Canine Heartworm Disease

Life Cycle

Canine heartworm disease is caused by long, slender, 10-14 inch spaghetti-like worms that are most frequently found in the right side of the heart and adjacent lung arteries. Heartworms can live for several years in a dog's body. The female worms are capable of producing millions of larvae called microfilaria, which circulate in the infected dog's bloodstream.

The life cycle of the heartworm cannot be completed without an intermediate host, the mosquito. A mosquito can bite an infected dog and ingest some microfilaria along with the blood. These ingested microfilaria incubate and undergo changes within the mosquito's body for 10-30 days and then enter its saliva. When the mosquito subsequently bites a dog, these infective larvae are deposited into the dog's skin. The larvae develop for 3-4 months in the dog's body and eventually migrate to the right side of the heart and adjacent large blood vessels. In another two months the larvae have developed into mature heartworms and being to reproduce. The entire cycle can now repeat itself. Heartworm disease cannot be transmitted to another dog by an exchange of blood; a mosquito must be involved as an intermediate host of the parasite.

Symptoms

The signs of heartworm disease will vary depending on the severity and duration of the infestation. In the early stages, no clinical signs of the disease will be evident. However, as the condition progresses, signs of cardiac failure will gradually become apparent. Animals in the more severe stages will usually exhibit signs of coughing, weakness, listlessness, and lack of stamina.

Diagnosis

A heartworm blood test is one of the most effective ways to detect the presence of heartworms. We test a small blood sample, taken from a dog's leg vein, using a heartworm antigen test. This test will show a positive result if microscopic particles (antigens) shed by the heartworms are present in the blood sample. These antigens are not detectable in blood until 5-7 months post-infection. If the test were to show a positive result, additional tests will be recommended. These tests will include chest x-rays, liver and kidney function tests, and possibly an electrocardiogram. These tests are necessary to determine the degree of advancement of the disease and will aid in recommending the best form of therapy.

The American Heartworm Association recommends annual testing for all dogs. It is also recommended to test before changing preventative products. Testing is recommended to ensure product efficacy (though failures are extremely rare, they do occur) and to detect infections that may have occurred due to missed doses. It is very important to make sure the medication was effective in preventing the disease before clinical signs become apparent. The heartworm blood test also enables us to screen your pet for exposure to Lyme disease.

Treatment

Treatment can be attempted in dogs which are found to be otherwise in good health. Depending on the extent of the disease in the patient, several protocols for treatment can be considered. Most commonly, treatment will involve multiple injections of an arsenic derivative which will destroy the adult worms over time. Ivermectin will be administered to kill the microfilaria. Strict confinement is required during two months of the course of treatment. Many animals will become ill during the treatment. Often they become lethargic, lose their appetite, and may develop some coughing. Some deaths during therapy have been reported in the veterinary literature.

Monthly treatment with Heartgard Plus (ivermectin) will result in a slow kill (up to two years) of adult heartworms. However, this treatment carries with it significant risks for the animal's health and is not currently recommended except in extenuating circumstances.

Prevention

Due to the potentially toxic nature of the drugs used to treat heartworm disease, the high expense of treatment, and the severity of the disease, we encourage you to take proactive measures to prevent the condition. Considering the life cycle of the parasite, two means of prevention are available:

1. Controlling the mosquito population and limiting mosquito exposure.
2. Preventing the larvae from reaching adulthood.

Controlling mosquito populations is important for both human and animal health. However, it is rarely 100% effective. Protection is most practically achieved through the use of monthly preventative medications which kill the infective larvae after a mosquito has deposited them into the skin. The most popular types of preventative medication are administered once a month year round. These products are also effective in controlling the most common intestinal parasites of dogs: hookworms and roundworms. These parasites are also infective to people, particularly children and elderly. In the past we have recommended seasonal use of preventative but due to the mild nature of recent winters, and the public health benefits of controlling intestinal parasites, we now recommend year-round administration of preventative medication starting in puppyhood. This is particularly important for clients who take their dogs to the southern states during the winter months.

You may be familiar with Interceptor and Heartgard Plus preventative. We have also added Sentinel to our inventory. Having a broad selection enables us to tailor a complete parasite control program to each animal's needs. Please view Parasite Prevention and discuss a plan for your pet with your veterinarian. All of the preventative medications listed above are usually well tolerated by dogs except in animals which are already infested with heartworms. Infected dogs can have a serious reaction to the medication. For this reason, it is very important to perform a heartworm antigen test prior to starting preventative in animals over seven months of age.

Feline Heartworm Disease

Feline heartworm disease is caused by the same parasite that is responsible for canine heartworm disease, Dirofilaria immitis. A cat must be bitten by a mosquito carrying the infective larval stage of the worm in order to become infected. Unlike dogs, however, cats are more resistant to heartworm infection. Once infected, the life cycle is similar to that in the dog with a few exceptions cats tend to have fewer adult worms (usually only one or two) and the worms have a much shorter lifespan in cats as compared to dogs.

The symptoms of heartworm disease in cats are also very different from those seen in dogs. Some cats may show vague signs like lethargy, chronic vomiting, chronic episodes or coughing, weight loss, or anorexia. Some cats may have acute episodes of collapse or respiratory distress and can even suddenly die.

Because the signs of feline heartworm disease are so vague and nonspecific, it is much more difficult to diagnose the disease in cats. A new heartworm test has recently been developed to help diagnose feline heartworm disease. It is a blood test that tests for antibodies to the adult heartworms. Other methods of diagnosis include x-rays and echocardiogram (ultrasound of the heart).

Treatment of infected cats is very risky and may be hazardous to the cat's health. Since infected cats usually only carry one or two heartworms, treatment is usually just supportive care using medications to alleviate the symptoms until the worms die on their own. Even with supportive care however, heartworm infected cats may still die.

Dogs are the main source of heartworms. Indoor cats are at less risk of being bitten by an infected mosquito than cats that go outdoors. There is a monthly heartworm preventative available for cats, but since cats are more resistant to infection than dogs, and heartworm disease is not very common in our area, we are not routinely recommending the preventive for cats.