Heartworm preventive products have never been “100% effective”, but they have been pretty close. It is interesting to note that in the drug approval process, testing is done in a (relatively) small number of Beagles in a laboratory setting, using only 100 microfilariae to infect the subjects. This is a far cry from the outdoor Labrador Retriever getting infected with thousands of microfilariae each month. In a way, I’m surprised that in years past we have seen almost 100% efficacy under field conditions.
As far as failure rates in one product versus another, it was the consensus of all present that there is no heartworm preventive that can be singled out as being worse or better than the rest.
Timing of the medicine dose does matter. Does “once per month” equal every 30 days? Maybe not. Maybe a client gives the medicine on July 4 and August 20. If you give the medications more than 30 days apart, risk of infection increases. More than 45 days apart increases the risk even more. Failing to give the medicine year-round in endemic areas dramatically increases risk of infection. Client compliance is obviously a factor, but I doubt that we have experienced a sudden and dramatic change in the numbers of clients who “cheat” on their medication, versus years past.
Merial (the makers of Heartgard-30) has done a market research study showing that many dog-owners lack awareness of the severity of the disease and of the expense and complexity of treating it. They don’t realize it is fatal, and believe it is easily and inexpensively treated “with a pill”. Thus they lack incentive to bring their dogs in and start preventive medicine. Here the company is looking at the “unchurched” for marketing purposes.
I doubt that this data is as applicable in endemic areas like ours as it is where the parasite is less problematic. Rarely (unlike 30 years ago) do I now find clients who have “never heard of heartworms.” And in considering the treatment process, I personally encounter more people who believe that the disease is always fatal and untreatable (which is certainly not true).
Merial’s proposed solution to this lack of awareness (found in their surveys among the “unchurched”) is to start a new ad campaign that diverges from the old “warm fuzzy” approach, to a new “scare hell out of them” approach. This ad appears to be the beginning. It says: “Preventing heartworm disease is easy. Treating it, sadly, is anything but.”
Again, this lack of awareness is not something that I am concerned with in my own patient population. We work strenuously to educate our clients. While there are certainly those with misconceptions, there are few who "never heard of it" these days. It certainly doesn't explain the cases of apparent failure of year-round preventive medication.
Immune system participation is implicit (but not fully understood) in the degree of natural resistance to infection, performance of the preventive medication, and in the outcome of the treatment process. This doubtless accounts for some variation in success in all these areas. Dogs just aren’t all the same.
Unfortunately, no strategy was recommended to increase the effectiveness of preventive medication. Clinical experience in our practice has shown no advantage to dosing more frequently or with higher doses (dramatically higher doses in some cases).