Canine Heartworm Disease

The mosquitoes are here! Unfortunately, that means that Heartworm disease is here too. And since I have had the displeasure of having to treat a patient for the disease just this past week, I thought it would be a pertinent topic to discuss.

What is Heartworm Disease?

Heartworm disease (dirofilaiasis) is a serious and potentially fatal disease in dogs and is transmitted by mosquitoes. Contrary to what many individuals believe, just because a dog spends most of the day indoors does NOT mean he/she cannot get heartworm disease. Mosquitoes get into the house....I know this all to well as I am attacked by the critters regularly! It is also a disease that threatens dogs (and cats) throughout all seasons and in all parts of these United States and Canada, especially in warm weather regions.

Heartworms are found in the heart and blood vessels of the lungs (namely the pulmonary arteries) of infected dogs. They live up to 5 years and during this time a female worm can produce millions of offspring (microfilaria). These immature worms cannot complete their life cycle in the dog. This is where our friend the mosquito comes in. The female mosquito bites an infected dog and ingests these immature worms. The worms then develop in the mosquito and are now called infective larvae. They are injected into another unsuspecting dog when the mosquito takes a meal and develop into adult heartworms. After maturing in about 2-3 months they will start reproducing. The disease cannot be spread directly from an infected dog to a healthy dog without the mosquito.

What do Heartworms do to the dog?
Adult heartworms can cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the motion/action of heart valves. Blood supply to other organs is reduced, mostly in the lungs, liver and kidneys leading to malfunction and failure.

Many infected dogs may not show signs of disease for as long as 2 years, but usually by the time these signs are seed the disease is well advanced. Signs can vary depending on number of worms, where they are and how long they have been present. The most obvious signs are a soft, dry cough, shortness of breath, weakness, restlessness, lethargy, and exercise intolerance. All signs are most obvious after exercise. Some dogs may even faint. Severely infected dogs can die suddenly during exercise or excitement.

Young heartworms circulate through the body in smaller blood vessels and can block blood flow and deprive oxygen and nutrients to organs. Mostly the liver and lungs are affected.

How is Heartworm Disease diagnosed?
In most cases diagnosis can be made via a simple blood test that can many times be run right at your local veterinary office or can be sent to a veterinary laboratory. Other diagnostic tests are essential to determine if an infected dog can tolerate heartworm treatment. Further blood tests to check for young heartworms, check liver, kidney and other organ function, radiographs (x-rays) of the heart and lungs, Electrocardiogram (ECG/EKG) to check for abnormal heart rhythm and Echocardiogram (ultrasound of the dog's heart) to look at heart chambers and even see worms in the heart are a very important of diagnosis and treatment in the infected dog.

How is Heartworm Disease treated?
There is some risk involved in treating dogs with heartworms, although fatalities are now more rare then they used to be. One older drug used to treat heartworms contained arsenic, so toxic effects and reactions were more common. A newer drug is now available that does not have such harmful side effects, so we can now successfully treat about 90% of heartworm cases. This drug is given by injection twice, 24 hours apart. It kills adult worms. In more serious infections these injections may be given 30 days apart. Your veterinarian can discuss in more detail the follow up instructions once an infected dog is treated as well as side effects that may occur. Other drugs may need to be given to these dogs to treat additional symptoms of heartworm disease. Some dogs have lifelong problems with heart disease and failure even after the worms are gone.

Dogs that are far advanced in the disease usually cannot be successfully treated and usually succumb to organ failure within a few weeks or months.

How can Heartworm Disease be prevented?
There are many effective and safe heartworm prevention products available through your veterinarian. Most of them are chewable tablets or are in beefy treat form. There is also a topical form that can be applied. Generics are even now available providing a cost effective way to prevent this disease from affecting your best friend! Monthly administration is very important and most veterinarians recommend giving the prevention year-round. Even if given year round it is still important that your dog be tested periodically as advised by your veterinarian because no medication is 100% effective. Dogs that are only on prevention during warm weather months must be tested every 6-12 months before re-starting the medication. Re-starting medication without testing can be harmful or even deadly to a dog that is unknowingly infected, so it is very important to call your veterinarian if you have skipped even one month of prevention. Puppies are usually started on prevention without testing first because their potential for exposure prior to giving medication is very low.

So PLEASE everyone....,make sure your best friend is protected from this deadly disease. Get your dog tested and on prevention!! Dang those pesky mosquitoes!!