The bottom line first: one mosquito bite is all it takes, and prevention is far easier and cheaper than treatment. Once a month is the golden rule.
Heartworm is a parasite transmitted by mosquitoes. When a mosquito that has fed on an infected animal bites your dog, larvae enter through the skin and mature over roughly six months, eventually settling in the heart and pulmonary arteries. Adult worms grow long and thin like strands of noodles, clogging blood vessels and straining the heart — left untreated, the disease can progress to heart failure. Remember: dogs cannot pass it to each other directly. It spreads only through mosquitoes, which is why the warm mosquito months call for extra caution.
In the early stages there are usually no symptoms at all. Signs appear gradually as the worms mature. The most common ones are:
These symptoms overlap with other heart and respiratory conditions, so the first step is getting your dog tested at a vet clinic as soon as you notice them. By the time symptoms show, the infection has often progressed considerably.
Preventives come as chewable tablets, spot-on drops for the back of the neck, and injections given at the clinic — most are given once a month. They work by eliminating larvae picked up over the previous month, so you must keep them up from when mosquitoes first appear until one month after mosquito season ends. That said, because mosquitoes survive indoors, many vets now recommend year-round prevention regardless of the season. Picking the same date each month makes it easy to stay on schedule. If you'd like to manage vaccine and preventive schedules together, try our dog vaccination calculator.
If you paused prevention over the winter or missed a few months, don't simply restart on your own — have your vet run an antigen (kit) test first. Giving a preventive to a dog that's already infected can carry a risk of adverse reactions. Adult heartworm infection is treatable, but treatment takes months, costs far more, and requires strict exercise restriction that's hard on your dog. A few minutes of prevention each month beats months of treatment every time.
Yes. Mosquitoes get indoors through gaps in window screens and open doors, so even a dog that rarely goes on walks can be infected. It's safest not to skip preventives for indoor dogs either.
Don't restart on your own — have your vet run a heartworm test first. Giving a preventive to an already-infected dog can carry a risk of adverse reactions, so your vet will test first and decide when it's safe to resume.
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This content is general information and does not replace veterinary diagnosis. Always consult a vet for heartworm testing and preventive prescriptions.