Reviewed for accuracy on June 24, 2019, by Dr. Katie Grzyb, DVM
Heartworm disease is a serious and potentially fatal disease that can affect dogs if they are left unprotected. As a pet parent, you need to know the following information to help keep these parasites away:
What heartworms are
What causes heartworms in dogs
How to tell if your dog has heartworms
How to prevent heartworm disease in dogs
What Is Heartworm Disease in Dogs?
Dogs suffering from heartworm disease are infected with the organism Dirofilaria immitis, a nematode (roundworm) commonly referred to as the heartworm.
The severity of heartworm disease in dogs is directly dependent upon the number of worms present in the body, how long they’ve been there and the response of the dog’s body.
In regions where Dirofilaria immitis is endemic, dogs without prescription heartworm medicine are very likely to develop heartworm disease. The heartworm is mainly prevalent in geographic areas with tropical and subtropical climates.
It is commonly found along the Atlantic and Gulf Coasts and up through the Ohio and Mississippi river basins.
The presence of Dirofilaria immitis is not limited to these areas, however. Dogs have been diagnosed with heartworm disease in all 50 states.
What Causes Heartworms in Dogs?
Heartworms are spread through the bites of mosquitos that carry the infective heartworm larvae. These larvae then migrate through the dog’s body until they reach the heart and blood vessels within the lungs, a process that takes approximately six months.
The larvae continue to mature in the dog’s heart and lungs—an adult heartworm can grow to be about 12 inches long. These adults reproduce and release immature heartworms, known as microfilariae, into the dog’s bloodstream.
When a mosquito bites an infected dog, the microfilariae can enter the mosquito’s body, mature, and then be passed on to another dog, thereby continuing the heartworm’s life cycle and spreading the disease to the next host.
Which Dogs Are Most at Risk for Heartworm Disease?
Risk factors associated with heartworm disease in dogs include:
Living in endemic regions
Exposure to mosquitos
Lack of proper preventative heartworm medication
Most dogs in the United States have the first two risk factors, making heartworm preventatives the only way to mitigate your dog’s risk. Preventative heartworm medications should be given to all dogs as directed by your veterinarian.
What Are the Signs and Symptoms of Heartworms in Dogs?
Common signs and symptoms of heartworm disease in dogs include a cough, exercise intolerance and poor body condition, but symptoms can vary depending on the severity of the infection.
Classes of Heartworm Disease
Heartworm disease is divided into four classes that increase in severity. Here are the symptoms for each.
Dogs with Class I heartworm disease are often asymptomatic, meaning they exhibit no visible symptoms, or may only exhibit minimal signs, such as an occasional cough.
Signs of heartworms in Class II dogs typically include coughing and intolerance to a moderate level of exercise.
Class III heartworm symptoms include a generalized loss of body condition (weight loss, greasy or dry hair, loss of muscle), more extreme exercise intolerance, labored breathing and a potbellied appearance associated with fluid accumulation in the abdomen as a result of right-sided heart failure.
Dogs with Class IV heartworm disease have a condition known as caval syndrome, which is caused by the presence of so many heartworms that they block the flow of blood into the heart. Treatment for dogs with Class IV heartworm disease is aimed at comfort, as the disease has progressed too far to treat.
How to Tell If Your Dog Has Heartworms
A veterinarian can run a quick blood test to screen a dog for heartworms. These tests are routinely run both on dogs who are suspected of having heartworm disease and to monitor dogs who are on preventative heartworm medications.
A positive screening test should be confirmed with another type of test before a definitive diagnosis is made. The second test is used to confirm that the first antigen test was truly positive and to rule out microfilariae in the bloodstream.
Additional tests that are routinely run on dogs with heartworm disease include a blood chemistry panel, a complete blood cell count, a urinalysis and chest X-rays. These, and possibly other tests, are necessary to plan an appropriate treatment of heartworms in dogs and to determine a dog’s prognosis.
What’s the Prognosis for Dogs With Heartworm Disease?
For dogs who do contract heartworm disease, the prognosis is good for mild to moderate cases with appropriate and timely heartworm treatment. Dogs with more severe cases may suffer from serious short- and long-term complications associated with the disease and its treatment.
The American Heartworm Society has established the gold standard for the treatment of heartworm disease in dogs. Their treatment recommendations are created for EVERY heartworm-positive dog, whether Class I or Class IV.
Further treatment may be required for the higher stages—this includes anti-inflammatory medications, hospitalization with oxygen therapy, and in some cases, surgical intervention where the worms are removed from the heart.
Treatment is extended and usually takes about three months to complete. It isn’t until nine months after starting treatment that the veterinarian can confirm that the dog is heartworm-negative via antigen testing.
Thus, strict exercise restriction is required during the entire nine-month treatment process.
How Are Heartworms in Dogs Treated?
Dogs with heartworm disease will initially receive any treatments needed to stabilize their condition. They will then be given medication to kill circulating microfilariae, and most will undergo a series of three injections over a month’s time to kill adult heartworms in the heart and lungs.
Hospitalization is necessary when these injections are given, and possibly at other times, so that your veterinarian can watch closely for side effects. Prescription pet medications like prednisone and doxycycline are also typically prescribed to reduce the chances that the dog will react badly to the death of the heartworms.
Pain medication and anti-nausea medication are often used as well because the injections can cause significant discomfort and stomach upset.
Other treatments may be needed based on an individual dog’s condition. Without treatment, most cases of heartworm disease in dogs are eventually fatal.
Surgical Removal of Heartworms for Class IV
If a dog has caval syndrome, a surgical procedure will be necessary to remove adult heartworms from the right heart and pulmonary artery by way of the jugular vein. However, surgery is a real risk in these dogs due to their compromised heart and lung function. Most dogs with caval syndrome die regardless of treatment.
The procedure involves general anesthesia. A surgical instrument is placed into the jugular vein to remove the adult worms from the heart itself. It is effective, but it does not remove the adult worms from the pulmonary arteries.
Thus, it is necessary to perform the injection protocol that is recommended for all heartworm-positive dogs after completing the surgery to ensure that all of the worms are killed.
How to Manage a Dog With Heartworms
Restricting exercise before, during and after treatment of heartworms in dogs is absolutely vital to treatment’s success. Severely affected dogs may need to be kept in a crate to limit activity.
A test for the presence of adult heartworms should be done approximately six months after treatment is complete to check for a continued presence of Dirofilaria immitis. If the test is positive, the treatment can be repeated.
Dogs who have been treated for heartworm disease also need to receive preventive medications since they can be reinfected.
How to Prevent Heartworm Disease in Dogs
Heartworm disease in dogs is preventable with monthly heartworm medicine as prescribed by your veterinarian. Your dog must be tested for heartworms before preventative heartworm medication can be prescribed, especially if a dose was ever skipped or given late.
There are a number of heartworm preventatives that are safe, highly effective and commonly used. All products labeled to kill heartworms are only available by prescription, so you will need to talk with your veterinarian to find the best one for you.
Heartworm preventatives are not 100 percent effective, particularly if they are not used per label instructions or if doses are missed. Therefore, routine heartworm screening is recommended so that the disease can be caught early, when treatment is safest and most effective.
Treatment of heartworms in dogs is expensive and always carries some risk to the dog. It is certainly better to prevent the disease than to deal with its consequences. It costs about the same to prevent heartworms for a dog’s entire life as it does to treat the disease just once.
Featured Image: iStock.com/miodrag ignjatovic