Heartworm and other filarial worms
Also called Dirofilariasis, microfilaria infection
A mosquito-borne worm infection in which adult worms live in the heart and lung vessels of dogs and cats. Early disease may cause no signs, and prevention is far easier than treatment.
Cause and spread
- Cause
- The heartworm Dirofilaria immitis, and other filarial worms whose larvae (microfilariae) swim in the blood.
- How it spreads
- By mosquitoes that carry the larvae from one animal to another.
Signs to watch for
- No signs in early infection
- Cough and tiring easily on exercise
- Weight loss and poor condition
- Breathing difficulty and, later, heart failure with a swollen belly
- Sudden collapse in severe cases
Many diseases look alike. Only a laboratory test can tell them apart, so please see a veterinarian for any sick animal.
How it is diagnosed
Heartworm is usually detected with a test for worm antigen together with a test for microfilariae in the blood, as recommended by the American Heartworm Society. Both are used because antigen tests can miss some infections and microfilaria tests can find others; microfilaria testing matters most when prevention history is unknown. PCR can detect microfilariae in a blood sample.
- Confirms the diseaseMicrofilaria PCRDetects microfilariae in blood. It does not replace the antigen test that your veterinarian uses to look for adult worms.
- Supports the diagnosisWet Film Examination (Blood)A drop of fresh blood can show microfilariae moving.
- Supports the diagnosisComplete Blood Count (CBC)Shows anaemia and other blood changes.
Prevention and control
- Give year-round heartworm preventive medicine prescribed by your veterinarian
- Test dogs before starting a preventive and test regularly, and after any missed doses
- Reduce mosquito exposure
Where this information comes from
- Canine Heartworm Guidelines (2018) · American Heartworm Society
General information for animal owners and veterinarians, last checked October 2026. It does not replace the advice of your veterinarian, who should decide which tests and treatment your animal needs.