Heartworm Disease, caused by the parasite Dirofilaria immitis
, was once considered a disease of the southern United States. But it is now found in every state in the continental United States. A heartworm infection can affect both dogs and cats, but the disease is a bit different in each species.
In dogs, the immature larval stage of the worms are deposited into the dog's body by a mosquito. In 2-3 months they migrate through the bloodstream to the large blood vessels leading from the right heart to the lungs. There they grow into adult worms. As they grow, the heartworms can lodge in these blood vessels. If they are in large numbers, they also lodge in the heart. If untreated, dogs usually die of heart failure.
Cats' bodies react to heartworm infestation a bit differently. It takes about one month longer for the larvae to develop into adults. The heartworms affect the respiratory system more than the heart, causing "HARD" - Heartworm Associated Respiratory Disease. Both the larvae and adults can cause severe inflammation. Additionally, the larvae may migrate to other tissues of the cat's body, such as the brain or the eye.
The first outward signs of heartworm disease may not be apparent until a year after infection and may begin simply as a soft cough. He will be reluctant to exercise, and he may have a decreased appetite and weight loss. His quality of life can severely diminish, and as heart failure occurs, the dog can die. Severity of disease depends on the number of heartworms present.
Infected cats have signs that mimic many other diseases. These include coughing and difficulty breathing. You may also see vomiting, fainting, seizures, blindness, loss of appetite and weight loss.
If a dog has heartworm disease, an immature stage of the worms, called "microfilariae," will be present in 80% of individuals. These can be identified microscopically by your veterinarian. The most common blood test however detects certain proteins on the adult female worm. Another blood test detects antibodies the pet's body has made in an attempt to kill the heartworms. This test is most commonly used in cats. If a test is positive, other confirmatory tests including x-rays, ultrasound, and additional laboratory tests are performed.
In dogs, heartworm disease can be treated, but treatment can result in complications, sometimes fatal ones. The adult worms are killed by injections of arsenic-containing compounds. The treatment must be done carefully to avoid drug toxicity or complications resulting from the dead or dying worms.
These complications can occur when the dead or dying worms obstruct blood vessels to the lungs. Dogs must be hospitalized during treatments and must be kept quiet (cage rest) for at least four weeks after treatment. The treatment will kill the adult worms, but damage already done to the heart and lungs will remain. In severe cases, the worms may need to be surgically removed from the heart.
The good news is that there is no need for your pet to have to endure heartworm disease or its treatment, because it is so easy to prevent. A heartworm prevention program is effective and simple, and consists of three parts:
- Regular Blood Testing for Dogs - This ensures your dog is free from heartworms before he begins or continues on his preventive medication. Your veterinarian will advise you as to the recommended frequency of regular blood tests. Interpreting test results in cats is more problematic and you should discuss testing with your veterinarian.
- Preventive medication - This means administering a heartworm preventive to your pet year round, regardless of the mosquito season. Prevention for dogs includes monthly preparations like
Iverhart Max, Trifexis, or
Sentinel (which includes flea prevention).
also offer a monthly feline heartworm preventive.
- Reducing your pet's exposure to mosquitoes - This means making your
pet's environment less hospitable to mosquitoes. This decreases the risk of your pet being infected with heartworm in the first place.
Heartworm prevention is safe, easy and inexpensive compared to treating a dog or cat after worms have matured into adults. While treatment for heartworm disease is possible in dogs, it is a complicated and expensive process, taking weeks for infected animals to recover and there is often permanent damage to the heart. For cats, there is no effective treatment to kill the worms. By investing in a preventive medication, you will spare your pet from this deadly disease and its complicated treatment. Talk to your veterinarian today to start a prevention program for your pets.
Learn what every pet owner needs to know about heartworm prevention:
visit our Special Heartworm Disease Information Section!