Canine Heartworm Disease

Heartworm disease is a parasitic disease caused by a worm, the heartworm, which is transmitted to dogs through a mosquito bite. It's also known as canine dirofilariasis or "heartworm disease."

This parasite settles inside this organ, but also in the arteries leading to the lungs and liver, causing serious problems. Below, we'll look at how to identify this condition, how to treat it, and above all, how to prevent it.

What is heartworm disease?

This chronic, progressively worsening parasitic disease is characterized by adult heartworms living in the heart — specifically in the right chamber and the pulmonary vessels.

It's transmitted by mosquitoes and occurs worldwide. It's more common in warm climates, since its spread depends on mosquito behavior. These mosquitoes can belong to the genera Culex, Aedes, Anopheles, Culiseta, and Coquillettidia, among others.

Its incidence currently appears to be rising, which has drawn attention to the need for prevention. This is especially important in areas where mosquitoes are abundant.

The heartworm's life cycle

The heartworm's scientific name is Dirofilaria immitis. Understanding its life cycle helps us understand the preventive measures. It's a relatively long cycle (generally 7 to 9 months) and starts when a susceptible mosquito becomes infected by feeding on the blood of a microfilaremic (infected) host.

This mosquito now carries infective heartworm larvae in its mouthparts, and when it bites the dog, these larvae burrow under the skin until they reach the bloodstream. In the dog's body, they undergo a first transformation between 1 and 12 days later. They stay in this stage for about two months, at which point they molt into the immature worm stage.

These are the ones that migrate to the heart. If there are males and females, they'll reproduce, producing microfilariae a few months later. These can live for years in the dog's bloodstream. Mosquitoes pick them up from there when they feed, and inside the mosquito they molt into infective larvae, restarting the cycle. Each stage of the heartworm can be susceptible to different drugs.

Where the heartworm settles

We highlight where the heartworm can settle because this explains the symptoms we'll be able to notice in our dog. So, the immature worms settle in the right ventricle of the heart and in the pulmonary arteries. When the worm count is high, they also move into the right atrium. And in very heavy infestations, worms can be found in the vena cava and hepatic veins.

With worms in the pulmonary arteries, what's known as pulmonary thromboembolism can occur, from blocked blood flow and clot formation. This can also cause heart failure. Worms in the vena cava or hepatic veins cause what's called vena cava syndrome, which causes liver failure.

What's more, every stage of the worm harbors endosymbiotic bacteria — that is, bacteria that live inside these worms in a mutually beneficial relationship. This is important to keep in mind for treatment, since if these bacteria aren't treated first, killing the adult heartworms could cause a massive release of these bacteria, triggering major inflammatory reactions and serious health consequences for our dogs.

Characteristics of the heartworm

Keeping in mind the appearance and reproductive behavior of heartworms makes it easy to picture the enormous damage they can cause a dog. Adult heartworms can grow up to 30 cm (about 12 inches) long and live around 5 years. In dogs with heavy infestations, around 250 worms have been found.

When they reproduce, each worm can produce about 5,000 microfilariae per day, which live in the bloodstream for up to 3 years. When a mosquito ingests them, they molt inside it in under 10 days in the climates most favorable to them.

Which dog breeds get heartworm disease?

The domestic dog and some wild canids are the usual definitive hosts for the heartworm and, as such, act as the main reservoirs of infection. But it can also affect other animals, such as cats and ferrets.

Since this disease is closely tied to its vector — that is, the insect that transmits it — dogs in areas with more mosquitoes present are at higher risk of infection, since they're more exposed to bites. So, large-breed dogs that live outdoors get sick at higher rates.

This disease is endemic in Spain, and among the areas with the highest prevalence we could highlight the Canary Islands, southern Spain, major river basins, and some irrigated farming areas, due to their high humidity.

Symptoms of heartworm disease in dogs

The symptoms of this disease depend on the number of worms present in the animal, their location, and the dog's size. If there are few heartworms, the dog may show no symptoms at all. Either way, we should watch for clinical signs such as the following:

• Fatigue.
• Exercise intolerance.
• A chronic, non-productive cough.
• Weight loss.
• Tachypnea (rapid breathing) and dyspnea (difficulty breathing).
• Fainting (syncope).
• Coughing up blood in dogs with thromboembolism, hemoptysis (coughing blood from the mouth), and epistaxis (through the nostrils).
• Dogs with liver failure will show jaundice, yellowish coloring of the skin and mucous membranes, ascites, bleeding, and anemia. They can collapse and die.

It's important to know that when a dog becomes infected, symptoms don't appear until about 6 months later, which is how long it takes these worms to mature.

How is canine heartworm disease diagnosed?

The first thing to keep in mind is that any dog living in a risk area without mosquito protection is at risk of carrying heartworms. By taking a blood sample, the vet can run a test to detect the parasite's presence, through serological detection of circulating antigens, specifically an antigen produced by female worms.

False negatives can occur if there aren't yet any adult worms (which are what's detected), if there are no females in the blood, or if there are only a few worms. However, these tests are highly specific, and there's no possibility of cross-reactions with other parasites, so false positives are very rare and only occur due to technical errors in running the test.

Circulating microfilariae can also be identified, though their absence doesn't mean the dog is healthy, since there could be only adult worms present. A chest X-ray can sometimes be useful, allowing the severity of the infection to be assessed.

It may show enlargement of the right ventricle and the pulmonary arteries if there's a significant number of worms in the area. An electrocardiogram or echocardiogram are other tests that also provide useful information, allowing the size of the heart chambers, possible pulmonary hypertension, and even the speed, direction, and characteristics of blood flow to be assessed.

Blood and urine tests will also be run, which may reveal anemia, abnormalities in the coagulation profile, altered liver enzymes, or kidney function markers.

Treating heartworm disease

Treatment is very delicate and complex, since the worms cause a lot of damage but, once they die, they get carried through the bloodstream, potentially blocking blood flow at different points and causing serious injury. That's why the vet needs to carefully assess each case and choose one of the possible treatments.

The goal is to eliminate the adult worms and microfilariae, put preventive measures in place, and avoid complications caused either by the heartworms themselves or by their death. This also involves first eliminating the bacteria associated with these worms mentioned earlier, using specific antibiotics. Another issue is that the drugs used come with side effects that can even be serious. The vet usually gives them by injection or orally.

The animal may need to be kept in strict rest. There are cases, especially in older dogs, where treatment is not advisable. Others will need surgery to remove the adult worms. After treatment, tests are repeated to confirm the dog is free of heartworms.

How to prevent canine heartworm disease

The vet, based on the region we live in and our dog's characteristics, will recommend the most suitable prevention plan.

On the market you'll find products that prevent mosquitoes from biting our dog. Depending on the area, these can be used year-round or only during mosquito season. Generally, from April through October, the mosquitoes that transmit canine heartworm disease are especially active. For this reason, extra care should be taken with our dogs during these months.

If we suspect our dog might have heartworms, it's a good idea to run a test before starting these preventive products, in case they're already infected. Since nothing will be 100% effective, we should also follow measures to minimize our dog's exposure to mosquitoes as much as possible.

Mosquitoes feed at dusk, so we'll protect our dog by keeping them indoors during that time. Mosquito breeding areas should be avoided, such as standing water or damp environments. Environmental repellent products can also be used.

Strict, consistent prevention is important, especially in endemic areas, since even a single missed or delayed dose of our preventive treatment could cost our companion an infection.