Heartworm Prevention Information

April is National Heartworm Prevention Month.



An article written in March of 2013 recently made a reappearance on the internet and has been pickup up on by various different internet chat sites including Facebook. It is a discussion about the effects of heartworm medications and dosage schedules and has some recommendations that are not supported by the American Heartworm Society or the veterinarians at our clinic. I feel that it is important to share with our clients current relevant information about all aspects of their pet’s care including the prevention of heartworm disease..

OK, so here is my view of the use of heartworm preventatives in dogs. It is true that in perfect conditions, the preventative works retroactively for 5-6 weeks depending on the patient as well as larval stage activity of the developing heartworm. Studies have shown, however, that this is quite variable and to make a blanket recommendation of giving every dog the medication every 6 weeks is really pushing the envelope. Even in the best of circumstances it leaves no margin of error for an owner forgetting to give the medication or giving it late due to multiple causes. This also make a patient even more vulnerable (more prolonged unprotected time) if there is an unknown loss of a dose (dog gets medication, dog goes outside and eats something stupid, dog barfs medication up, entire dose interval no longer covered).

The biggest cause of “failure” of the preventative is lack of compliance by the owner. This is not meant to be judgemental but even in this age of technology it is challenging to always remember to give the preventative on time. By giving the medication monthly, the challenge of compliance is lessened as the owner can link giving the medication to other things that are done monthly. It is much easier for our clients to think, “Oh, it is the first (or second, or third) of the month. Have I given HW medication yet?”. When the interval is every 6 weeks, this becomes more complicated and if a dose is given late, there is a period of lack of protection. For most of my clients, this is a real problem so giving them a reasonable interval (29-31 days) make it more likely that the medication is actually going to get into the patient.

The medications that we are giving, whether it is Heartgard or Sentinel, are present in the patient for approximately 12 hours. They are considered extremely safe at the doses recommended and are effective because they interfer with several larval stages in the developing heartworm. The medications ARE NOT present in the patient for a month (or 6 weeks), thus from a physiologic point of view, whether the 12 hours happens every 4.5 weeks or every 6 weeks, the benefit to the patient is pretty minimal. To say it in a different way, the preventative doesn’t last 30 days, we just give it every 30 days to wipe out any new larva that may have been introduced.

Finally, the use of the medication year-round is recommended for several reasons:
1. Again, compliance is much easier when there is no stopping and starting every year.

2. The changing weather patterns that we are seeing has made our New Mexico “winters” very mild recently with several stretches of multiple days well above freezing in January and February. Trying to tell clients when it is safe not to give preventative is impossible at this time.

3. The monthly preventatives that we are using have broad-spectrum anti-parasitic effects to help prevent the spread and infestation of our patients with common zoonotic intestinal parasites (parasites that can infect people). Because our dogs spend time out and about at parks, on trails and generally living the good life, it is important that we do our part in helping to prevent the spread of intestinal parasites in the environment that we share with children and other pets.

So, in theory it might be OK (in perfect conditions) to give these preventatives every 6 weeks but the only real benefit is cost (8-9 doses vs 12 doses a year) . Many of my clients now get their medication online or at Costco so I have little to gain from recommending monthly dosing. I firmly believe that the benefit of giving the medication monthly in my patients outweighs any financial benefit as the year-round, monthly schedule makes it much more likely that we will prevent this nasty disease. The incidence of heartworm disease in central New Mexico has increased almost 10-fold in the last decade so this is a real threat. Please feel free to contact our office if you have any questions about medications and Heartworm Disease.

Barb McGuire DVM

Barb McGuire DVM