Heartworm Disease: Prevention Is the Only Real Cure

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm transmitted exclusively by mosquito bites, that matures into foot-long adults living in the pulmonary arteries and right side of the heart. A single infected mosquito can transmit larvae that take roughly six months to develop into reproductively mature adults, by which point significant vascular damage may already be underway. Unlike most parasites, heartworm has no simple over-the-counter cure once adult worms are established.

The Six-Month Journey From Bite to Adult Worm

When a mosquito carrying infective L3 larvae bites a dog, the larvae migrate through subcutaneous tissue for several weeks before entering the bloodstream and travelling to the pulmonary arteries. There they mature into adults over four to six months, mate, and begin producing microfilariae that circulate in the blood, ready to infect the next mosquito that feeds.

Adult worms cause damage primarily by provoking inflammation and endothelial proliferation within the pulmonary arteries, restricting blood flow and eventually straining the right ventricle, a process that can progress silently for one to three years before signs appear.

Why Symptoms Lag Behind Infection

• Persistent, soft cough that worsens with exercise
• Reduced stamina and reluctance to exercise as before
• Weight loss despite a normal or increased appetite
• Abdominal swelling from fluid accumulation in advanced disease
• Fainting episodes or sudden collapse in caval syndrome cases

Dogs with a light worm burden can appear entirely healthy for a long time, which is exactly why annual testing matters even in dogs on preventives, since missed doses or resistant strains can allow infection to establish undetected.

Testing Before You Treat

Diagnosis relies on an antigen test detecting proteins from adult female worms, typically combined with a microfilaria concentration test to assess whether circulating larvae are present, since some dogs on partial prevention carry adults without microfilaraemia. Chest radiographs and echocardiography stage the severity of cardiopulmonary damage and help plan safe exercise restriction during treatment.

Dogs testing positive should also be screened for concurrent tick-borne disease and organ function, because the adulticide treatment protocol places additional strain on the liver and kidneys.

The Arsenic-Based Treatment Protocol

Treating established heartworm disease is a multi-month process designed to kill worms gradually and minimise the risk of fatal embolism from dying worms lodging in the lungs.

• Melarsomine injections given in a staged three-dose protocol
• Strict cage rest for weeks before, during and after treatment
• Doxycycline to target Wolbachia bacteria that heartworms depend on
• Ongoing monthly macrocyclic lactone prevention throughout the treatment course

Monthly Prevention Is Non-Negotiable

Strict exercise restriction is arguably the single most important part of treatment, since physical exertion increases pulmonary blood flow and the risk that dying worm fragments cause fatal thromboembolism, meaning even a normally active dog needs crate rest for a month or more. Most dogs recover well with a full staged protocol, though dogs with heavy worm burdens or pre-existing heart disease carry a higher risk of complications during treatment.

Because reinfection is entirely possible after successful treatment, dogs must resume monthly heartworm prevention as part of the same protocol rather than treating the disease as a one-off event.

When to seek help urgently

• Sudden collapse, weakness or dark urine (suggestive of caval syndrome)
• Laboured breathing at rest following any recent heartworm treatment
• Coughing up blood-tinged fluid or froth
• Marked abdominal distension with lethargy

Heartworm disease is preventable with a monthly product but genuinely dangerous to treat once established, so consistent, year-round prevention discussed with your vet is far safer than relying on treatment after the fact.