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Cherokee Animal Clinic
P O Box 416
665 Johnson Street
Rusk, TX 75785
Clinic Hours:
Monday - Friday 7:30 am - 5:30 pm

What Is Heartworm Disease?

Heartworm disease is one of the major health problems of dogs in the US and throughout the temperate and tropical areas of the World. This disease is caused by worms (dirofilaria immitis) that may grow to be 14" long adults that live in the right side of the heart and the arteries of the lungs. Heartworm infection can cause potentially serious damage to these arteries. This may eventually lead to heart failure and in servere cases, damage to other organs such as the liver and kidneys. In extreme cases, a dog can be infected with several hundred heartworms.

The risk of infection varies in different communities; however, heartworm disease is a threat to unprotected dogs in most areas of the 48 contiguous states and Hawaii. Heartworms are spread from animal to animal by mosquitos. Infected coyotes, foxes and wolves can also be sources of infection. Cats and ferrets are susceptible but to not contribute significantly to spreading the infection.

Heartworm Life Cycle

Adult female heartworms release their young, called microfilariae, into the dog's bloodstream. Mosquitoes become infected with microfilariae while taking a blood meal from an infected dog. During the next 10-14 days the microfilariae mature to the infective larval stage within the mosquito. When the mosquito bites another dog, cat, or other susceptible animal, the infective larvae enter through the bite wound. It then takes a little over 6 months for the infective larvae to mature into adult worms that may live for 5-7 years in the dog. Microfiliaria cannot mature into adult heartworms without first passing through a mosquito.

Signs of Heartworm Disease

Heartworms tend to accumulate gradually, sometimes over years, as a result of repeated bites from mosquitoes with infective heartworm larvae. Clinical signs of heartworm disease may not be recognized in the early stages. Recently infected dogs and those that are lightly infected generally exhibit no outward signs of the disease. More heavily infected dogs may eventually show clinical signs such as a mild, persistent cough, reluctance to move about or excercise, reduced appetite, and weight loss.

The worst cases may also develop heart failure, most often recognized by a "swollen belly" caused by accumulation of fluid in the abdomen. One particularly serious but relatively uncommon form of the disease is known as "caval syndrome" - a form of liver failure. This occurs in some heavily infected dogs. These dogs rapidly become very weak and their urine turns dark brown. This life-threatening situation requires prompt surgical removal of the worms.

Detecting Heartworm Infection

Detection of infection in apparently healthy dogs and confirmation in those that are sick is usually made with blood tests for microfilariae or a heartworm substance called an "antigen". Neither test is consistently positive until about 7 months after infection has occurred. Testing dogs less than 7 months of age is unnecessary.

Confirming evidence of heartworm infection may be obtained by examining x-ray and/or ultrasound images of the heart and lungs. These 2 tests are used primarily for evaluating dogs known to be infected.


Usually, all but the most advanced cases of heartworm disease can be successfully treated. A thorough physical examination and additional tests as needed prior to treatment allow assessment of the risk involved.

Adult heartworms are killed using a drug called an adulticide that is injected into the muscle. A series of injections are given. Treatment may be administered on an outpatient basis, but hospitalization is usually recommended. When the dog is sent home, exercise should be limited to a leash walking for the duration of the recovery period, when can last from one to two months. This decreases the risk of partial or complete blockage of blood flow through the lungs by dead worms.

Reinfection during treatment is prevented by administration of a heartworm preventative. These preventives may also eliminate microfilariae if they are present. Dogs in heart failure and those with caval syndrome require special attention.


There are a variety of options for preventing heartworm infection, including daily and monthly tablets and chewables, a monthly topical and an injectable administered by one of our doctors that provides protection for six months. All these methods are extremely effective and when administered properly on a timely schedule, heartworm infection can be completely prevented. As guardian, it is YOUR responsibility to faithfully maintain the prevention program we set up after consulation.

These medications interrupt heartworm development before adult worms reach the lungs and cause disease. Heartworm prevention is easy, safe, and inexpensive compared to treating a dog after the worms have matured into adults.

Call us for a program to keep your pet 'Worm Free'!!

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