CONDITION
Heartworm Disease in Dogs
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 symptoms. This delayed presentation means that by the time clinical signs appear, significant cardiovascular damage may already be present.
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 episodes
Severe infestations may cause episodes of weakness or collapse, particularly during activity.
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: The dying worms can cause inflammatory reactions; strict rest during treatment is typically required.
Slow-kill approaches
Ongoing preventive medication may gradually reduce worm burden over extended periods.
Trade-offs: Takes considerably longer and allows continued cardiovascular damage during the treatment period.
Surgical removal
Physical extraction of worms may be considered in severe cases with large worm burdens.
Trade-offs: Reserved for cases where other approaches carry unacceptable risk.
Supportive care
Managing symptoms 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"
Mosquitoes can enter homes, and even limited outdoor exposure provides opportunity for transmission.
"Prevention is unnecessary in cooler climates"
Transmission patterns can be unpredictable, and year-round prevention provides consistent protection.
"Treatment is simple and quick"
Treatment requires multiple stages, strict rest periods, and careful monitoring over several months.
Understanding your dog's risk factors, including geographic location and lifestyle, provides context for discussions about prevention strategies. For dogs already diagnosed, maintaining detailed observations during treatment helps monitor recovery progress.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS