Heartworm disease

This potentially serious parasitic disease can cause heart failure and other complications. In most countries where the disease occurs, preventative treatment is given to pet dogs to ensure they do not become infected.

©Jorge Guerrero

The disease, unlike the name suggests, is caused by a parasitic worm called Dirofilaria immitis which lives primarily in the pulmonary arteries and not the heart. The life-cycle of this parasite involves both dogs (where it can cause serious disease) and mosquitos, which act as a vector, transmitting worm larvae between dogs and spreading infection. Cats and ferrets can also be infected.

Heartworm is an important disease in several parts of the world, including:

• Parts of Europe (not UK, except in imported dogs)
• USA, Central and South America
• Australia
• Japan

The tiny immature forms of the worm (larvae) enter a dog via a mosquito bite. They are carried in the blood and settle in the pulmonary arteries. Here the larvae develop and grow into adult worms. The presence of the worms in this location can lead to serious symptoms.

Mild signs may show as tiring and an inability to cope with strenuous exercise; there may be a cough. More advanced or heavier infections can result in heart failure:

• Fatigue
• Breathing difficulties and cough, sometimes with blood
• Abdominal swelling due to fluid accumulation
• Weight loss and poor overall condition.

The disease may be strongly suspected from clinical history and tests such as X-ray and ultrasound. Infection can be confirmed either by a blood test that detects antigens (foreign proteins associated with the parasitic worm), or by finding tiny worm larvae known as microfilaria in a blood sample.

Infected dogs can be treated with a combination of drugs designed to kill adult worms and microfilaria and minimize complications. These drugs are given by injection and by mouth. Side-effects and toxicity may occasionally occur during treatment, either due to the drugs or in response to effect of the drugs on the worms. As the worms are killed an allergic shock reaction develops in some dogs. Exercise restriction during the recovery period is ESSENTIAL for minimizing complications. If side-effects occur, additional treatment may be needed to support the dog and further treatment may need to be delayed to allow the dog time to recover.

If the animal has already developed heart failure when the disease is diagnosed additional treatment may be needed.

In areas where infection is present, preventative medication (ivermectin, milbemycin, moxidectin or selamectin), usually given monthly under veterinary prescription, is used to guard against heartworm infection. This prevention can be started in puppies at around 6-10 weeks of age. If your dog does not live in, or travel to, an area where heartworm is present they cannot catch the disease. Dogs in the UK are not currently at risk and preventative treatment is not routinely required.

The recent discovery of the Asian tiger mosquito (Aedes albopictus) in southern England may change recommendations if it becomes establised. This mosquito is a known vector of heartworm and has been migrating northward through Europe over the last decade.