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  • The Village Vet

Let's Talk: Heartworms

Most pet owners have heard of heartworms, but may not understand what they are, how they're transmitted, or what can be done to prevent them from becoming residents in your pet's heart. Three weeks into 2020 and there have already been dogs diagnosed with heartworm disease in our county, highlighting the importance of year round protection. In January of 2019, Springfield, MA ranked in the top 10 nationwide for the largest percentage increase in heartworm positive diagnoses. This shows us that heartworms are here to stay, so let's talk about heartworms.


Just as the name implies, heartworms are worms that live in the heart... at least in the adult stages. These worms can grow up to 12 inches, but how do they get there? Here's a diagram that shows the lifecycle of heartworms (from of the Companion Animal Parasite Council):

We start with an infected animal - which can be domestic and wild dogs and cats, coyotes and even marine mammals. A pesky mosquito comes by and takes a bite from the infected animal, picking up the "microfilaria" that live in the blood stream (these are little baby heartworms that haven't started to grow up yet).

From there, the microfilaria actually start their development in the mosquito. Over the course of 2 weeks, the babies grow into the next stage, called "larva". After they reach this stage, they're ready to finish their development, and the next time the mosquito bites a dog (or other suitable animal) it will deposit these larva into that new host.

These young larva continue to develop in the dog for approximately 2 months before they migrate (travel) to the heart and finish developing into adults. Once you get more than one adult worm in the heart, they can start to multiply and make new babies (microfilaria) which travel through the blood stream and the cycle starts again. This can happen just 6 months after the dog was first bit by the mosquito.


Luckily, protecting your pet is easy. These days, heartworm prevention tablets are flavored and pets love to eat them - many think they're getting a special treat - and they only need to be given once every 30 days! Have a picky pet or one with severe food allergies and can't (or won't) eat the flavored tablets? No worries - there are topical preventatives that are just as effective. But wait... are the preventatives safe? Yes! Even collie breeds with MDR-1 mutations who are sensitive to ivermectin can safely take Heartgard Plus or Tri-Heart Plus, since the doses needed for heartworm prevention are so low... but if you're still worried there are non-ivermectin alternatives that can be prescribed.

Heartworm preventatives will work in the first 2 months after infection to remove the larva from the tissues (preventing them from traveling to the heart and becoming adults). This is why it's so important to test your dogs annually - if they're negative, we want to get them on preventative right away and keep them on preventative year round. If they're positive, then we need to prevent new adult heartworms from forming while we start the treatment process.

Oh no - we missed a dose! It's okay, I understand - we've all been there (yes, even I have missed a dose before). If you forget a month of preventative, make sure to give the next 2 doses on time to "catch up" for that missed month. If you miss several months in a row, you should give your vet a call for advice on when to plan for the next heartworm test and how to get started back on preventatives. Do you need help remembering the dose each month? You can put a recurring reminder in your phone, or adding a calendar event each month on the same date to remember. Don't forget to use the stickers that come with your preventative to mark on your calendar when you last gave the heartworm pills.

One last note about preventatives, and it's that they are recommended all year round. It's difficult to predict when the weather may warm up, and if the mosquitoes come out in March, but you don't start giving preventatives until May, then it may be too late to protect your pet. Monthly preventatives are <$10 per dose... but treatment can cost well over $1000. (As an added bonus, the monthly preventatives also help eliminate some common intestinal parasites your dog may pick up while walking outside, such as roundworms.)


The first step is to confirm the positive diagnosis. If an in-house test comes up positive, then a second test is sent out to a reference lab to confirm, or sometimes it's possible to see the microfilaria (baby heartworms) under the microscope. Either way, it's important to confirm the diagnosis before starting the treatment protocol.

Every once in a while a dog will test positive, even though they've been on preventatives. Some adult dogs who weren't previously on preventatives (such as rescue pups) may be negative on their first test but then come up positive on their test a year later - this is because the test checks for adult heartworms, and it can take up to 6 months for the adults to develop. In extremely rare cases, some pets that have been on preventatives their entire life may have a positive test result, but if you've been purchasing 12 doses/year from your veterinarian and have been getting annual tests, the manufacturer may cover all or part of the costs of treatment. This is one of the reasons that annual heartworm testing is recommended for all dogs, even while on preventatives (as a bonus, the test also checks for common tick borne disease, such as Lyme disease).


The American Heartworm Society's recommended treatment for heartworms lasts for approximately 7-9 months after the initial diagnosis. Once the diagnosis is confirmed, the dog is prescribed an antibiotic to kill bacteria that can live in some of the heartworms, called Wolbachia. This bacteria can cause inflammation once we start to kill the adult heartworms, which is why it's best to kill the bacteria first. During this time we also start on anti-inflammatory medication and safe heartworm preventatives to keep more adults from forming. This is also when we start start exercise restriction (sorry, but no running, jumping or rough play until we get a negative test result).

Two months after confirming the diagnosis is when the first adulticide (adult heartworm killing medication) is administered. This is a medication called "melarsomine", which is injected deep into the muscles in the dog's back. After the first injection, we wait 1 month and then administer the final 2 injections (given 24 hours apart). This 3-injection protocol is 98% effective in eliminating adult heartworms.

Approximately 4-6 months after the last injection is when we can perform a heartworm test to make sure the treatment was successful. During this waiting period, don't forget to continue giving the monthly preventatives! We don't want any new adult heartworms to develop.


That's great! The heartworm test can be performed during your housecall visit (the results are received before it's over) and we'll pick the right preventative for your pet. Have more questions? Don't worry - there's no question too big or too small, we'll discuss them all during your appointment.

Let your pet's heart be filled with love... not worms!

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