Little Creek Veterinary Clinic

Which heartworm preventative is right for your pet?

     If you’ve ever skipped giving your pet its heartworm medicine because you figure last month’s dose will carry over, you could be making a deadly mistake.

     Pet owners might assume that the heartworm preventative medication stays in the pet’s body all month long, providing constant protection against parasites.  But that’s not how the medications work.

How does it work?
     When you give your pet its dose of heartworm preventative each month, it immediately goes to work killing off heartworms that have infected the body in the prior 30 days.  The medication then leaves the body, often 24-48 hours after dosing.  Your pet is then left “unprotected” for 30 days until you give the next dose.

     It is important to remember to give the medication on time, because worms that are allowed to grow past a certain stage will not be killed by the medication; instead they will continue on to maturity.  Have a look at this example:

April 4………..your pet receives its monthly heartworm preventative dose, which is designed to kill heartworm larvae acquired since the previous dose in March.

April 7………your pet is bitten by a mosquito and is infected with heartworms.

May 4………your pet is due for its monthly dose, but you have forgotten to give it; the larval infection is now 27 days old and is still in the range to be removed by the medication.

May 17……the larval infection is now 40 days old, but you haven’t remembered the dose yet.

June 4…..the larval infection is 58 days old; many have become juvenile worms and will outlive the heartworm dose that you remember to give today.  In another few weeks, the juvenile worms will enter the heart and lungs where they will mature and reproduce.

What if I keep giving the medicine?
     Eventually, the mature heartworms will produce babies (we call them “microfilariae.”)  Here’s where it gets tricky:  the monthly heartworm medicine also kills the microfilariae.  That might sound like a good thing, and in a way, it is:  without circulating microfilariae in the blood, your dog will not pose an infection risk to other pets (cats generally don’t exhibit this stage, so they do not pose a risk to other animals.) 
     Unfortunately, the absence of this stage of disease also produces false negative test results.  Because there are different ways to test for “baby” heartworms versus adult heartworms, it is crucial that you tell your vet if your pet has missed any heartworm doses, especially in the last six months or more.  With this information, the vet can choose the proper test to determine whether your pet has the disease.

Which heartworm preventative do you recommend?
     Our clinic carries several types of preventatives, depending on the needs of your pet.
     For cats, we carry only one brand:  Revolution
     For dogs, we carry HeartGard Plus, Iverhart, and Sentinel

Can I buy heartworm preventative over the counter?
     No.  In Virginia, heartworm preventatives are considered prescription drugs.  Your pet must have an exam and blood test by a veterinarian before starting this medication.  If you find heartworm medications sold at stores or online without a prescription – steer clear!  These can be imported fake medications.  The manufacturers of the legitimate brands will not honor their guarantees if the medications are not purchased from a veterinarian.
********************************************************************************************
Questions?  Use the contact form on this blog or leave a note in the comment section.