Heartworm in Dogs in India: The Mosquito Season Nobody Plans For

The cough was so small that Anju almost missed it. One soft, dry sound as Chotu got up off the floor in the morning, the kind of noise you put down to dust, or to a bit of water going the wrong way. He was six, a Labrador with a good appetite and a bad habit of stealing slippers. Nothing about him looked ill.

What she noticed three weeks later was the walk. He used to drag her up the lane. Now he stopped halfway, sat down on the damp tar, and looked back at her as if asking whether they could just go home.

A cough you can ignore, and a dog who has quietly lost interest in his walk. That combination can mean a dozen different things. One of them is a worm that lives in the heart and the lungs, and it is spread by the same mosquitoes that have been keeping you awake since the rain stopped.

Why Heartworm in Dogs in India Peaks After the Rain

Heartworm is not passed from one dog to another. It is delivered by a mosquito, which makes it a seasonal problem in a country that spends four months filling every bucket, tyre, tank and fold of tarpaulin with standing water.

The cycle is simple enough to picture. A mosquito bites a dog that is already infected and drinks in microscopic larvae circulating in that dog's blood. Those larvae spend roughly ten to fourteen days developing inside the mosquito, and warm weather speeds the process up. When that mosquito next bites your dog, it leaves infective larvae behind in the skin.

September and October are when this arithmetic turns against us. The rain has stopped, the water has not drained, and mosquito numbers are at their peak. Your dog does not have to go anywhere unusual for this to happen. A balcony in Kochi, a society compound in Pune, a verandah in Guwahati. One mosquito is the entire story.

Indian veterinary surveys have found infected dogs across many parts of the country, with higher rates reported from the humid northeast and from coastal belts. It is not a rare foreign disease. It is simply one that most pet parents here have never had explained to them.

The Worm That Takes Half a Year to Show Up

Here is what makes heartworm so easy to miss, and so frustrating for vets. Nothing happens for a long time.

The larvae that go in under the skin take roughly six to seven months to grow into adult worms and settle in the blood vessels of the lungs and the right side of the heart. By then they are long, thin and very much alive, and adults can survive in a dog for about five to seven years.

Through most of that stretch, the dog looks fine. Early signs, when they finally arrive, are the sort of thing you explain away: a soft dry cough, a bit less enthusiasm on walks, tiring faster than he used to. It is only later that the picture turns obvious, with laboured breathing, weight loss, a swollen belly from fluid, fainting, and in the worst cases sudden collapse during exertion.

There is a severe form where a heavy load of worms shifts backwards into the heart and the large vein returning blood to it. Dogs with that can pass red or brown urine and become suddenly, dangerously anaemic. It is an emergency, and it is the end of a road that started with a cough nobody thought twice about.

So the honest summary is this. By the time heartworm looks like a serious illness, it has usually been in your dog for years.

What Your Vet Is Looking For, and When

The main test is a blood test that looks for antigen, a protein shed by adult female worms. It is quick and it is reliable, with one important catch: it only turns positive around seven months after the mosquito bite, because that is when the worms are mature enough to shed anything. Testing a puppy that was bitten last month will tell you nothing useful.

Your vet may also put a drop of blood under the microscope to look for the circulating larvae directly. If either test points to infection, a chest X-ray and a scan of the heart help work out how much damage has already been done, which changes how treatment is planned.

This timing is also why a vet will want to test before starting a preventive. It is not an upsell. Preventives kill the young larval stages, and in a dog already carrying a crowd of circulating larvae, that sudden die-off can trigger a serious reaction. Test first, then prevent. That order matters.

Why the Cure Is Harder Than the Illness Sounds

Treating an established infection is possible, and dogs do come through it. But it is not a course of tablets and a pat on the head.

Adult worms are killed with a specific injectable drug given by a vet, usually alongside an antibiotic that targets bacteria living inside the worms themselves. The difficult part comes after. Dead worms break up and travel, and the fragments can lodge in the blood vessels of the lungs. So the dog has to be kept genuinely still for weeks. No stairs if you can help it, no games, no long walks, no excitement at the gate.

Anyone who has tried to keep a young Labrador on strict rest for a month will tell you how that goes. It is the single hardest part of the treatment, and it is the part that most often goes wrong at home.

Compare that with prevention. A monthly preventive, given through the year, stops larvae before they can ever become adults. Several drug families do this job, given as a tablet, a chew or a spot-on. Your vet chooses which one, and there is a real reason for that: some herding breeds carry a gene variant that makes them sensitive to certain doses of drugs in the ivermectin family. This is not a decision to make from a pet shop shelf or a WhatsApp group.

Three Things Worth Doing Before Diwali

If you take nothing else from this, take these.

• Book the test. Ask your vet for a heartworm antigen test at your dog's next visit, especially if your dog is over a year old and has never been on a preventive.
• Start the preventive and set a reminder. The gaps are what cause failures. A monthly alarm on the same date works better than good intentions.
• Cut the mosquitoes down at home. Empty the plant saucers, the cooler tray, the terrace bucket, the puddle on the sunshade that nobody can see. Be careful with coils and repellents around pets, and keep cats well away from products meant for dogs.

Cats Get It Too, and So, Rarely, Do People

Cats are not immune. Most worms that reach a cat never make it to adulthood, but even a small number can cause real trouble in a cat's small lungs, and there is no approved adult worm treatment for cats. Prevention is genuinely the only good option there, and that is worth asking your vet about if your cat sits at an open window all evening.

People come up in this conversation too, and the answer deserves care rather than alarm. A related worm, spread by the same mosquitoes, has been reported in rare human cases in India, usually as a lump under the skin or near the eye. You cannot catch it by hugging your dog or cleaning up after him. The mosquito is the link, not the animal, which means that reducing breeding sites around your home protects everybody in it. You can read more on our heartworm disease in dogs page.

Back to Chotu

Anju took him in. The blood test came back positive, the X-ray showed changes in his lungs, and the next four months were slow, expensive and dull for a dog who loves nothing more than a run. He came through it. He is nine now, and he still steals slippers.

What stays with her is not the treatment. It is the sentence her vet said at the first visit, gently and without blame: this was preventable for about the cost of a month of dog treats.

If your dog is not on a monthly preventive right now, this is the month to fix that. The mosquitoes outside your window are not waiting for a better time, and neither should you.

The cough is never the beginning. It is the part of the story you can finally hear.

Every dog deserves a vet who knows their history and a pet parent who books the appointment before the symptoms arrive. Find trusted vets, groomers and pet care near you at fluffypet.care.