This is another area that is getting a good, hard second look by dog experts. Many people feel that it is unnecessary to give an insecticide (poison) every month to our dogs in the remote chance that they will get infected with heartworm.
While heartworm does occur, it is usually in immunocompromised pets and outdoor dogs in areas that are highly infected with mosquitos.
For heartworms to develop in the mosquito, the temperature MUST be 57 or above day and night for 30 days. If you live in a hot part of the country, then this is a concern. But many of us don't live in areas where this type of heat occurs on a monthly basis.
Here's a couple websites that explains the heartworm cycle ... and the risks and benefits of monthly treatments.
It has also been proven that heartworm preventative stays in the system for 45 days, NOT 30 days. So it can safely be given every month and half, not monthly. It is labeled for monthly use because the pharmaceutical and veterinary industries decided that people would probably forget to administer it if it was labeled for 45 day use.
I am NOT saying to NOT give heartworm preventative. It is something you will have to decide for yourself. But with all the studies coming out on the likelihood of it occurring in healthy dogs, it certainly deserves a good hard look and weighing the risks and benefits.