Heartworm Disease

Heartworm disease is a major health problem in dogs in the United States and throughout certain areas of the world. Although it is easily preventable with a relatively inexpensive medication, it unfortunately persists in this country perhaps due to a lack of information on the part of pet owners or an unwillingness to provide basic veterinary care for their pets. Rescue workers, especially in this part of the country, have to deal with heartworm-positive dogs all too often. It used to be that heartworm disease was seen mainly in coastal regions and the South, but it is now reported in all 50 states. The American Heartworm Society now recommends year-round prevention, even in areas where the possibility of infection is considered seasonal.

Dogs do not “catch” heartworms from other dogs. People cannot get heartworms directly from their dog. The heartworm, which is an actual worm or parasite that has the scientific name Dirofilaria immitis, undergoes a complicated life cycle. It all starts with a fertilized adult female heartworm (which is living inside the blood vessels or heart of a dog or other host) giving birth to her young, which are called microfilariae, into the dog’s bloodstream. A mosquito then bites the dog and ingests some of these microfilariae along with its blood meal. It takes 10 to 14 days for these microfilariae to mature inside the mosquito to the infective stage. Then, when the mosquito bites another dog, it can leave these infective larvae at the wound site to migrate into the dog’s subcutaneous tissues. It takes over six months for these larvae to develop into adult heartworms, at which time they take up residence in pulmonary (lung) arteries and sometimes even in the right chambers of the heart. The male heartworms are four to six inches in length and the females 10 to 12 inches. Their lifespan in the dog averages five to seven years. Note that the microfilaria cannot mature into an adult heartworm without first passing through a mosquito.

Once the heartworms have become established inside the dog, the results can vary from no apparent signs of disease (a sedentary dog that has a low number of adult worms) to an acute onset of life-threatening disease (usually a more active, often working, dog with a so-called high worm burden). Veterinarians normally categorize heartworm disease patients into several classes. Class 1 represents early infection with no clinical signs or just a cough, with no changes on radiographs. Class 2 is moderate disease, with a cough, exercise intolerance, and abnormal lung sounds when the doctor listens with a stethoscope. Class 3 represents severe disease; this dog may have a cough, difficulty breathing, exercise intolerance, abnormal heart and lung sounds, an enlarged liver, fluid in the abdomen, and syncope (temporary loss of consciousness). Class 4 or Caval Syndrome is the most severe category; this patient has a sudden onset of weakness and lethargy and will have an orange or brown colored urine due to hemoglobin being broken down and released into the bloodstream. These dogs have a very guarded prognosis.

Whether a dog can be successfully treated for heartworms will depend on a number of factors, including which of the above classes the dog is in when diagnosed with heartworms, how long the dog has had heartworms (rescuers rarely will know this), what damage has already been done by the heartworms, and other presenting health problems. The heart and lungs are the major organs affected by heartworms, but the liver and kidneys can also sometimes be damaged. The majority of dogs, except for the most advanced cases, can be safely treated for their adult heartworms.

The goal of heartworm treatment is to eliminate adult heartworms with a minimum of side effects and complications to the patient. At present, only one drug is approved by the FDA for the elimination of adult heartworms in dogs. This adulticide is melarsomine dihydrochloride or Immiticide, which is an organic arsenical compound. It is administered by deep intramuscular injection into the lumbar muscles in the lower back of the dog. Until recently, it was recommended to be given as two injections 24 hours apart to Class 1 and 2 patients, and Class 3 patients were to receive a split-dose regimen involving one injection followed in about a month by two more injections 24 hours apart. Now the American Heartworm Society recommends that all patients receive the split-dose technique, and most veterinarians are following that advice. The benefit is that the heartworms are killed more slowly, and the chances of a thromboembolism or clot in the lungs are lessened.

The remains of dead worms, along with lesions in the blood vessels of the lungs, will cause some degree of thromboembolism, hopefully mild, so that the dog exhibits only some minor coughing and gagging post-adulticide treatment. Should the dead worms be numerous and the vessel injury severe, widespread obstruction in arteries can occur. The dog in this case will show more severe signs, such as fever, coughing up blood, pale gums, breathing difficulty, and potentially even sudden death. The risk of clot formation is the main reason why dogs MUST be severely exercise restricted for at least four weeks after each phase of treatment; this means no running, playing, excitement, getting overheated, etc. See our guide on Heartworm Treatment for more information.

The other problem most often seen from adulticide treatment is local injection reaction. It is common for most dogs to be sore in the lower back area for several days after treatment. The treating veterinarian will normally send medication to deal with the soreness and tissue inflammation from the injections. Rarely, neurological complications have been seen from Immiticide treatment, which in most cases resolve with aggressive care.

Treatment for heartworms is neither inexpensive nor fun, for the patient or the owner/rescuer. How much easier and cheaper it is to prevent rather than treat heartworms! There are several monthly oral products, usually chewable and palatable to the dog, as well as some topical spot-ons. These products also protect against a variety of other parasites, such as intestinal worms and in some cases external parasites also. A six-month injectable heartworm preventative is once again available, with some restrictions, after being pulled off the market in 2004. In all cases, it is imperative that the dog be tested for heartworms before being started on any preventative. First, some preventatives can cause complications if given to a microfilaria-positive dog. Also, the sooner a dog is diagnosed, the sooner he can be evaluated for treatment, decreasing the time that damage can be done by the heartworms. Along with a lack of information, a lack of owner compliance contributes to the number of heartworm-positive dogs. The dog simply must receive his preventative, whatever form has been prescribed by his veterinarian, at the frequency it is meant to be given, or he is at risk for heartworm disease.

The heartworm preventatives available today are safe, effective, and cost-friendly. There are no “natural” remedies that are effective. It is not enough to keep mosquitoes away from the dog, as dogs still go outside to eliminate, and mosquitoes come inside as well. Long-haired dogs are not safe; mosquitoes will bite them on the nose or hairless inner ear flap or belly. All of us who love and work with dogs should spread the word and try to educate and inform those we encounter who do not understand the horrors of heartworm disease.

The American Heartworm Society provides up-to-date information on this subject. Go to American Heartworm Society.

With thanks to Nancy Wilhelm, DVM for her generous contribution. Used with permission.