05/13/2026
What is heartworm?
Heartworms are parasites that live in the body in the blood stream and heart. The adult worms live in the heart, while the baby worms (microfilaria) circulate through the blood stream.
How does my animal get heartworm disease?
When a mosquito bites an animal with active heartworm disease, the mosquito picks up some of the baby worms (microfilaria) in that bite. The microfilaria mature into an infective stage in the mosquito until it bites another animal. In that second bite, the microfilaria are transferred to this new animal. Inside the new animal, the microfilaria mature over ~5-6 months into adult worms. The adult worms settle in the heart continue to produce more microfilaria in this animal.
Why do you need to test my animal before starting heartworm prevention?
An animal needs to be tested before starting heartworm prevention because the prevention kills the microfilaria when they are young enough (within 2 months of a mosquito bite), and if there is a high burden of microfilaria, as they die off from the prevention, they can cause your pet to go into anaphylactic shock.
Why do you say there’s a 6-month “lag” on the test?
A heartworm test only detects an adult female heartworm. Since it takes about 6 months for the microfilaria to mature into adult worms in your pet’s body, there is a 6 month “lag” from the time your pet was bitten to the time they will show up positive on a test.
Can there be false positives and false negatives on the test?
Yes! There can be both false positives and false negatives. False positives occur due to an error in the individual test itself. Therefore, we also perform confirmatory testing before starting heartworm treatment. False negatives can occur if your pet is being tested “too early” (i.e. the microfilaria haven’t developed into adult worms yet), or if your pet has an adult male population only since the test only detects adult female worms. Having a male only population is very uncommon, but more commonly occurs in cats since cats tend to have small numbers of worms in their heart (due to their smaller size).
