Heartworms are the most life-threatening canine parasites. They reside in the dog’s heart and pulmonary arteries, causing heart failure and eventually death.

Transmission and life cycle

Heartworms are transmitted by mosquitoes. When an infected mosquito bites a dog, it passes the parasite’s larvae to the dog. These larvae migrate until they reach their final site (heart and pulmonary arteries) in about 3-4 months; here they grow to maturity (macrofilariae) within 3 months and start producing larvae (microfilariae) which can survive for about 2 years in the bloodstream. When a mosquito bites an infected dog, it picks up these larvae and can transmit the infection to other dogs.


Dogs are considered the definitive host for heartworms (Dirofilaria immitis). However, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) and humans as well. When a mosquito carrying infective heartworm larvae bites a dog and transmits the infection, the larvae grow, develop, and migrate in the body over a period of several months to become sexually mature male and female worms. These reside in the heart, lungs, and associated blood vessels. Even as immature adults, the worms mate and the females release their offspring (microfilariae), into the blood stream. The time elapsed from when the larvae enter the dog until the offspring can be detected in the blood (pre-patent period) is about six to seven months. The male heartworms (four to six inches in length) and the females (10-12 inches) become fully grown about one year after infection, and their life span in dogs appears to be at least five to seven years.

Symptoms of Heartworm Disease

Over time, the presence of the adult worms in heart and pulmonary arteries causes an inflammation and thickening of the walls of the blood vessels which leads to an increase in blood pressure and in the cardiac effort to push the blood in these vessels. As a consequence, the dog may develop heart failure, which can eventually cause death. Clinical symptoms usually appear only when the disease has reached a very severe stage. Initial signs include sporadic coughing, tiredness, and weight loss. With time, this cough becomes chronic and is accompanied by difficult respiration, particularly during and after exercise, and mild anemia. In advanced cases, the dog may collapse after even light physical exertion. Most dogs eventually develop congestive heart failure.

Heartworm disease may cause combinations of medical problems within the same dog including dysfunction of the lungs, heart, liver and kidneys. The disease may have an acute onset but usually begins with slow barely detectable signs resulting from a chronic infection with a combination of physiologic changes. Dogs with a low number of adult worms in the body that are not exercised strenuously may never have overt signs of heartworm disease. The heart and lungs are the major organs affected by heartworms in dogs with varying degrees of clinical signs.

Clinical Signs Associated with Canine Heartworm Disease

Early InfectionNo abnormal clinical signs observed
Mild DiseaseCough
Moderate DiseaseCough, exercise intolerance, abnormal lung sounds
Severe DiseaseCough, exercise intolerance, dyspnea (difficulty breathing), abnormal lung sounds, hepatomegaly (enlargement of the liver), syncope (temporary loss of consciousness due to poor blood flow to the brain), ascites (fluid accumulation in the abdominal cavity), abnormal heart sounds, death


The diagnosis of canine heartworm disease depends upon the following:


  • an accurate patient history
  • the recognition of varied clinical signs
  • the use of several diagnostic procedures that may include:
    • blood (serological) testing
    • microfilarial detection and differentiation
    • physical exam and clinical laboratory tests X-ray (radiology)
    • angiography
    • ultrasound (echocardiography)
    • in the worst case scenario, examination after death (necropsy)

Treating Heartworm Infection

Most dogs infected with heartworm can be successfully treated. The goal of treatment is to kill all adult worms with an adulticide and all microfilariae with a microfilaricide. It is important to try to accomplish this goal with a minimum of harmful effects from drugs and a minimal degree of complications created by the dying heartworms. Heartworm infected dogs showing no signs or mild signs have a high success rate with treatment. Patients with evidence of more severe heartworm disease can be successfully treated, but the possibility of complications and mortality are greater. The presence of severe heartworm disease within a patient in addition to the presence of other life-threatening diseases may prevent treatment for heartworm infection.

Heartworm Prevention

In contrast to treatment, heartworm prevention is safe, easy and effective. Before prophylactic treatment is started, dogs should be tested for heartworm infection. Those found to be infected have to be treated against adults and microfilariae, before a prevention program is started. Preventatives are usually administered monthly on a year-round basis. Most heartworm prevention medications are also effective against all other common dog parasites (roundworm, whipworm, and hookworm) thus ensuring complete protection for your pet. There are monthly tablets (Trifexis®, Heartguard®, Interceptor®, Sentinel®), and monthly topicals (Revolution®). All of these methods are extremely effective and when the drugs are administered properly on a timely schedule, heartworm infection can be completely prevented.

Facts About Heartworms:
  • All dogs and cats, regardless of age, sex, size or breed, are at risk wherever mosquitoes are present. It takes only ONE infected mosquito to infect your pet.
  • Even when you do not have mosquitoes in your area, if you take your pet with you to areas where mosquitoes are present, your pet is at risk.
  • Heartworm Disease can kill. – Effective, easy-to-use preventive medication in a once a month flavor tab form can save your pet’s life.

U.S. Map of 2010 Incidence of Heartworm Disease



Canine heartworm infection is widely distributed throughout the United States. Heartworm infection has been found in dogs native to all 50 states. All dogs regardless of their age, sex, or habitat are susceptible to heartworm infection. The highest infection rates in dogs (not maintained on heartworm preventive) are observed within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Regions where heartworm disease is common have infections diagnosed in dogs as young as one year of age, with most areas diagnosing infections primarily between the ages of three and eight years. Although there are differences in frequency of infection for dogs in different regions of the U.S., all dogs should be considered at risk, placed on prevention programs and frequently examined by a veterinarian.

Since the beginning of 2005 until now (Aug 27, 2012), veterinarians in Los Angeles County have reported 96 cases. Of these, 12 cases were in cats and 84 in dogs. The majority of the cases (73%) had no symptoms at the time it was diagnosed. In the other cases, symptoms included cough (19%), tiredness (12%), and heart failure (6%). There are more heartworm cases in the county that are not reported.  The Companion Animal Parasite Council reports that 176 dogs tested positive for heartworm in LA County in 2011.

Dog vs. Cat

ParasiteDirofilaria immitisDirofilaria immitis
Transmission/td] [td] Mosquito Mosquito
Suseptibility to infectionVery high – virtually 100% of   dogs exposed to infective  larvae become infectedLower than dogs – 61-90%   of cats exposed to infective  larvae become infected
Longevity of worms 5-7 years 2-3 years
Ectopic Infections (other than typical locations)OccasionallyNot common
Number of wormsNot uncommon to find > 30Usually < 6, most commonly  1-2 worms
Single-sex infections in moderate to high areasUnusualCommon

Very common (80-90%)


Can last years, even after
death of adult worms


Seen in only 20% of cats

Lasts about 1 month

Organ with greatest pathologyHeart and lungsLungs
Clinical importance of small worm burdensLittle clinical importance,   depending upon size of dog  and exercise levelPotentially fatal
DiagnosisRelatively simpleComplex
Treatment1 compound approved

Complications manageable

None approved

High risk of complications

  Compounds for prevention1 compound approved

Complications manageable

None approved

High risk of complications

Compounds for prevention1 compound approved

Complications manageable

None approved

High risk of complications