We receive a lot of questions and confusion when it comes to testing for heartworm disease. There are so many situations where we recommend running a heartworm test that seem to make very little sense. It all boils down to an understanding of how testing works, the recommendations of the American Heartworm Society - which is considered best standard of care, and how heartworm prevention actually prevents heartworm disease.
How does heartworm testing work?
There are two main heartworm screening tests that are done in clinic. The first is the most common, which is an antigen test. The antigen test detects a protein that is released by mature adult female heartworms. This antigen can be detected at the very earliest at 6 months after a dog was exposed to heartworm larva from an infected mosquito. The antigen test cannot detect male heartworms, larval stages that are circulating in the system, nor microfilaria (baby heartworms that circulate in the blood). The second most common test run in clinic is a microfilaria test where we look for baby heartworms in a drop of blood under the microscope. In most cases, this test lets us know that not only have we seen the antigen from the female heartworms, but also that they are reproductively active. Presence of microfilaria means your dog can be a reservoir for heartworms to be passed to other dogs through mosquitos.
What are the recommendations of the American Heartworm Society for testing?
The American Heartworm Society recommends testing for heartworms prior to starting or continuing heartworm prevention in any of the following situations:
- puppies and dogs older than 6 months that have never been on prevention
- 6 months after starting prevention (with consistent dosing)
- before changing heartworm prevention products
- 6 months after changing heartworm prevention products (with consistent dosing)
- in situations of non-compliance - that is, missed doses or inconsistent administration of heartworm prevention
- 6 months after testing for non-compliance
The American Heartworm Society recommends testing for heartworms on an annual basis for all dogs that receive consistent heartworm prevention as directed by the label instructions for the heartworm preventative they are on. That is, if dogs receive monthly heartworm prevention every 30 days as directed year round, they only need a heartworm test once a year.
How does heartworm prevention prevent heartworm disease?
There are two common forms of heartworm prevention - a long lasting injection and monthly prevention. The long lasting injection is administered either every 6 months or every 12 months and works continuously to kill any infective larva that are deposited by mosquitos. Because it is stored in the body, it does not matter at what point during the effective treatment time a dog is exposed to heartworm larva, it will kill the stage 3 infective larva (and any stage 4 larva that survive) and prevent them from maturing. Monthly heartworm prevention does not typically stay in the system and continue to work, rather, it kills stages 3 and 4 heartworm larva that are in the system at the time of administration. With monthly dosing, anything a dog has been exposed to since the last dose will be effectively killed before it can mature. Unfortunately, missed and late doses can result in losing the window of effective control which can allow larva circulating in the system to mature beyond the point where preventative is effective in killing them. Monthly prevention must be administered every 30 days for effective control.
There is a window of 1.5-6 months after exposure to heartworm larva where prevention is not effective and disease is not detectable. Heartworm prevention can have negative health effects when administered to dogs that are positive for heartworm disease; therefore, it is important to ensure they are negative prior to starting prevention or resuming prevention after missing or late doses. Because of the window where disease is not detectable, testing 6 months after starting or resuming prevention ensures that any exposure during that window is accounted for and we are not risking dogs by providing prevention if they are potentially positive.
In a situation where a test turns up positive, we must verify the test by sending the blood sample to a reference lab for confirmation. In the case of dogs that have been on heartworm prevention and end up with a positive test, manufacturers require that all appropriate dosing and testing steps have been taken according to best care practices to make a claim for product guarantees which may cover part or all of the cost of heartworm treatment.
Still have questions? Call us today and we'll be happy to help.