Heartworms are blood-borne parasites that live in the heart and blood vessels of infected dogs.  The female worm is 15-36 cm long & 5mm wide. The male worm is half that size, and one infected dog can have as many as 300 worms inside its body.

How do dogs contract heartworms?

Heartworms infect domestic dogs, as well as wolves, foxes, and coyotes.  They can also occur in cats (domestic and wild), ferrets, and California sea lions - and are spread from one infected animal to another mosquitoes.

The female heartworm produces millions of offspring called microfilaria. These offspring live mainly in the bloodstream's small vessels and cannot complete their life cycle in the dog.  Heartworms develop in the mosquitoes to their infective form, and are then passed onto dogs. This disease is only spread when mosquitoes are present.  The offspring are picked up by the mosquitoes, when they bite the dog, develop in the mosquitos, and then spread to other animals.  When fully developed, the infective offspring enter the bloodstream and move to the heart and blood vessels where they grow to maturity in two to three months and start reproducing, thereby completing the full life cycle.

It takes a number of years before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in 4 to 8 year old dogs. The disease is very seldom diagnosed in a dog less than one year of age because the young worms (larvae) take up 5-7 months to mature after infection.

What do heartworms do to the dog?

Adult heartworms cause disease by clogging the heart and major blood vessels leading from the heart.  They interfere with valve action of the heart.  By clogging the main blood vessels, the blood supply to the other organs of the body is reduced particularly affecting the lungs, liver and kidneys leading to malfunction of these organs.

Most dogs with heartworm disease had no signs, and the disease is detected by routine heartworm blood tests. The more athletic and active the dog, the earlier the signs are seen. The first signs are often lethargy, decreased activity, and coughing. As the disease worsens, breathing rate and effort increase. Some dogs have fainting episodes when stressed. With advanced disease and heart failure, fluid build-up in the abdomen and weight loss may occur.  If large numbers of adult heartworms obstruct blood flow in the main arteries, lethargy, weakness, loss of appetite and fever may occur. The membranes of the mouth may be pale or yellow from red blood cell destruction and anemia.

How is heartworm diagnosed?

Two blood tests can be run to diagnose heartworm disease in your dog. A serological test for antigens to adult heartworms is the most common test because it detects the antigens (proteins) produced by adult heartworms. This test can also result in false negatives in early infections because only the female heartworm produces the antigen. A blood sample can also be examined to check for microfilariae (heartworm offspring). If microfilariae are seen, the test is positive. The number of microfilariae seen gives us a general indication of the severity of infection.

If a dog is positive for heartworms, chest x-rays are done to look for heart and lung changes. An echocardiogram (heart ultrasound) is done in dogs with heart failure and to assess the heart.  Prior to treatment, routine laboratory tests are usually recommended. If other problems, such as liver or kidney disease are found, they are treated first, because the drug used to kill adult heartworms can harmfully affect the kidneys and liver.

How are dogs treated for Heartworm?

There are some risks involved in treating dogs with heartworms, although deaths are rare. New drugs allow successful treatment of more than 95% of dogs with heartworms. Depending on the severity of the disease the longer the treatment may be.   The first dose of medication is usually and injectable to kill all adult heartworms. Complete rest is essential after the first treatment to prevent acute death caused by the decomposing adult heartworms. Treatment of the microfilariae usually occurs one month after the initial treatment and your dog will need to stay in the hospital overnight for observation.

4-6 months after adult heartworms are treated; a blood test is done to determine whether all adults were killed. A few dogs may remain positive and require repeated adulticide treatment. After successful heartworm treatment, preventative therapy is often given year round or at least during mosquito season. An annual or biannual heartworm test is done to ensure that preventative is working.

How do I prevent heartworm disease?

Yearly heartworm testing is important to ensure that the heartworm as not affected your dog as well as using heartworm prevention medications given monthly during the time when mosquitos are prevalent. The best medicine for this disease is prevention. If you have any further questions regarding heartworm testing and medications it is always important to consult your veterinary team. Lexy