Dog - Heartworm Disease

Heartworm is a disease that primarily affects dogs. Cats can be affected but then the disease is usually less severe in the cat. Today the disease has worldwide distribution particularly the United States, Australia and most of southern Europe. Dogs imported into the UK from these areas or dogs which have visited these areas under PETS and have returned, have been found to be infected.

What causes canine heartworm disease?

It is caused by a nematode (roundworm) called Dirofilaria immitis. Heartworms are usually found in the right side of the heart and adjacent pulmonary (lung) vessels of infected dogs. They are long slender worms, (20–30 cm). The male is about half the size of the female. In severe infections a dog may have as many as 300 adult worms in the heart and adjacent blood vessels.

How does the dog get infected?

The immature larvae (microfilaria) are transmitted by a bite from an infected mosquito. The microfilaria, produced by the adult worms, (Dirofilaria) circulate in the dog’s blood stream and are ingested by the mosquito when it bites the dog. These immature forms develop in the mosquito for up to a month and then migrate to the mouth parts. They are then ready to complete the next part of the life cycle when the mosquito bites another susceptible dog. After months, or even years, the dog ends up with heart worms in the right side of the heart and adjacent blood vessels which in turn produce more microfilaria.

What signs does the dog show?

Many dogs infected with heartworm will not show any signs for two years or more. Usual signs include weakness during exercise, coughing, weight loss and occasionally right sided heart failure or sudden death. Since the worms can block the vessels taking blood to other organs, liver, lungs and kidneys can also be affected.

The microfilaria, (immature worms), circulate in the bloodstream and can block blood flow in the smaller vessels. This in turn can cause problems to the tissues those vessels supply. The lungs and liver are primarily affected and signs vary from coughing, when the lungs are affected, to jaundice, anaemia and general weakness if the liver is deprived of its essential circulation.

How is heartworm infection diagnosed?

The signs can be attributed to other diseases but blood tests are available to check for both adult worms and also microfilarial infection. Sometimes diagnostic imaging, e.g. x-rays and ultrasonography, etc., together with ECGs may be needed for confirmation.

How is the condition treated?

Treatment for both adult worms and microfilaria is possible but can be associated with side effects. However we will discuss this with you fully. This is one of those occasions where prevention is better than cure.

Prevention of heartworm

Heartworm affecting dogs in the UK has increased with the relaxation of quarantine regulations and the introduction of PETS. (Please see handout.) If travelling to an area where heartworm is prevalent please discuss preventative treatment with us well in advance. Effective preventative measures are available but they should be started at least 3-4 weeks before travelling and continue according to the manufacturers’ recommendations. This usually includes at least a month after your return.

If your dog has been in a heartworm area without preventative treatment please call us to discuss blood tests since infections are much easier and safer to treat before major symptoms arise. Please bear in mind that these could take months or even years to develop.

If my dog is infected, is treatment likely to work?

It is more effective if treatment starts early before any signs are apparent. Even in dogs with advanced heartworm, treatment can result in a dog with renewed vigour and vitality, improved appetite and obvious weight gain.

Please contact us if you have any concerns.