Years ago (1990s if my memory is correct) the heartworm preventative injection was pulled from the market in the United States as there were problems with it. I would not use it on any of my dogs or rescue foster dogs and would rather stay with the safer oral monthly products.
If you are paying $175.00 for six months of heartworm preventative, there are lots of less costly options for you. I use Interceptor and can get it for about $56.00 USD for six months for my German Shepherd. I order it on-line and my vet authorizes the prescription.
There are many folks who use the 1% injectable cattle Ivermectin and give it orally to their dogs (non-Collies). I used this for years before going to the Interceptor and just drew it up in a syringe, popped off the needle and squirted the liquid in the back of the dog's throat. A treat followed and no problems for many years with a multi-dog household.
I am sure that others will chime in with other options.
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge