Heartworm and lungworm in dogs – symptoms, tests and treatment

Overview

D. immitis, sometimes called American heartworm, is a canine parasite found particularly in southern and south-eastern Europe. Adult worms live mainly in the pulmonary arteries, rather than inside the gut like ordinary intestinal worms.

An adult heartworm can grow to around 30 cm long. The worms can live in a dog for 5–7 years, so an untreated infection may persist for a long time.

Adult French heartworms are around 14–20 mm long, making them much smaller than D. immitis. D. immitis infections have also been diagnosed in imported dogs in northern Europe.

One report from northern Europe described French heartworm diagnosed in a dog imported from France as early as 2005.

The subcutaneous worm Dirofilaria repens is a different parasite from heartworm, although it too uses mosquitoes as vectors.

Causes

Mosquitoes are part of the heartworm life cycle

Mosquitoes acquire early heartworm larvae when feeding on blood from an infected canid. The larva must develop inside the mosquito before it can establish an adult-worm infection in another dog.

Infective larvae enter the dog through the mosquito's bite wound. A dog therefore does not acquire an adult heartworm infection simply by being near another dog.

Development inside the mosquito needs warmth: larval maturation stops below 14°C. Larvae can first be detected in the dog's blood around 6 months after infection at the earliest, and usually after 7–9 months.

French heartworm cycles through molluscs

A. vasorum needs a slug or snail as an intermediate host in its life cycle. A dog can also become infected by eating a frog carrying larvae.

Larvae that hatch in the lungs are coughed up to the throat, swallowed and eventually passed in the faeces. Shedding of new larvae usually begins 6–8 weeks after infection.

An interval of 28–108 days from infection to shedding has also been described, so an early sample may be taken before detectable shedding begins. Without treatment effective against the parasite, infection can last for the dog's remaining lifetime.

French heartworm infection may also be possible through larvae released from molluscs into the environment, for example in puddle water.

Risk factors

Travel to a heartworm-endemic area and importation from one are important parts of the exposure history. Mosquito activity and local climate affect the possibility of D. immitis infection.

Across much of endemic Europe, the transmission season is roughly April to October, but local climate changes these limits. Eating slugs, snails and other possible intermediate hosts increases the risk of French heartworm.

Foxes play a role in maintaining and spreading French heartworm in the wild. The parasite is not limited to France: it occurs in local endemic areas in several European countries.

A routine preventive worming product does not necessarily cover every internal parasite, so the product's spectrum of activity matters.

Typical symptoms

A small heartworm burden may initially cause no signs, even though worms are already present.

• A persistent cough can be associated with lung changes caused by either parasite.

• Your dog becomes breathless or tired sooner than before during an ordinary walk.

• Breathing becomes difficult or faster as the disease progresses.

• Your dog loses weight or its appetite declines.

• With French heartworm, even a small wound may bleed for an unusually long time.

• Black stools, coughing up blood or bleeding beneath the skin may be associated with the bleeding disorder caused by French heartworm.

• Abdominal swelling may result from right-sided heart failure.

A resting respiratory rate above 30 breaths per minute in a dog with D. immitis is an abnormal finding. French heartworm can also cause no signs at all, so an absence of symptoms does not rule out infection.

Complications and consequences

Heartworms damage the pulmonary arteries and can raise pulmonary blood pressure. This increased pressure can strain the right side of the heart and lead to heart failure.

In caval syndrome, a large worm burden obstructs blood flow through the valve on the right side of the heart. This acute complication can cause death within 2 days without prompt worm removal.

French heartworm can cause severe bleeding into the lungs and blood-clotting disorders. A. vasorum larvae migrating elsewhere in the body can cause signs involving the brain or eye, for example.

Treatment of mild heartworm disease often goes well when exercise is restricted, but severe disease and other underlying conditions complicate treatment. Untreated French heartworm infection can end in sudden death.

The immune reaction associated with heartworm can damage the kidney's filtering structures and cause protein loss in the urine.

Diagnosis at the vet

Heartworm is sought in blood

When D. immitis is suspected, adult-worm antigen testing is combined with testing for circulating larvae, called microfilariae. The antigen test mainly detects a protein released by adult female worms.

Antigen can first be detected around 5 months after infection at the earliest. In practice, testing is often timed around 7 months after possible exposure.

A negative test in a recently arrived imported dog may therefore need repeating later. False-negative results are also possible with a low worm burden or an infection consisting only of male worms.

A positive antigen result is confirmed by further testing before treatment begins. Radiographs and ultrasound assess damage to the lungs, pulmonary arteries and heart to help plan treatment.

The colour intensity of a positive rapid test cannot reliably indicate the dog's worm burden. Microfilariae found in blood are identified to species because they may belong to a parasite other than D. immitis.

Radiographic findings support suspicion but do not confirm a current heartworm infection without parasite-specific testing.

French heartworm requires different tests

A. vasorum can be sought in fresh faeces using the Baermann method, which identifies the parasite's larvae. Samples from 3 consecutive bowel movements over 1–2 days are recommended because larval shedding varies.

Baermann samples are kept separate and examined as fresh as possible. A blood test for A. vasorum antigen and PCR testing can also complement the investigation.

Blood clotting is also assessed when bleeding signs are present. Routine faecal flotation detects larvae less reliably than a Baermann examination directed at this parasite.

Treatment

Heartworm treatment and rest are planned together

Your vet prescribes parasite treatment and monitoring according to the species identified, disease-related damage and your dog's general condition. D. immitis treatment aims to eliminate both adult worms and developing larval stages.

A severely ill dog is stabilised before treatment targeting adult worms. Melarsomine may be used to treat adult heartworms.

Doxycycline targets Wolbachia bacteria living inside the worms and reduces tissue damage associated with dying worms. The medicine used against larval stages belongs to the macrocyclic lactone group.

Dying worms can block pulmonary blood vessels, so treatment carries a risk of pulmonary thromboembolism. Exercise, excitement and overheating are restricted throughout treatment and recovery.

Strict exercise restriction usually continues for another 6–8 weeks after the final melarsomine treatment. Symptomatic embolic complications often appear within 7–10 days, but have been reported between 5 days and 4 weeks after treatment.

In caval syndrome, worms that have moved into the heart may need urgent removal in a procedure. Antigen testing to assess treatment success is usually performed 9 months after the final melarsomine treatment.

Acetylsalicylic acid is not recommended to prevent thromboembolism in heartworm infection because convincing evidence of clinical benefit is lacking. Prolonged larval treatment alone is not recommended as the first-choice method of eliminating adult heartworms.

French heartworm treatment is a separate plan

A. vasorum is treated with an effective antiparasitic medicine such as moxidectin, milbemycin oxime or fenbendazole. Treatment duration depends on the medicine selected.

Severe breathing difficulty or bleeding may require oxygen and supportive treatment to control haemorrhage. Rest is also needed when treating this parasite, for at least 2–3 days and, depending on the situation, throughout treatment.

In one imported dog treated in northern Europe, lung radiographic changes remained after 6 months despite the dog being clinically well.

Prevention

Before travel, parasite protection is planned according to the destination and your dog's exposure. Heartworm prevention uses medication effective against larvae and measures to reduce mosquito exposure.

Protecting an already infected dog from mosquitoes also reduces transmission of the parasite to other animals. French heartworm prevention includes reducing the eating of slugs and snails and exposure through hunting.

Suitable preventive parasite medicines are also available for this parasite in areas where infection risk is significant. Tests chosen according to travel or import history are needed even for a dog without signs.

Providing clean drinking water reduces the need to drink from puddles that may contain parasite larvae.

Important for dog owners

Record the countries and places where your dog has lived or travelled, and the last possible exposure date. Bring previous test results and precise details of parasite medication given. The words heartworm test on paperwork from abroad do not always identify which species or testing method was used.

Arrange faecal sampling before collecting anything so samples reach the correct test while fresh. Monitor coughing, breathing, exercise tolerance and any bleeding. Count breaths during quiet rest and record changes so they can be compared at follow-up.

If your dog is being treated for D. immitis, plan how the whole family will manage rest. Include carers, visitors and situations that usually excite your dog. Do not shorten the agreed rest period when signs improve or start additional worming treatment outside the vet's plan.

Frequently asked questions

Are heartworm and French heartworm the same parasite?

They are different roundworms with different transmission routes: D. immitis spreads through mosquitoes, and A. vasorum usually through slugs or snails. Diagnosis and treatment selection are also specific to the parasite species. A heartworm blood test does not replace a faecal sample for French heartworm.

Can a dog infect another dog through ordinary contact?

D. immitis needs a mosquito for development, so an adult-worm infection does not spread directly through ordinary contact. The usual French heartworm life cycle involves mollusc intermediate hosts, which transmit infection when eaten by a dog. Foxes and other wild canids can maintain the parasite's cycle in the environment.

Is a negative blood test immediately after returning from abroad enough?

A recent infection may be missed because detectable antigen is only produced once the worm has developed. D. immitis testing is usually timed around 7 months after possible infection, and the vet plans any necessary repeat tests alongside previous results.

Why must my dog rest when it no longer coughs?

Dying heartworms and the inflammation they cause can block pulmonary blood vessels even after your dog has begun to feel better. Exercise restriction reduces the risk of heart and lung complications during treatment. Rest continues for the recovery period prescribed by the vet, even if outward signs improve sooner.

Can people catch these parasites from a dog?

D. immitis can exceptionally infect people through mosquitoes and cause a lung granuloma, for example. French heartworm has no known equivalent human health risk and should not be confused with other Angiostrongylus species that infect people.