[JeffreyT]

You may be interested in this article from Dr. Karen Becker.
http://healthypets.mercola.com/sites/healthypets/archive/2010/08/03/why-havent-pet-owners-been-told-these-facts-about-heartworm.aspx

And here is more detailed info from Jasmusen…
http://www.dogs4dogs.com/blog/2009/05/13/heartworm-medication-safety/