This is a common question. You give your dog heartworm prevention medication every month, so why should you get him tested once a year? Your dog is not showing any signs of heartworm disease, he’s not acting sick, so why test him?
Heartworm disease can be a very serious disease, and if untreated it can be fatal.
Dogs can be infected with heartworms by a mosquito bite. Teenage heartworms live in the blood, and migrate to the heart. Once they are in the heart, they set up camp and grow up to become adult heartworms. Then the adults make baby heartworms (microfilaria). The next time this dog is bitten by a mosquito, the mosquito is infected with baby heartworms and can go on to infect other dogs.
The initial mosquito bite and infection are not accompanied with any signs of disease (except maybe a little itching at the bite). If a dog is infected with heartworms, it can take months before he starts to show clinical signs of heartworm disease. The signs do not develop until the adult heartworms are large enough to cause heart problems. The worms block the blood vessels that take blood away from the heart to the lungs. They can also get in the way of the normal heart valves. Here is a diagram of a dog’s heart that is infected with heartworms.
Image from ePetHealth.
Signs you may notice in a dog with heartworms are:
- Shortness of breath
- Loss of stamina
Once a dog is showing these signs of heartworm disease, treatment is necessary to kill the heartworms. However, this treatment can also be dangerous for a dog that is sick.
Okay, we know that the disease can be serious, but why bother testing if you are using a monthly heartworm preventative?
The term “heartworm preventative” is a little bit misleading. The medication you give your dog once a month does not prevent them from getting bitten by a mosquito and getting infected with teenage heartworms. The medication, if given appropriately, actually kills these teenage heartworms every month so they can’t grow up to become adult heartworms.
Are you sure that your dog is taking every pill you give him? If your dog takes the “treat” away and buries it, or vomits shortly after eating his pill, he will not be protected against heartworms. The medication needs to be in the body to work! Even missing just one month makes your dog more susceptible to becoming infected with heartworms.
Not all medications are 100% effective. If a dog is infected with a large number of teenage heartworms from multiple mosquito bites, the preventative medication may not be able to kill all of them, which could result in infection.
What’s the benefit of testing every year?
If the heartworm preventative does fail, or if you forget a dose, or if your dog doesn’t actually get the medication, annual heartworm testing can detect disease before your dog is showing clinical signs. If we can detect heartworm disease in this subclinical stage (before the dog looks sick), treatment of heartworms is much safer for your dog. The same medications are used to treat subclinical heartworm disease as are used to treat clinical (showing signs) heartworm disease. But if your dog still looks and feels healthy, the risks to your dog’s health are much less.
Testing is especially important if you do not use heartworm preventative year-round. Yes, there is a “mosquito season.” But that season changes every year, and it is different in every area of the country. When we have mild winters, mosquito season really never ends. If you wait to give heartworm preventative until you see the first mosquito of the season, you have waited too long. It only takes one mosquito bite for a dog to be infected with microfilaria, and if you don’t start the preventative medications until two months after that, your dog has a very high risk of developing heartworm disease. Testing every year can diagnose heartworm disease early. Early diagnosis means early treatment, and a healthier dog!
What about cats?
Cats can also be infected with heartworms, although it is not as common as in dogs. Testing for heartworm disease in cats is not as easy as in dogs – the same tests won’t work in both dogs and cats, unfortunately. There are monthly heartworm preventatives available for cats. Ask about heartworm prevention for cats at your next visit.
When was your dog last tested for heartworm disease?
Check out our Heartworm Disease Resource Page for even more information about heartworms.