Heartworm in Dogs

A few weeks ago I got a call from one of my dog walking clients. She wanted to let me know that her dog was being treated for heartworm. I was familiar with the need to prevent heartworm in dogs, but had never encountered an infected dog. She explained that she had rescued her dog last summer so she didn’t know she had been infected at the time. There was no medical history provided when the dog was surrendered, so they did not know that the previous owner had not taken precautions to prevent the disease.

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She went on to explain that the medical process was lengthy and that her dog would not be feeling up to par, so we would have to keep an eye on her for the next several months as she goes through the many steps involved in curing her.

I listened to what her needs were, followed the instructions with regards to her walks and any additional instructions that were required. So far, she seems to be doing well. There was a period of time where she had trouble eating, followed by some vomiting and some unusual fatigue, but overall, she is responding well, thankfully. Still, I had so many questions that I needed answered. I knew it was spread by mosquitoes, but how? She was a rescue from a professional agency. Why didn’t the agency catch this when they prepared her for adoption? How long and effective is the cure? How common is it? I decided to learn more.

How is it transmitted?

In short (and unpleasant) terms, a mosquito bites and infected dog and ingests the worm larvae. These worms grow inside the mosquito for about 2 weeks. At this time, if the mosquito bites another dog, it will regurgitate the ingested worms and some manage to crawl through the hole made by the bite. They then work their way into the dog’s blood stream. It takes up to 3 months before they are big enough and strong enough to break through a vein and land in the pulmonary artery (lung), where they multiply. It takes about 7 months for the worms to reach adulthood. An adult female can be approximately 15 inches and the male, approximately 5 inches. At this size they can reach the heart from the pulmonary artery. If the dog survives, the worms can live 5 to 7 years inside the body.

Heartworm in dogs will cause excessive fatigue with only mild exercise, coughing, loss of appetite and weight loss.

How is it detected?

Blood work to detect heartworm can only be used if the female worms have reached adulthood. At this stage they produce a protein in the blood that is detected by an antigen test. If the infection is still in the early stages, there would be no symptoms and no protein for antigen test to detect. This explains why a dog that was tested in rescue situation may go undetected. In this case, the dog had probably been infected only weeks before adoption. The symptoms, which include cough, becoming easily fatigued after mild exercise, loss of appetite and weight loss would not be apparent until long after the initial mosquito bite.

How is it cured?

The cure is a very long process; months, in fact. It begins with the dog being injected with a drug that kills the worms in the heart and vessels. Often, there are a series of injections. The first two are given a month apart and then two more over the following two days. During this time, as the worms die and decompose in the body, the dog will be given antibiotics to combat any infection that could arise. The dog may have a cough for several weeks afterward. The dog will also be given a drug to kill the baby worms that are beginning to grow in the body. Through most of this process, the dog is to be kept calm and have minimal to no exercise.

How common is Heartworm Infection in dogs?

According to Dwight Bowman, MS, PhD, 1 out of every 200 dogs is diagnosed as being infected with heartworm annually.



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