Heartworm preventative recommendations for Sighthounds by Dr. W. Jean Dodds, DVM, is and has been available via her popular Pet Health Resource Blog.  If you missed it -- here is the abbreviated info from her her article "Heartworm: A real and present danger"

I took parts from the section "Quick Guide to Heartworm Preventives" to share with Sighthound fanciers.

• Foremost, I recommend administering heartworm preventive every 45 days instead of every 30 days, but only if this interval is strictly adhered to and if it’s difficult to keep track with a reminder calendar, then your dog may need to stay on the medication every month.  
• Animals that have been taking monthly preventives for a relatively long time may develop subsequent product intolerance. Other preventatives should be sought out or rotated (Please see her blog and other articles for more details on alternatives).
• Dogs affected with autoimmune diseases and their immediate relatives should receive only plain daily heartworm preventive (Dimmitrol = diethylcarbamazine). This drug can be found in Canada. For more information about dogs who have autoimmune diseases, chronic thyroid or liver conditions, please visit my blog post, “General Recommendations for Heartworm Preventives.”
• Trifexis is an oral preventative for fleas and heartworm. Trifexis contains spinosads which are contraindicated in epileptic or seizure prone dogs and should not be given to these dogs. Unfortunately, this is generally unknown and should be shared with your veterinarian, friends and family.
• Plain milbemycin oxime (Interceptor) is preferable as a heartworm preventive for certain groups of dogs, like sighthounds, smaller white breed dogs and those prone to seizures. (My bold font)

For my Wolfhounds, I now use Interceptor as my preferred brand of heartworm preventative and I administer it every 45 days throughout May to December.