Heartworm Preventative Considerations
It is still my opinion that dogs in the Chicago-land area do not need to be on HW preventative 12 months each year. By December we have no mosquitoes and the disease cannot be transmitted unless there are mosquitoes and these have to be infected mosquitoes. If you go down South throughout the winter then, by all means, stay on the preventative.
When a monthly HW medication dose is given, its microfilaria-killing activity reaches back 30 days (actually a little more than 30 days). This means that if you give a dose on December 1st, your pet is protected back to November 1st (or to mid-October, according to some studies). Another example - if you give a dose May 1st your dog is protected back to at least April 1st.
Heartworm testing: Feel free to test your dog during any month of the year. However, we still recommend that heartworm preventative be given from May 1st to December 1st. The biology of the mosquito has not changed in 50 million years (only veterinary clinic management procedures!).
There is emerging data to suggest that mosquitoes are developing resistance to our heartworm preventatives, this makes a yearly test even more imperative. More on this at new data emerges!
Very important!!- Some people who have their heartworm tests run in, for example, June, July, August, September, etc, wait until that time to start the preventative medication. This is wrong! (and may be one valid argument for giving the medication year round, for those people who cannot think or remember). Regardless of when your dog is tested, start medication around May 1st.