A great point was brought up by Kathy on my last post. Unlike those of us in the Midwest she is fortunate to live in a part of the country where many parasites are virtually non existent. However, times are changing. Data provided by the American Heartworm Society shows that heartworms have been diagnosed in all 50 states now and incidence of heartworm positive dogs are on the rise as shown in the following maps.
Another reason is the increased mobility of our human and canine populations. As people travel with their pets, relocate for jobs or other reasons they are bringing diseases with them to new parts of the country. With the increase in popularity of dog sports and shows people are traveling more widely and more frequently with their dogs on a regular basis. Think of what happens when a national event is held in a heartworm endemic area? Thousands of dogs travel from all over the country and may be exposed to things they ordinarily would not see in their home area.
Furthermore dogs are bought, sold and shipped all over the country. This includes rescue dogs who are frequently moved across many state lines to areas with better shelters, more resources, etc. The most significant example of this is the dogs rescued after hurricane Katrina. These dogs came from a heartworm endemic area (as you can see on the map) and were literally moved all over the country-taking their heartworms with them.
Finally there have been some very limited reports of heartworm resistance or preventative failure not linked to client compliance. These failures are very geographically limited at this time but they are being very closely investigated by parasite experts as spread of this strain (or strains) of heartworms could be catastrophic in our canine companions.
As a side note I saw a disturbing number or anti-heartworm preventative websites while looking for graphics to use in this post. These people are concerned with putting unnecessary drugs and toxins in their pet. I wonder if they realize that if their pet contracted heartworms it would need to be treated with very painful, very toxic injections? I'm not saying there isn't a a place for natural or holistic treatments but really, this isn't it. One website advocated giving the preventatives every 6-8 weeks instead of every 4. This is not a good idea. Does heartworm efficacy stop directly at 30 days. No, but it may very well drop to less than adequate levels by the 6-8 week mark. Each product varies and you take a huge risk by using it only every 2 months. Trust me, we don't sell it as a once monthly product to make money-it's because that is what is best for your pet. And finally, don't fall prey to the my pet is indoors/long-haired/etc and can't get heartworms myth. Remember when I got Lyric she was just 2 years old, long-haired, indoors and heartworm positive.
So, talk to your vet, determine your risk, and come up with a plan that works for you and your pet. Personally, no matter where I lived I would use year round prevention. Partly because I don't want my pet to ever go through treatment and partly because the preventative I use also protects against a wide range of intestinal parasites that can easily be picked up many places.