There is a potentially emerging problem here in the US....potential resistance of heartworms to current preventative medicines. While it's not time to panic, there is reason for concern.
One of the worst areas in the US for heartworm disease is the lower Mississippi River region. The presence of water as well as high temperatures and humidity throughout the year make it an excellent breeding ground for the mosquitoes that carry heartworm disease. In the last few years there have been several cases reported of dogs who had been consistently on prevention yet still developed heartworm disease. It's gone beyond a couple of questionable cases to the point that top parasitologists are investigating the possibility of resistance.
All of this fear is currently speculation and hasn't been confirmed. It's also only been a worry in certain areas of the Mississippi where heartworm disease is the highest. There is enough prevalence of the parasite in this area that breeding and reproduction is rapid, allowing many generations in a short period of time. If anywhere was going to show a resistant population of heartworms, this would be it. However, before we panic and run around like Chicken Little, there is still much study and research to be done.
In a worse case scenario, development of resistance to current preventions would be a major health issue. Unfortunately, only about 56% of American dogs are on consistent heartworm prevention, much less than it should be. However, those dogs are well protected. If medicine starts being ineffective, even these dogs could be at risk. Then you would have pet owners who are doing the right thing suddenly learn that their pets have a potentially fatal disease that is expensive and somewhat risky to treat. There is the potential for a real canine health crisis if this turns out to be true and the resistance strains spread to other parts of the country.
This is definitely a topic to continue watching closely.