Heartworm Disease Overview

Posted by Aaron

Wednesday, June 17, 2009


The link above will give you lots and lots of good information on this subject. The pictures above are small, I know. I don't have any larger pictures. Maybe someday I'll get a good heart to get pictures of.

I'm going to post a separate blog on preventative choices, feline heartworm disease, and heartworm treatment.

Here are a few common myths and answers. There's discussion below.

Myth: Heartworms are found in the stool. Your dog gets heartworms by coming in contact with another dog's stool.

Fact: Heartworms are exactly that - they are found in the heart. The larvae are found in the blood stream and they are transmitted by mosquitoes.

Myth: Only dogs get heartowrm.
Fact: We used to think so. We were WRONG. Cat's do get heartworm. So do people.

Myth: My dog is indoors, so I don't need preventative. Same argument for cats.
Fact: Turns out this is absolutely false as well.

Myth: Preventatives are over the counter. My vet is just being a pain in the butt about requiring me to come in.

Fact: See my previous posts on the importance of regular physical exams. Preventatives are all PRESCRIPTION products. This means we can't just sell it over the counter or we get huge fines and risk loosing our license. We're not being a PItA. We're following federal drug law.

Here's the skinny on heartworm disease.

These worms start out as baby larva in an infected dog. These larva are referred to as L1 larva. A mosquito comes along, takes a blood meal that includes the L1 larva and flies away. The larva grow and develop into L2 and then L3 larva. Same mosquito bites another dog and those little L3 larva come down the mosquito's mouth and climb into that tiny hole and into the dog (cat, human, wolf, fox, etc). The larva then continue to devlop into L4, L5, then adult worms. As L5 and adults they take up residence in the right side of the heart and the pulmonary artery leaving the right side of the heart and into the lungs. You must have both males and females to reproduce, but once you do - the life cycle begins over again. Adults live for about 5 years in the dog.

The canid is the definitive host - meaning they are the host the parasite is intended for. Dogs, fox, coyote, wolf are all routinely infected. Other species can be infected with the parasite, but the body kills it off before it can become an adult. In people, it's usually killed off in the skin but it is possible for it to migrate to the lung and form a little granuloma before it's dead.

We used to think that cats were an uncommon host for the parasite - we were incorrect. See my companion post about cats and heartworm disease.

It takes about 4 or 5 months for a worm to go from larva to adult. The preventatives only work during the first 30-45 days of this period. Tests used by your veterinarian are only positive when there are adult female heartworms present. It is therefore possible to have worms in the heart and a negative test if either 1) they are only 4 months old or 2) you have all males or less than 3 females. 100 males, 1 female = NEGATIVE test! (This is uncommon, but it happens!)

The timing on this is important because this means there is a period of time between when a larva can be killed and when the test would become positive. For this reason, a pet that has been off preventative cannot be considered negative until a repeat test is done 5-6 months later.

What happens if I don't treat or prevent heartworm disease? Honestly, a dog can probably tolerate a small worm burden (1 or 2 worms) for a lifetime. The will have changes in the blood vessels in the lungs and will have chronic bronchitis-type problems. Most dogs with the disease don't just have one or two worms and therefore get progressively get sicker and sicker with hearts that fail and lungs that don't work. They may develop clots in the lungs and can suffer sudden death. Untreated heartworm disease kills. It's pretty nasty.

Next posts will be on prevention, treatment, and feline heartworm disease.



Anonymous said...

Now I can just print this out and tape it to the counter.