as a haven for mosquitos. Today heartworm disease has infiltrated all areas of the country due to increased travel opportunities. Additionally, heartworm disease can infect other mammal species including wolves, coyotes, foxes and sea lions. In response to the spread of this disease, the American Heartworm Society was founded. They have done extensive research, and educated veterinarians and the public about the disease.

Regarding the specifics of the disease, adult female heartworms living in an infected pet give birth to microscopic offspring in the infected animal’s blood stream. These newly created parasites, called microfilaria, inhabit the animal’s blood-vascular system, and living off the nourishment of the blood of its host. When a mosquito bites and takes blood from an infected animal then it sucks up these microfilaria too, and the microfilaria transform into larvae. Subsequently, when the mosquito bites a different dog, cat or other susceptible mammal, the larvae are deposited on the surface of the skin of the bitten animal. The larvae then enter the animal’s body through the opening made by the mosquito bite wound. Once inside a new host, it takes approximately six months for the larvae to mature into adult heartworms that produce offspring, which starts the life-cycle all over again. This is a very complex process utilized by nature for survival of this species of parasite. Heartworm disease is not contagious, so a dog cannot catch the disease from being near an infected dog. Heartworm disease is only spread through the bite of a mosquito. Mature heartworms can live for 5-7 years in dogs and 2-3 years in cats. Although uncommon, it is possible for humans to contract heartworm disease from the bite of an infected mosquito.

Heartworm Disease in Dogs

In the early stages of the disease, dogs may show only minor symptoms. In fact, they may not show any symptoms at all. Depending on the severity of infection and the current health of the dog, moderate to severe clinical symptoms may appear. Dogs that are usually active may show fatigue, loss of appetite, weight loss and a persistent cough. As the disease progresses increased numbers of adult heartworms may develop, which creates blockages of blood and can cause a life-threatening cardiac collapse. The dog may develop a swollen abdomen due to the accumulation of fluid in the abdominal cavity. There is also a possibility that large numbers of mature heartworms will block the flow of blood within the heart chambers and connecting vessels. This causes a condition known as caval syndrome. This syndrome reveals itself through a sudden onset of labored breathing, pale gums and dark bloody urine. Very few dogs survive without immediate surgical removal of the heartworm blockage.

Heartworm Disease in Cats

Heartworm disease is atypical in cats, and exhibits different symptoms than in dogs. Symptoms of heartworm disease in cats can either be very subtle or very dramatic. Symptoms include coughing, lack of appetite, weight loss, occasional vomiting, chronic asthma-like attacks, occasional seizures or accumulated fluids in the abdomen. Unfortunately, sometimes the first sign of heartworm disease in cats is sudden death.

Have Your Pet Tested for Heartworm Disease

If the above symptoms are observed, call your veterinarian for a heartworm disease test. If local news media reports that heartworm disease has become endemic in your geographic area then you should have your dog tested. The test is a blood test that veterinarians can examine in their own offices, although some do send the blood samples to a laboratory. This test is usually included in the dog’s annual preventive examination. Puppies under 7 months of age can be started on heartworm medication without needing a test, because it takes six months for the puppy that was bitten by an infected mosquito to develop adult heartworms. However, the puppy will need a test 6 months after starting on the preventive medication with annual testing thereafter. Dogs who have not been taking a heartworm preventive medication, or have missed a scheduled treatment should be tested prior to being administered the preventive care. The preventative heartworm medication can cause blockages of blood vessels with dead heartworms and microfilaria, which will result in many health problems.

According to the American Heartworm Society all pets should be tested annually regardless of your geographic location or the fact that your dog and/or cat is in the house all the time. Consult with your veterinarian to see what is best for your pet. Both your pet and your checkbook will thank you!

Treatment for Heartworm Disease

Treatment of dogs with adult heartworm infestation is a complex and expensive proposition. The veterinarian must confirm the diagnosis with two tests. The first test is a blood test. After finding the presence of heartworms, the veterinarian will restrict exercise and may need to stabilize the dog’s general health before starting the medication. The extent of activity and time needed to bring the dog into acceptable condition to justify giving it the medication is determined by the severity of the infestation and the collateral damage that the disease has caused. This process can take up to six months or more before the dog is able to withstand the treatment. The treatment medication for adult heartworms in dogs is an injectable arsenical medication (melarsomine dihydrochloride) and can be damaging to the dog itself if not given under strict control. There is no approved treatment for cats. If cats test positive for heartworm disease, consult your veterinarian for the best plan to manage their symptoms and sustain a positive quality of health for the remainder of their life. The American Heartworm Society provides guidelines for developing the plan for treatment of heartworms.

Preventative Care for Heartworm Disease

It’s very important to administer all heartworm preventative care on a strict schedule. The heartworm larvae can turn into adult heartworms in as little as 51 days, and then your veterinarian will need to administer treatment for the adult heartworms. The schedule for heartworm preventative care is monthly for oral and topical products and every 6 months for the injectable product. Remember to follow this strict schedule to prevent any major health problems in your pet.

Thanks for reading, and be sure to check back for more articles from Bruce W. Little, DVM on Veterinarians.com, and follow him on Twitter @DrBruceLittle!