This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
See also http://wrbu.si.edu/www/culicidae/heartworm.html.
Symptoms may not appear until a full year has passed since
infection. Because of this, the disease is often mistaken for
another problem. The most persistant sign is a soft, deep
cough. After exercise, the cough may be so severe that that the
dog faints. Weight loss, discharge of bloody sputum,
listlessness, and weakness are also common (from Carlson &
The rest of the information on heartworms was adapted from a
very informative post by Kristin Thommes who posted it March 5,
The Heartworm Lifecycle
Start with an infected dog. This dog has adult heartworms
living in its pulmonary arteries (they crawl into the heart
after the dog dies). Female worms mate with male worms and
produce microfilaria (first stage larva, L1, or a "baby"
heartworm). The microfilaria enter the circulation of the dog.
When this infected dog with circulating microfilaria is bitten
by a mosquito, the mosquito will ingest 1 or 2 microfilariae.
If the mosquito ingests more larvae than this, it will die!
In the mosquito, the microfilariae (L1) will molt twice, to the
L2 and then the L3 stage. At the L3 stage, the larvae migrate
to the mosquito's mouthparts. Then when the mosquito bites a
dog, the larvae are deposited ON the dog's skin and then crawl
into the bite wound left by the feeding mosquito. If a mosquito
with the L1 or L2 larval forms bites a dog, they will NOT be
transmitting heartworms to the dog. Likewise, if the L1 forms
are not removed from the dog's circulation by a biting
mosquito, they will die off. The L1 stage does NOT "mature"
into adult worms in the dog. So, the L3 larvae that crawl into
a dog bitten by a mosquito will develop in the dog's
subcutaneous tissues to L4 and finally L5 life stages. These
then enter the venous system and enter the heart. They travel
to the pulmonary arteries and become full-fledged adult worms,
ready to reproduce.