By Robin Brennen Heartworm is a potentially fatal disease caused by a blood-borne parasite known as Dirofilaria immitis. This parasite is transmitted through the bite of an infected female mosquito; mosquitos are infected by feeding off a dog that has heartworm. Don't worry, pet owners; heartworms are very rarely transmitted to humans. We are not their natural host. Infective larvae enter the dog's body when the mosquito bites the dog. They migrate into the bloodstream and move to the heart maturing, mating and reproducing microfilariae (heartworm babies) within six to seven months. It is the microfilariae that, in turn, are consumed by the female mosquito, making her a future inoculator. Since transmission requires the mosquito as an intermediate host, the disease is not spread directly from dog to dog. Spread of the disease therefore coincides with mosquito season, which can be year-round in many parts of the United States. Heartworm disease is diagnosed in every state. It usually takes several years before dogs show clinical signs of infection. The worms can grow to 12 inches in length and live five to seven years! A dog can actually have hundreds of these icky worms living inside the heart at one time. Adult heartworms cause disease by clogging the heart and major blood vessels leading from the heart. By clogging the main blood vessel, the blood supply to other organs of the body is reduced, causing them to malfunction. The most obvious clinical signs of heartworm disease are a soft, dry cough, shortness of breath, weakness, exercise intolerance and fainting. All of these signs are most noticeable following exercise. In advanced cases, congestive heart failure occurs and dogs have a difficult time breathing from fluid accumulation in their lungs. A simple blood test can detect the presence of heartworm far in advance of them showing clinical signs of disease. This blood test can easily be performed by your veterinarian. If your dog tests positive, your vet will recommend some additional tests to assess the stage of disease, and from there, a recommended course of treatment can be suggested. Treatment is aimed at killing the adult heartworms and microfilaria. It is done in stages to minimize reactions. Treatment consists of injections to kill the adults, oral meds to kill the microfilaria and a combination of antibiotics and anti-inflammatory medications aimed at controlling secondary problems. In addition, if the dog exhibits signs of advanced disease, medications may be given to control or ameliorate those symptoms. During treatment, it is imperative that the dogs be strictly rested, as the death of the adult worms and microfilaria can cause bad reactions. With this disease, an ounce of prevention is truly worth a pound of cure. Treatment for heartworm is very expensive ($1,000 and up) and can have side effects that are just as dangerous as the disease itself. Preventing infection is the best course of action and can be done with a prescription for a monthly medication from your veterinarian. Your veterinarian will require a blood test first before prescribing the preventative medication, as putting your dog on preventatives without knowing its heartworm infection status can be dangerous. The monthly cost of this oral or topical medication is less than a burger and fries at your favorite fast-food restaurant. Prevention should be maintained year-round, as these medications also protect your dog from intestinal parasites. Some topical preparations also contain mosquito repellent to further diminish exposure. Most people don't realize that heartworm disease affects cats as well, but their infection rates are much lower. They are an atypical host and often only have one or two worms living in their heart. Symptoms in cats are not due to worm burden but rather an allergic reaction when the worms naturally die off. Cats manifest the infection with symptoms similar to asthma (coughing, wheezing, shortness of breath). Some cats will have nondescript symptoms of malaise or vomiting. However infection can cause sudden death in otherwise healthy-appearing cats. Interestingly, a recent study from North Carolina University found that 25 percent of cats infected with heartworms were totally indoor cats. There is monthly preventative medication for cats and, recently, a blood test has been introduced that can be performed by your veterinarian. Treatment options are very limited at this time, but knowing the infection status of your cat can be helpful. Robin Brennen is chief of veterinary services and vice president of operations at the Animal Hospitals at Bideawee.
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