Heartworm Disease

Because heartworms cause damage slowly, signs often don't appear until the disease is moderately or severely advanced. This is part of what makes prevention so critical — by the time you notice something is wrong, there can already be significant damage.

In dogs:

• Mild persistent cough, especially after exercise
• Exercise intolerance — tiring quickly or refusing to exercise
• Rapid or labored breathing
• Weight loss despite a normal or increased appetite
• Bulging chest or pot-bellied appearance (fluid accumulation in advanced cases)
• Collapse or sudden death in severe or "caval syndrome" cases, where a large mass of worms obstructs blood flow through the heart

In cats (Heartworm-Associated Respiratory Disease / HARD):

• Intermittent coughing or wheezing, sometimes mistaken for asthma
• Sudden, acute respiratory distress
• Vomiting unrelated to eating
• Neurological signs (rare), lethargy, or sudden death
• Cats may show no signs at all until a sudden crisis occurs

Any dog that lives in or has visited a heartworm-endemic area (which now includes California) and hasn't been tested recently should be screened, regardless of signs.

Heartworm is transmitted exclusively by mosquitoes — it cannot be passed directly from pet to pet. Any dog or cat exposed to mosquitoes is at risk, and there is no time of year in Southern California when mosquito exposure is zero.

• Geographic risk: heartworm is present throughout the continental US, including California. The Los Angeles basin has documented cases, and risk is elevated in areas near standing water, riverbeds, and wetlands
• Outdoor lifestyle: dogs that spend time outdoors, especially at dawn and dusk when mosquitoes are most active, face the highest exposure
• Indoor cats are not immune: mosquitoes readily enter homes; heartworm in cats is frequently diagnosed in animals that are described as entirely indoor
• Lapses in prevention: missing even one or two monthly preventive doses can open a window during which infection can occur and establish itself before the next dose
• Travel: pets that travel to high-prevalence regions (Gulf Coast, Southeast US) carry that exposure back with them

Dogs and cats are tested differently, and understanding the distinction helps set expectations for what testing can and cannot detect.

• Antigen test (dogs): a blood test that detects proteins produced by adult female heartworms; this is the standard annual screening test for dogs and can be run in-house in minutes. It becomes positive approximately six to seven months after infection.
• Microfilaria test (dogs): a blood smear or modified Knott's test detects the microscopic larvae (microfilaria) that adult female worms release into the bloodstream; useful for assessing the burden and for distinguishing true infection from occult (antigen-positive only) disease
• Antibody test (cats): cats mount an immune response to heartworm larvae, so antibody testing detects exposure earlier than an antigen test; antigen testing in cats should be paired with antibody testing because adult worms in cats may all be male (and thus antigen-negative)
• Thoracic radiographs and echocardiography: for confirmed positive animals, chest X-rays and ultrasound (echocardiogram) reveal the degree of lung and heart disease and guide treatment planning. Our in-house imaging makes this a same-visit process.

Prevention is the treatment of choice — full stop. If heartworm disease is diagnosed, the treatment approach depends on species, disease severity, and the pet's overall health.

Prevention (dogs and cats):

• Monthly oral or topical macrocyclic lactones (ivermectin, milbemycin, selamectin) kill heartworm larvae before they can develop. These are safe, affordable, and highly effective when given consistently.
• Injectable moxidectin (ProHeart) provides six or twelve months of protection from a single injection — ideal for pets whose owners struggle with monthly compliance

Treatment of confirmed heartworm disease in dogs:

• Strict exercise restriction: physical activity causes worms to lodge more deeply in pulmonary vessels; affected dogs must be kept calm and confined for the full treatment period
• Doxycycline: antibiotic treatment targets Wolbachia, a symbiotic bacterium inside heartworms that contributes to inflammation and worm survival
• Melarsomine (Immiticide) injections: an arsenic-based drug injected into the deep back muscles to kill adult worms; a multi-injection protocol over weeks is the standard
• Prednisolone: a corticosteroid to manage pulmonary inflammation as worms die
• Monthly preventives during and after treatment: to prevent new infection and kill circulating larvae

Heartworm disease in cats:

• There is no FDA-approved adulticide treatment for cats. Management focuses on controlling inflammation and supporting the cat through respiratory crises with corticosteroids and supplemental oxygen. Some cats survive to natural worm death (one to three years); others do not. This is a compelling reason to prevent rather than treat.

If your dog hasn't had a heartworm test in the past year, or your pet isn't on consistent prevention, schedule an appointment now. Testing is fast and in-house, and starting prevention is straightforward. If your dog or cat is showing respiratory signs — cough, labored breathing, exercise intolerance — and heartworm hasn't been ruled out, bring them in for evaluation. Early detection dramatically improves outcomes compared to late-stage disease. Book an appointment and let us protect your pet year-round.