Do we really need to test our dogs for heartworm every year?
The simple answer is probably not.
Here are the basics that we use at Mitchell Animal Hospital to determine if a patient needs a heartworm test;
1) Has the pet missed any doses of the preventive medication during the previous season?
2) Has the pet traveled into areas that have a high incidence of heartworm such as the southeastern US or Mexico?
If you answered yes to either of these questions, then you should have your pet tested for heartworm.
If you have missed doses of preventive medication in the previous year, traveled to high incidence areas, or have a dog that has not been tested and had at least 1 negative test (usually our 1 year old patients that didn't get prevention when they were very young), then you should test your dog for heartworm.
A word of caution. We have been seeing more cases of Lyme disease at the clinic in the past couple of years. This disease is transmitted by ticks. The test that checks for heartworm also tests for 3 tick borne illnesses as well, so even though they may not need to be tested for heartworm, tick borne illness should not be ignored.
So that is the short and simple answer. If you are the type of pet owner that wants all of the research and information, then keep reading. If you just want the facts, then you don't need to read on.
Heartworm testing and prevention started over 30 years ago. The initial medications that we used for prevention had effect on both immature and mature stages of the heartworm parasite. These pills were given daily for 6 months. When a patient has heartworm and undergoes treatment to kill the adult phase of the parasite living in the heart, there is a risk of an anaphylaxis type reaction as the worms die, which could be very dangerous to the pet. For this reason, pets were all heartworm tested before the medication was started to ensure that there was no risk of adult worms being killed by the medications.
About 30 years ago, once a month medications were released which we still use today. These medications only affect the immature stages of the parasite for about 40 days after infection, then lose effect. This means that the risk of having adult worms dying and causing complications is no longer present. If a dog were to have traveled to a very high incidence area however and is carrying a huge larval burden in it's blood stream, then as these larva are killed off by the medication, a reaction could occur. This type of infestation is not seen in dogs residing in this part of North America.
In 1997 Mitchell Animal Hospital recognized that we needed to change our heartworm testing protocols based on the fact that we did not necessarily need to test before starting the medications. As a result we changed to every 2 year testing at the time of their annual check up rather than in the spring.
This changed again around the year 2000 after a report on heartworm testing protocols in Ontario was published by a team of veterinary epidemiologist and parasitologists. In their study, they looked at what is called positive predictive value. Essentially, when we have a positive heartworm test, how confident are we that we have a true positive and not a false positive? What they found was that if the dog has already had 1 negative test, was on prevention properly and did not travel into high incidence areas, that if we test and get a positive test, then that test will be a true positive only 3.6% of the time. That is 96.4% of the time, the test will be wrong if it comes up positive. Given this fact, we see no reason to test dogs for heartworm that are deemed extremely low risk.
If you would like to read a PDF version of the study, click here.