Heartworm Disease (Canine)
| PrintHeartworm disease is a parasitic disease of dogs that is spread by mosquito bites. The adult worms are large and live in the heart and lungs, causing disease both by their physical obstruction of blood flow and by the immune response they induce in the dog's body. Although easily preventable, once established the treatment for heartworm disease can have serious side effects.
Common names: Heartworm disease
Scientific names: Dirofilariasis
Diagnosis
Signalment
All dogs not receiving a preventive are susceptible, with male dogs and those being housed outside in mosquito endemic areas more commonly affected. The typical age at diagnosis is between 3 and 8 years.
Incidence/prevalence
Incidence and prevalence vary widely depending on geographical area, with incidence of up to 45 percent in the most affected regions of the United States, with most of North America having infection rates of 5 percent.
Geographic distribution
In the United States, the Mississippi River Valley, Atlantic-coast area and gulf-coast region have the highest rates of infection. The disease is found in Japan, Australia, South America, Southern Europe, Southeast Asia and the Middle East as well.
Clinical signs (primary, most to least frequent, scientific term, synonyms)
Often asymptomatic, cough, occasionally bloody (hemoptysis), exercise intolerance.
Clinical signs (secondary, most to least frequent, scientific term, synonyms)
Pulmonary hypertension, Congestive heart failure, Ascites (fluid in abdomen), liver dysfunction, kidney dysfunction.
Causes (scientific, common term)
Dirofilaria immitis (heartworms).
Organ systems affected (most to least affected)
Heart, Lungs, Kidneys, Liver.
Diagnostic tests
Chest radiographs (X-rays), Heartworm antigen blood test Direct exam of blood for microfilaria.
Differential Diagnosis
Primary heart disease (congestive heart failure, right sided), Primary lung disease (pulmonary hypertension, lung cancer, pneumonia), Dipetalonema reconditum.
Overview
Heartworm disease is a serious disease of dogs and similar canids caused by the nematode worm Dirofilaria immitis. Common in much of North America, its distribution seasonally and geographically follows the distribution of mosquitoes.
Microscopic immature worms infect dogs through mosquito bites and migrate through the body tissues, eventually reaching adulthood and settling in the pulmonary arteries and right side of the heart. There they complete their life cycle, breeding and releasing larvae, known as microfilaria, into the bloodstream. A mosquito feeding on the dog's blood can then ingest microfilaria and continue the lifecycle.
The worms can reach up to 12 inches in length and number over 100 per dog in severe cases. The physical presence of the worms can be enough to cause pulmonary hypertension and congestive heart failure. More often, however, the dog has a lower number of worms in its system and can be completely asymptomatic.
Over the worm's 5-7 year life, the inflammation caused by the dog's immune reaction to substances secreted by the worms can cause damage to the lining of the heart, vessels of the lungs and, less often, the kidneys.
Treatment
Home care
Heartworm disease is easily prevented with monthly treatment with any of several commercial products (e.g., Heartgard Interceptor Revolution). Given the risk of side effects and the high cost of the treatment, all dogs in endemic areas are advised to take heartworm prophylaxis.
Professional care
The current recommendations for treatment depend on the clinical stage of disease, so a thorough examination is required before starting therapy, including chest radiographs (X-rays) and blood work. Hospitalization is required during treatment, which includes a series of injections of an arsenic-based compound called melarsomine (Immiticide). Extremely serious heartworm disease may require surgical removal of the worms.
Treatment is complicated by the size and preferred location of the worms. Any substance that instantly killed all the worms would cause large numbers of dead worms to lodge in the vessels of the lungs, potentially killing the dog. Melarsomine kills the worms slowly, giving the dog's body a chance a to remove the dead worms gradually. Even so, heartworm treatment has a relatively high incidence of side effects, including toxic-drug and allergic reactions. After the adults are killed, the microfilaria must be removed with a separate drug, usually three to five weeks after the adulticide therapy.
Action
It is important that all dogs undergoing treatment be monitored in a veterinary hospital for complications. Upon discharge the dog must be prevented from exercise that would place any strain on the lungs until it has a chance to recover. When the microfilaricide drug is given, the dog should be observed closely in a veterinary hospital for at least several hours afterward to monitor for signs of anaphylaxis, which can be triggered by large numbers of dying larvae.
Outcome
If caught early in its course, heartworm treatment carries an excellent prognosis. The risk of life-threatening complications both from the disease and the treatment is related to number of worms present, the length of infection, and the overall health of the dog and host immune factors.
References/Additional Readings
Bowman, Dwight D. Parasitology for Veterinarians. 6th edition. Philadelphia: W. B. Saunders Company, 1995: 217-220.
Ettinger, Stephen J. Textbook of Veterinary Internal Medicine. 4th edition. Philadelphia: W.B. Saunders Company, 1995; 1046-1061.
Nelson, Richard W. Small Animal Internal Medicine. 2nd edition. St Louis: Mosby, 162-175.
Urquart, G. M. Veterinary Parasitology. 2nd edition. London: Blackwell Sciences Ltd. 88-91.
Author
Colin Dwyer, DVM
Editor
Steven Hansen, DVM, MS, MBA, DABVT, DABT
Copyright 2007. The American Society for the Prevention of Cruelty to Animals (ASPCA)
Please note that PetVet should only be used as a guide and in no way should be substituted for licensed veterinary care.
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