CONDITION
Heartworm Disease in Cats
Heartworm disease in cats occurs when parasitic worms transmitted by mosquitoes settle in the blood vessels of the lungs and, less commonly, the heart. Unlike in dogs, where large numbers of adult worms may develop, cats typically harbour only a few worms, and many infections resolve without the worms reaching maturity. The condition behaves differently in cats than in dogs, and even a small worm burden can trigger significant inflammatory reactions in the lungs. Owners often arrive at this topic because their cat has developed respiratory signs—coughing, difficulty breathing, or episodes that resemble asthma—or because heartworm has been mentioned as a possibility during investigations for these symptoms. In some cases, cats show vague signs such as reduced appetite, weight loss, or vomiting, or the condition is discovered incidentally during testing for other reasons. Sudden collapse or death can occur, though this is uncommon. This page explores what signs may be observed, what is happening in the lungs and heart when heartworm is present, how the condition is investigated, and what approaches exist for managing affected cats and reducing future risk.
Why this matters now
Heartworm disease can affect cats of any age, though signs often become apparent in young to middle-aged adults, typically between one and six years old. Indoor cats can be affected as well as outdoor cats, since mosquitoes may enter the home. Geographic risk varies with mosquito populations, and in the UK the condition is uncommon compared to endemic regions such as parts of southern Europe, the Mediterranean, and North America, though travel or import history may be relevant.
The timeline from infection to observable signs is measured in months. After a mosquito bite introduces larvae, these may migrate through tissues for several months before reaching the pulmonary arteries. Some infections resolve during this early phase without the cat showing signs. In cases where worms do reach the lungs, inflammatory changes may begin well before the worms mature, and signs can appear suddenly or develop gradually over weeks to months. The death of even a single worm can trigger acute worsening.
Signals & patterns
Early signals
Intermittent coughing or wheezing
The cat may cough occasionally, or produce sounds during breathing that resemble wheezing or asthma. These episodes can be sporadic and may initially be mistaken for hairballs or minor respiratory irritation.
Increased respiratory effort
Breathing may appear slightly more laboured at rest or after mild activity, with the chest or abdomen moving more visibly than usual. This change can be subtle and may come and go over days or weeks.
Reduced activity or lethargy
The cat may spend more time resting, show less interest in play, or tire more quickly than before. This shift in behaviour is often gradual and may be attributed to other causes initially.
Occasional vomiting
Vomiting unrelated to eating can occur, sometimes without other obvious digestive signs. The vomiting may be sporadic and is not always associated with food.
Decreased appetite or weight loss
The cat may eat less consistently or show less enthusiasm for meals, and weight loss may become noticeable over weeks. These changes are nonspecific and can overlap with many other conditions.
Later signals
Persistent difficulty breathing
Respiratory distress may become more pronounced, with open-mouth breathing, rapid breathing, or obvious effort at rest. This represents a significant change from earlier intermittent signs.
Collapse or sudden weakness
In some cases, a cat may suddenly become weak, collapse, or be found unresponsive. This can occur without preceding signs and may follow the death of a worm within the pulmonary vessels.
Neurological changes
Rarely, signs such as disorientation, seizures, or difficulty walking may appear if larval worms migrate to locations outside the lungs. These signs are uncommon but can be severe.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a detailed history, including travel, time spent outdoors, and the pattern of respiratory signs. Physical examination may reveal abnormal lung sounds or, less commonly, heart murmurs, though many affected cats have unremarkable findings on auscultation. Because the signs often overlap with other respiratory conditions, particularly feline asthma, a combination of imaging, blood tests, and specific heartworm assays is usually needed to build a picture of what may be present.
Radiography (chest X-rays)
Heartworm antigen testing
Heartworm antibody testing
Echocardiography (ultrasound of the heart)
Complete blood count and biochemistry
Options & trade-offs
Management is tailored to the individual cat and depends on the severity of signs, the presence of identifiable worms, and the stage of infection. Because no licensed treatment exists in cats to kill adult heartworms safely, the focus is often on supporting the cat through the inflammatory response and managing respiratory signs while the worms complete their natural lifespan, which is typically two to three years (roughly half the 5-7 years typical in dogs). Different combinations suit different households, depending on the cat's temperament, the owner's capacity for monitoring, and the balance of risk and intervention the owner finds acceptable.
Supportive care and monitoring
This involves regular observation at home and periodic veterinary review to track respiratory signs, weight, and general condition. No specific treatment is given for the heartworm infection itself, and the approach relies on the cat's immune system managing the worm burden over time. Anti-inflammatory medication or bronchodilators may be used intermittently if respiratory signs worsen.
Trade-offs: This approach avoids the risks associated with medical intervention but requires acceptance that signs may fluctuate, and acute worsening can occur if a worm dies suddenly. It is less suited to cats with severe respiratory distress or those in whom close monitoring is difficult.
Corticosteroids to manage inflammation
Prednisolone or other corticosteroids may be given to reduce the inflammatory response in the lungs and pulmonary arteries. The dose and duration are adjusted based on the severity of signs and the cat's response. This approach does not eliminate the worms but can lessen the tissue damage and respiratory symptoms triggered by the immune reaction.
Trade-offs: Long-term corticosteroid use carries risks including increased thirst and urination, weight gain, and in some cases immune suppression or effects on other organ systems. Some cats tolerate oral medication poorly, and the anti-inflammatory effect may mask worsening disease in rare cases.
Bronchodilators and respiratory support
Medications such as terbutaline or inhaled bronchodilators may be used to open the airways and ease breathing, particularly in cats whose signs resemble asthma. Oxygen therapy may be provided during acute episodes. These measures address the symptoms of airway constriction and respiratory distress without targeting the underlying heartworm infection.
Trade-offs: Bronchodilators are most helpful when airway narrowing is a prominent feature, but they do not reduce inflammation or affect worm survival. Inhaled treatments require training and equipment, and not all cats tolerate the delivery method.
Preventive medication to reduce future risk
Monthly preventive treatments, typically containing selamectin, moxidectin, or other macrocyclic lactones, can be started to kill larval stages introduced by new mosquito bites, reducing the risk of additional infections. Some formulations may also gradually reduce the burden of immature worms already present, though they do not reliably eliminate adult worms.
Trade-offs: Preventives are generally well tolerated but require consistent monthly administration, which can be challenging in some cats. They do not address the current adult worm burden, and their benefit is most apparent in preventing further infection rather than resolving existing disease.
Surgical removal of worms
In rare cases where worms are visible on echocardiography and accessible within the heart or major vessels, surgical or interventional removal may be considered. This is a highly specialised procedure performed at referral centres and involves significant anaesthetic and procedural risk.
Trade-offs: Surgery can provide immediate relief if worms are causing obstruction or severe right-sided heart dysfunction, but the risks are substantial, and not all worms are accessible. The procedure is rarely performed in cats and is typically reserved for cases where other approaches have failed or are not feasible.
Common misconceptions
"Heartworm disease only affects dogs, so cats do not need preventive treatment."
Cats can be infected by the same mosquito-borne larvae that cause heartworm in dogs, though the disease behaves differently in the two species. Even indoor cats are at some risk, and in areas where heartworm is endemic, preventive treatment is often considered for both dogs and cats in the same household.
"A negative heartworm test means the cat definitely does not have heartworm."
Heartworm tests in cats have limitations. Antigen tests detect only mature female worms, and antibody tests indicate exposure rather than current active infection. A cat with a low worm burden, male-only worms, or immature stages may test negative on one or both assays despite having heartworm disease.
"If a cat has heartworm, treatment with the drugs used in dogs will cure the infection."
The adulticidal drugs used to kill heartworms in dogs are not considered safe in cats, because the sudden death of worms can trigger severe, life-threatening inflammatory reactions in the lungs. Management in cats typically focuses on controlling inflammation and supporting the cat while the worms die naturally over time.
Related conditions
Heartworm Disease in Dogs
Heartworm disease in dogs is caused by the same parasite and transmitted in the same way, but the infection behaves quite differently—dogs typically harbour larger numbers of worms that survive to adulthood, whereas cats often mount a stronger immune response that limits worm development and may clear the infection spontaneously.
Eosinophilic Bronchopneumopathy
Both conditions can cause respiratory signs driven by eosinophilic inflammation in the lungs, and distinguishing between them may require specific heartworm testing, as the clinical picture—coughing, difficulty breathing, and airway inflammation—can appear similar.
Pulmonary Thromboembolism
Heartworm disease in cats can trigger thromboembolism when worm fragments or inflammatory reactions lead to clot formation in the pulmonary arteries, and the sudden onset of severe respiratory distress or collapse seen in some heartworm cases may reflect this mechanism.
Pulmonary Hypertension
The inflammatory response to heartworm larvae and adult worms in the pulmonary arteries can contribute to increased pressure in the lung vessels over time, particularly in cats with persistent infection or those that have experienced repeated larval exposure.
Restrictive Cardiomyopathy in Cats
Cats with underlying heart disease, including restrictive cardiomyopathy, may be at higher risk of developing blood clots that can complicate heartworm infection, and both conditions can present with respiratory signs that may overlap during investigation.
Cats with respiratory signs often benefit from investigation into overlapping conditions such as feline asthma, which may coexist with or mimic heartworm disease. The broader context of metabolic health, including how the heart and lungs respond to chronic stress, can inform conversations about long-term monitoring and the balance of intervention. In households where heartworm is a consideration, the question of preventive medication and the circumstances in which it may be useful is often worth exploring over time.