eartworm disease – the ugly truth

Did you know that the number of heartworm positive dogs in the state of Massachusetts has more than doubled from 2011 to 2015?  As per the Capcvet.org incidence maps – there were 940 dogs that tested heartworm positive in 2011 while in 2015 that number more than doubled and rang in at a staggering 1,951.  Well you might say – well that’s for all of Massachusetts, what about here in Norfolk county?  In 2011 Norfolk County had only 85 heartworm positive dogs, last year in 2015 that more than doubled to 233!!!  We are definitely appreciating this increase at both Medfield Veterinary Clinic and Westwood Veterinary Care.   We are seeing more heartworm positive dogs this year than we have previously – and we are seeing them all throughout the year, not just seasonally!! This makes heartworm prevention more important than ever.  Our moto here at Medfield Veterinary Clinic and Westwood Veterinary Care is “ONCE A MONTH, YEAR ROUND, NO EXCEPTIONS, NO BREAKS!”

So now that we know that the incidence of heartworm disease is on the rise, what is heartworm disease?  As the name implies, heartworms are quite literally worms that live in the heart (quite disgusting if you ask me).  Heartworm disease is transmitted by mosquitos carrying the larvae.  Mosquitos pick up the heartworm larvae by feeding on a heartworm positive animal – which might mean a domestic dog or wild animals such as coyotes or foxes!  Thus even if we as a community are 100% about our prevention, we can’t eradicate heartworm disease as our wild canine friends can harbor infestations.  Now if you give your heartworm prevention “ONCE A MONTH, YEAR ROUND, NO EXCEPTIONS, NO BREAKS!” it kills any of the larvae that a mosquito might have injected into your dog in the past month.  That is why it is very important to keep giving it every month.  This means that even when you are not seeing mosquitos you should still give the medication – you are playing clean-up of anything that might have happened in the prior 4 weeks.  If you wait too long in between doses (6 or more weeks) – there is a chance that the heartworm larvae will have molted into a new stage of life in which they can no longer be killed by your pet’s monthly heartworm prevention!!  It’s much safer (and more cost effective) to give your pet their prevention every month than it is to treat a heartworm infection, so get into the habit!

So what should you do if your pet has been off heartworm prevention?  First please call our offices to make an appointment for a heartworm test.  It is very important to test all pets for heartworm disease prior to restarting prevention.  If the test comes back negative, we recommend restarting prevention “ONCE A MONTH, YEAR ROUND, NO EXCEPTIONS, NO BREAKS!” We would also recommend having your pet retested for heartworm disease 6-7 months after restarting prevention.  The reason for this is the heartworm antigen test which is used to diagnose heartworm infestations in dogs relies on female adult worms living in the heart!  Given the life cycle of the heartworm, this takes about 6-7 months from the time of mosquito bite to the time of positive heartworm test.  We also recommend testing any newly adopted dogs from heavy heartworm areas (ex. the south) at time of adoption (or 6-7 months of age if they are puppies) and again 6 -7 months later.

Have any questions or concerns about heartworm disease?  Give us a call or stop on by Medfield Veterinary Clinic or Westwood Veterinary Care we would love to answer any questions you might have!

-Dr. Sara Buckley

Medfield Veterinary Clinic and Westwood Veterinary Care are proudly serving the communities of Westwood, Norwood, Walpole, Dover, Dedham, Canton, Sherborn, Millis and Medfield.

If you would like a more visual approach to heartworm disease – please refer to the heartworm life cycle handout on The American Heartworm Society’s website to better understand how and when your pet’s prevention is effective https://www.heartwormsociety.org/images/pdf/AHS2014HWLifeCycleCLIENTCLR.pdf.

Other heartworm resources include:



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