Here is another article about the times of the year to give heartworm medication since it has to be 60 degrees for the mosquito to inject the worm and so, in Chaicago, for instance you only have to give it certain months of the year when it is warmer. I found it intriquing. They also use Black Walnut instead of drugs as a preventive.
http://www.clickersolutions.com/arti...lated_Info.htm