Yes: the Companion Animal Parasite Council says every dog and every cat should be on heartworm prevention all year, and the American Heartworm Society says the same for dogs, cats and ferrets. Mosquitoes pass heartworm on, and it has been found in all 50 states. A dog can be treated over several months, but no drug is approved to remove the worms from a cat or a ferret, so for them everything depends on prevention.
Heartworms are foot-long worms that live in the heart, the lungs and the blood vessels around them. The American Heartworm Society describes the cycle: a mosquito bites an infected animal, such as a dog, a coyote or a fox, and takes up baby worms with the blood. Over 10 to 14 days they grow inside the mosquito, and at its next bite they are left on the skin and go in through the bite wound. About six months later, the society says, they are adult worms.
The American Veterinary Medical Association (AVMA) says mosquitoes are the only way heartworm travels, so a pet cannot catch it directly from another animal. The FDA says people cannot get heartworms from their pets.
The American Heartworm Society's guidelines for dogs, revised in 2024, call for a yearly test for every dog over seven months old: one blood test for a protein that adult female worms give off, and one for baby worms.
The first test cannot turn positive until about five months after the bite, so the guidelines see no justification for testing a dog younger than seven months. Such a puppy can start a preventive without a test, the society says, and is tested six months later, six months after that, and then yearly.
An older dog is tested before it starts and again six months later. The label of Heartgard Plus, one of the monthly preventives, says:
The label adds that the product does not work against adult heartworms, and the guidelines say testing first finds an existing infection without delay. The FDA explains the danger: if baby worms are in the dog's blood, a preventive may make them die suddenly, setting off a shock-like reaction and possibly death.
The yearly test stays even when no dose is missed, because a dog can spit out or vomit a pill or rub off a liquid, the society says. After a missed dose, it says to call the vet, restart the preventive and test again six months later.
The American Heartworm Society says most infected dogs can be treated successfully. The Merck Veterinary Manual says a dog that moves little may show few or no signs even with many worms. The society's guidelines set the treatment out in stages:
| When | What happens |
|---|---|
| Day 0 | A second test of a different kind confirms the result. The dog starts a preventive and four weeks of the antibiotic doxycycline, and its activity is restricted. |
| Days 31 to 59 | A month of waiting follows the antibiotic. |
| Day 60 | The dog gets the first of three injections of melarsomine, the only drug the FDA has approved to kill adult heartworms in dogs. Strict rest begins, which means a crate indoors and a leash outdoors. |
| Days 90 and 91 | The second and third injections are given a day apart. Strict rest continues for 6 to 8 weeks. |
| Day 120 | A blood test looks for baby worms, and normal activity returns gradually over four weeks. |
| Day 365 | Both heartworm tests are repeated, 9 months after the last injection. |
The guidelines call rest essential, and the AVMA gives the reason: as the worms die they can block blood vessels in the lungs, and physical effort raises the risk. Giving only a preventive and waiting for the worms to die, called slow kill, is not recommended by the society or the Companion Animal Parasite Council.
Very many worms can suddenly block the blood flow in a dog's heart. The society's signs are sudden labored breathing, pale gums and dark, bloody or coffee-colored urine, and it says few dogs survive without prompt surgery. That is an emergency: see the emergency page for dogs.
Yes, although the American Heartworm Society says the cat is not the worm's usual host: most worms do not survive to become adults, and a cat with adult worms typically has one to three. Even the young worms damage the lungs.
The signs the society lists are coughing, attacks that look like asthma, occasional vomiting, poor appetite and weight loss. In some cats the first sign is sudden collapse or sudden death.
The society's guidelines say no single test finds every case in a cat, so vets use two blood tests together: an antigen test for adult female worms, and an antibody test that shows the cat has been exposed to the young worms. The FDA says a negative result does not rule heartworm out.
No drug is approved to remove heartworms from a cat, and the AVMA says the drug used for dogs is unsafe for cats, so prevention is the only protection. The guidelines cite one study in which about 25% of the cats found with adult heartworms were described by their owners as indoor cats.
The American Heartworm Society says ferrets are extremely susceptible, even as indoor pets, and that one worm can cause serious disease or death. The FDA says blood tests for heartworm are generally unreliable in ferrets and no drug is approved to treat it in them, but two products, both containing imidacloprid and moxidectin, are approved to prevent it.
Every three years the American Heartworm Society maps the heartworm tests that clinics and shelters report for dogs. Its 2025 map covers that year's tests.
Texas topped the list for the first time, followed by Louisiana, Arkansas, Mississippi and South Carolina. Tennessee, Alabama, North Carolina, Florida and Georgia complete the ten highest states.
The society says heartworm has been found in all 50 states, and its maps since 2001 are here: Heartworm incidence maps. The Companion Animal Parasite Council's county maps are linked from the page on flea, tick and heartworm protection.
The American Heartworm Society calls preventives relatively inexpensive and treatment costly and difficult. The FDA says treatment is expensive because it takes several vet visits, blood tests, x-rays, a hospital stay and a series of injections. None of these sources gives a national price, so there is none here: see What does a vet bill cost?
Two things are not worth paying for: a heartworm test for a puppy younger than seven months, and any product sold without a prescription as heartworm prevention. The society says preventives are sold only by prescription and that there is no such thing as a natural one.
The AVMA's 2026 calendar lists April as National Heartworm Awareness Month, a good time to look up the date of your pet's last test.
If something here is out of date, tell us at hello@petipedia.io and we fix it the same day.
Sources read on 2 October 2026; every number is as the source gives it. The American Heartworm Society's guideline files say Revised 2024 on the cover and carry the date 19 June 2025 in their file names.