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
Options & trade-offs
Last reviewed: Invalid Date ·