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Infectious Diseases & Parasites

Serious Heartworm Disease on the Rise: Is Your Dog Protected?

Canine heartworm is a life-threatening disease. Find out the latest on this parasitic disease and how to protect your dog with three easy steps.

By Krista Halling DVM CCRP DACVS

Heartworm disease is a potentially deadly parasitic condition affecting dogs worldwide, with its prevalence and geographic distribution increasing in recent years. In this article, we explore the latest on heartworm disease from the American Heartworm Society and how you can protect your dog with three easy steps.

The heartworm lifecycle

Heartworm is caused by the parasitic worm Dirofilaria immitis, which is transmitted from dog to dog through the bites of infected mosquitoes. The lifecycle of the canine heartworm requires a dog, a mosquito, and environmental temperatures warm enough to favour mosquitoes feeding and the development of heartworm larvae.

Adult heartworms, measuring up to 30cm (12 inches) long, live in an infected dog’s heart and the adjacent blood vessels. These adult heartworms reproduce, resulting in many larvae – called microfilariae – in the dog’s bloodstream.

Along comes a mosquito

When a mosquito bites an infected dog, the mosquito ingests these heartworm larvae along with the dog’s blood. In the mosquito’s gut, the larvae mature to what is known as stage-three larvae and after 2 weeks they move to the mosquito’s mouthpiece. The mosquito, by playing this essential role in the development of a heartworm, is called the intermediate host.

How dogs becomes infected with heartworm

When the above mosquito bites another dog, the heartworm larvae are deposited from the mosquito’s mouthpiece to the dog’s skin. These larvae then migrate into the small skin wound created by the mosquito bite, then into the dog’s bloodstream, eventually reaching the heart and blood vessels in the lungs.

Over the course of several months, the larvae mature into adult heartworms, which can grow up to a foot long. These adult worms can live for several years in dogs, causing life-threatening damage to the heart, lungs, blood vessels and often kidney and liver.

Signs of heartworm disease

In the early stages, dogs may not show any signs of heartworm disease. However, as the disease progresses and the worms grow and reproduce and the heart and blood vessels become damaged, signs become apparent. Common signs include:

  • Coughing: one of the earliest and most common signs.
  • Fatigue and low exercise tolerance
  • Enlarged abdomen or swollen limbs: as the heart failure develops, fluid may start to accumulate in the dog’s abdomen and limbs

Diagnosing heartworm disease

Heartworm disease is diagnosed through a simple blood tests that detects the presence of heartworm proteins or microfilariae (larvae) in your dog’s blood. Annual testing is recommended to ensure early detection.

If a dog tests positive for heartworm, additional tests to confirm the diagnosis, plus additional diagnostics such as chest x-rays, heart ultrasound and additional bloodwork may be required to assess the extent of the infection and the damage to the dog’s organs.

Treatment of heartworm disease

Treating heartworm disease is a life-threatening process in itself, that requires veterinary supervision. The primary treatment involves administering medications to kill the adult worms and all larvae. The reason this is a risky procedure for a dog to endure is that the dying worms can cause blood vessel blockages and allergic reactions.

During treatment, dogs must be kept calm and have their activity restricted. Exercise can increase the risk of complications from the dead worms blocking blood flow to lungs and other tissues. After the treatment period, follow-up blood tests are necessary to ensure all the heartworms have been eliminated.

Three easy steps to protect your dog

Preventing heartworm disease in your dog is effective and much safer and than treating a heartworm infection. Here are the current recommendations by the American Heartworm Society:

  1. Give year-round preventive medication: this works by killing the stage of larvae that was deposited by the mosquito into your dog, before they develop into adult heartworms. Preventive medication should be administered all months of the year, since very few urban microclimates get cold enough to arrest the heartworm lifecycle. Your veterinarian can discuss the medication that is most suitable for your dog.
  2. Reduce your dog’s exposure to mosquitoes: since no preventive medication can guarantee 100% efficacy, it is important to reduce your dog’s chance of being bitten by mosquitoes. Effective methods include keeping your dog indoors (or inside a tent) during peak mosquito biting hours, using a pet-safe insect repellant on your dog, eliminating free-standing water, and adventuring in climates that have low mosquito populations.
  3. Have your dog tested annually for heartworms and microfilariae even if you are giving your dog heartworm preventive medication. It is recommended to test yearly for both mature heartworms (by testing for heartworm proteins) and microfilariae larvae in your dog’s blood, since heartworm is a life-threatening disease and your dog may not show signs until the disease has reached an advanced stage. These simple blood tests let you catch and treat heartworm disease early.
Take measures to prevent a mosquito bite

While heartworm disease in dogs is a serious and potentially fatal condition, it is a preventable disease. By taking these proactive steps, you can ensure your furry friend lives a healthy and heartworm-free life.

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