This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
When a dog is tested for heartworms, a sample of blood is
drawn. The blood cells are lysed and the remaining sample is
examined microscopically for the presence of microfilariae.
(This is the Knott's test or Filter test, depending on how it's
done). So, if no microfilariae are seen, the dog is diagnosed
as being heartworm negative and you can restart medication.
Because of the development that the larvae must go through
prior to becoming adult worms and reproducing, it takes, on
average, 6 MONTHS from the time a healthy dog is bitten and
infected until the dog has circulating microfilariae. This
means that a heartworm test done less than 6 months since a dog
was bitten and infected will be *negative.* Dogs that have been
taking Heartgard present another problem in the detection of
heartworms. Heartgard will cause adult female worms already
present in the dog to become sterile, so the females will not
produce any microfilaria. Heartgard will not kill any adult
worms. The adult worms cause heart problems with dogs who have
heartworms, NOT the microfilariae. It is the adult worms that
we are really attempting to protect the dog from when we use
So if a dog is on Heartgard and is tested for heartworms using
the Knott's test, chances are the dog will test negative even
if there are adult worms present. There is a different, more
expensive test for dogs who may have sterile worms. It uses a
blood sample to test for antigens produced by the adult
heartworms. If the dog has heartworm antigen, it has a greater
than 99% chance of having heartworms. This test should be used
on any dogs that are on Heartgard since they will not have
microfilariae in their bloodstream. Likewise, if there are only
low numbers of circulating microfilariae, the Antigen test will
give a positive result where the direct Knotts (Filter) test
may be negative. Just like the standard Knotts test, the
Antigen test will be negative if the dog was infected less than
6 months ago.
It is therefore very important for those dogs on the monthly
medication to be tested with the Antigen test rather than the
Can another dog can get heartworm by coming in contact with an
infected dog's blood? (transfusion, bite)
No. If a dog was infected and had circulating microfilaria, and
these microfilariae were transplanted into a healthy dog via a
transfusion, the healthy dog would NOT get adult heartworms because
the lifecycle could not be completed within the body of the dog. A
mosquito is needed for development from the L1 to the L3 stage.
Could a pregnant bitch with heartworms give them to her own
No, for the same reason as above, you need the mosquito for the
intermediate stages between microfilarae and adult worms. While the
placental barrier will keep the microfilarae out, even if this
barrier broke down (which can happen), the pups will not be
How do those medications work anyway?
There are basically 2 types of medication available that will help
to prevent adult heartworm formation in dogs that are negative. One
type is the daily medication Diethlycarbamazine (DEC). It works by
killing any larvae that have crawled into the dog from the mosquito
within approximately the past 36 hours. DEC kills L3 larvae. Once
they molt into L4's, DEC will not kill them and these larvae may
develop into adult worms.