General Information
Heartworm disease is “endemic” (always present) on the Gulf Coast of Texas. The disease is caused by the heartworm Dirofilaria immitis. These “spaghetti size” worms live in the right chamber of the heart, in nearby arteries, and in the lungs. The presence of these large worms leads to dramatic changes in the heart and lungs producing coughing and gagging, especially exercise related, and loss of stamina as the most common symptoms.
Transmission
Heartworms are transmitted from dog to dog by mosquitoes. A mosquito will feed on a dog, which is infected with heartworms, ingesting large numbers of the baby heartworms (microfilaria). The larvae are then injected under the skin of another dog during a later feeding. The larvae migrate through the body for approximately 4 months before reaching the heart. Heartworms can grow to 10-14 inches in length when mature.
Diagnosis
Female heartworms usually produce a large number of microfilaria, which can be detected by examining a blood smear under the microscope. Twenty five percent of heartworm positive dogs have no microfilaria in circulation, and therefore appear to be heartworm free. If there is suspicion of heartworm disease, an occult blood test, which tests for the presence of adult heartworm antibodies, can be performed.
Prevention
There is a variety of once a month medications available to prevent heartworms. Both Heartgard (a chewable beef treat) and Interceptor (a chewable tablet) are labeled for the prevention of heartworms and intestinal parasites. Sentinel is a chewable tablet that is labeled to prevent heartworms, intestinal parasites and help reduce the flea population. There is also a topical preventative, Revolution, which is especially convenient for cats. Revolution is a liquid that is simply applied to the back of the neck once a month and is labeled to prevent heartworms, fleas, ticks, mange, ear mites and intestinal parasites in cats.
Treatment in Dogs
It is best to avoid the need for treatment by administering a preventative. Heartworm positive dogs require laboratory screening, chest x-rays, and a urinalysis to evaluate the treatment regimen and prognosis. If your dog is diagnosed as having heartworms, we hospitalize for 3 days for testing and treatment to kill the adult “spaghetti size” worms present in the heart. It is imperative that the dog must have strict rest for 6-8 weeks post heartworm treatment! It takes 6-8 weeks for the adult worms to dissolve and leave the heart and lungs. This is the period when complications to the heartworm treatment can arise. Strict rest and air conditioning are your best bet to help prevent post treatment complications. A second drug is given 1 month later to kill the microfilaria in the blood. After the microfilaria are killed, the dog is safe to resume once a month heartworm preventative.
Heartworm preventative must NEVER be given to any dog that has microfilaria since a severe reaction may occur. Thus, it is critically important that all dogs be tested prior to administering heartworm preventative or if they have missed their preventative for more than 2 months.
Heartworm Disease in Cats
Cats are susceptible to heartworms but they tend to have more natural resistance to them than do dogs and ferrets, therefore heartworms are much less prevalent in cats. However, if a cat does contract heartworms the consequences are severe and deadly. In fact, sudden death may be the first and only sign in some cats. Infestation can cause respiratory symptoms of coughing, wheezing and difficulty breathing. Since heartworm prevention is so easy now with the once-a-month drops or chewable tablets it is best to cover both indoor and outdoor cats with protection. Feline and ferret heartworms are difficult to diagnose and there is no treatment unlike dogs. Therefore, prevention is the only way to go. For ferrets, we generally give a once-a-month oral liquid since there is nothing on the market labeled for ferrets.
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