What is heartworm?
Updated By PetymFacts checked against official sources on
Quick answer
Heartworm, Dirofilaria immitis, is a long worm that lives in the heart and lung blood vessels of dogs and, less often, cats. It is spread by mosquito bites and is widespread in tropical regions, including parts of India. Signs include coughing, tiring on exercise and weight loss. It can be prevented, and dogs should be tested yearly.
Also called heartworm disease, Dirofilaria immitis, dirofilariasis, canine heartworm.
Key facts
- Dirofilaria immitis is a filarial worm of dogs, and cats, transmitted by mosquitoes. Tropical Council for Companion Animal Parasites
- Heartworm is widespread in tropical and sub-tropical regions. Tropical Council for Companion Animal Parasites
- A study in Guwahati and Aizawl found heartworm in 11 to 18% of dogs tested, depending on the test, and called the region endemic. The Scientific World Journal (2015)study
- Culex, Aedes and Anopheles mosquitoes can all spread it. The Scientific World Journal (2015)study
- TroCCAP advises testing dogs for heartworm every year, whether or not they are on prevention. Tropical Council for Companion Animal Parasites
- Both indoor and outdoor cats can be infected. Merck Veterinary Manual
How does a pet get heartworm?
A mosquito bites an infected dog and picks up tiny larvae from its blood. After they develop inside the mosquito, it bites another dog or cat and passes them on. The larvae migrate through the body and reach the heart and lung arteries as young adults from about 70 days; CAPC puts full maturity in dogs at about 6.5 months, when the worms are 15 to 18 cm long.
Anything that brings mosquitoes brings risk. In India that means the monsoon and the months after it, stagnant water near homes, and the Culex, Aedes and Anopheles mosquitoes that also carry human diseases. A study in Guwahati and Aizawl found heartworm in stray, pet and working dogs, with strays the most infected.
How does heartworm differ in dogs and cats?
Dogs are the main sitter. Cats are infected less often, but the disease can be more sudden in them.
| Dogs | Cats | |
|---|---|---|
| Signs Tropical Council for Companion Animal Parasites | Cough, tiring on exercise, weight loss, lethargy | Vomiting now and then, lethargy, coughing, shortness of breath |
| Serious outcome Companion Animal Parasite Council | Raised pressure in the lung arteries can lead to right-sided heart failure | Sudden collapse can happen |
| Testing Companion Animal Parasite Council | Yearly blood test, whether or not on prevention | CAPC advises combining two kinds of blood test, as one alone can mislead |
| Prevention Companion Animal Parasite Council | Regular preventive medicine from the vet | CAPC: all cats, whatever their lifestyle, on year-round prevention |
How is heartworm diagnosed and treated?
A vet takes a blood sample and tests for heartworm antigen or looks for the larvae under the microscope. Antigen tests can miss light or male-only infections, so vets sometimes combine tests. In dogs, treatment to kill adult worms is possible but demanding: CAPC advises at least a month of strict exercise restriction. The medicine used in dogs is fatal to cats, so infected cats get supportive care, and surgical removal carries risk.
That is why vets push prevention: it is far easier and safer than treatment. Petym does not name medicines or doses; your vet will choose a preventive that suits your pet and its other parasite control.
Is heartworm common across India?
Data are patchy. The Guwahati and Aizawl study confirmed it in stray, pet and working dogs in the north-east, and Petym found few published studies of heartworm in pet dogs from other states. Because heartworm spreads wherever infected dogs and mosquitoes meet, ask your vet whether prevention is advised where you live.
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Questions owners ask
Is heartworm present in India?
- Yes. A study in Guwahati, Assam and Aizawl, Mizoram found heartworm in 11 to 18% of dogs tested, depending on the test, and described the area as endemic. Tropical guidelines say heartworm is widespread in tropical regions, though Indian data from other states are limited. Ask your vet about the risk where you live.
Can humans get heartworm from dogs?
- Not directly from a dog: heartworm needs a mosquito to pass it on. Human infection is rare; CAPC notes it causes small lung nodules that can be mistaken for tumours or tuberculosis. Preventing mosquito bites and protecting dogs lowers the risk for everyone in the home.
What are the first signs of heartworm in dogs?
- Early infection often shows nothing. As worms build up, a dog may develop a cough, tire quickly on walks, lose weight and become lethargic, according to tropical guidelines. Because signs come late, a yearly blood test is the way to catch it.
Do indoor cats need heartworm prevention?
- The Merck Veterinary Manual notes that both indoor and outdoor cats can be infected. CAPC advises year-round prevention for all cats whatever their lifestyle. Ask your vet whether it is advised where you live in India.
Keep reading
Related terms
More in vaccines and parasites
- BabesiosisBabesiosis is a disease of dogs caused by Babesia, tiny parasites that live in and destroy red blood cells. It is spread mainly by ticks, and in India one species, Babesia gibsoni, also spreads through dog fights and blood transfusions. Signs are anaemia with pale gums, weakness, jaundice and red or brown urine. Dogs that recover can relapse.
- Booster vaccineA booster is a repeat dose of a vaccine that tops up or confirms a pet's protection. Puppies and kittens get a booster at about 26 weeks after their first course. After that, modern core vaccines for distemper, parvovirus and panleukopenia protect for years, while leptospirosis, kennel cough and most killed vaccines need a booster every year.
- Brown dog tickThe brown dog tick, Rhipicephalus sanguineus, is the most common tick on dogs in the tropics, including India. Unlike most ticks it lives and breeds indoors, in homes and kennels, as well as outdoors. It spreads ehrlichiosis and one form of babesiosis, the infections Indian owners call tick fever, so tick control is needed all year.
- Core vaccinesCore vaccines are the vaccines every dog or cat should have, wherever it lives, because they protect against widespread, serious diseases. For dogs they cover distemper, adenovirus and parvovirus; for cats, panleukopenia, herpesvirus and calicivirus. In India rabies is core for both, and WSAVA also treats leptospirosis vaccine as core for dogs where the disease is endemic.
- DewormingDeworming is giving a pet medicine, chosen by your vet, to kill intestinal worms such as roundworms, hookworms and tapeworms. Puppies can be born with roundworms and catch more from their mother's milk, so deworming starts in the first weeks of life. Some dog and cat worms can infect people, especially children, so regular deworming protects the family too.
- DHPPi vaccineDHPPi is the combined core vaccine for dogs printed on many Indian vaccine cards. The letters stand for distemper (D), hepatitis from canine adenovirus (H), parvovirus (P) and parainfluenza (Pi). DHPPiL adds leptospirosis (L). It is the same vaccine group that US cards call DHPP or DA2PP, and puppies start it at 6 to 8 weeks.
- EhrlichiosisEhrlichiosis is a disease of dogs caused by Ehrlichia canis, a bacterium carried by the brown dog tick. Signs appear 1 to 3 weeks after infection: fever, dullness, bleeding such as nosebleeds or bruising, and enlarged lymph nodes. Some dogs seem to recover but carry the infection silently and later develop a severe chronic form.
- Flea and tick preventionFlea and tick prevention means regularly protecting a pet from these parasites with a product your vet recommends, plus checking the coat and treating the home. Tropical guidelines say dogs and cats in places like India should be protected all year, including indoor pets. Use products licensed for the species, and never put a dog product on a cat.
- FleasFleas are small, wingless, blood-feeding insects. The cat flea, Ctenocephalides felis, is the most common flea on dogs and cats worldwide, and the main flea on dogs studied in India. Fleas cause itching, flea allergy, tapeworm and, in heavy infestations, anaemia in puppies and kittens. Most of the flea life cycle happens in the home.
- FVRCP vaccineFVRCP is the core combined vaccine for cats. It protects against feline viral rhinotracheitis (herpesvirus), calicivirus and panleukopenia. Indian cards often show a brand name from the vial sticker instead, such as Tricat. Kittens start at 6 to 8 weeks, with doses every 2 to 4 weeks until 16 weeks or older, then a booster at about 26 weeks.
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Sources
Each source was opened and read on the date shown. How Petym checks its sources
- Tropical Council for Companion Animal Parasites: Canine guidelines: heartworm (opens in a new tab)
- Merck Veterinary Manual: Heartworm disease in cats (opens in a new tab)
- Companion Animal Parasite Council: Heartworm (guidelines) (opens in a new tab)
- The Scientific World Journal (2015): Prevalence and molecular epidemiological data on Dirofilaria immitis in dogs from northeastern states of India (opens in a new tab)