Heartworm disease in dogs, an explanation, diagnosis and treatment
I can remember being devastated as a second grader to find out that our family dog had heartworms and was dying and there was nothing we could do. That was 1966. Thankfully, much has changed since then. Today, treatment for heartworms is available and better yet, it can be prevented. Heartworms are an internal parasite of both dogs and cats. These worms live in the bloodstream and cause life threatening disease. Heartworms are a leading cause of lung and heart disease in dogs and dogs are dying of this disease every day. Heartworm disease is caused by the parasite, Dirofilaria immitis. It is transmitted by mosquitoes and has been found in every state in the US. It only takes one mosquito bite for your dog to get heartworms and, without prevention; there is a 100% chance your dog will get it.
The way your dog becomes infected is the stuff of nightmares. You see, dogs are the reservoir of infection. This means that there are many dogs out there that are infected and when a mosquito bites an infected dog, it ingests immature heartworms known as microfilaria. Microfilaria is the larval form of heartworms that change within the mosquito to reach the third stage. It is the third stage that can infect your dog. When the infected mosquito bites your dog, it deposits third stage microfilaria on your dog’s skin and the microfilaria enters the dog through mosquito’s bite wound. The microfilaria then travel or migrate through the tissues (meaning skin, muscle, connective tissue) and mature to become a juvenile adult or fifth stage larvae. This process takes about 50-70 days in the dog. Sometime between 70 and 110 days after the dog is infected, the larvae reach the blood stream and end up in the arteries of the lung (pulmonary arteries). These juveniles then grow up to produce microfilaria of their own in about 6.5 months after the dog is infected.
Heartworms cause the most harm just by being present in the major arteries of the lung, the pulmonary arteries (pa). They wreak havoc in these arteries and the entire dog suffers. It happens on many levels and here is a list of the damages.
If you are one of the lucky ones, your dog can be diagnosed before any symptoms occur. Just in case you were wondering what the symptoms were, here is a list:
The Diagnosis of heartworm disease in dogs:
The>Before treatment is performed, we strongly recommend some basic work to assess the dog’s general condition, the severity of the infection, establish a prognosis, and to be sure there is no hidden condition which could result in unexpected complications to treatment. This includes a physical examination, a second heartworm test to confirm the diagnosis, chest x-rays, and a complete blood count and biochemical profile. Chest x-rays are important to assess the severity of changes in the lung and whether or not right sided heart disease exists. Dogs with mild to moderate disease have a very good prognosis. Dogs with severe disease are more likely to experience complications and represent higher risk patients. Pregnant dogs or dogs with kidney or liver failure or other serious underlying illness will not be treated.
Until recently, the treatment of heartworm disease was associated with a high rate of serious complications. Now we have a superior treatment available, Immiticide (melarsomine dihydrochloride). This drug was originally developed for use in higher risk patients, but because of it’s safety and efficacy, it has become the preferred treatment no matter what the severity of disease. The drug is administered by deep intramuscular injection. Most cases receive a staged treatment where the dog gets one injection, then 30 days later the dog gets two injections 24 hours apart. The dog is typically hospitalized for 2-4 days to allow assessment, treatment, and observation.
A word of caution. Any dog receiving even a single injection of Immiticide must be rested for at least 30 and preferably 45 days after the injection. That means dogs receiving staged treatment like mentioned above must be rested a minimum of 60 days. This is extremely important. It is absolutely essential that the dog be strictly confined for a minimum of 4 weeks after Immiticide administration to reduce the chance of thromboembolism (explained in this paragraph). The more severe the disease, the more strict the confinement. For most cases, they must be confined to the house or a small pen with no running, no playing, and no jumping. The dog can be leashed walked to do its business, but must immediately return to confinement. The purpose of this period of rest is to allow the lungs to heal. The treatment kills the adult heartworms, and it takes some time for body to clean up the mess that dead heartworms cause. The most severe complication to treatment is pulmonary thromboembolization when a chunk of heartworm breaks away and obstructs the blood flow to a portion of the lung (remember heartworms live in the arteries of the lung). Beginning at about 5 to 10 days post treatment, thromboembolism can occur any time for the next 4-6 weeks. Symptoms of thromboembolism include fever, panting, cough, and even collapse. It is most definitely life threatening. Thromboembolization is more likely if there is severe pulmonary arterial change; evidence of thromboembolism previous to treatment; severe infiltration of the lung; evidence of heart disease; or severe symptoms such as a heavy cough, respiratory distress, weakness, weight loss, or exercise intolerance. Treatment of pulmonary thromboembolism includes rest, oxygen, the careful administration of intravenous fluids, rest, and prednisolone. Symptoms, if they are not lethal, typically resolve within 2 weeks. The two main causes of pulmonary thromboembolism are lack of proper confinement or severe disease.
Dogs diagnosed with Vena Cava Syndrome are treated by surgical removal of the heartworms via the jugular vein on an emergency basis.
Once the dog has successfully completed treatment, heartworm prevention is started. Hopefully in your case, you can start your dog on heartworm prevention before infection occurs.