Heartworm in Dogs: What Every Owner Should Know

Maybe your veterinarian just called with a positive heartworm test. Or you are weighing a year of prevention for a dog who mostly lives on the couch. Either way, heartworm in dogs runs on a slow clock. About six months pass between the mosquito bite and worms grown to adult size. It takes at least six months after infection before a blood test can find them. And a preventive given tonight deals with larvae the dog already picked up, not with next month’s mosquitoes.

Those three delays are why the testing rules look fussy, why treatment takes most of a year, and why one missed dose matters. Heartworm comes from a mosquito, and it does its damage quietly for months before a dog looks sick. It is far easier to prevent than to treat, so the sections below follow it in the order it happens.

Month Zero: It Always Starts With a Mosquito

A mosquito picks up microscopic baby worms when it bites an infected animal, then passes them on as larvae the next time it feeds. That is how the infection develops, and it cannot happen without the mosquito. Heartworm is not contagious between dogs, so your dog cannot catch it from an infected dog at the park.

Heartworm disease has been diagnosed in all 50 states, so it is not something you can rule out by where you live. Indoor dogs are not exempt either, because infected mosquitoes come inside. That is where the couch-dog argument falls apart: one mosquito in the bedroom is enough.

Months One to Six: Worms Growing in a Dog That Looks Fine

Over about six months those larvae grow into adult heartworms: foot-long worms that live in the heart, the lungs and the blood vessels between them. They can survive there for five to seven years. For most of that stretch the dog in front of you looks entirely normal, which is what makes the disease so easy to miss.

The test carries the same delay. It takes at least six months after infection for a dog to test positive, so a negative result says nothing about a mosquito bite from last month. The usual screen is an antigen test, which looks for heartworm proteins in a small blood sample. The current heartworm guidelines pair it with a check for microfilariae (from the Greek for small and the Latin for thread), the baby worms circulating in the blood.

That delay sets the whole testing schedule, and it explains rules that otherwise look arbitrary. Dogs over seven months old should be tested every year, even when they get prevention all twelve months, because the test is how you know the prevention is working. A puppy under seven months can start prevention without a test and gets tested six months later. If a screening test does come back positive, your veterinarian will confirm it with a second, different test before anyone talks about treatment.

When the Dog Finally Looks Ill, and the Emergency Form

When something does finally show, the first changes are easy to blame on age or a lazy week. You might notice a mild cough that will not go away, less interest in exercise, tiring after moderate activity, a smaller appetite, or weight loss. In the early stages many dogs show few signs or none at all. As the disease advances it can lead to heart failure, sometimes with a belly swollen by fluid. The signs arrive too late to work as a warning. Do not wait for a cough to ask about testing.

One form of this ignores the slow timeline completely. When a large number of worms suddenly blocks blood flow through the heart, it is called caval syndrome, and few dogs survive it without prompt surgery to remove the worms. The signs are sudden labored breathing, pale gums, and dark, bloody or coffee-colored urine. If you see those, or your dog collapses, go to the emergency vet immediately instead of waiting to see whether it passes.

The Treatment Calendar: Four Weeks, Then Days 60, 90 and 91

Treatment is a months-long plan rather than a single visit. In the recommended adult-worm treatment plan, a veterinarian starts a heartworm preventive and four weeks of the antibiotic doxycycline. Then comes a series of three injections of melarsomine, the drug that kills the adult worms, around days 60, 90, and 91. A dog who is already sick may need to be stabilized first, so your dog’s condition may change the plan. A follow-up antigen test about nine months after the last injection checks that the worms are gone.

The hardest part for most households is not the drugs. Physical exertion increases the rate at which heartworms damage the heart and lungs, and the risk of treatment complications rises with how active the dog is. So activity is restricted from the day of diagnosis, becomes strict once the injections start, and stays strict for six to eight weeks after the last one. Ask your veterinary team exactly which walks and what confinement are safe, because “take it easy” means something very different to a two-year-old Labrador than it does to you.

Set that plan beside the alternative. Prevention is a chew or a spot of liquid once a month, at a cost you can predict. Treatment is hard on the dog and on the budget. The drugs can cause serious complications, and the plan means months of strict confinement for an animal that does not understand why. Preventives also do not kill adult heartworms, so once the worms have grown up there is no cheap way back.

Monthly Prevention, and What One Missed Dose Does

Most preventives are given monthly, as a chewable tablet or a liquid applied to the skin, and one is an injection your veterinarian gives every 6 or 12 months. They all work the same way: they eliminate the immature larvae your dog picked up recently. None of them protects forward against next month’s mosquitoes, which is why they have to be given strictly on schedule, all twelve months of the year. Each product has its own age, weight, and health requirements, so ask your veterinarian which one fits your dog.

If a dose is late or missed, even one, your dog may have been unprotected. Do not just quietly restart. Call your veterinarian first, because giving a preventive to a dog that already carries adult heartworms may be harmful. The recommended approach after a lapse is to test right away, test again six months later, and then go back to yearly testing. The second test exists because of that same six-month delay: an infection from the gap will not show up on the first one. I would rather a dog be tested twice than treated once.

If you also need a plan for fleas, ticks or intestinal worms, a whole-pet parasite prevention plan covers them together.