Dogs
The American Heartworm Society recommends regular testing and consistent prevention; treatment requires veterinary staging and supervision.
INTERNAL PARASITE OWNER GUIDE
Heartworms
Dirofilaria immitis
Mosquito-transmitted worms that primarily affect the pulmonary arteries and can cause severe heart and lung disease in dogs, cats and ferrets.
Internal parasiteHuman-health concern: Indirect
THE IMPORTANT DIFFERENCE
Knowing the name is not the same as confirming the diagnosis.
Many parasites produce similar signs, and some are microscopic or hidden inside the body. A veterinarian can choose the right examination, testing and species-safe prevention plan.
What does it actually look like?
Adult heartworms are long, thin, pale worms inside the cardiopulmonary circulation and cannot be identified by looking at an animal. Immature circulating stages, called microfilariae, are microscopic.
Seeing an unusual speck, worm, skin change or stool pattern does not confirm the species. Save a photograph or specimen only if it can be done safely and contact your veterinarian.
How does the parasite complete its life cycle?
- 01A mosquito feeds on an infected animal and ingests microscopic microfilariae.
- 02Larvae develop inside the mosquito.
- 03A later mosquito bite deposits infective larvae into a new susceptible host.
- 04Larvae mature over months, eventually reaching the pulmonary circulation; detection timing depends on stage and testing method.
Which animals can be affected?
DogsCatsSmall mammals
The exact parasite species, exposure route and appropriate treatment may differ between these animals.
Cats
Even a small number of worms or immature stages can cause serious respiratory disease, and the canine adulticide treatment is not used in cats.
Small mammals
Ferrets can be susceptible and should have an individualized prevention discussion with an experienced veterinarian.
How does an animal become exposed?
- An infected mosquito bite; indoor animals are not completely protected from mosquito exposure.
- Heartworms are not spread by sharing bowls, touching an infected pet or ordinary pet-to-pet contact.
What might an owner notice?
- Coughing, exercise intolerance, weight loss or reduced energy in dogs.
- Coughing, asthma-like episodes, vomiting or sudden collapse in cats.
- Severe breathing difficulty, fainting or collapse is an emergency.
Animals can have a parasite without showing every listed sign. Difficulty breathing, collapse, severe weakness, pale gums or rapidly worsening illness require urgent veterinary attention.
How does a veterinarian diagnose it?
- Canine antigen testing together with microfilaria testing when recommended by the veterinarian.
- Cats may require a combination of antigen and antibody testing, imaging and clinical interpretation; negative results do not always exclude disease.
- Testing and prevention plans follow species, age, product history and exposure risk.
Diagnostic choices depend on the suspected parasite, stage of infection, animal species and clinical history.
How can risk be reduced?
- Maintain veterinarian-recommended heartworm prevention consistently throughout the year where advised.
- Perform regular heartworm testing according to veterinary and American Heartworm Society guidance.
- Contact the clinic promptly after a missed or delayed preventive dose rather than guessing at a safe grace period.
Medication safety matters. Never assume a dog product is safe for a cat, rabbit, bird, reptile, horse or other animal. Confirm the exact species, weight, dose and label with a veterinarian.
Can this parasite affect people?
Human-health relevance: Indirect
People do not catch heartworm by touching or living with an infected pet. Rare human infection can occur through an infected mosquito, so the meaningful connection is shared vector exposure rather than direct pet-to-person spread.
Speak with a physician about personal medical concerns, especially during pregnancy, immune suppression or significant exposure. Ask your veterinarian what precautions are appropriate for the animal and household.
TRUSTED SOURCES
Sources behind this guide.
Companion Animal Parasite CouncilEvidence-informed parasite guidelines, life cycles, diagnostic recommendations and public-health considerations. ↗Centers for Disease Control and PreventionHuman parasitic diseases, zoonotic transmission, prevention and public-health guidance. ↗American Heartworm SocietyHeartworm transmission, prevention, testing and species-specific owner education. ↗
This educational guide does not replace veterinary examination, diagnostic testing, individualized treatment or human medical advice.