Heartworm in Dogs: Mosquito-Borne and Preventable

Heartworms are parasitic worms that live in a dog's heart and the blood vessels of the lungs. The disease is spread by mosquito bites, so in hot, humid climates with plenty of mosquitoes, every dog is at risk. The worrying part is that it progresses silently; by the time signs are obvious, the heart and lungs are already damaged.

How heartworm spreads

The cause is the worm Dirofilaria immitis. The transmission cycle goes like this:

1. A mosquito bites an infected dog and picks up tiny larvae from its blood.
2. The larvae develop inside the mosquito for a period of time.
3. The mosquito bites another dog and passes the larvae in through the bite.
4. The larvae migrate through the dog's body and, after a few months, reach the heart and pulmonary arteries, growing into adult worms as long as noodles.
5. Adult worms can live for years, releasing new larvae into the blood and continuing the cycle.

Dogs don't infect each other directly. A single infected mosquito is enough, so indoor dogs and apartment dogs are still at risk.

Signs of disease

In the early stage, dogs usually show no signs. As worm numbers grow and the disease goes on, you may see:

• A dry, persistent cough, especially after exercise.
• Tiring quickly, reluctance to walk, lying down a lot.
• Rapid or labored breathing.
• Weight loss, poor appetite, a dull coat.
• A gradually swelling belly from fluid buildup.
• Severe stage: collapse, pale gums, dark urine. This is an emergency.

Because signs appear late, the only way to detect it early is a blood test at a veterinary clinic.

Prevention: far easier and cheaper than treatment

Treating an infected dog is long, expensive and risky, because dying worms in the blood vessels can cause blockages. Prevention, by contrast, is fairly simple.

Heartworm preventives

• They come in several forms: chewables, spot-on treatments and injections. Some products also cover intestinal worms, ticks and fleas.
• The product, how it's given and the schedule for each dog are decided by your veterinarian based on age, weight and health. Always read the product label carefully.
• Where mosquitoes are present year-round, prevention usually needs to continue consistently all year, without gaps.
• Before starting prevention in an adult dog that has never had it, your vet will usually test to make sure the dog isn't infected. Giving preventives to a heavily infected dog can cause a dangerous reaction.
• Puppies start prevention young, at the age your veterinarian recommends.

Reduce mosquitoes around your dog

• Remove standing water around the house: plant saucers, old tires, jars and water troughs.
• Fit mosquito screens on doors and keep your dog indoors in the evening.
• Avoid walking your dog in grassy areas or by ponds at dusk, when mosquitoes are most active.
• Don't use human mosquito repellents on your dog. Some ingredients are toxic to dogs. Use only dog products as directed by your veterinarian.

Regular testing

Ask your veterinarian about periodic heartworm testing, especially if you've ever missed doses of prevention or have just adopted a dog with an unknown history.

At the visit, tell your vet which preventive your dog uses, when it was last given and whether any doses were missed. Bringing the box or a record helps a lot. A phone reminder is a simple way not to forget. Canine heartworm very rarely causes illness in people, so the main concern remains your dog’s health.

When to see a vet

• Your dog has a persistent cough, tires quickly or breathes fast with light activity.
• Your dog is losing weight, has a swelling belly or a dull coat.
• Collapse, severe breathing difficulty, pale gums or dark urine: this is an emergency.
• You've missed your dog's prevention for a while: ask your vet what to do and whether a retest is needed.
• A newly adopted dog, or an adult dog that has never had heartworm prevention: get a checkup and test before starting.