Heartworm is a very serious parasite that resides in the major blood vessels around the heart and within the heart itself. They breed and multiply within the heart, and release infectious larva into the bloodstream. It can cause major heart and lung problems, as the worms grow and fill the heart and vessels. It can be very hard and take a long time to treat and cure, with very serious potential complications. Testing is inexpensive and easy, with just a small amount of blood being tested to look for larva. Yearly testing is highly recommended for all dogs over the age of 6 months and is required to receive a prescription for heartworm prevention. Heartworm prevention is available for dogs in both topical and oral formulations and often prevents other internal and external parasites, such as fleas, roundworm, and mites. While heartworm historically is rare or absent in southwestern Ontario, it is on the rise as hotter temperatures are encouraging the breeding and proliferation of mosquitos, and the disease spreads from the United States, where it is much more common.

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What are the symptoms of heartworms in a dog?

Since the worms reside in the heart and major vessels around the heart and lungs, the symptoms are often related to these organs. They include coughing, resistance or inability to exercise for long, fatigue, weight loss and occasionally loss of appetite. These symptoms often get worse as the disease progresses, and will eventually lead to heart failure, but it is possible to have little to no symptoms for a long period of time; this is why it is important to test your dog yearly to catch the disease in its early stages.

How do dogs get heartworm?

Heartworm is a parasite carried through the bite of the common mosquito. After the mosquito bites an infected animal (which can include domestic dogs, foxes, coyotes, and wolves), it carries the blood with the heartworm larva in its body. The larva enters the bloodstream when the mosquito bites a second animal, and the larvae are transferred into the bloodstream. They then grow and once adults, reside in the heart and major blood vessels around it to start creating larvae of their own.

What are the treatment options for heartworms?

The current treatment protocol for heartworm involves two months of heartworm preventative and antibiotics to kill the immature worms and prevent worsening of the disease. Then, three injections of an adulticide called melarsomine are given over the course of one month to kill the adult worms in the heart. The dog must be in the hospital for monitoring for these injections since complications can arise as the worms die and travel throughout the body, through the blood vessels. After the treatment is finished, rechecks and testing for heartworms and their larvae must occur before the patient is considered cured. The entire process takes about 9 months.

In rare cases, surgery to remove the worms directly from the heart and major blood vessels can be performed.

Why is recovery and heartworm treatment challenging?

Throughout the entire treatment process, the pet must be kept calm and activity restricted. Keeping the animal in a small room or even crated at all times may be necessary, as increased activity, heart rate, or breathing rate can increase damage to the heart and major blood vessels. Short, leash-only walks are also required, as is keeping the dog out of hot temperatures. This must continue throughout the treatment process, and up to 2 months after the last adulticide injection, which means about 5 months of these new restrictions!

Serious complications are also possible due to the death of the parasites in the heart and bloodstream. The body can overreact to these “foreign invaders,” which is why antihistamines and steroids are often prescribed. The deceased worms can also block blood vessels that lead to the lungs, which can be life-threatening. This is the reason very strict exercise restrictions are put in place, as it greatly reduces the risk.

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