Dogs get heartworms only through the bite of an infected mosquito. They do not catch heartworm disease by sharing water, touching another dog, or living near an infected animal. The difficult part is that an infected dog may appear healthy for months while the larvae mature and begin damaging blood vessels in the lungs.
How Heartworm Infection Starts
The heartworm life cycle requires both an infected animal and a mosquito. Adult female heartworms living in an infected dog or wild canid release microscopic offspring called microfilariae into the bloodstream.
A mosquito picks up the microfilariae when it feeds on that animal. Inside the mosquito, they develop into infective larvae. Under suitable environmental conditions, this development usually takes about 10 to 14 days.
When the mosquito later bites another dog, the infective larvae are deposited on the skin and enter through the bite wound. This means even an indoor dog can be exposed if a mosquito enters the home. Heartworm infections have been reported in dogs in all 50 U.S. states.
What Happens Inside the Dog
The larvae do not travel directly to the heart immediately after the mosquito bite. They first develop and migrate through tissues under the skin. Young worms eventually enter the bloodstream and can reach the pulmonary arteries, the vessels carrying blood from the heart to the lungs, as early as about 70 days after infection.
Full maturation takes much longer. It generally takes about six to seven months for infective larvae to become adult heartworms capable of reproducing. Adult worms usually live in the pulmonary arteries and may extend into the right side of the heart when the infection is severe.
Why Early Heartworm Infection May Have No Symptoms
During the first months, immature worms may not create changes that an owner can easily see. Even after adult worms are present, a dog with a low worm burden may have no obvious symptoms.
Symptom severity depends on more than the number of worms. The length of the infection, the dog’s activity level, existing heart or lung problems, and the body’s inflammatory response can all affect what the owner notices. A highly active dog may show exercise intolerance earlier because physical activity increases demands on damaged pulmonary vessels.
How Heartworm Symptoms May Progress
Heartworm disease does not always follow a perfectly predictable order. However, these patterns can help owners recognize when a change needs veterinary evaluation.
| Possible stage | What an owner may notice | What may be happening |
|---|---|---|
| Early infection | No visible symptoms | Larvae are developing and migrating through tissues. |
| Mild established disease | Occasional cough or tiredness after moderate activity | Adult worms and inflammation are beginning to affect pulmonary blood vessels. |
| Moderate disease | Persistent cough, shorter walks, reluctance to exercise, reduced appetite, or weight loss | Blood flow through the lungs is becoming more restricted, increasing strain on the heart and lungs. |
| Advanced disease | Difficulty breathing, weakness after mild activity, poor body condition, or a swollen abdomen | Pulmonary hypertension and right-sided heart failure may be developing. |
| Caval syndrome | Sudden labored breathing, pale gums, dark red or brown urine, weakness, or collapse | A large mass of worms is interfering with blood flow through the heart. |
A mild cough or reduced stamina does not prove that a dog has heartworms. Respiratory infections, airway disease, heart conditions, pain, obesity, heat, and poor conditioning can cause similar changes. Symptoms should guide an examination, not replace heartworm testing.
When Heartworm Tests Can Detect Infection
A blood test performed soon after a mosquito bite may be negative even if infection has occurred. Antigen tests detect proteins released mainly by adult female heartworms. These proteins may become detectable about five months after infection, although detection can take longer in some dogs.
Tests for microfilariae usually cannot detect infection until adult worms have matured and reproduced, generally around six months or later. Because of this delay, a veterinarian may recommend repeat testing after a lapse in prevention or a possible exposure.
Dogs should continue receiving regular heartworm tests even when they stay on prevention. The American Heartworm Society and the Companion Animal Parasite Council recommend annual testing because doses can be delayed, oral medication may be spit out or vomited, topical medication may not be applied correctly, and no preventive is completely effective.
What to Do After a Missed Preventive Dose
Do not automatically double the next dose. Contact the veterinarian who prescribed the product and provide the medication name, the scheduled date, and how late the dose is. Recommendations can differ among monthly oral products, topical medications, and long-acting injections.
A veterinarian may advise restarting prevention, testing immediately, and repeating the test several months later. Continue following the prescribed schedule rather than waiting for coughing or fatigue to appear. Preventives work against immature stages and are not a substitute for treatment when adult worms are already present.
When a Dog Needs Prompt Veterinary Care
Arrange a veterinary examination when a dog develops a persistent cough, tires much faster than before, repeatedly stops during normal walks, loses weight without explanation, or has noticeable breathing changes.
Labored breathing, pale gums, dark urine, sudden severe weakness, collapse, or a rapidly enlarging abdomen require urgent veterinary care. These signs can occur with advanced heartworm disease, but they can also indicate other life-threatening heart or respiratory problems.
Louie’s Prevention Routine
Louie, a two-year-old Yorkshire Terrier, follows a veterinarian-directed heartworm prevention schedule. His routine is not based on whether mosquitoes are visible or whether he appears healthy. The practical goal is to give each dose on schedule and maintain regular testing instead of waiting for symptoms.
The Main Point
Heartworm disease begins with an infected mosquito, develops silently for several months, and may not cause obvious symptoms until adult worms have already affected the pulmonary arteries. Prevention should be maintained year-round according to the product directions and the veterinarian’s advice. Regular testing remains necessary because symptoms alone cannot confirm or rule out infection.