Heartworm Disease

Companion Animals August 27, 2014 Print Friendly and PDF

Heartworm disease (Dirofilaria immitis) is a preventable but serious and potentially fatal disease. It is caused by a parasitic worm that primarily infects members of the canid family, including domesticated dogs and wild canids, such as coyotes, wolves, and foxes. The infection is not limited to dogs, as it has been documented in other mammals as well, including cats, ferrets, raccoons, opossums, seals, sea lions, and rarely humans. 

History

According to the American Heartworm Society, the first published description of heartworm in dogs in the United States appeared more than 100 years ago in an issue of “The Western Journal of Medicine and Surgery.” The earliest case of heartworm disease in cats was documented in the early 1920s.

Transmission

Heartworms are classified as roundworms and spread through the bite of an infected mosquito. When a mosquito bites an infected animal, juvenile heartworms (microfilariae) enter into the mosquito and within weeks develop into infective larvae. The infective larvae can then be transmitted to another mammal when the mosquito takes its next blood meal.  Once the infective larvae reach a suitable definitive host, they migrate through the animal’s body eventually reaching the blood vessels of the lungs. Many times, the infected animal will develop a noticeable cough as a result of the worms and the damage caused to surrounding tissues. The worms reach adulthood in approximately six months. They can cause severe tissue, blood vessel, and in rare cases, neurological damage, which in turn reduces the ability of the heart to pump efficiently, resulting in lung and heart disease. When animals begin to show signs of illness due to the infection, it is termed "heartworm disease". Although effective preventative methods are available, clinical cases of the disease continue to be reported worldwide and have been documented in all 50 United States, with southern states having a higher prevalence of infection.

Diagnosis

Numerous diagnostic tests are available to detect the presence of adult heartworm infection. The most common test detects the presence of adult female worms by testing for antigens or antibodies. Other tests assist with determining the severity of the disease and the best treatment plan, including: microscopically viewing microfilariae in a blood sample, radiographs (x-ray), echocardiogram (heart ultrasound), and a blood profile.

Prevention

The best news about heartworm disease is that it is 100 percent preventable by the regular administration of a FDA-approved adulticide. A variety of preventatives are available and can be given daily, monthly, or every six months.  There are also various options for administration route including oral, topical, and injectable. Since the preventatives available are not meant to eliminate a current heartworm infection, regular heartworm testing is necessary to make sure a pet is free of the worms, or microfilariae, before administering prevention. The FDA recommends annual testing for dogs and cats. Also, keeping yards and other areas around houses or kennels clear of stagnant, freestanding water where mosquitoes breed is helpful.

Heartworm Disease in Dogs

Adult heartworms can survive for five to seven years in dogs. Clinical signs in dogs depend on the severity of infection. In recent or mild infections, little to no signs of illness may be present. As the disease progresses, there may be a loss of appetite, difficulty breathing, a noticeable cough, lethargy, and reduced tolerance to mild exercise.

In dogs, four stages of the disease include:

  1. No clinical signs.
  2. Cough, usually when lying down or just getting up in the morning or after exercise.
  3. Cough, exercise intolerance, and difficulty breathing.
  4. Sudden onset of fatigue and weakness, occasional blood in the urine and sputum may be noticed.

Treatment in dogs is time consuming, expensive, complicated, and comes with many risks, which is why prevention is always best. An adulticide is used to kill the adult worms and microfilariae. The process can take weeks and is usually done in a series of treatments to avoid overloading the dog's body. It is important that while going through treatment the dog remains rested and, best case scenario, hospitalized and monitored by professionals. As treatments are being administered, worms are dying and being broken down in the dog’s lungs; therefore, other drugs, besides the melarsomine (Immiticide®), are administered to combat resulting inflammatory reactions.

Heartworm Disease in Cats

The prevalence of infection in cats is between five to 10 percent of that in dogs in any given area. Unlike dogs, it is rare that cats have microfilariae circulating in their blood; therefore, it is unlikely that an infected cat will transfer the heartworm infection to another mosquito. Adult worms are capable of surviving for several months to years in cats. Signs of possible heartworm disease in cats include intermittent vomiting not associated with eating, weight loss, respiratory distress, coughing, and, in rare cases, sudden death. Many times, the clinical signs in cats are mistaken for feline asthma or allergic bronchitis when they are due to a syndrome known as HARD, or heartworm associated respiratory disease. In cats, the infection is usually self-limiting and there are no current safe medical treatments for infection or disease in cats. Supportive treatments are used to reduce the inflammatory response and combat the negative effects of the disease. In rare cases, the worms are removed through surgical excision. Surgery is a high-risk procedure and reserved for the most severe cases.

Heartworm Disease in Ferrets

Laboratory studies of heartworm infection in ferrets have shown that they are highly susceptible, with infection and recovery rates similar to those in dogs and higher than those in cats. Common clinical signs in infected ferrets include coughing, sneezing, anorexia, and lethargy. Microfilaremia is transient in nature, similar to the infection in cats.  Presently, canine prophylactic pharmaceuticals are used to prevent infection, having been shown to stop infection.  Treatment has not yet been confirmed and there is no current FDA-approved drug for prevention or treatment.

Courtnye Jackson D.V.M. and Oye Ajifolokun D.V.M.

References:

American Heartwom Society
AVMA: heartworm disease
Heartworm disease manifested by encephalomyelitis and myositis in a dog: Cooley AJ, Clemmons RM, Gorss TL; Journal of the American Veterinary Medical Association 1987 190(4): 431-432.
Feline heartworm diases: A clinical review: Annette L. Litster, Richard B. Atwell  Journal of Feline Medicine and Surgery, April 2008 volume 10 no 2 137-144.
Feline Hearworm disease: Solving the Puzzle: Byron L. Blagburn, Ray Dillion March 1, 2007 dvm 360.
Clinical significance of feline heartworm disease: Ray Dillion; Vet Clinic North Am Small Anim Practice 1998 Nov 28 (6) 1547-65.
Dirofilariasis in the domestic ferret: John W. McCall; Volume 13 Issue 2 (1998) 109-112; Clinical Techniques in Small Animal Practice.
Imidacloprid plus moxidectin to prevent heartworm infection (dirofilaria immitis) in ferrets: R. Schaper, J. Heine, R.G. Arther, S.D Charles, J. McCall; Parasitol Res (2007) 101: S57-S62.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.