Heartworm disease (dirofilariasis) is a serious and potentially fatal disease in dogs and cats. A heartworm prevention program is very important and consists of chewable tablets given monthly (Heartgard, Interceptor), and a new topical mosquito repellent “Advantix” applied monthly. Be advised that Advantix is not a heartworm preventative.
Frequency and transmission.
Heartworm disease occurs all over the world. Indiana is among the 10 top states for the frequency of the disease. An intermediate host, the mosquito, is required for transmission. The mosquito becomes a carrier of microlarvae by biting tn infected animal. Transmission of heartworm larvae to another pet is accomplished when the infected mosquito bites another animal.
How heartworms get into the heart.
Adult heartworms live in the heart and pulmonary arteries of infected dogs. They survive up to 5 years and, during this time, the female worm produces millions of young (microfilaria). These microfilaria live in the bloodstream. These microfilaria cannot grow to adulthood in the dog. However, they will be ingested by a female mosquito during a blood meal. The mosquito becomes a carrier and the microfilaria develop into infective larvae. The mosquito infects another pet and the infective larvae enter the bloodstream of the pet and migrate to the heart and adjacent vessels. There, the larvae grow to maturity in 2 to 3 months (adult heartworm) and start reproducing the microlarvae. The female heartworm is 6 to 14 inches long and 1/8 inch wide: the male is about half the size of the female. One dog may have as many as 300 worms.
Effects on the dog.
Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They also interfere with the valve action in the heart. By clogging the main blood vessels, the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs. Most dogs infected with heartworms do not show any signs of disease for as long as 2 years. Unfortunately, by the time signs are seen, the disease is well advanced and serious damage is done to the heart as well as other organs. The most obvious signs are: a soft, dry, chronic cough, shortness of breath, weakness, nervousness, listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when some dogs may even faint. Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In advanced cases, irreversible congestive heart failure may be apparent and the abdomen and legs will swell from fluid accumulation. There may also be evidence of weight loss, poor circulation, and anemia. Severely infected dogs may die suddenly during exercise or excitement.
Cats can also be infected with the heartworm disease through a mosquito bite. The infective larvae will lodge in the lungs rather than the heart as it is for dogs. In the lungs, the adult worm will develop. The most common signs will be a dry cough mimicking asthma or a hairball irritation. The cat can also die suddenly.
Our hospital is using a Heartworm SNAP test performed on site; it requires 2 drops of blood and it takes 10 minutes to have the result. Our policy is to require a heartworm test every other year for dogs for their own protection. Some preventive programs, such as Heartgard (Merial, Co.), will guarantee their preventive medication only if the dog receives their medication once a month, year round and has a yearly test. If the test is positive for heartworm, further testings will be necessary to determine the treatment plan. Chest X-Ray to assess the heart damage, and blood test to assess damage to other organs are routine. Electrocardiogram and ultrasound of the thorax may be recommended. For the cat, a different Heartworm test is performed and is sent to another laboratory. Same diagnostic procedures are recommended.
There is some risk involved in treating dogs with heartworms. In the past, the drug used to treat heartworms contained arsenic, so toxic effects and reactions occurred somewhat frequently. Now a newer drug, named Immiticide (Merial, Co.), is available and does not have the toxic side effects of the old one. Determination of the severity of the disease is assessed by the results of the diagnosis plan. According to the severity, two injections are administered 24 hours apart or 4 months apart The dog has to be hospitalized for close monitoring. With the treatment, the adult worms die in a few hours and start to decompose. As they break up, they are carried to the lungs, where they lodge in the small blood vessels and are eventually reabsorbed by the body. This is a dangerous period, and it is imperative that the dog be kept quiet and not be allowed to exercise for at least 1 month following the treatment. Prompt treatment is essential if the dog has a significant reaction in the weeks following the initial treatment. If a dog shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, depression, you should notify your veterinarian promptly.
Six to eight weeks after the treatment, an occult blood test is performed to detect any microfilaria present. If the test is positive, oral ivermectin is given to the dog. If the test is negative, the Heartworm Snap test should be repeated 6 months later, as the microfilaria have the time to develop into worms. In dogs that are severely affected, treatment of specific organs should be addressed. The most common is the congestive heart failure and it may require a lifetime treatment.
The cost of the treatment varies accordingly to the weight of the animal. The cost ranges between $350 to $750.
Prevention is the KEY wrod! A heartworm prevention program should start at the puppy stage and no later than 6 months of age. If the dog is older, a heartworm test should be performed prior to the start of the prevention program. Different drugs are used as prevention. These include the monthly administered Heartgard, Interceptor, and Sentinel. We recommend to give the preventive drug on a year round basis for three reasons: mosquitoes are here very early in spring and stay with us till late fall, the drug protects your dog against intestinal parasites, and the Merial Company guarantees its product if your dog receives the drug monthly and has a heartworm test yearly. Cats should receive the monthly Heartgard or the monthly topical Revolution. We recommend the preventive program for inside cats also as mosquitoes always find their way into our houses. Heartworm is a very serious disease and it is easy to prevent it though very difficult and risky to treat.