Heartworm in Dogs

What is heartworm?

Each year in the United States some 250,000 dogs get heartworm. Heartworm is a devastating disease that can kill your dog. The sad fact about these statistics is that heartworm is preventable. So what is heartworm, how do dogs get it and how is it treated?

Heartworm Facts
Heartworm is caused by a parasite that scientists have given the obscure name Dirofilaria immitis that is found pretty much in the entire United States. This is a worm that lives primarily in the right side of the heart, giving the disease its name. While heartworm infects primarily dogs, it is also found in cats, wolves, coyotes, and foxes. There have even been a few documented cases of infections in humans. But don't worry if your dog has heartworm disease-its not directly transmissible from your dog to another dog or to a human. Heartworm is transmitted by mosquitoes. So, anywhere you find mosquitoes there is a risk of heartworm disease. As we will see shortly, the development of the heartworm is temperature sensitive so in colder locales heartworm transmission is seasonal.

Heartworm starts with a bite from an infected mosquito. The bite of the mosquito injects several heartworm larvae into the bloodstream of the dog, and they begin a process of maturation. These immature worms are called microfilariae, but we will refer to them as teenagers (easier to grasp). Over a period that lasts about 4-6 months, the heartworm larvae develop into small adult worms which can live in a dog for a period of up to 7 years.

Once they reach adult form, the heartworms move into a vein and head straight for the heart. There they become able to reproduce, getting ready for the next stage in the heartworm life cycle-you guessed it-this stage is having babies. A female heartworm can give birth to 5,000 young per day which circulate in the bloodstream of an infected dog. They can do so for up to 3 years! And what are they waiting for? You've probably figured this out-they're waiting for another mosquito to come along and bite the dog. The mosquito takes up some of the baby worms when it bites an infected dog, and becomes ready to transmit the infection to a new dog.

Climate Matters
It turns out that a warm climate is vital for the development of the baby heartworms. To infect a new dog, they have to become teen-agers. This is a process that requires the temperature to stay balmy-if it dips below 57 degrees F for just a couple of hours it won't happen. This process also takes several days. As a result heartworm infection is not found in Alaska and parts of Canada, and is seasonal in many of the lower 48 states. But during the summertime when temperatures routinely stay above 57 degrees even at night, you need to be on alert for heartworm. The daytime temperature has to be generally 80 degrees or above for around two weeks for the babies to transform into teenage heartworms (this can happen more rapidly if the climate is warmer).

Heartworm Disease and Symptoms
Unfortunately, in most cases heartworm disease doesn't exhibit too many symptoms. It depends on the load of adult worms that the dog has. If the number of adult worms infecting the dog is low, then the worms tend to live in the pulmonary arteries (arteries in the lung) and in the right side of the heart. A large number of worms, as you might imagine, can cause some serious health problems based on where they are living. Early signs of heartworm disease include a soft cough and a dog that tires easily after exercise. You can see how these symptoms might be overlooked in a lot of dogs, in fact they might be missed entirely in a sedentary dog. Worse than this most dogs don't show any symptoms at all.

In an advanced case, the large heartworm burden causes a lot of problems. You might have a dog that faints, shows signs of weight loss, and coughs up blood. An advanced case can lead to a condition called congestive heart failure that can kill the dog.

Since most dogs don't show symptoms, and most that do have mild symptoms that could be mistaken for something else, the only way to diagnose the condition is with a heartworm test. You should have all of your dogs tested for heartworm. This is done with a blood test which looks for antigens that come from the female heartworms in the dog.

Heartworm Treatments
If a dog tests positive, the first step is to estimate how advanced the infection is. Blood work might be indicated to determine the status of kidney and liver function, and x-rays are in order to assess lung and heart damage.

Based on the results of these tests, your veterinarian will prescribe a treatment for your dog. This treatment begins with an injection of an arsenic based medication designed to kill the heartworms. It is given in the muscle. Currently, the drug of choice is one called immiticide (or Melarsomine Dihyrdochloride). The drug kills the worms, and unfortunately this process leads to complications. The dead worms can circulate into the ends of the pulmonary arteries, where they can cause a pulmonary embolism. This is a condition whereby blood flow is blocked in the lungs, and it can lead to death.

There are two steps taken to reduce the risk of this condition developing. First, most veterinarians treat the worms gradually. This is done by giving the medication in two steps. An initial dose is given to begin killing the worms, then a second dose is given about a month later to finish them off. Secondly, you can reduce the risk by keeping your dog sedentary. Generally you will have to keep your dog resting in a confined space such as a dog crate for a period of about 6 weeks or 42 days. If the dog stays resting and confined he is much less likely to have complications from the treatment. This allows the body to break down the heartworms after they die, and avoid having pieces of dead heartworm end up in the terminal pulmonary arteries where they can cause problems. The third week after the initial treatment can be the worst period, and unfortunately many dog owners let their guard down. They feel guilty keeping their dog confined for such a long period, the dog seems OK, and they let the dog out for some exercise and then an embolism results. If your dog is undergoing heartworm treatment don't fall into this trap. Keep him confined for the entire 6 week period.

Once the treatment is over, you can gradually resume exercise and activity with your dog. If your dog is young and/or has a light infection load, his chances for a full recovery are good. Dogs with advanced heartworm cases may require surgery to clear the heart out or repair damage.

Heartworm Prevention
The best and easiest way to deal with heartworm is to prevent it in the first place. A dog which tests negative should be put on a preventative medication. It has been found that heartworm preventatives are 99% effective. In fact, the missing 1% actually results from dog owners missing a dose or giving the medication irregularly. If you stick to a dosage schedule and give your dog a correct dose, chances are nearly 100% he will not become infected.

One popular preventative on the market is called Heartgard (ivermectin/pyrantel). This is given to your dog once a month. It resembles a kind of meaty dog treat, and most dogs eat it without hesitation. In addition to preventing heartworm, it treats hookworm and ascarid, so its a good medication to have your dogs on. It may save their lives. If your dog does test positive for heartworm and is treated, he will have to be on heartworm medication for the rest of his life. But start now, and get your dog on preventative before he has a chance of becoming infected.

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