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Heartworm (Dirofilaria immitis) is spread between animals by mosquito bites. It is most common in dogs but can also affect cats and ferrets. Heartworm is a major problem in North America and is also present in Southern Europe and Australia. It can be fatal.

Some strains of the worm are adapting to cooler climates, so we may see this disease in the UK in the not-too-distant future.

Signs of disease

The worms have a very long life-span (5-7 years) so signs may take a long time to become apparent.

  • exercise intolerance or lethargy may be the first sign noticed
  • coughing
  • sudden collapse

Diagnosis and treatment

A blood test is required to detect the presence of the worms in the blood stream. Several tests may need to be performed to confirm the result.

Treatment is risky, if too many of the adult worms are killed off at once, the dead worms may get trapped in the capillaries of the lungs, leading to respiratory failure and death, therefore hospitalisation and strict rest is required for at least 2-3 weeks after treatment.

After the adults have been killed using the above method, different treatment is used to kill the larvae. Again, this is risky and can lead to a severe allergic reaction to the dead worms. Animals should be observed for 8 hours after treatment.


Prevention is by far the best method, considering the danger of the disease and the treatment. If started before infestation, it is very safe and effective, however it needs to be given every month while in an at-risk area.

Prevention of mosquito bites can be acheived by using the products mentioned here. Prevention of heartworm can be acheived by monthly administration of the following. Products should be used under the direction of your veterinary surgeon.

  • Ivermectin – does not kill larvae but stops them maturing. Can be toxic to some strains of Collies.
  • Selamectin (Stronghold) – ‘spot-on’ treatment that can be used on cats and dogs. Also kills fleas, roundworms, biting lice, sarcoptic mange and ear mites
  • Mibemycin oxime (Milbemax, Program Plus) – tablets available for use in dogs and cats (Milbemax only). Also provide protection against gastrointestinal worms and fleas (Program Plus only).
  • Moxidectin (Advocate) – ‘spot-on’ treatment available for cats, dogs and ferrets. Also protects against fleas, mites, roundworms and lungworm

Further resources

  • American Heartworm Society – further information about the disease and its prevalence across the USA.
  • Know Heartworms – information about heartworm in cats.


Author: Helen

Published: 2011-11-01

Last modified: 2011-11-01


This website should not be used as a substitute for personal professional advice and if your animal is sick, I would urge you to book an appointment with your veterinary surgeon as soon as possible. I will endeavour to keep the information on this site as accurate and up to date as possible, however I cannot be held liable for problems occurring from incorrect use of this information.

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