When Pets On Heartworm Preventive Get Heartworms

Sometimes pet owners administer their pets’ heartworm preventive right on schedule, but they get heartworms anyway.

There are no perfect medications.

One of my staff members came to me with a surprise a few months ago, “Dr. Randolph, I have something to tell you.”

Her lower lip began to tremble.

“I’m pregnant.”

She took her birth control right on schedule, but it failed. The “syndrome” is a lot more common than you might think.

Likewise, some pets have heartworm preventive failure.

Each manufacturer has his own statistics, but the breakdown rate is between one and two percent overall.

Of course, medication is not always to blame.

I recall the now-hilarious two-decades-old story from the time of daily heartworm preventives. A little Dachshund came to us for his routine semiannual heartworm test and it came up positive. With trepidation, I asked his owner whether she had failed to give any of his Filaribits.

“No!” was her adamant reply. “He loves them! I give him the little chewable tablet and he marches off with it to eat it behind the couch.

We discussed the concept of failure rates of medications and she accepted that the little fellow had gotten heartworms, that we would treat him, and he would have an excellent chance of full recovery.

My prediction did, indeed, come to pass and there were no complications during the heartworm treatment. In six months he would be back for another test to ensure that all of the adult heartworms had been killed by the treatment.

Three months passed, and I got a phone call from his owner.

“I decided to paint my living room and do a thorough spring cleaning at the same time,” she began. “My first step was to move all of the furniture into the middle of the room and cover it with plastic. When I moved the couch, you wouldn’t believe the size of the pile of Filaribits there. I thought he was going back there to eat them, he was actually hiding them behind the couch!”

Mystery solved. Filaribits absolved.

In 2009 almost noone uses daily heartworm preventives, preferring the monthly schedule. Furthermore, the failure rate of modern heartworm preventives is lower than older, daily medications. Still, they have to be given on schedule, and some people have trouble with that.

“But, Dr. Randolph, it’s not late, I just gave it five weeks ago.”

Monthly heartworm preventives are not “every-five-weeks” heartworm preventives. Nor are they “every-six-weeks” heartworm preventives. Pick a day in the month and give it on schedule.  Every month.

Every monthly heartworm preventive comes with calendar stickers to remind you to administer it to your kitty or pooch in a timely fashion.

Most manufacturers also offer a free phone-call service and a free e-mail service to remind you to stay on time. Take advantage of those! It can’t hurt to use all three, stickers, phone call and e-mail. You simply don’t want your pet becoming infected with heartworms!

Another common failure scenario is the “two people doing a one-person job” problem. Mom gives the heartworm preventive sometimes, Dad gives it sometimes. The problem arises when Mom thinks Dad gave the medication, when, in fact, neither of them gave it. Then they are both surprised when the heartworm test comes up positive.

Summarized suggestions for staying heartworm-free:

  1. All dogs and all cats need monthly, year-round heartworm preventive.
  2. Take advantage of all manufacturers’ reminders: calendar stickers, automated phone calls and free e-mails.
  3. Assign one responsible family member to the job of administering the monthly heartworm preventive.
  4. If you are using an oral heartworm preventive, actually observe the pet ingest the medication.

See you tomorrow, Dr. Randolph.


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