What do I need to know about Heartworm in Dogs and Cats and what prevention options do I have … ?

Heartworm is unfortunately very common here in the tropics. It is currently something of an epidemic locally with over 50 infections detected in the first 6 months of 2013 in unprotected dogs presented to our practice. It is carried and spread by Mosquitoes who are a vital part of the worm’s lifecycle. In summary, when an infected dog is bitten by the mosquito and the mosquito sucks the dogs blood, it also sucks up tiny immature heart worms. These heart worm larvae mature in the mosquito’s salivary glands, and then are released into your dog when the mosquito feeds on it. These microscopic worms then migrate through your dogs tissues and blood until they arrive in the heart and lungs, where they live for the rest of your dogs life. Over time they grow and become quite large – 15-25 cm long and about as thick as spaghetti. They can number from a single worm to as many as 250, but a typical burden is 5-20 worms. As adult worms they mate and produce microscopic larvae for mosquitoes to suck up again, completing the life cycle and putting other dogs at risk.

The heart worms cause severe inflammation within the arteries of the lung and the heart valves and muscle itself. This inflammation results in irreversible damage to the heart and lungs over time, ultimately causing heart failure and is fatal if not treated.

The key is prevention of infection – We recommend the annual injection (Proheart SR12) as the best protection against Heartworm, which we start with our puppy vaccinations and continue into their adult life.

If prevention has lapsed for more than 3 months, a heart worm test will be advised, as early detection, before clinical disease develops, is vital. This early detection will allow us to prevent the inflammation and damage to the heart and vessels, and little or no harm will have been done.

When prevention has been intermittent or lapsed for a long time (>6 months), heart worm testing will be required. If your dog is infected and there are clinical signs of heart disease, more work up with chest X-rays and Blood Tests will be done, and treatment to kill the adult worms will be commenced.

Treatment is expensive and does have some risks. For this reason, regular prevention, using either a monthly or annually administered product is vital for the long term health of your dog.

What about Heartworm for Cats? Are they at risk?

Cats (even those that live indoors) are at risk for heart worm. The infection rate is lower than in dogs, as the worms are adapted to living in the blood, hearty and blood vessels of dogs rather those of cats.

When the infection rates in dogs are as high as they are currently, the risk to cats increases dramatically as there are more mosquitoes carrying infective larvae. Unfortunately, heart worm disease in cats is more severe than in dogs in many ways:

  • Diagnosis is difficult as the testing is poorly sensitive in cats
  • Cats cannot tolerate high worm burdens at all – a single heart worm is sufficient to cause death, and most burdens are only 2-5 worms.
  • There are rarely any warning signs that your cat has heart worm disease – sudden death with no pre-existing illness is the most common presentation, in other cases the only symptom is intermittent vomiting


  • there is no treatment option for heart worm infected cats – prevention is their only protection

There are few preventative options for cats (monthly Advocate is the best option) and we strongly recommend prevention. Advocate also provides excellent protection against intestinal worms and fleas, and also prevents ear mites.