At our hospital, we recommend Interceptor as our main heartworm preventative. Interceptor also acts as a once monthly dewormer for intestinal parasites, and eliminates the need for fecal analysis to treat and prevent worms. Our prices on Interceptor are very affordable:
2-10 lbs - $3.00/pill or $36.00/year supply
11-25 lbs - $3.50/pill or $42.00/year supply
26-50 lbs - $4.25/pill or $51.00/year supply
51-100 lbs - $6.25/pill or $75.00/year supply
Pets over 100 lbs should take one 51-100 tablet and the cooresponding tablet to meet their weight class every month

If your dog is not already on heartworm prevention, they should be tested in March or April. Once tested, all dogs should start and maintain on the appropriate dose of Interceptor every month. This is the only way to truly prevent your dog from getting heartworms, and the only way to stop the spread of heartworm in Colorado.