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What’s the Big Deal About a Simple Mosquito Bite?

Now more than ever, a simple mosquito bite may mean serious trouble for you and your sporting dog. With all the “buzz” about West Nile Virus (WNV) and mosquitoes, no wonder many dog owners are concerned. To put some of the concerns about WNV in dogs to rest, dog owners should understand that their canine friends are very resistant to the virus and very few dogs develop any clinical signs. HOWEVER, a more prevalent and serious threat to our canine friends does exist…the dreaded heartworm disease!

Heartworm disease is found all over the world. In the U.S.A., the disease has been found most commonly in the East, South, and Midwest areas. With time, the disease is spreading to every area of the country. Heartworm disease is one of the most serious infectious diseases encountered in canine medicine. Do not let your hunting buddy be one of the thousands of cases of heartworm diagnosed each year in the United States!

What Causes the Disease:Heartworm disease is caused by the parasite Dirofilaria immitis. The parasite begins its life cycle as microscopic organisms called microfilariae. Microfilariae enter a dog’s circulatory system through the bite of an infected mosquito. Once the infective larvae enter the blood stream and tissues of the dog, they grow and mature into adult stage worms. After this phase, they begin to migrate to the heart and lungs. Adult worms can measure as much as 27 cm in length.

Clinical Signs: The most common signs of heartworm disease include coughing, weight loss, fever, and a lack of energy. Additionally, signs of more progressed disease include difficulty breathing, fluid in the abdomen, and swelling of the extremities. If the worm has progressed to the heart, severe weakness, collapse, and sudden death can occur. In some cases, heartworm disease may cause damage to the kidneys; consequently, an animal will have increased thirst and need to urinate.

Disease Transmission: Heartworm disease is indirectly spread from animal to animal through the bite of a mosquito. Any other method of sharing blood or blood products between dogs, such as blood transfusions, may also cause disease transmission. Heartworm disease is not transmitted by direct animal to animal contact.

Diagnosis: If any animal is suspected of having heartworm disease, a simple and accurate test may be performed in a veterinary clinic or diagnostic laboratory. In rare situations, tests on some animals with heartworms produce a negative test result. If this occurs, additional testing may be necessary. Because treatment is most successful if the diagnosis is made early, routine screening of healthy dogs for heartworm disease is highly recommended. In dogs that have heartworm disease, radiographs and ultrasound of the heart can be used to help determine the extent of damage caused by the adult worms.

Treatment: Older methods of treating this problem involved using a product that had some very severe side effects. Because of this, the animal was hospitalized and carefully monitored. Newer and safer medications have since been introduced that are not as dangerous. However, it is important to understand that treating any dog with heartworm disease does carry risks, and some animals have died during treatment. Treatment consists of initially killing the adult worms in the heart. Injections of specific drugs with this capacity are first administered. As the worms die, their bodies must be absorbed by the animal’s body. Because the dead worm bodies may fragment with any strenuous activity, strict rest for the dog is encouraged for the next 4-6 weeks. These fragments can act as deadly blood clots, possibly causing heart attacks, strokes, and a variety of other problems. After this phase of treatment, another medication must be administered to destroy the microfilariae. After another waiting period, the dog is retested for heartworm. If the dog tests negative, it is placed on a preventative program.

Prevention: Once a dog is determined to be heartworm free, any of the heartworm preventative products may be recommended. These products are very safe for most animals. The most common heartworm prevention products include Interceptor® (milbemycin oxime), Heartgard® (ivermectin), Revolution® (selamectin), plus others. These products are usually given once a month and are dosed according to the animal’s weight. Depending on the product, they should only be given to dogs that are at least 6 weeks of age or weigh at least 2 pounds. There is also a product called ProHeart 6® (moxidectin) that can be given as an injection that provides six months of protection from heartworm.

Precautions: All animals should be blood tested before any preventative measures are taken. Placing a dog already infected with heartworm disease on any type of prevention may cause serious problems in that animal!

This article was written by the veterinarians at, Dr. Cody W. Faerber and Dr. S. Mario Durrant. Become a subscriber to Infovets Canine Manual in CD-Rom, online, or printed form and have instant access to this and hundreds of other articles, pictures, and video on canine health. Visit or call toll free 1-877-424-7838 to learn more about all that Infovets has to offer!

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