Are heartworms REALLY a thing here in Pennsylvania?

I have written about heartworms before, but in the context of animals down in Nicaragua on my mission trips. Recently, I have been having this same conversation multiple times in a row, and we have had a couple of patients who are on preventative, but have tested positive! This being said, I feel compelled to clear a few things up for our clients and followers regarding heartworms.

Let’s start this lesson with a quick, and exciting, life cycle recap – here is how heartworms work: the worm larva (called a microfilaria), lives in a mosquito. The mosquito bites a dog and transmits the microfilaria to the dog. These little buggers then travel through the body and spend the next SIX MONTHS growing, landing in the heart – which is how they get their name. After 6 months they are adults, so then start producing microfilaria of their own. So when a mosquito bites them, the process starts all over again.

Why the important distinction about them taking 6 months to mature? Let’s say your dog takes heartworm preventative part of the year. You stop it in September, and plan to restart in April as it gets warmer. What if a mosquito bites your dog in October (as you have seen, this October was still VERY warm) and transmits the microfilaria….and then you don’t restart preventative until April? That’s right…as you have JUST learned, there are now adult worms in your dog’s heart – and it is TOO LATE for the preventative to protect your pup.


Let’s also take a quick moment to talk about the preventative. One of the reasons people talk about not feeling comfortable using it, is for the long-term effects on their pet’s bodies. That, ‘they aren’t comfortable putting those toxins in their dog.’ But the good news is that is not how heartworm preventative works – it is literally just a dose of deworming. A broad spectrum dewormer, that covers for heartworms (as well as lots of other critters like roundworms, hookworms, and whipworms!). It works by killing any worms currently in the body – so it doesn’t have long lasting effects, and it doesn’t STAY in the body, and it is only effective against the microfilaria/larva of the heartworms, not the adults! This is why it is SO important that we stay on this preventative and give it every month! We must kill the baby worms before they can become big worms and are much more difficult and dangerous to kill.


So back to the lifecycle stuff…now that we understand why it is important to give it every month, I want to also make a point about if we miss a few months. There is about a 3-month window when the worms are just microfilaria and our preventative will definitely kill them. After that, they have started to mature and we may not catch them! So if you miss a few months, and then we restart the preventative – ideally we should be testing for heartworms 6 months from when you restarted the preventative. This is the safest, and most accurate way to make sure that our pup is protected now, but that we also follow up to make sure none squeaked through in that window of opportunity.

If you’ve made it this far (I fully recognize this is not a very exciting topic – but it is important!!), god bless you…but also, you may be wondering – how often does this really happen here anyway? Well, I can tell you in our little practice, we treat an animal with heartworms almost every week, or every couple of weeks. Most of them are from down south – they have a HUGE problem down there with warmer weather, more dogs living outside, less preventative use, etc. BUT, the number of animals testing positive who live up here, or aren’t always on preventative is slowly creeping up. I know of 5-6 dogs from PA who have only lived here, who we have treated in the past year! Is this a huge issue here? No…well, not yet. The incidence maps are creeping up with more and more animals further north contracting heartworms over time. It’s going up enough that I think this topic definitely warrants a conversation with your veterinarian about it. The preventative is great for multiple types of parasites, some of which ARE very common here, and can be transmitted to us (ew!) – so good to use anyway. Plus, heartworms can be deadly – so also good to protect against it, even if the risk isn’t huge! And if you are really not comfortable using the preventative, then consider testing annually – screening them for it is a great way to be able to take action if they were positive, before it starts causing heart disease or heart failure. (and as an added bonus here, the screening test also tests for Lyme disease, which is RIDICULOUSLY common here!)

The moral of the story, is that heartworms are more of a thing here then they were 10 years ago! And the preventative is a super safe, inexpensive way to keep our pets protected from this parasite, as well as many others! Food for thought….and discuss it with your veterinarian when in doubt!


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