Q. Can giving a dog heartworm preventative to a dog that has heartworms make them sick?
JESSICA L DESROSIERS
A. YES. Never give a heartworm medication to a dog positive for heartworm unless instructed to do so by a vet.
Heartworm medications require a negative test prior to administration. This is due to the fact that giving a heartworm medication to a dog that is positive may kill off the adult worms too quickly. This can cause worms to become lodged in smaller blood vessels in the body which may lead to loss of blood to vital body parts and organs.
If your dog is heartworm positive, treatment usually involves a slow administration of medications to kill the worms under veterinary monitoring. Some vets may recommend hospitalizing a dog during their first treatment so that emergency care can be given if there is an adverse reaction or a worm becomes lodged in a vessel. A more recent procedure is to do a day-long "fast" die off of worms under monitor to prevent worms from repopulating that may occur during a longer term treatment.