Max was about a year old when he was adopted into his forever family from the rescue that found him on the streets. He was current on his vaccines, had no evidence of heartworms or intestinal parasites, and he was started on monthly heartworm preventative. As long as Max remained happy and healthy, we would only need to see him once a year for his annual exam, vaccinations, and heartworm test.
A year later, Max did just that. His owners told us he was doing great at home – he was the perfect addition to their family. His nose-to-tail physical exam was completely normal. We updated his vaccines and took a few drops of blood for his heartworm test. This year, however, he tested positive.
How is that possible? Max’s owners had him on monthly heartworm prevention, they never missed a dose. How did he test positive this year?
Heartworms are transmitted through the bite of an infected mosquito. The mosquitoes inject the dog with immature heartworms that go on to develop into adults in the heart over the next 6-9 months. The adults produce more immature heartworms that the next mosquito picks up when it feeds, and the life cycle is completed. It is also important to have a brief understanding of how annual heartworm tests diagnose disease and how monthly preventatives keep our pets safe. Heartworm tests detect the presence of the adult parasite, not the immature worms. This is why, once the dog is infected, it will take 6-9 months for the test to be positive. Monthly heartworm preventatives are responsible for eliminating the immature worms to prevent them becoming adults.
With this understanding, we know Max became infected while he was a stray. He tested negative at his first visit because it was too early for the heartworm test to detect the disease. Luckily, because Max’s owners were giving the monthly prevention, he was not considered infectious to other pets. In other words, there were no immature worms circulating through his body for a mosquito to pick up and infect another dog. Left undiagnosed or untreated, the disease will eventually progress into heart failure as the parasite will not allow the heart to function properly.
Max’s owners chose to pursue heartworm treatment, even though there are some considerable risks involved. His disease was carefully staged with bloodwork and chest x-rays to make sure he was a good candidate for the treatment – and fortunately, he was. Over the course of two months, Max sailed through his treatment with flying colors. He was administered injections in his lower back aimed at eliminating the adult worms and sent home on medications to control inflammation and pain. The most important component of the entire treatment was that Max was to take it extremely easy while the adult parasites were dying. If he were to become too active, a worm could become dislodged from his heart, travel to his lungs, and cause a series of complications, sometimes fatal. Max’s owners did a great job of keeping him calm and a month after treatment, when he tested negative for heartworms, Max sprinted through the parking lot to the car to go chase the rabbits.
Every dog with heartworm disease is different and certain factors need to be considered before pursuing treatment including the age of the dog, any underlying illness or disease, or if the dog is showing any symptoms such as coughing or exercise intolerance. Not every dog will be a good candidate for treatment and treatment is risky. This should all be discussed with your veterinarian when deciding on a treatment plan. In order to avoid putting Max through treatment in the future, his owners are still diligent about giving him his monthly heartworm preventative and testing him every year.