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Heartworm is a parasitic roundworm (Dirofilaria immitis). It primarily infects dogs, cats and ferrets, although it can also infect a wide variety of wildlife. It is a potentially fatal parasite, however, it is preventable. Humans have become infected, although this is rare and does not usually cause illness.



Mosquitoes are the only way that heartworms can be transmitted from animal to animal. When mosquitoes bite an infected animal and ingest their blood, young heartworms (microfilariae) enter the mosquito’s digestive system. Within 2 to 6 weeks, the microfilariae inside the mosquito develop into infective larvae. They then migrate to the mosquito’s salivary glands. These infective larvae can then be transmitted to another animal when the mosquito bites to get its next meal of blood.

The period between the initial infection when the dog or cat is bitten by an infected mosquito and the development of the microfalariae into adults living in the tissues of your pet usually takes 6 to 7 months. This is known as the “prepatent period”.

In cats, the microfilariae live for only a month in the vascular system and are seen in only 20% of infected cats (compared to 80 to 90% in dogs). This means that infected cats are unlikely to transfer the heartworm to another mosquito – in stark contrast to infected dogs.

The larvae introduced by the mosquito grow for 1 to 2 weeks under the skin at the site of the mosquito bite. Then, they migrate to muscles in the chest and abdomen, and 45 to 60 days after infection, molt to the next larval stage. Between 10 to 15 weeks after infection, the larvae enter the blood system where they are transported through the heart to reside in the pulmonary artery of the lungs.

During the next 3 months, the larvae (or immature worms), continue their development to become adults. Adult female worms can grow up to 14 inches in length and males about 2 inches shorter. The growing and feeding worms damage the blood vessels which causes a decrease in the efficiency of the heart and moving blood around the body. Occasionally, adult heartworms migrate to the right side of the heart and even to the large cardiac veins in severe infections. This can result in severe lung and heart disease. Animals showing signs of illness from adult heartworm infection are said to be suffering from heartworm disease. Death can occur as the result of congestive heart failure. Adult heartworms can have a long lifespan; they may survive for 5 to 7 years in dogs and 2 to 3 years in cats.

At approximately 5 to 7 months post-infection, if both adult females and males are present, they will mate and produce new microfilariae. The host’s immune system raises a strong response to these microfilariae, which unfortunately, can cause damage to other organs. The life-cycle then continues when a mosquito bites an infected pet and itself becomes infected by the microfilariae.


At one time, heartworm in the U.S. was confined to the southern states, but has now spread to nearly all locations where mosquitoes are found. It is also found in many other countries. All dogs are susceptible to heartworm infection and both indoor and outdoor cats are at risk of the disease. If you plan to visit different areas of the country with your dog or cat, or another country, consult your veterinarian about the risk of heartworm in the area you intend to visit.

Because mosquitoes are the vector of this disease, any animal which comes into contact with mosquitoes should be tested. This also includes pets that might go outside only occasionally.



Dogs with recent or mild heartworm infections may show no signs of illness. However, once the adult worms have developed in the lungs, your dog may cough, have difficulty breathing, become lethargic or lose its appetite; he may also tire rapidly after only light or moderate exercise.

Your veterinarian will have available to them, numerous diagnostic tests to detect the presence of adult heartworms. These are infections that have occurred at least 6 months previously. The presence of adult female heartworms can be detected with an antigen test which is very accurate in dogs. Exposure to heartworms can be detected by antibody tests. To confirm the diagnosis and evaluate the severity of the disease, your veterinarian may conduct other tests such as chest radiographs (x-rays), a blood profile, or an ultrasound of the heart (echocardiogram). These will also help determine the best treatment plan for your dog.


Symptoms of heartworm disease in cats include those often associated with other diseases. These include respiratory distress, gagging, coughing and vomiting. Symptoms associated with the first stage of heartworm disease, when the heartworms are carried to the pulmonary arteries, are often mistaken for feline asthma or allergic bronchitis, when in fact, they are actually due to a syndrome newly defined as Heartworm Associated Respiratory Disease (HARD). Cats may die suddenly from heartworms, although this happens rarely.

The diagnosis of heartworm infection in cats is more difficult than it is with dogs. Several tests may be required to assess the likelihood of heartworm infection. Even then, the results may be inconclusive. Usually, both antigen and antibody tests are conducted for cats to increase the probability of detecting heartworms.



As usual with most medical problems, prevention is better than cure. However, if your dog does contract a heartworm infection, treatment is available. Treating dogs for heartworms carries with it a substantial risk, but, if your dog is in good health and the instructions of your veterinarian are followed, this will considerably reduce the possibility of serious complications.

The aim of treating heartworm is to kill both the adult worms and microfilariae, as safely as possible. Successful treatment means there will be heartworms dying inside your dog’s lungs. Therefore, during treatment, hospitalization may be required initially. Upon going home, the dog should be rested (limited to leash walking) for a period, to decrease the risk of partial or complete blockage of blood flow through the lungs by dead worms. Other medications may be required to reduce the body’s inflammatory reaction to the worms being broken down in the lungs.


At present, there is no effective and safe treatment for heartworm infection in cats. If your veterinarian diagnoses heartworms in your cat, they may also recommend medications to reduce the inflammatory response and the resulting disease. Another course of action may be surgery to remove the heartworms. Cats are more resistant hosts to heartworm than dogs, and are often able to rid themselves of infections. Unfortunately, many cats react severely to the dead worms as they are cleared by the body which can result in a physiological shock reaction, a life-threatening condition.



Removing heartworms by surgery is a high-risk procedure for both dogs and cats. It is typically reserved for only severe cases, however, this approach may be necessary to give your pet the best chances of survival.


Heartworm infection is almost 100% preventable in dogs and cats. There are several heartworm medications available which can be obtained in a variety of formulations. Based on your pet’s risk factors and life-style, your veterinarian will recommend the best preventative, but of course, you will have to remember to administer the medication for it to work!

The preventatives do not kill adult heartworms. They also do not eliminate heartworm infection or prevent signs of heartworm disease if heartworms are already present in the animal’s body. Therefore, it is recommended that a blood test for existing heartworm infection is conducted to assess the pet’s current heartworm status, before beginning a prevention program. It is more difficult to detect heartworms in cats, therefore, additional testing may be necessary to make sure there is no infection during any course of prevention.

Once treatment has finished, testing must then be repeated at the correct intervals. Six months after starting the preventative treatment, a test should be performed to confirm that your pet was not infected prior to starting the prevention – it should be remembered that tests only detect adult worms. From then, heartworm tests should be conducted on an annual basis to ensure your pet does not subsequently become infected. This will also check the appropriate amount of medication is being administered. Some pets develop a heartworm infection despite year-round treatment with a preventive; having regular tests is the most effective way to keep your pets protected.

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