Heartworm disease happens, even in Colorado.  While the incidence is lower than in other parts of the country, it is a life threatening disease easily prevented and difficult to treat.  We recommend April through September Heartworm preventative and annual testing.  Many heartworm preventatives also provide protection against  more common intestinal parasites which can be transmitted to humans and pets.  For this reason, we recommend that puppies take heartworm preventative year round.

http://www.heartwormsociety.org  contains everything you always wanted to know about heartworm disease.