When should I get my dog tested for heartworms?

Any time is a good time to test your dog for heartworms. However, in our local climate, spring tends to be a better time to test. To understand why, one must look to the lifecycle of the heartworm.

The lifecycle of the heartworm (Dirofilaria immitis) is divided into 6 stages, L1 to L6. The L stands for larval stage. The L1 stage, also called microfilaria, are essentially the new born worms. These larvae circulate in the blood of the host dog and are consumed by mosquitoes when feeding. Inside the mosquito, the L1 matures into an L2 and finally into an L3. The L3 larvae are the infective stage that the mosquito transmits into a new host when feeding.

Once inside the host, the L3 and L4 stages mature in the tissues around the mosquito bite until becoming young adults (or L5). The L5 move into the blood vessels and find their way to the right side of the heart and the arteries that carry blood to the lungs. There, they mature into adult heartworms (or L6).

The maturation process in the mosquito takes approximately 30 days at an average daily temperature of 64 degrees Fahrenheit. The rate of maturation is temperature dependent. The maturation process in the dog takes about 6 months. Our current tests are able to detect the microfilaria or antigen from adult females. Therefore, we cannot detect infection during the L3, L4 or L5 stages in the dog.

Because heartworms are transmitted from host to host by mosquitoes, the transmission season varies with the mosquito season and climate conditions. In our area this means that most years the earliest transmission could take place is late May. Transmission rates will increase over the summer as mosquito numbers increase and temperatures allow the larval maturation process to proceed more quickly. Therefore, transmission rates are likely to be highest during the summer months and into early fall.

The mosquitoes that have been around longer are more likely to carry heartworm larvae which have matured to the infective stage. The mosquito season declines rapidly in the fall with freezing temperatures. The end of the season varies from year to year, but usually lasts through at least early November.

The activities of the heartworm preventatives must also be taken into the equation for determining a better time for testing. The most common heartworm preventatives are very effective at killing the L1, L3 and to some extent the early L4 larvae. They will not kill the later L4, L5 or adult heartworms stages. However, most of the common heartworm preventatives will shorten the life and stop, or at least decrease, the reproductive ability of adult heartworms.

When taken together, these factors suggest that in our area, the better time to test is in the spring. During this time, our ability to detect an infection from the previous season is high and it is before the start of the new heartworm season. For dogs on year round preventatives, the need for annual testing remains, but the seasonality for testing becomes less important.

The American Heartworm Society, www.americanheartwormsociety.org, recommends annual testing and preventative administration 12 month out of the year. Not only does this decrease the risk of your dog developing heartworm disease, most heartworm preventatives contain medications to treat intestinal parasites. Therefore, your dog is less likely to contaminate your home environment with parasites that can cause problems for people.

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