Heartworm Disease in Dogs: A Complete Guide
Heartworm disease is one of the most serious parasitic infections a dog can acquire, capable of causing severe heart and lung damage and, if untreated, death. The causative parasite — Dirofilaria immitis — is not currently endemic in the United Kingdom, but this does not mean UK dogs are safe from the risk. Dogs travelling to continental Europe, and dogs imported into the UK from affected regions, are at meaningful risk of infection. Every dog owner planning to travel abroad with their pet should understand this disease.
The Parasite: Dirofilaria immitis
Dirofilaria immitis is a filarial nematode — a thread-like roundworm — that lives as an adult in the pulmonary arteries and right side of the heart of infected dogs. Adult worms can reach lengths of 25–30 cm and live for five to seven years inside the host. A single dog can harbour dozens of adult worms simultaneously, causing progressive damage to the heart, blood vessels, and lungs.
The disease is not transmitted directly from dog to dog. It requires an intermediate host — the mosquito — to complete its lifecycle.
Transmission: The Role of the Mosquito
When a mosquito feeds on the blood of an infected dog, it ingests microscopic larval worms called microfilariae, which circulate in the blood. Inside the mosquito, the microfilariae develop through larval stages over a period of around two weeks (depending on environmental temperature). When the mosquito then feeds on another dog, it deposits the infective larvae onto the skin, where they enter through the bite wound.
Once inside the new host, the larvae migrate through the body's tissues over a period of months, eventually reaching the heart and pulmonary arteries where they develop into adult worms. This migration period — from the time of infective bite to the presence of adult worms — takes approximately six months and is known as the prepatent period. During this time, the infection cannot be detected by standard blood tests.
Once adult worms are present and reproducing, the female worms release microfilariae into the bloodstream. These microfilariae can be detected in a blood sample, confirming active infection.
Is There a Risk in the UK?
Heartworm is not currently endemic in the UK. The combination of climate, mosquito species present, and the absence of a significant reservoir of infected dogs means that transmission in the UK itself is not occurring in meaningful numbers. However, there are two important scenarios in which UK dogs face genuine risk:
- Travel to endemic regions: Dogs travelling with their owners to Southern Europe — including Spain, Portugal, Italy, Greece, and France south of the Loire — are at risk during warmer months when mosquitoes are active. The Mediterranean basin has significant heartworm endemicity.
- Imported dogs: Dogs brought to the UK from endemic regions, whether as rescue dogs or purchased animals, may already be infected. Testing at the time of importation is strongly advisable.
Clinical Signs of Heartworm Disease
The clinical signs of heartworm infection depend on the worm burden and the duration of infection. Lightly infected dogs, or those in the early stages, may show no symptoms at all. As the disease progresses, signs develop progressively:
- A mild but persistent cough
- Exercise intolerance — the dog tires more quickly than expected
- Reduced appetite and weight loss
- Laboured breathing
- Swelling of the abdomen due to fluid accumulation (ascites)
- Collapse in severe cases
In severe infections — sometimes called caval syndrome — dogs can deteriorate rapidly, with sudden onset of laboured breathing, pale gums, and collapse due to obstruction of blood flow through the heart by a mass of worms.
Diagnosis
Several diagnostic tests are available:
- Antigen test: Detects proteins produced by adult female worms. This is the primary screening test and is highly accurate once adult worms are present. It will not detect infection during the prepatent period.
- Microfilariae detection: A blood smear or filter test can identify microfilariae in the blood of infected dogs. Not all infected dogs have detectable microfilariae (occult infection), so this test is used alongside antigen testing.
- Chest X-rays and echocardiography: Used to assess the degree of cardiac and pulmonary involvement and guide treatment decisions.
Treatment: The Melarsomine Protocol
Treatment of established heartworm infection is a serious undertaking that carries inherent risks and requires strict management. The treatment protocol typically involves several stages:
Stabilisation
Before adulticide treatment begins, dogs with significant clinical signs are stabilised. Inflammatory conditions in the lungs are treated with corticosteroids, and exercise is restricted. Microfilariae may be reduced using macrocyclic lactones to reduce the risk of a severe reaction when adult worms begin to die.
Adulticide Treatment with Melarsomine
Melarsomine is an arsenical compound administered by deep intramuscular injection into the lumbar (lower back) muscles. It kills adult heartworms. The standard protocol uses a split dosing approach:
- A single injection is given initially
- One month later, two further injections are given 24 hours apart
This split protocol is considered safer than giving all doses at once, as it allows the worm burden to be reduced gradually, lowering the risk of complications from dying worms blocking the pulmonary blood vessels.
Strict Exercise Restriction
Strict rest is arguably the most critical component of heartworm treatment. As adult worms die following melarsomine injections, their dead bodies are carried by the bloodstream into the pulmonary arteries, where they are gradually broken down. Increased heart rate and blood flow from exercise significantly increases the risk of a fatal pulmonary embolism during this period. Dogs must be kept calm and confined — leash-only toilet trips for a minimum of six to eight weeks following the final injection.
Prevention
For dogs travelling to endemic regions, preventive treatment is essential and straightforward. Preventive medications work by killing the larval stages of heartworm before they can develop into adults. They must be given regularly throughout the period of risk — before, during, and after travel.
- Moxidectin: Available as a monthly spot-on or an injectable long-acting formulation. Highly effective against heartworm larvae. Often combined with other parasite treatments in a single product.
- Milbemycin oxime: An oral tablet given monthly, also effective against heartworm larvae and often combined with treatments for other parasites.
Prevention must begin before the dog's first exposure to potentially infected mosquitoes and should continue for at least one month after returning from an endemic region, to ensure that any larvae acquired just before departure are eliminated.
If you are travelling to Southern Europe or the Mediterranean with your dog, discuss heartworm prevention with your vet well in advance of your trip. Prevention is safe, simple, and far preferable to the prolonged, risky, and costly process of treating established infection.