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Heartworm disease (Dirofilariasis) is a serious and potentially fatal disease in cats and dogs throughout the world. The female worm is 6-14 inches long, and an infected dog may have as many as 300 in the heart or adjacent blood vessels. Cats usually have much fewer worms, but as few as 2 worms can cause severe disease and even death.
The disease is not spread directly from pet to pet. An intermediate host (the mosquito) is needed for transmission. Dogs serve as the reservoir for heartworms, such that a mosquito biting an infected dog can spread the disease to other dogs and cats. Although cats do become infected with heartworms, the worms cannot complete their life cycle in the cat. The number of dogs infected and the length of the mosquito season are important factors in the prevalence of the disease in any given area. In the United States, infection rates in dogs are 15-20%, but can approach 50% in some areas. The prevalence in cats is about 5-15%; in Knox County, OH, the infection rate is almost 10%. Recent scientific information has shown no statistical difference in indoor versus outdoor cats.
Adult heartworms live in the heart and adjacent large vessels of infected cats and dogs. The worms survive up to 5 years, and during this time the female produces millions of young worms (microfilariae) in the dog. Cats usually do not support microfilariae. These microfilariae live in the bloodstream, mainly in the small blood vessels. They cannot grow into adults without passing through the intermediate host (the mosquito). The microfilariae are therefore not infective (cannot grow into adults) in the dog, although they do cause other problems. The microfilariae cannot be transmitted directly to another cat or dog at this stage of their development.
As many as 30 species of mosquito can serve as the intermediate host. The female mosquito bites the infected dog and swallows the microfilariae during the blood meal. The microfilariae develop further for 10-30 days in the mosquito and then enter the saliva of the mosquito. The microfilariae are now called infective larvae, because at this stage of development they will grow to adults when they enter a cat or dog. The mosquito bites a cat or dog and the infective larvae in the mosquito's saliva are deposited under the skin. Here they develop further for 3-4 months.
When fully developed the infective larvae now enter the bloodstream and move to the heart and adjacent blood vessels, where they grow to maturity in 2-3 months and start reproducing, thereby completing the full life cycle.
Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver, and kidneys.
Most cats and dogs infected with heartworms do not show any signs of disease. Unfortunately, by the time signs are seen the disease is well advanced. The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver, and kidneys.
In dogs, the most obvious signs are a soft, dry, chronic cough; shortness of breath; weakness; nervousness; listlessness; and loss of stamina. All of these signs are most noticeable following exercise, which causes some dogs even to faint. In advanced cases, where there is congestive heart failure, there may be swelling of the abdomen and the legs from fluid accumulation. There also may be evidence of weight loss, poor condition, and anemia. Severely infected dogs may die suddenly during exertion or excitement.
In cats, the clinical signs are often vague and intermittent. Some of the signs are difficulty breathing, coughing, vomiting, sudden collapse, or blood clots (thromboembolism). Unfortunately, the most common presentation in clinically normal cats is sudden death.
In most cases, whether there are signs of the disease or not, a diagnosis of heartworm disease can be made by a relatively simple blood test in dogs. A blood sample is examined under the microscope for the presence of microfilariae or an antigen test is used to detect the presence of adult worms in the heart. Dogs should be tested annually and, depending on the preventative that they are on, might need to be tested before they start their preventative. It is important to check with your veterinarian about your dog's testing schedule. In clinically normal cats, however, the available tests are difficult to interpret and are generally not performed annually. If your cat is ill, your veterinarian may suggest a heartworm test to rule out this disease.
In cases of a positive test, your veterinarian may recommend some or all of the following procedures before treatment is started:
X-rays will often show heart enlargement and swelling of the large arteries leading to the lungs from the heart. This helps to assess the level of existing damage to the heart and lungs.
Electrocardiogram (ECG) is a pen tracing of the electric currents generated by the heart. It is used to determine the performance of the heart and the extent of disease in the heart muscle.
Cardiac Ultrasound is used to visualize damaged valves and to assess the contractility of the heart muscle. Often the worms themselves can be visualized by ultrasound.
Blood chemistries and urinalysis including complete blood cell counts and blood tests for kidney and liver function help determine the general condition of the cat or dog and whether they can tolerate treatment.
There is some risk involved in treating dogs with heartworms, although fatalities are rare. There is currently no approved treatment for heartworm disease in cats, other than oral steroids and good supportive care. In dogs, treatment involves hospitalization and treatment to destroy the adult worms, complete rest for 1 month following treatment, subsequent treatment to destroy the microfilariae, and retesting for both microfilariae and adult heartworms. In cases of heartworm disease with advanced kidney or liver disease, or where the reactions to medication used to treat the adult heartworms is too severe, heart surgery can be performed to remove as many of the adult worms as possible. Because such dogs are in poor condition the survival rate is not good. However, surgery is sometimes the only alternative left to try to save the dog's life. In dogs with severe heartworm disease, it may be necessary to treat them with antibiotics, special diets, diuretics (to remove fluid accumulations), and digitalis to improve heart function prior to treatment for the heartworms. Dogs in which there is severe heart disease may need lifetime treatment for the diseased heart even after the heartworms are destroyed.
The number 1 way to protect your pet from the potentially fatal effects of heartworm disease is to use a preventative. Chewable, topical (skin application), and injectable products are available for both cats and dogs. Ask your veterinarian about your preventative options.