Heartworm Disease in Dogs

Heartworm disease is an infestation of a parasitic worm (Dirofilaria immitus) that initially lodges in the lungs but with severe enough infections can back-up through the heart, where it gets it's name. This parasite is transmitted by multiple species of mosquitoes in a larval stage and is a prevalent disease found throughout the world. Testing for this disease can be performed with a simple blood test. If a dog tests positive for heartworm disease and is not treated, the disease can be fatal. Heartworm disease is a preventable disease.

Heartworm infection in dogs has been diagnosed around the world and has been found in all 50 states. Because the disease is transmitted by mosquitoes, changes to the climate and environment allowing more mosquitoes to thrive has increased the transmission of this disease. While the transmission of heartworm disease does decrease in winter months in northern climates, it never reaches zero due to the ability of mosquitoes to over-winter in our houses and buildings. The quantity of pet travel, especially from highly endemic areas has also increased the spread of the disease.

A mosquito transmits the disease by first taking a blood meal from a host who has immature heartworms (microfilaria) circulating throughout it's bloodstream. The microfilaria will mature in the mosquito to larvae where they become infective, and then are transmitted to a new animal by the mosquito. The transfer of these larvae is immediate with a blood meal of a naïve animal. These larvae mature in the animal and travel to the pulmonary vasculature (blood vessels in the lungs). With progression of disease, these larvae mature to adults and can occupy larger blood vessels, entering the pulmonary artery and even the right side of the heart. The life cycle of the heartworm can be as long as 9 months, once mature they will reproduce many microfilaria making the animal infectious, perpetuating and spreading the disease if intervention does not occur.

Screening for heartworm disease is very important. There are many different testing options available; the one most often used and most sensitive for screening is an antigen test to evaluate for any adult heartworms. These tests detect mature adult female heartworms, and the earliest this test can be performed is 5 months following infection. Tests are also available to test for immature heartworms (microfilaria) and can be performed as early as 6 months following infection. Annual testing should be an integral part of your pets annual examination, as early detection and early treatment carries a better prognosis. Every dog should be tested if they either; do not recieve monthly preventatives, if there are gaps in treatment (for example if you treat seasonally or if you treat intermittently), or in the case where you change the brand or type of heartworm preventative. If a dog is already on a preventative (especially preventatives in the macrocyclic lactone class), the most reliable method of testing is antigen testing, as preventative administration can interfere with testing for microfilaria.

If a dog tests positive for heartworm disease, confirmatory testing and staging of the severity of disease is recommended. It is important to remember that when developing a plan for pre-staging as well as treatment protocols that every patient and client are unique and every situation is different. Staging of severity may include any combination of the following tests:

  1. Thoracic radiography: This modality looks at major cardiac and pulmonary structures and evaluates for damage caused by heartworms themselves, which can include increased heart size or changes seen in the pulmonary parenchyma and pulmonary vasculature. Pulmonary vessels can appear enlarged, toruous, and often truncated. With severe disease, enlargement of the right heart can occur.
  2. Echocardiography: Adult heartworms can often be visualized on echocardiography (ultrasound of the heart). This diagnostic modality can provide definitive evidence of heartworm disease and can also evaluate the structure and function of the heart as well as indicate pulmonary blood pressures. In mild disease, heartworms are often limited to peripheral branches of the pulmonary arteries, making them impossible to detect with ultrasound. With severe disease or a high worm burden, heartworms are often present in the main pulmonary artery, interlobar branches or within the right side of the heart.
  3. Routine Complete Blood Cell count and Chemistry Profile: These pieces of bloodwork can detect elevations in organ enzymes as well as changes to red and white blood cells in the blood stream.

Patients affected by heartworm disease may be completely asymptomatic (free of any signs of infection) and the disease is frequently picked up on routine screening. Other patients with mild disease may have an occasional cough, increased respiratory rate or effort, deceased appetite, lethargy, fever, or other general findings associated with illness. Occasionally those who are more significantly affected will have what's referred to as “caval syndrome” in which blood flow is obstructed to the right side of the heart and can be evidenced by jugular venous distention. Other signs of severe infections are lethargy, difficulty breathing, pale mucous membranes, and weakness, coupled with hematologic changes including hemoglobimenia and hemoglobinura. In some cases, surgical extraction of the worms is necessary and should be considered as part of immediate care along with stabilization of the patient. Manual extraction of the heartworms is ideal to reduce the risk and improve the treatment process. This is done under sedation or anesthesia, and should only be performed by a skilled veterinarian or veterinary specialist with great experience.

The longer a patient has heartworm disease, the greater the potential for damage to the cardiovascular and pulmonary systems and the greater the risk to the patient, including death. Treatment for heartworm disease is multi-modal, meaning that there are multiple medications and treatment strategies involved.

During the course of treatment, there are a few things that can help to predict potential complications. These include things such as the activity level of the dog, how much pulmonary vascular disease/damage is present, and the severity of the infection. Exercise restriction during treatment is paramount to the success of treatment, and one of the single most important things that a pet owner can provide. High levels of energy during the course of treatment can cause dislodging of the worms and cause parasitic pulmonary thromboembolism. Exercise, over-exertion, excitement, and an increase in body temperature secondary to any of these things can cause significant, and potentially fatal, complications.

The key point is that canine heartworm disease is a preventable disease. Monthly oral or topical preventative medications are used on a year-round basis. There are many different types of heartworm preventatives. The major class of preventatives include macrocyclic lactones, including ivermectin, milbemycin, moxidectin, and selamectin). These preventatives work to kill microfilaria, larvae, and some (although not all) adults. For this reason, use of heartworm preventative administration as a treatment for heartworm disease alone is not always advised, as adults can continue to survive. Because the transmission of heartworm disease never reaches zero, year-round heartworm prevention is recommended.

There are some breeds of dogs that, due to a deficiency with P-glycoprotein, may have sensitivities to certain specific preventatives as well as other medications including anti-depressants, antibiotics, and cardiac medications. It is still critical to administer a preventative to these dogs. Even though some compounds may cause sensitivities in certain dog breeds, there are alternatives that have been show to be safe in all dog breeds.

It is ideal to start preventative as early as possible in puppies, and usually is begun when the puppy is at least 8 weeks of age. Due to the lag time for detection and the life cycle of the heartworm, it is reasonable to wait to test a new puppy until they are 7 months of age, but starting preventative early and consistently is the best way to keep our pets disease free and healthy.