CONDITION
Heartworm Disease in Dogs
Heartworm disease is an infection caused by a parasitic worm (Dirofilaria immitis) that, once mature, lives in the large blood vessels of the lungs and, in advanced cases, the heart itself. The worms are transmitted by mosquitoes, and it can take many months after a bite for the parasite to develop into its adult form. Heartworm is not endemic in the UK, where cases are seen in dogs that have travelled or been imported, but it is widespread in parts of southern and eastern Europe and other warmer regions. Many dogs with heartworm show no outward signs for a long time. When signs do appear, they often relate to the heart and lungs: coughing, tiring more easily on walks, or changes in breathing. In some cases, the first sign is an unexpected finding during routine screening or investigation for something else. The severity of signs tends to reflect how many worms are present and how long the infection has been established. This page explores what heartworm infection can look like in day-to-day life, what happens inside the body as the parasite matures, how the condition is identified through testing and imaging, and the range of approaches used to manage it. It also describes prevention strategies, which are particularly relevant for dogs travelling to or living in regions where heartworm is endemic.
Why this matters now
Heartworm disease develops slowly over several months following infection. Dogs may be infected for six months or longer before adult worms mature and begin producing signs. This delayed presentation means that by the time clinical signs appear, significant cardiovascular damage may already be present. The slow course can be interrupted by sudden deterioration, such as caval syndrome or clots in the lung arteries when worms die.
Untreated heartworm disease tends to worsen progressively as the worm burden increases and cardiovascular damage accumulates. Early stages may show minimal signs, while advanced disease can lead to heart failure and other serious complications.
Signals & patterns
Early signals
Mild persistent cough
A soft, dry cough that may initially be attributed to other causes can be an early indicator of heartworm-related lung changes.
Reduced stamina during exercise
Dogs may tire more quickly than expected or seem reluctant to engage in previously enjoyed activities.
Occasional breathlessness after activity
Recovery from exertion may take longer than it used to, with the dog appearing winded.
Later signals
Persistent cough with exercise intolerance
Coughing becomes more frequent and may be accompanied by obvious difficulty with normal physical activity.
Abdominal swelling
Fluid accumulation in the abdomen can occur as the heart struggles to manage blood flow.
Collapse, or sudden weakness with dark urine
Heavy worm burdens can cause fainting or collapse after exercise or excitement. A separate, sudden pattern is caval syndrome, in which a mass of worms moves into the right side of the heart and obstructs blood flow: severe lethargy and weakness come on abruptly, with laboured breathing, pale gums and dark red-brown urine from the breakdown of red blood cells. The American Heartworm Society guidelines state that this course usually ends fatally within 2 days unless the worms are removed surgically. The cause of a collapse cannot be judged at home.
Sudden laboured breathing or coughing up blood
During long-standing infection, the natural death of many worms can cause clots in the lung arteries, with a sudden onset of severe breathing difficulty and coughing up blood. ESCCAP describes this as life-threatening.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis involves confirming the presence of heartworms and assessing the extent of cardiovascular damage. Testing approaches can detect different life stages of the parasite.
Antigen test
Microfilaria test
Chest radiographs
Echocardiography
Options & trade-offs
Management typically involves elimination of the parasites, though the approach and timing may vary based on disease severity and individual factors.
Adulticide treatment
Injectable medication administered to kill adult worms over a series of treatments.
Trade-offs: Dead worms lodge in the lung arteries and cause clots (pulmonary thromboembolism), which the American Heartworm Society describes as an inevitable consequence of successful treatment. Where signs develop, they are a low-grade fever, cough, coughing up blood or worsening right-sided heart failure, usually within 7 to 10 days and occasionally as late as 4 weeks after the injections. A high level of activity during treatment and for 6 to 8 weeks after the last injection is one of the most significant factors in these complications, so strict exercise restriction continues throughout that period.
Slow-kill approaches
Ongoing preventive medication may gradually reduce worm burden over extended periods.
Trade-offs: Preventive medication alone has been shown to take more than 2 years of continuous use to eliminate 95% of adult worms, and damage to the lung arteries continues throughout. The American Heartworm Society does not recommend this approach.
Surgical removal
Physical extraction of worms may be considered in severe cases with large worm burdens.
Trade-offs: This is the approach described for caval syndrome, in which worms have moved into the right side of the heart and are removed through the jugular vein. Once the dog has recovered from surgery, the standard medical treatment course is started, with the adulticide injections given later in that course, to clear the remaining worms.
Supportive care
Managing signs and stabilising cardiac function before and during treatment.
Trade-offs: May be necessary to reduce treatment risks in severely affected dogs.
Common misconceptions
"Indoor dogs cannot get heartworm"
In regions where heartworm is endemic, mosquitoes can enter homes, and even limited outdoor exposure provides opportunity for transmission.
"Prevention only matters for dogs that live in warm countries"
Heartworm is not endemic in the UK, so the risk for UK dogs comes from travel to, or import from, endemic regions. ESCCAP guidance for dogs travelling to an endemic area is preventive treatment starting within 30 days of arrival, with the last dose within one month of leaving. For stays of no more than one month, a single treatment soon after returning home is described as sufficient.
"Treatment is simple and quick"
Treatment requires multiple stages, strict rest periods, and careful monitoring over several months.
Related conditions
Angiostrongylus (Lungworm)
Angiostrongylus (lungworm) can produce respiratory and cardiac signs that overlap with those of heartworm, including coughing and exercise intolerance, though the parasites differ in life cycle and the patterns of lung damage they cause. Both conditions may be considered when a dog presents with unexplained respiratory signs and a history of exposure to parasites.
Pulmonary Hypertension
Heartworm infection can lead to pulmonary hypertension as adult worms obstruct and damage the blood vessels in the lungs, increasing the resistance to blood flow and forcing the right side of the heart to work harder. In some cases, pulmonary hypertension may be one of the first measurable changes detected during investigation of heartworm disease.
Congestive Heart Failure in Dogs
In advanced heartworm disease, the burden of parasites and the resulting damage to the heart and lungs can eventually lead to congestive heart failure, with fluid accumulating mainly in the abdomen as the right side of the heart loses its ability to pump efficiently. The two conditions share many overlapping signs, including coughing, breathlessness, and reduced stamina.
Glomerulonephritis
Chronic heartworm infection can trigger immune responses that may, in some cases, lead to glomerulonephritis, as immune complexes formed in response to the parasite are deposited in the kidneys' filtering units. This relationship means that dogs with long-standing heartworm disease may develop protein loss in the urine and progressive kidney changes.
Leishmaniasis
Leishmaniasis and heartworm disease are both vector-borne infections more commonly encountered in dogs with travel histories to southern Europe and other warm regions, and both can cause chronic, multi-system illness that may take months to become apparent. Awareness of one condition often prompts consideration of the other in dogs with appropriate exposure history.
Understanding a dog's risk factors, including travel history, country of origin and lifestyle, provides context for discussions about prevention strategies. For dogs already diagnosed, maintaining detailed observations during treatment helps monitor recovery progress.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS