My two-cents on heartworm prevention | Boulder veterinarian

Aug 12, 2020

My two-cents on heartworm prevention | Boulder veterinarian

I find that most dog and cat owners on the Front Range fall into three categories when it relates to heartworm prevention and testing; 1) those who are all in, 2) those who are a bit skeptical and may only treat for six months out of the year (and are reluctant to test annually because they use prevention when mosquitos are out), and 3) those who aren’t worried and go without.

I remember being a dog owner prior to becoming a veterinarian.  I was probably in that “2” category above, even leaning “3” when it came to heartworm prevention (it should be noted that I’m wired as a skeptic).  I also thought annual testing was a waste of money – that testing every year was a little bit of a scam.  Even coming out of vet school, I began practicing in Colorado and we hardly saw it unless the dog was imported from the Southern U.S.  So, I get where the 2’s and 3’s are coming from.  I’m not judging.

But I do want to share what I’ve seen in Colorado over the past 5-10 years, which is more heartworm.  Data shows there’s more heartworm too.  A lot more.  A recent article in Parasites & Vectors by Jason Drake and Rudolph S. Parrish makes this very clear. Here are the highlights

  • From 2014-2017, more than 114,000 dogs (25,000-30,000 per year) were imported into Colorado by over 130 animal shelters and rescue organizations. The majority of these dogs were shipped to Colorado from states with higher heartworm prevalence.
    • New Mexico made up over 30% of these imported dogs
    • Texas and Oklahoma (combined) made up about 50% of these imported dogs
    • So 80% of our imported dogs during this time were coming from either NM, TX or OK (much higher heartworm prevalence areas than Colorado)
    • When they compared the national heartworm prevalence to Colorado’s, Colorado’s rate of increase is over 3x higher than the national increase rate.
      • So when comparing 2017 to 2013 and the % increase in heartworm prevalence, the National US increase in heartworm disease is about 21% and Colorado’s increase in heartworm disease is 69%
    • Out of the 130 rescue or shelter organizations who participated in this survey of the study, only about 1/3 of these organizations:
      • Tested, treated OR provided a single dose of heartworm preventative!

    The above isn’t very positive, and the typical exam room explanation of all that info can sound like a sales pitch to sell prevention.  And again, I get that sentiment.  Heartworm used to elude me because I couldn’t see it (you can see adult worms on ultrasound, but that’s beside the point).  The baby worms are microscopic, it takes a long time for them to develop, and dogs with mild infections can be asymptomatic.  In cats it’s a little different and I’ll outline that in a minute.  Things like arthritis are easy.  “See your dog limping, and hear that crunch in his hip?  Yeah, that’s arthritis.”  Same with dental disease. “See that rotten tooth? We should pull that and take an x-ray* to make sure it’s all gone.”  But for a lot of clients, when the vet starts speaking about microfilariae, L3 – L5 larval stages, and macrocycline lactones, they go cross-eyed and drop off.

    So, here’s a quick attempt to break it down without confusing you:

    -A mosquito bites an infected dog or a coyote (yup, coyotes).  The mosquito then bites another dog and injects heartworm babies.  Babies grow for six months into adults.  Dogs have been found with 100s of worms.

    -The oral heartworm meds work well, but only for 48ish hours.  It kills any baby worms that may have been injected over the past month.  If you miss a dose, the med may not work, and your dog can get adults worms.

    -You have to use two different types of tests; one for babies another for female adults, and one doesn’t work for the other.  And there’s not an inexpensive way to test for male adults.  The routine 10 years ago was to only test for female adults, which told you nothing about the presence of those baby worms.   Unfortunately, if you only test every couple of years those worms can multiply by magnitudes.

    -Year-round medication is highly recommended.  Dogs can get bit late in the year, and mosquitos can come out earlier than many people think.  And again, if you miss-time your medication, it may not work.

    -We can treat it, but it’s expensive, it’s not always successful, and it sucks for you and your dog.

    – I had one patient that lived in Frisco, CO that got heartworm (and Frisco’s altitude is 9100 feet).  Yes, mosquitos live at altitude too.

    So, here’s my super-quick recommendation;

     -Test for baby worms and adult worms every year (yes, you need two types of tests)

    -Give prevention year round

    I see heartworm more than other diseases in our “core” vaccination protocols (with the exception of parvo perhaps, but it’s close).

    A brief note on cats;

    -Cats can get it too.

    -Cats aren’t a natural host for heartworms, so they very rarely have adult worms, and babies are only present in extremely small numbers.

    -Because adult worms are rare in cats, and there are usually so few baby worms in cats, it’s extremely difficult to diagnose (and often isn’t).  X-rays* and echocardiograms (“heart ultrasounds”) are good methods of testing, but you’ll be out $500+ financially every year to test and most people can’t afford that.  Understood.  But we can do it if you choose.

    -There is an antibody test but there’s only a 50% chance there’s an ongoing infection.  Often a positive only means exposure, not disease.

    -Even though cats aren’t their natural host, it royally screws up their lungs and they end up looking asthmatic.  And then they usually don’t survive.

    -The med that treats heartworm in dogs can’t be used in cats.


    All of this is wordier than I intended, but it’s hard to explain heartworm without all the verbiage above.  That’s part of why our skeptics check-out when vets try to explain the disease.  It’s also probably why people are bugged when we simply say; “you should do this and that, and it’s an added cost.”  Most people want options, and I’m one of them.

    One thing for now is true: in the odds game, your dog or cat probably isn’t going to get heartworm.  But the same applies to not wearing your seat belt.  You’re probably going to be ok, but a big part of that is out of your control (oncoming traffic, drunk drivers, deer…I’m from Pennsylvania, and there’s a lot of deer crossing the road.  I’ve hit two, both at night. At night you can’t see them until they’re in the headlights).  And with heartworm or car accidents, they can be life changing if not ending.

    One more note; There is an injection that provides year-round protection.  The med is contained in these little teeny-tiny balls that slowly dissolve in a dog’s fat.  It’s pretty slick, and it’s what I use for my dogs.  The oral meds work well too.  Just don’t miss a dose.

    That’s my two cents on heartworm.  The recommendation in seven words; test annually and use prevention year-round.

    *I’ve put an asterisk next to the word x-ray because in the veterinary world we call them radiographs, not x-rays.  Technically radiograph is a better term, but I think it’s a bit much.  People know what x-rays are.  They usually don’t know what a radiograph is.

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