Here is the thread I refered to earlier:
http://www.yorkietalk.com/forums/sic...lp-please.html
Has some relavent info in it but Kiwi had other problems too, mainly with being little. More serious HW discussion at the end of the thread.
Here is one link about the doxy /wolbachia and HW not able to reproduce (contains links to PubMed Studies):
DogAware.com Articles: Heartworm Treatment in Dogs