Dog Heartworm: Identify, Understand and Prevent

Dog heartworm (also referred to as “heartworms”, “heartworm disease”, or “dirofilariasis”) is a disease caused by a parasitic worm called Dirofilaria immitis.

Heartworm is incredibly dangerous for dogs because the worms can clog up the heart and block the blood vessels. In particular, heartworms can build up in the pulmonary artery that carries blood from the heart to the lungs. This restricts blood and oxygen circulation to the rest of the body, which is very harmful to other organs like the kidneys and liver.

Very severe cases of dog heartworm can cause a life-threatening condition called Caval syndrome. As a huge mass of worms fills the right chambers of the heart, they start to push back into a large blood vessel called the vena cava. Blood can no longer flow into the heart through this vein, which can quickly lead to heart, lung, kidney and liver failure, and death.

How do dogs get heartworm?

Heartworm is passed between animals via mosquito bites only. In an infected animal, the adult heartworms produce offspring called microfilariae, which circulate in the blood. When a mosquito bites the infected animal, they also consume the microfilariae.

The microfilariae grow inside the mosquito and become “infective”, or able to cause disease. When the mosquito bites another dog, it deposits the infective microfilariae into the new dog’s system. There, they mature into adult heartworms and make their way to the heart and lungs, where they mate and produce more microfilariae to continue the cycle.

The more mosquitos in the area, the greater the risk of dog heartworm. The highest risk is in hot, humid climates and in/around:

• Coastal regions.
• Bodies of water, e.g. lakes and streams.
• Wetlands, swamps and marshes.
• Flood-prone areas, e.g. flood plains and river banks.

There is also a seasonal risk in the warmer summer months when mosquito activity peaks. Dogs that have free access to outdoor areas are more likely to be bitten than dogs who spend most of their time indoors, although indoor dogs can and do get heartworm too.

Heartworm prevention for dogs

Dog heartworm is usually treatable, but it can cause serious long-term health problems and reduced quality of life for your dog. For that reason, heartworm prevention for dogs is the best course of action.

Your vet can recommend or prescribe a medication called a heartworm preventative. These are available in various formats; tablet or topical forms are given once a month, all year round, while injections are given by your vet every 12 months.

If you choose to use the monthly tablets or topicals, it is extremely important not to miss any doses. If you do, let your vet know so they can assess your dog’s risk of infection. Depending on how many doses were missed, your vet may suggest performing a heartworm test before recommencing the preventative.

Yearly heartworm injections are often preferred as they can be given by your vet at the same time as your dog’s annual vaccinations, making it easy to remember.

Other heartworm prevention strategies include:

• Spraying your pup with a dog-safe, vet-approved mosquito repellant when they go outdoors.
• Scheduling your walks outside of dawn and dusk hours when mosquitos are most active.
• Avoiding areas with stagnant water, like ponds.
• Removing any areas of standing water, such as buckets, from your garden to prevent mosquito breeding.

Heartworm symptoms in dogs

Common heartworm symptoms in dogs include:

• A soft, dry cough.
• Shortness of breath even when resting.
• Rapid breathing.
• Exercise intolerance, e.g. struggling to walk.
• Weakness and lethargy.
• Disorientation or confusion.
• Loss of appetite and weight loss.
• Swelling in the stomach and/or legs.

The severity of a dog’s symptoms will depend on:

• How long the dog has been infected.
• How many worms are present and where.
• How much damage the heartworms have done to the organs.

This means that your dog might not show many obvious signs of heartworm in the early stages, but the worms can still be causing damage to the heart and lungs. Knowing the signs means you can spot heartworm as early as possible, minimize the health effects, and give your dog the best chance of recovery.

Treating heartworm in dogs

Your vet will want to do a series of tests to discover if your dog has heartworm, how advanced their condition is, and where in the body the worms are. These tests will usually include a physical examination, blood tests, and imaging tests like x-rays or ultrasounds. (Note that it can take some time after initial infection before these tests can accurately detect dog heartworms).

If it is confirmed your dog has heartworm, your vet’s priorities will be to remove any adult worms from the body and remove any microfilariae from the blood. This can often be done with a course of heartworm medicine for dogs, but more serious cases may also require surgery. The treatment course is complex and can lead to serious complications, including blockage of major blood vessels by the killed worms, which is why preventing heartworm infection in your dog is always preferred over treatment.

Nobody wants to see their dog suffer from heartworm. Regular preventatives and annual check-ups can go a long way towards protecting them. If you have any questions and concerns about heartworm and your dog, don’t hesitate to get in touch with your vet. They’re always happy to give you expert, tailored advice to keep your canine friend happy and healthy!