If you live in an area where heartworms are uncommon then this likely is of little importance. But here in the thick of heartworm country this is a daily dilemma of veterinarians and dog owners.
Many, many clients wonder why, if heartworm preventatives are so reliable do they need to spend money on an annual test to see if their dogs are still negative. After all, why shell out all that money (although some prevntatives are very reasonably priced) if we still have to run the test? And when heartworm tests range from $12-$40 depending on the test and the clinic and a person has multiple dogs, this seems a reasonable question to ask.
So, here are some reasons...
Heartworm preventatives are prescription drugs. There are some legal and ethical reasons that a veterinarian may not want to allow purchase of a prescription drug without first knowing the dog's heartworm status. In fact, certain preventatives can cause an adverse reaction if given to a heartworm positive dog.
Many companies will offer some sort of guarantee of their product and pay for all or part of the treatment if your dog contracts heartworms while on their product. Part of this guarantee requires annual heartworm testing.
While these products are all very good, no drug is 100% effective all the time and annual testing ensures that if there is any failure of the drug we catch it early. If you wait until your dog is clinical from heartworm disease then his chances at full recovery and less than if caught early. Also, heartworms are doing damage on a microscopic level even early in the disease so the sooner we know about it the better.
Resistance to heartworm preventatives is a fairly new, and geographically limited topic. And depending on who you ask, the issue may or may not be actual resistance, but poor owner compliance, etc. However, it is something we need to keep in mind.
Owner compliance is something that is a big issue-usually more so from the veterinarian standpoint that the owners. Typical scenario is that the owner claims to have kept their dog on prevention all year, but the records show that not enough prevention was purchased. Possible to have gotten some somewhere else, leftover from a dog that passed away etc, but trust me, those exceptions cannot possibly account for the number of times this situation actually comes up. Other situation is that when asked if owner needs more prevention they say they don't because they still have some. Again records would show that they should have used it all up by now indicating that they missed a few months.
And finally, there are the freak things that can happen such as the dog spitting out the pill when the owner isn't looking, jumping in the pool or lake right after a topical is applied, etc. And I know everyone will say that doesn't happen, they watch their dog, and those are great hypothetical situations, but I have a real world example of just such a situation. In fact, it's what brought this topic to the forefront of my mind.
Last month I gave all the dogs their heartworm prevention as usual. Lyric got a Heartgard chewable tablet. Eighteen hours later, she vomited up the whole pill, entirely undigested. Luckily I was home and also it was February. But what if I hadn't been there to see it, or it happened outside? In fact, I don't know that something like hasn't happened before. So it really served to remind me, and now hopefully my readers and clients how important annual heartworm testing can be.