Test Required After Dog Misses A Dose Of Heartworm Preventive

“Good morning, Kristi and Stephanie, I need a refill of Revolution heartworm preventive for Bowsie, please.”

“We can take care of that for you, Mrs. Stephens. Has Bowsie missed any doses of her monthly Revolution?”

“Yes, we just moved into our new house and I lost track of time during the transition. She missed last month. Does that matter?”

If your pet has been off his heartworm preventive and has gotten heartworms he must be tested before restarting heartworm preventive.
If your pet has been off his heartworm preventive and has gotten heartworms he must be tested before restarting heartworm preventive.

“Yes, ma’am, if Bowsie has contracted heartworms during the time she was off her monthly heartworm preventive and the restarts it, she could have a bad reaction that could even be fatal. Indeed, I see in our computer system that your purchase history reflects that you would have administered your last dose of Revolution month-before-last. For Bowsie’s safety she needs to have a blood test to determine whether she has any heartworms now. If the heartworm test is negative we can dispense more Revolution and get her back to excellent protection from heartworms as well as fleas!” said Stephanie.

“Bowsie loves to go bye-bye in the car, so I have her outside right now. Does Dr. Randolph have time to do the test?” inquired Mrs. Stephens.

“As it happens,” Kristi offered, “we have a 30-minute gap before our next appointment that you can fit right into.”

We performed Bowsie’s heartworm test, and, good news, the result was negative. However, that was not the end of the story, so we had one more discussion.

“Even though today’s test is negative, there is a limitation to any heartworm test, and that is that during the time Bowsie has been off of her heartworm preventive, there could be heartworms that are in her body now that are just not old enough to show up on this test. Furthermore, they could be too old to be stopped by restarting her Revolution (or any other heartworm preventive) now, which we will do. So, be sure to administer Bowsie’s Revolution every month for the next six months. Then, when you get a reminder in the mail for a followup heartworm test in six months, if that test comes up positive, we will be able to catch those heartworms early before they have a chance to do a lot of damage. If the test is negative then, we will be very grateful that heartworms didn’t sneak in during the time she was off heartworm preventive.”

“Just remember,” I added, “Bowsie is not out of the woods until she has that followup heartworm test in six months.”

“I understand,” said Mrs. Stephens. “Could Stephanie put the first dose of Revolution on for me now?”

“We will be happy to do that. And, we’ll get Bowsie another treat afterward.”

Bowsie’s situation is a scenario that plays out every single day in every veterinary practice in America. People are busy and forgetful. They lose track of the stickers that come with their heartworm preventive, they overlook the monthly automated e-mail reminder from Pfizer Animal Health, they forget about the automated phone reminder. They unintentionally put their pets at risk for heartworm disease.

Heartworm preventives are prescription medications for a good reason. The federal Food and Drug Administration (FDA) sets regulations for medications that have the potential for harm as well as good. Heartworm preventives really can kill your dog if they are used improperly, so the FDA limits dispensing of heartworm preventives to veterinarians, trained medical professionals who understand both the benefits and the risks.

So, when your pet’s doctor informs you that your pet must have a heartworm test prior to restarting his heartworm preventive, please comply cheerfully. Don’t ask him to “overlook it just this one time,” or “I know my dog doesn’t have heartworms” (not even your veterinarian knows until he has the test results!). He puts your dog’s health first, and has taken an oath to do what is best for his patients.

See you tomorrow, Dr. Randolph.



  1. Also, remind pet owners that if they are switched from a “swallow” heartworm pill to a “chew” heartworm pill the dog MUST chew the pill or the preventive will not work. A dog can not swallow a must chew heartworm pill. No one told me this and the fine print was hidden away in pages of small print directions that came with the medicine. My dog suffered.

    • The Heartgard Plus package insert says, in part, ” Because most dogs find HEARTGARD Plus palatable, the product can be
      offered to the dog by hand. Alternatively, it may be added intact to a small amount
      of dog food.The chewable should be administered in a manner that encourages the
      dog to chew, rather than to swallow without chewing. Chewables may be broken into
      pieces and fed to dogs that normally swallow treats whole.”

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