Heartworm Causes and Treatment


What causes heartworm?

“Heartworm” is a potentially fatal disease caused by infection with the parasitic roundworm Dirofilaria immitis. The worm generally infects the chambers of the heart and the large blood vessels entering and leaving the heart. Heartworm typically affects dogs, although cats are occasionally diagnosed with the disease as well.

How is heartworm transmitted?

Heartworm is transmitted by mosquitoes. The mosquito transfers larvae into the dog when it feeds. These larvae migrate to the heart, where they develop into adult worms. The adult female worm gives birth to microscopically small worms (microfilaria) which circulate in the dog’s bloodstream.

In order to continue the life cycle, a mosquito must ingest blood infected with microfilaria, which can then develop into infective larvae inside the mosquito. The disease can not be transferred from dog to dog – the mosquito is required as an intermediate host.

What are the symptoms of a heartworm infection?

The symptoms of heartworm are variable – they become more severe with higher worm burdens.

1. Mild cases – asymptomatic or minimal signs such as chronic coughing

2. Moderate cases – coughing, difficulty exercising

3. Severe cases – coughing, laboured breathing, wasting/weight loss, difficulty exercising, fainting/collapse, and fluid accumulation in the abdomen and legs. Severe cases will often progress to death if left untreated because of the extensive damage done to the heart, lungs, blood vessels, liver and kidneys.

How can I prevent my pet from being infected?

If you live in a region where heartworm is endemic, consult your veterinarian about the best method of prevention for your pet. There are a number of prophylactic treatments for dogs at risk. Although some only treat heartworm, other treatments will control fleas and/or intestinal worms.

The following are only a few of the many treatments currently on the market:

• Diethylcarbamazine citrate (Dimmitrol) – daily tablet that prevents heartworm
• Ivermectin (Heartguard) – monthly tablet that prevents heartworm (Heartguard Plus also controls hookworms and roundworms)
• Milbemycine oxime (Interceptor) – monthly tablet that controls  heartworm and roundworms
• Milbemycine oxime (Sentinel Spectrum) – monthly tablet that prevents heartworm, fleas, roundworms, and tapeworms
• Moxidectin/Imidacloprid (Advocate) – monthly spot-on that kills fleas, prevents heartworm and roundworms, and controls sarcoptic mange and lice
• Selamectin (Revolution) – monthly spot-on that controls fleas, ear mites, and sarcoptic mange in addition to preventing heartworm

What happens if I forget to give my pet its heartworm prophylaxis?

If you forget to give your animal its daily or monthly tablet, do not continue as usual. Your pet must have a blood test to check for microfilaria (young worms found in the bloodstream) before you can resume treatment.

How is heartworm diagnosed?

A veterinarian will diagnose heartworm based on a history of lack of prophylaxis, physical examination, blood test, x-rays, and ultrasound. In areas where heartworm is common, you should have your dog regularly tested for heartworm.

How is heartworm treated?

Depending on the severity of infection, your veterinarian may recommend a number of changes to treat the disease.

1. Medication: your dog may have to be hospitalized during administration of medicine that kills adult worms. Some dogs may also require follow-up treatment with a medicine that kills the young worms (microfilaria).

2. Activity: exercise is usually for a few weeks following administration of medication that kills adult heartworms

3. Diet: dogs that are in heart failure will need a sodium-restricted (low salt) diet

Treatment of heartworm can be long and difficult. However the prognosis is usually good for mild to moderate cases. There is a higher risk of complication for severe cases, which is why it is important to have your dog tested regularly for heartworm and to administer heartworm prophylaxis if you reside in an endemic region.

(photo: zevotron)