Heartworm is a parasite spread by mosquito bites: microscopic larvae enter your dog's bloodstream and, over months, mature into worms living in the heart and the arteries of the lungs. Yes, your dog needs prevention — veterinarians recommend it for dogs in nearly every region, because a single bite from an infected mosquito is all it takes.
A Slow, Silent Kind of Damage
What makes heartworm disease so insidious is its timeline. After the bite, larvae develop quietly for roughly six months before they can even be detected by standard tests. During that time your dog shows nothing — he runs, eats, and naps as usual — while the worms grow, sometimes to a foot long, in the worst possible neighborhood.
As the burden builds, the heart and lungs strain. Early signs are subtle: a soft cough, tiring faster on walks a dog used to finish easily, a little weight loss. Later stages can bring serious breathing difficulty and heart failure. The tragedy owners describe most is that the disease was preventable the entire time, and that the first obvious symptom arrived only after real damage was done.
Prevention and Testing Work as a Pair
Prevention products — given on the schedule your vet directs, typically monthly and year-round — kill the larvae at an early stage before they can mature. They are not a treatment for adult worms; they stop new infections from taking hold, which is why starting and staying consistent matters more than the specific brand conversation.
Testing is the other half. Vets generally test annually even for dogs on prevention, because no routine is perfect: a dose spat out unseen, a month that slipped during a move, a puppy started late. A test catches a breakthrough early, when your vet has the most options. Puppies begin prevention young and are tested once they are old enough for the test to be meaningful — your vet will set that first schedule.
If your dog has missed prevention entirely, do not just restart and assume all is well. Tell your vet about the gap so testing can be timed correctly; a dog can carry early infections that a test will only reveal months after the bite.
A Word on Indoor Dogs and Winter
Owners sometimes ask whether a mostly indoor dog, or a northern winter, buys an exemption. Mosquitoes get indoors easily, and heartworm has been diagnosed in all fifty states; unseasonably warm spells also stretch mosquito season beyond the calendar owners picture. Most vets consider the risk low-cost to prevent and high-cost to face, and recommend simply not gambling.
Keep the History Together
Heartworm care is a years-long paper trail of test dates, results, and prevention history, and it fragments across clinics the moment you move or visit an emergency vet. Keep test results and prevention history together in KinsLe's Pet Passport — the records packet for vaccines, medications, allergies, and documents, shareable with vets and sitters — so nothing gets lost between visits. A new vet can see at a glance that your dog has been tested and protected continuously. Keep his Pet Passport current in KinsLe, and explore more pet answers on KinsLe whenever a parasite question has you second-guessing.