Heartworm disease is an important disease in many parts of the world. The disease affects dogs in the United States, Central and South America, Australia, Japan, and Europe (but not the UK). Dog heartworm can lead to serious health complications and preventative medication is recommended for dogs living or travelling to areas where heartworm is widespread.

You might still be wondering, what are heartworms in a dog, how do dogs get heartworms and what are the signs? This guide will cover these questions as well as treatment and prevention of heartworms in dogs.

What Is Heartworm Disease in Dogs?

Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. The disease is also known as cardiopulmonary dirofilariasis. However, the name heartworm is a bit misleading as the worm primarily lives in the blood vessels that carry blood from the dog’s heart to the lungs (pulmonary arteries), rather than the heart. But canine heartworms can migrate to the heart which leads to heart failure and death if there are large numbers in the dog.

Clinical symptoms may become apparent when the worms die or if blood vessels are obstructed by blood clots or worm fragments. Heartworms can live for 5-7 years inside an infected dog and grow to 30 cm in length. Fortunately, humans are not affected by Dirofilaria immitis.

So How Do Dogs Get Heartworm?

Mosquitoes are the vectors of the parasite – meaning they transmit the worm to the dog. When a mosquito bites an infected animal, like a dog, fox, coyote, or wolf, it can pick up tiny larvae of heartworms called microfilaria.

When infected mosquitos bite another dog, these immature forms of the worm enter the blood and travel to the pulmonary arteries. The infective larvae grow to adult worms in these arteries and damage the arterial walls, disrupting blood flowing and causing serious problems for the dog. Typically, once the immature worms (larvae) reach 6 months of age, they will mature into adult heartworms.

What Are the Signs and Symptoms of Heartworms in Dogs?

If the number of worms in the dog is low and at an early stage, the dog may just show mild signs like being tired and reluctant to do any strenuous exercise. They may also have a cough. However, if there are heavier canine heartworm infestations or more advanced infection, the dog may suffer from heart failure. Owners should look out for the following heartworm symptoms in dogs:

• Fatigue
• Breathing difficulty
• Cough – may be blood tinged
• Swelling of tummy due to accumulation of fluid
• Weight loss and loss of condition

If you live in an area where heartworm is commonly found, annual testing for canine heartworm is advised. If you’ve recently travelled to a heartworm area and suspect your dog may have been infected, take your dog to the vet for heartworm testing as early diagnosis is important. Your vet may suspect that your dog has heartworm by noting your dog’s clinical signs and history, but blood tests can confirm diagnosis. An antigen test that detects the proteins associated with the heartworm can be used.

Your vet may also take X-rays and ultrasound scans of the heart and lungs. If there are large numbers of heartworms in a dog, the adult heartworms can migrate from the pulmonary arteries to the heart. Echocardiography (ultrasound of the inside of heart) may be used to check if any adult worms are present in the heart.

Large numbers of heartworms can lead to the arteries to the heart to suddenly become blocked - a life-threatening condition called caval syndrome. If this happens, the dog will need prompt surgical treatment to extract the worms from the heart.

Heartworm disease can be classified into 4 stages:

• Class 1 – mild or subclinical heartworm disease
• Class 2 – moderate heartworm disease
• Class 3 – severe heartworm disease
• Class 4 – caval syndrome

Heartworm Treatment for Dogs

If your dog has signs of heartworm and has tested positive, your vet will recommend restricted exercise or cage rest and prescribe medication. However, it may be necessary for the dog to be stabilised before medication to kill the heartworms can be given. This may involve giving your dog intravenous fluids and medication to reduce lung and heart symptoms.

To treat heartworms, your vet will prescribe a combination of drugs that will kill both the adult worms and microfilariae. The medication can be given by injection or by mouth, but the treatment is complex, and some side effects can be seen. Some dogs can suffer from an allergic shock reaction as the worms are killed.

Treatment must always be carried out under veterinary supervision over several months, so severe reactions are less likely. If there are side effects, treatment will need to be stopped, and the dog given time to recover before treatment is restarted.

Dogs should always be restricted from exercise while they recover from heartworm disease. Exercise increases the amount of damage the adult heartworms can do to the heart and lungs so must be limited. If the dog has already been diagnosed with heart failure, then additional heart medication may be prescribed.

After heartworm treatment, the dog will be re-tested to make sure there are no more heartworms present. Heartworm treatment can be expensive and may require months of treatment, so heartworm prevention is important for dogs living in areas with infective mosquitoes. The disease progressively gets worse so the earlier treatment can begin, the better chance the dog will recover.

If a dog has been diagnosed with caval syndrome, surgical removal of the heartworms is necessary (using forceps), but the outlook is poor for the dog.

Heartworm Prevention for Dogs

Keeping your dog indoors at dawn and dusk when mosquitoes are most active will help to limit your dog’s exposure to mosquitoes. However, some dog owners may consider that this along with the use of approved mosquito repellents and environmental control methods is sufficient to eliminate the risk of their dog being bitten by an infected mosquito. These precautions may be useful but, unfortunately, they’re not enough.

Dog heartworm prevention is essential in areas wherever there are mosquitoes. Many vets also recommend that dogs are treated with preventative medication all year round – not just in the warmer months or ‘mosquito season’. This is because mosquitoes can be active all year long if the climate is warm enough.

Treatment can be given daily, monthly or every 3 months and can be started in puppies from 6-10 weeks. They can be given orally with chewable pills, or topically via ‘spot-on's’. The preventatives are relatively inexpensive and easy to give. There are also heartworm preventatives that only need to be given every 6 or 12 months by injection making it easier to make sure your dog is protected all year round.

Some preventatives only prevent heartworms, while others also prevent intestinal worms and other parasites including fleas, ticks, and mites – your vet will recommend the best preventative to suit your dog. Your dog will still need to be antigen tested for heartworm every year to check that the preventative you have chosen to give your dog is working.

Dogs in the UK are not at risk of being infected. This means they don’t need to be given heartworm prevention unless they‘re travelling to an area where there are likely to be mosquitoes. But, if mosquitoes become established here, the recommendations may change. The Asian tiger mosquito has been advancing north from southern Europe, so watch this space...

Heartworm disease is a serious condition and infected dogs can have lifelong heart damage. Heartworm preventatives are essential in countries where mosquitoes and heartworms are widespread. If you plan to travel to areas where mosquitoes are prevalent, ask your vet for advice about giving your dog preventative medication before you go to reduce the risk of infection.

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