Heartworms

Spring is right around the corner! Now is a great time to make sure your pet is up to date on heartworm prevention. We will discuss fleas and ticks in the next posts, so stay tuned for that.
 
So what is heartworm disease, and why should you worry about it?
 
Heartworm Life Cycle:
Heartworms are parasites that are transmitted by mosquitoes. The mosquitoes inject baby heartworms (L3, or 3rd stage larvae) under the skin of a cat or dog. These baby heartworms grow and eventually migrate to the large blood vessels in the lungs (pulmonary arteries) and sometimes the caudal vena cava (a large vein entering the heart from the abdomen). Here, over the course of about 6 months, they grow to be adult heartworms, several inches long. As long as there are at least 1 male and 1 female heartworm, those adult heartworms will reproduce and form more baby heartworms (L1, 1st stage larvae, or microfilaria). The L1 larvae are what a mosquito picks up as it feeds on an infected dog. In the mosquito it goes through its L2 stage, and is eventually passed on to another dog or cat as the L3. Because the heartworm has to go through a mosquito host to become an adult, an infected dog cannot directly infect other pets. Although the other pets living with an infected pet are at a higher risk of infection because it is more likely that there are infected mosquitoes nearby.
 
Here is a diagram of the heartworm life cycle, as well as a video of Microfilaria under a microscope.
 
Heartworm disease:
Heartworm disease is the medical condition we see in infected pets. The disease is very different in dogs versus cats. Almost all dogs that are exposed to heartworms and are not on preventative will get adult heartworms, and it is not uncommon that they are infected with large numbers of heartworms.

If they have a mild form of the disease, they may not show any clinical signs, and the only way to detect it is with a quick blood test that your vet can do at their clinic. With moderate to severe infections, the heavy worm burden causes the heart to work harder to pump blood to the rest of the body, and eventually leads to congestive heart failure. An affected dog may cough, show exercise intolerance, have difficulty breathing, and potentially a swollen abdomen from fluid accumulation.
 
Heartworm disease in the cat is very different. Cats have a much stronger immune response to heartworms than dogs. Very few heartworms develop into adults, and if they do they are smaller and have a much shorter lifespan than they would in a dog. However it is the cat’s immune response to the heartworm larvae that causes significant inflammation in the lungs, leading to permanent damage of the lungs. When this happens, affected cats show signs similar to asthma, with coughing and episodes of panting, fast breathing, or open-mouthed breathing. The lung damage does not reverse once the heartworms have been cleared from the body, so often the asthma-like symptoms remain for life.
 
Treatment:
So what can we do about it? Fortunately preventing heartworm disease is easy. There are many preventatives available to protect our cats and dogs from heartworm disease. You can find chewable tablets, soft chews, or topical products to apply to the skin. They often will protect your pet from other common parasites as well. These products do not prevent the dog from being bitten by infected mosquitoes. Instead, they basically deworm for any heartworms that your pet may have been exposed to in the month since their last dose. The trick is to give the prevention EVERY MONTH, because as the heartworms mature they get to a point where the preventative will not kill them. Talk to your vet about the best preventative option for your pet and lifestyle.
 
If your dog gets heartworms there is a treatment available, called Immiticide. It is a rather expensive drug that is injected into the muscle along the dog’s spine 2-3 times, depending on the situation and severity of the disease. Your vet will want to do some testing to determine your dog’s overall health status prior to giving the medication. Your pet will be on a steroid to reduce inflammation as the worms die, and often an antibiotic as well. He will need to be in a crate at all times for several months to prevent him from throwing a clot as the worms die.
 
Preventing the disease in the first place is much easier on the dog, as well as much cheaper for you. Unfortunately there are no approved treatments for cats with heartworm disease, so preventing heartworms is essential. Talk to your vet about getting your pet on a heartworm preventative that is right for you!
 
See www.heartwormsociety.org for more information.