Heartworm Disease

TL;DR. Heartworm disease is a common, potentially fatal mosquito-borne parasitic infection that damages the heart and lungs of dogs and cats, requiring prompt veterinary intervention but remaining highly preventable.

Adult heartworms primarily reside in the pulmonary arteries, restricting blood flow and damaging vascular tissue.

What is it?

Heartworm disease is a serious and potentially fatal condition caused by a parasitic worm called Dirofilaria immitis. These parasites are transmitted through the bite of an infected mosquito. Once inside the host, the larvae migrate through body tissues and eventually settle in the pulmonary arteries (the blood vessels carrying blood from the heart to the lungs) and the right side of the heart. As these worms grow up to a foot in length, they cause severe physical obstruction and trigger intense inflammation.

This inflammatory response leads to reactive vascular lesions, which scar and narrow the blood vessels. Over time, the restricted blood flow causes pulmonary hypertension (abnormally high blood pressure in the lungs). To pump blood through these narrowed, damaged vessels, the right side of the heart must work much harder. This chronic strain can ultimately lead to right-sided congestive heart failure. In severe cases with high worm burdens, the parasites can migrate upstream, blocking the tricuspid valve and the vena cava, resulting in a life-threatening emergency known as caval syndrome.

The disease can also affect other organ systems. For example, the immune system's response to the parasite can cause systemic complications, as noted in a leading veterinary internal medicine reference:

"Circulating immune complexes or possibly microfilarial antigens result in glomerulonephritis. Renal amyloidosis has been associated rarely with HWD in dogs. Although the caudal pulmonary arteries are the preferred site, worm migration upstream into the right heart and even to vena cavae is associated with heavy worm burdens. A massive number of worms can cause arteries, tricuspid valve region, or [vena cavae] blockages..." [2]

While dogs are the natural hosts for heartworms, cats can also become infected. However, the disease behaves differently in feline patients:

"Cats that develop mature HW infection generally have fewer adult worms than do infected dogs. HWs mature more slowly, fewer numbers of infective larvae mature to adults, and the adult life span is shorter in cats. However, live worms can persist for 2 to 4 years." [3]

In cats, even immature worms can cause significant damage, a condition known as Heartworm-Associated Respiratory Disease (HARD) or pulmonary larval dirofilariasis.

Causes & risk factors

The sole cause of heartworm disease is the transmission of Dirofilaria immitis larvae by mosquitoes. When a mosquito bites an infected animal, it ingests microscopic baby worms called microfilariae. These larvae mature inside the mosquito over a period of weeks, depending on environmental temperatures. When the mosquito bites another dog or cat, it deposits the infective larvae into the skin, where they begin their migration.

Any dog or cat living in or traveling to areas where mosquitoes are active is at risk. The disease is common and highly prevalent in enzootic regions, particularly warm, humid climates where mosquito populations thrive.

While there are no specific breed predispositions for heartworm disease, lifestyle and sex can influence risk. For instance, male dogs are statistically more prone to developing the most severe form of the disease:

"Factors other than worm burden alone are probably also involved in the development of the caval syndrome, including degree of pulmonary hypertension. Caval syndrome occurs more often in geographic areas where HWD is enzootic; up to 20% of dogs with HWD are estimated to be affected in some areas. Most dogs that develop caval syndrome are males. Often no history of HW-related signs exists." [1]

Additionally, researchers have noted that genetic variations in the parasites themselves may play a role in how the disease spreads, with some evidence pointing toward "genetic polymorphism leading to true parasite resistance" to common preventatives in select populations [3].

Signs to watch for

The clinical signs of heartworm disease vary widely depending on the number of worms present (worm burden), the duration of the infection, and the host's activity level. Active dogs often show more pronounced signs early on.

Common signs

• Cough: A persistent, dry cough is often the first noticeable sign, caused by inflammation in the lung tissue surrounding the arteries.
• Dyspnea: Difficulty breathing or labored breathing, especially after mild exertion.
• Exercise intolerance: Becoming easily fatigued or refusing to exercise.
• Tachypnea: Rapid breathing even while resting.

Occasional signs

• Ascites: Fluid accumulation in the abdomen, leading to a swollen or potbellied appearance, caused by right-sided heart failure.
• Jugular vein distention and pulsations: Visible swelling or pulsing in the large veins of the neck.
• Hemoglobinuria: Dark red or tea-colored urine, caused by the physical destruction of red blood cells as they pass through worm-congested valves.
• Acute collapse: Sudden loss of consciousness or inability to stand.
• Syncope: Fainting spells, often triggered by exercise or excitement.
• Cyanosis: Blue or purple discoloration of the gums and tongue, indicating a lack of oxygen.
• Hemoptysis: Coughing up blood.
• Bilirubinuria: Dark orange or yellow urine due to liver or red blood cell breakdown.
• Fever: Elevated body temperature from systemic inflammation.
• Pallor: Pale gums, indicating anemia or poor blood circulation.
• Anorexia and weight loss: Loss of appetite and progressive weight loss.
• Cachexia: Severe muscle wasting and physical decline.
• Weakness: General lethargy and lack of energy.

Rare signs

• Change in or loss of bark: Due to pressure on the nerves near the airways or larynx.
• Epistaxis: Nosebleeds.

Coughing, exercise intolerance, and labored breathing are common signs of progressive heartworm disease.

How vets diagnose it

Diagnosing heartworm disease requires a combination of blood tests, imaging, and physical evaluation. Because the signs can mimic other cardiopulmonary diseases, your vet will perform several targeted tests to confirm the diagnosis and assess the severity of the infection.

The Gold Standard: Adult Heartworm Antigen Test

The primary screening tool is the adult heartworm antigen test. This blood test detects specific proteins released by adult female heartworms. It is highly accurate, though it may return a false negative if the infection consists only of male worms or immature larvae.

Microfilaria Concentration Tests

If the antigen test is positive, or if infection is highly suspected, your vet will perform a concentration test to look for circulating baby worms (microfilariae) in the blood.

"Concentration tests are done using either a millipore filter or the modified Knott's centrifugation technique. Both techniques lyse the red blood cells and fix any existing microfilariae. The modified Knott test is preferred for measuring larval body size and differentiating D. immitis from nonpathogenic filarial larvae such as Acanthocheilonema (formerly Dipetalonema ) reconditum..." [5]

Diagnostic Imaging

• Thoracic Radiography (Chest X-rays): X-rays are vital for assessing the extent of damage to the lungs and heart. Your vet will look for enlarged, twisted pulmonary arteries and signs of inflammation or fluid in the surrounding lung tissue.
• Echocardiography (Heart Ultrasound): This imaging technique allows your vet to visualize the physical structure of the heart. In severe infections, adult worms can actually be seen as distinct "double-lined" structures within the right atrium, right ventricle, or pulmonary artery. Echocardiography is also critical for evaluating secondary complications:

"Secondary tricuspid regurgitation (TR) is common and its maximal velocity can be used to estimate the severity of the pulmonary hypertension... HWD should always be ruled out as an underlying cause in patients with pulmonary hypertension." [4]

• Electrocardiography (ECG): An ECG may be performed to monitor for cardiac arrhythmias (abnormal heart rhythms) caused by the physical presence of the worms and the resulting strain on the heart muscle.

Echocardiography allows veterinarians to visualize adult heartworms and assess heart function.

Treatment options

Treating heartworm disease is a complex, multi-step process that must be tailored to the patient's clinical class (ranging from Class 1, which is mild or asymptomatic, to Class 4, which is life-threatening caval syndrome). The primary goal is to safely eliminate the adult and immature worms while minimizing the risk of treatment-related complications, such as pulmonary thromboembolism (blood clots in the lungs caused by dying worms).

First-Line Therapies

• Melarsomine (Arsenical Antiparasitic / Heartworm Adulticide): This is the only approved medication that directly kills adult heartworms. It is administered via deep intramuscular injections into the lower back muscles. Because killing the worms too quickly can cause catastrophic blockages in the lungs, this treatment is typically given in a split-dose protocol spread over several weeks.
• Glucocorticoids (Corticosteroids): Your vet will prescribe a course of steroids before and during the adulticide injections. These medications help control the severe inflammatory response in the lungs as the adult worms die and break apart.
• Doxycycline (Tetracycline Antibiotic): Heartworms harbor a symbiotic bacterium called Wolbachia. Doxycycline is administered to eliminate these bacteria, which weakens the adult worms, halts their reproduction, and significantly reduces the inflammatory reaction in the lungs when the worms are killed.

Second-Line and Supportive Therapies

• Sildenafil (Vasodilator - PDE5 Inhibitor): If your dog has developed severe secondary pulmonary hypertension, your vet may prescribe sildenafil. This medication helps dilate the blood vessels in the lungs, reducing the workload on the right side of the heart and improving oxygen levels.
• Strict Activity Restriction: This is perhaps the most critical component of treatment. As the adulticide kills the worms, they die, fragment, and are carried by blood flow into the lungs, where they are gradually absorbed by the body. Any increase in heart rate or blood pressure from exercise can cause these fragments to clump, leading to fatal pulmonary thromboembolism. Your pet must be kept on strict crate rest and leash-walked only for elimination throughout the entire treatment process.

Surgical Intervention

For patients presenting with caval syndrome, medical therapy alone is insufficient and highly dangerous.

"A massive number of worms can cause arteries, tricuspid valve region, or [vena cavae] blockages..." [2]

In these critical cases, your vet or a veterinary surgeon must perform emergency surgical extraction, physically pulling the adult worms out through the jugular vein using specialized instruments.

Prognosis

The prognosis for pets with heartworm disease is generally favorable to guarded, depending heavily on the clinical class of the disease at the time of diagnosis:

• Class 1 & 2 (Mild to Moderate): Pets with mild signs (such as an occasional cough) or no signs at all have a highly favorable prognosis. Their pulmonary arterial disease typically improves considerably after successful adulticide therapy, and most return to a normal quality of life.
• Class 3 (Severe): Pets with advanced disease, showing severe exercise intolerance, weight loss, and significant lung changes, have a guarded prognosis. While treatment can still be successful, some degree of permanent lung or heart damage may persist.
• Class 4 (Caval Syndrome): The prognosis for untreated caval syndrome is extremely poor, with death typically occurring within 24 to 72 hours. Even with emergency surgical intervention, the prognosis remains guarded, though successful worm extraction can be curative if performed in time.

In cats, the prognosis is always guarded. Because there is no safe, approved adulticide medication for feline patients, treatment focuses on managing symptoms with supportive care (such as corticosteroids) until the worms complete their natural lifespan of 2 to 4 years.

Prevention

Heartworm disease is almost entirely preventable. Because treating an active infection is costly, physically demanding, and carries inherent risks, year-round prevention is the standard of care for both dogs and cats.

• Preventative Medications: Preventatives belong to the macrocyclic lactone class of drugs and are available as monthly oral chews, monthly topical spot-on treatments, or long-acting injections. These medications work by killing the immature larvae deposited by mosquitoes before they can migrate to the heart and lungs.
• Routine Screening: Dogs should be tested annually for heartworm antigens and microfilariae, even if they are on year-round prevention. This ensures that any lapse in administration or rare product failure is caught early before significant damage occurs.
• Mosquito Control: Minimizing your pet's exposure to mosquitoes by keeping them indoors during peak mosquito hours (dawn and dusk) and using pet-safe mosquito repellents can provide an extra layer of protection, though it should never replace medical preventatives.

When to call your vet

If your pet is not on a regular heartworm preventative, or if you have missed doses, you should schedule a screening appointment.

You must seek emergency veterinary care immediately if your pet exhibits any of the following red-flag signs of advanced heartworm disease or caval syndrome:

• Sudden collapse or extreme weakness
• Severe, labored breathing or gasping for air
• Pale, blue, or purple gums
• Coughing up blood (hemoptysis)
• Dark red, brown, or tea-colored urine (hemoglobinuria)

These symptoms indicate a critical cardiovascular crisis that requires immediate medical or surgical intervention.

Sources

• Small Animal Internal Medicine, pages 207, 208, 209, 216, 218.

My highlights & notes

New note