Heartworms have been found in all 50 states and can infect more than 30 species of animals, including coyotes, foxes, wolves, domestic cats, sea lions, ferrets and even humans. However, dogs are considered the primary host for heartworms.
When a mosquito carrying infective heartworm larva bites a dog and transmits the infection, the larva grow and migrate all over the dog’s body over several months. The mature male and female worms, measuring 4-12 inches in length, find their final resting place in the heart and lungs. This is where the adult worms mate and release their microfilaria, or offspring, into the blood stream.
These microfilaria will show up in the bloodstream 6-7 months after the initial mosquito bite occurs. Adult heartworms, left untreated, have a potential lifespan of about 5-7 years if the dog survives their damaging effects.
The severity of heartworm disease in your dog depends on the number of adults heartworms present, the age of the infection and the level of activity of your dog. The number of worms in a dog can range from one to 250! Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes.
The initial infection causes damage to the lower caudal pulmonary arteries. As the numbers increase the infection spreads to the right chambers of the heart. The very presence of the heartworms starts an inflammatory process in the arteries of the lungs. Later the heart becomes enlarged and weakened and congestive heart failure occurs. Occasionally, the heartworms will also descend into the caudal vena cava, the main artery to the lower body. If the heartworms are not removed surgically, this will cause sudden collapse and death within 2-3 days.
Diagnosis of Heartworms
Most cases of heartworms are diagnosed with a simple blood test done in the veterinary clinic. The test detects the presence of heartworm antigen. A positive heartworm antigen test indicates that there is at least one or more mature female heartworms that are at least 7-8 months old.
Physical Examinations to Access Heartworm Disease Levels
A heartworm positive dog with mild disease may appear perfectly normal on physical examination with a consistent heartbeat and clear lungs. As the disease progresses a cough may be the only obvious clinical sign in Stage 2, although x-rays may show the beginning of lung damage or heart enlargement. By Stage 3 your dog will have a cough, poor endurance for physical activity and abnormal lung sounds. The most severe state, or Stage 4, will include cough, a slow heartbeat, fever, exercise intolerance, difficulty breathing, abnormal lung sounds, an enlarged liver, temporary loss of consciousness, fluid accumulations in the belly, abnormal heart sounds and eventually death. X-rays, EKG’s and ultrasound are valuable in evaluating your dog’s state of involvement to prepare for treatment. Labwork will confirm liver or kidney damage.
Treatment of Heartworms:
Treatment of heartworms consists of 2 phases: the destruction of the adult heartworms and finally the elimination of the microfiliaria or larva from the blood. It is important to accomplish this with the minimum of harmful effects from the drugs and a tolerable degree of complications from dying heartworms being released into the blood stream.
If your dog is already symptomatic, tests will be run to access liver, kidney, heart and lung function to determine the safest course of treatment. Complications are more common with animals evaluated and found to be in Stage 3 or 4 of the disease or with animals that have concurrent health risks.
If your pet is found to be in Stage 1 or 2 (mild to moderate heartworm disease) with no clinical symptoms the adulticide treatment will be done during a simple 2 night stay in the hospital. Two injections are given deep in the muscles of the back over 24 hours. The second phase of the treatment to kill the remaining larva will be done 4 weeks later and include a recheck exam and 8 hour stay in the hospital. It is very important to understand that your dog’s activity must be SEVERELY restricted during the 6 weeks of treatment. Do not allow running, rough play, extended barking episodes, or other excitement. Go out in the yard with your dog to make sure he doesn’t run, using a leash if necessary.
If your dog is in Stage 3 or 4 (severe to critical heartworm disease) the initial adulticide treatment will be split into two sessions – getting one injection then two injections over a 24 hour period in 4 weeks. The second phase or larval elimination will be done 4 weeks after the last injection and your animal will be observed for 8 hours following treatment. Again, your dog must be strictly confined for the full 8 weeks of treatment.
Melarsomine (Immiticide) is the drug we use to eliminate adult heartworms. It is administered by deep intramuscular injections in the lumbar region. Melarsomine is an arsenic-based compound which can cause pain in the lower back muscles or make the dog feel nauseated. These symptoms should ease in an couple of days. Despite the dangers of giving an arsenic compound, with careful observation, 95% of all dogs make it through heartworm treatment.
Circulating microfilaria (larval heartworms) are treated with the same drugs used in monthly heartworm prevention products. However, your dog must be observed following this special microfilarial treatment for 8 hours, or just throughout the day. Again, the severity of the microfilarial infestation will determine the difficulty of the treatment and the possible side effects.
Complications of Heartworm Treatment:
Normal side effects of treatment include fatigue and pain at the injection site. Some patients will be very tired and sleep a lot for 2-4 days. Some have difficulty getting up because of the muscle soreness.
The important things to watch are the following:
Gum color – they should be pink. If they get red or white and your pet is listless, please call us.
A combination of listlessness, increased respirations, restlessness and coughing – Call us so we can begin Prednisone therapy to ease the inflammation.
Vomiting or any bloody discharge – Combined with listlessness, fever or pale gums this can be life-threatening. Please take your pet to the nearest emergency facility where immediate therapy including oxygen, IV fluids and cortisone injections will be started.
The primary complication of adulticide therapy is the development of pulmonary thromboembolisms which result from the obstruction of blood flow because of the dead worms or pieces of dead worms. If the treatment if effective, some degree of thromboembolism will occur.
Most heartworm treatment dogs will have a certain amount of coughing or gagging. The worms are dying and dissolving and are being passed through the bloodstream to the lungs. The dogs need to cough up the resulting phlegm, which is not the same as vomiting. If the coughing is extreme or uncontrollable, please call us.
Heartworm Treatment Follow-Up:
Six months following adulticide treatment, your pet should test negative by antigen testing if all the worms have been destroyed. Dogs that remain positive at that time will require an additional round of adulticide therapy. However, we ask that you start your preventative medicine such as Interceptor, Iverhart or Advantage Multi the month following the larvacidal treatment. Prevention of heartworms is safer and more economical. In Oklahoma, we recommend that all dogs be on heartworm prevention throughout the entire year. The added benefit to heartworm prevention is also the protection against many intestinal worms and/or fleas.
The information listed here is for information only and is no way intended to be used to diagnose or treat your animal. Your animal MUST be seen and diagnosed by a licensed Veterinarian.