ColleenC:Actually, hearworm meds don't stay in the system.
ColleenC is correct. Interceptor and Heartgard are out of the dog's system in 24-48 hours. The "preventatives" prevent adult heartworms by killing
immature heartworms. Heartworm "treatments" kill
adult heartworms and are very hard on the dog (requires arsenic compounds).
A dog will
not have microfilaria unless it already has adult heartworms. The microfilaria are the initial stage of the heartworm babies. The microfilaria develop into stage L3 inside a mosquito. A dog is infected with heartworms when the mosquito deposits an L3 into a dog's skin.
Heartworm "preventatives" will kill stage L3, L4, and some younger L5 immature heartworms.
If your dogs have immature heartworms that have gotten to the older L5 stage, there is no way to find that out right now. Wait 6 months and have your dogs tested for adult female heartworms (L6).
NOTE: A dog has to have
several adult female heartworms before that test will be positive for adults. Have the testing done just before you are due for a heartworm "preventative" to have the greatest chance of detecting microfilaria. They accumulate between doses. There is no test for adult male heartworms.