How soon after infection will a dog show signs of heartworm?

A dog can show signs of heartworm as early as about six months. They typically don’t show any symptoms before that, as the infestation isn't mature enough at that point.

It takes about six months from what I call a hot active mosquito that's carrying the heartworms. Six months after they bite your dog and inject those larval microfilariae heartworms into your dogs, it takes that six months for them to develop into that adult worm. So generally, it's anywhere from six to 12 months after they've been bitten; you may start noticing signs in your pet. And what we'll see in most cases are things like coughing and exercise intolerance. Maybe they're not as active as they used to be, or you're noticing some lethargy, changes in their appetite, or other things like that.

It can take several months to start seeing actual signs, as it takes anywhere from six to eight months for the larva to reach the adult stage. It can even take up to two years to see any signs.

It varies, as dogs often remain asymptomatic during the early stages of heartworm disease. It can take 6-12 months or even 12-24 months for symptoms to develop. The longer the infection persists, the more symptoms will start to appear.

It takes up to six months for the heartworms to fully develop into adults. Therefore, you're not likely to notice any signs until then. The symptoms also depend on the worm burden that the dog carries.