What Heartworm Is
Heartworm is a parasitic worm that lives in the heart and pulmonary arteries of dogs and cats. It spreads only through mosquito bites -- a mosquito that bites an infected animal can carry larvae to the next animal it bites -- and it isn't transmitted directly between pets or to humans. Any pet in an area with mosquitoes is at some risk, even one that lives entirely indoors.
From Infection to Adult Worm: About Six Months
After a mosquito bite introduces larvae, they migrate through skin and muscle tissue before reaching the heart and pulmonary arteries and maturing into adult worms roughly six months later. Because this incubation period usually causes no noticeable symptoms, an infection can go unnoticed for a long time -- which is exactly why regular prevention matters more than trying to catch it after the fact.
Symptoms in Dogs
Early on, dogs may show only a mild dry cough, getting tired more easily after exercise, or lower energy. As the worm population grows and the disease progresses, it can worsen into appetite loss, weight loss, difficulty breathing, and fluid buildup in the abdomen from heart failure. Treatment is possible but hard on the body and often costly, which is why preventing infection in the first place is by far the more reliable approach.
Why Cats Are a Special Case
Cats can have a serious reaction even with far fewer worms than it would take to affect a dog, and symptoms -- coughing, vomiting, difficulty breathing -- are easily mistaken for asthma or another respiratory issue. In some cases, cats show no warning signs at all before sudden respiratory collapse. Critically, there is no well-established treatment for heartworm in cats the way there is in dogs, which makes consistent prevention far more important than a plan to treat it after infection.
Why Testing Comes Before Starting Prevention
Heartworm preventives aren't designed to safely clear worms that have already matured into adults, so giving prevention medication to an animal that's already infected can trigger a dangerous reaction or simply mask the problem. That's why a blood antigen test is recommended before starting prevention for the first time, or restarting after a long gap -- follow your vet's guidance on when to test and when to begin.
Types of Prevention and How Often to Give Them
Options include a monthly oral tablet, a monthly topical spot-on treatment, and an injectable version given by a vet every six or twelve months. Many products also cover fleas, ticks, or intestinal parasites at the same time, so which format fits best depends on your pet's lifestyle and what other parasites you're already managing -- worth discussing with your vet.
What to Do If You Miss a Dose
Heartworm prevention only works fully when given on a consistent schedule -- skipping even a month or two creates a window where a mosquito bite could result in infection. If you miss a dose, don't double up on your own the next time; tell your vet how late you are so they can advise whether a retest is needed and when to safely restart.