The 'Slow Kill' Treatment & It's Flaws

Typical Heartworm Test
with a Light Positive Result
(Updated 3/31/16 as some folks are confused!) 
We recently passed Pebbles' one year Gotcha Day, which meant she was due for her annual vet visit, vaccines, and testing. It was a sad reminder that she has not yet been adopted, but at least I know, with me, she is safe.

Now, usually I don't inform you guys when the crew gets their vaccines, but this time...this time I wanted to do some educating. Pebbles is a perfect 'case', so here we go.

A typical canine heartworm test (for dogs, cats are a bit more complicated) has a negative, light positive, and heavy positive result option. (This does not necessarily indicate 'how many' but one could argue that it does. Their argument is that small amount of worms will produce less hormone than a large amount of worms. Regardless, discussing heartworm hormones is not necessarily important, here.)

A year ago, when Pebbles was first tested she had a very faint 'light positive' result. Okay, no big deal. This is not major...but, we weighed our options. At the time, she would not be able to handle the actual heartworm treatment. It would be too hard on her sickly little body, and so we opted for what folks call in the rescue/shelter world the 'Slow Kill Treatment' using Heartworm Prevention alone. Her original test was almost hard to tell it was positive because the 'light positive' dot was so faint.

Now, to debunk some pretty common and wide spread myths...this 'method' doesn't kill anything. (...and the crowd goes wild!) Let me explain why... ;)

Heartworm Prevention don't prevent heartworms either. Yes, yes...I said it. Heartworm Preventions prevent Heartworm Disease. At any given time, especially here in the Southern US, every dog on heartworm prevention will have baby heartworms (microfilaria) in their blood stream. These babies are growing and trying to find the heart so they can bunker down and grow up to adults. However, when you give your dog their heartworm prevention, it wipes out all the babies in the blood stream in one go.

Heartworm prevention works backwards; it's ultimately a dewormer, if you will. It doesn't keep medicine in the blood; it doesn't store little chemical assassins that sneak around and attack the babies as the mosquitoes deposit them. Instead, the day you give the pill...there is an onslaught in your dog's blood stream and all the baby heartworms are wiped out. That's it. Savage, right?

So, let's get back to the Slow Kill Treatment/Method, keeping in mind how Heartworm Prevention works. This 'treatment' has a variety of ways to do get to the same goal, like most things. Some give heartworm prevention as normal (once a month), some do it every two weeks, some do heartworm prevention as normal with antibiotics, or antibiotics every 2 weeks with monthly heartworm prevention, etc. I have seen it in any number of ways. NONE of these are a true 'treatment'. In fact, American Heartworm Societies are urging veterinarians to stop this nonsense because we are creating resisting worms. What will all these rescue and shelter groups do for their dogs when the heartworms are resisitant to heartworm prevention? Scary thought, right? (I have added a note about the use of Doxy at the end of this post.)

The Slow Kill Method basically keeps the heartworm burden from progressing, but not the actual disease caused by the adult worms. With any positive test, the dog already has adult worms hunkered down in their heart. These tests only pick up a hormone produced by adult female worms; heartworm preventions don't touch these worms. They can only attack a certain life stage of the entire life cycle. The idea is that you give the dog heartworm prevention and the dog has to 'out live' the adult worms.  While the prevention keeps the dog from getting more worms, the adult worms are wreaking havoc on the heart and lungs and surrounding tissues until they die on their own.

It's actually a misconception to call this method 'heartworm treatment', and we run into folks all the time who say their dog has been 'treated', and they, in fact, have not. They've just been giving Heartworm Prevention and crossing their fingers.

Most dogs who are 'light positive' will come up negative in a year or so because their burden is not that heavy and adult heartworms only live 5-7 years. So, the dog outlives the adult worms they already had and the heartworm pill kills off the babies each month. There are arguments that a dog less than 5-7 will have a positive test and become negative within a year, so how do you argue the preventions don't touch adult worms. This is a whole huge debate, and I will leave it as the animal's immune system and various other factors come into play when we start discussing parasites. So, back to what I was saying, after year or so, the test is now 'negative' and everyone is excited and call this new 'treatment' a life saver, when really...it wasn't. The heartworms are/were still in the heart the entire time damaging the heart and lungs while we wait for them to die off.

Now, don't get me wrong...I don't think rescues are doing 'wrong', by any means. These dogs are given a second chance and are kept from getting more severe heartworm burdens. It is FAR better, IMO, than to give nothing at all. However, calling it a 'treatment' is where I get a little sideways about it. BUT, that's for another time. ;)

For Pebbles, like I said, we opted for not treating her at the time and starting her on a safe heartworm prevention. Not all heartworm preventions are safe for positive dogs, so it's important to be talking with your veterinarian before you listen to whoever said whatever on the internet. ;) Again, the idea was to keep Pebbles' heartworm burden (the number of worms) in her heart from getting worse and to let her outlive the burden of adult worms that she had.

However, Pebbles Heartworm Test, this year, is WORSE than her original test. One could argue, (as I said, depending on the vet you talk to) Pebbles' burden is heavier as she has MORE hormone in her bloodstream, and now we are faced with some tough decisions. Actual Heartworm Treatment is no joke, not to mention cost. It can be anywhere from $500-$1500.00...and to think, it could have been prevented for less than $5 a month! But, I digress.

Best Pals

So, how is Pebbles' test showing she has MORE heartworm hormone even though she has been getting heartworm prevention religiously?

As I mentioned before, these preventions don't touch adult or 'teenage' worms. They ONLY combat a specific life stage. From the time the mosquito drops the microfilaria (baby microscopic heartworms) into the dog's bloodstream, it takes 6 months for them to mature enough to the adult stage to show on heartworm tests, IF they are female. There are a number of different life stages in that 6 months, and like I said, heartworm preventions DON'T tackle them all.

So, Pebbles had adult worms (she had a very faint light positive test, remember?), but she also had other life stages that weren't wiped out by her monthly heartworm prevention, because...

HEARTWORM PREVENTIONS ONLY COMBAT A SPECIFIC LIFE CYCLE. Thus, why this 'method' is NOT a true treatment for heartworm disease.

Phew. Sorry. I didn't mean to yell...but, seriously! For the love of all that is holy and true, don't miss a dose on your dog's heartworm prevention. These little suckers can slip through the cracks and here we are with Pebbles who is now needing legitimate heartworm treatment because SOMEONE before me...missed doses. I mean, truthfully, she probably never had heartworm prevention in her life before me, but you get my point.

Also, just for fun...here are some quick facts regarding Heartworms in the US:
  • Heartworm disease has been reported in all 50 states.
  • Heartworm indicators are found in the blood of 80%-90% of dogs.
  • One million dogs are estimated to be heartworm positive in the United States each year, but only 30% of them will actually be diagnosed with a heartworm infection by a veterinarian.
I know this is super long, but super informative. I hope it inspires some of you to talk to your vet about heartworms and heartworm prevention (regardless of what state you live in!) and of course, gives you a better understanding of what the 'Slow Kill Treatment/Method' actually does and where it's faults and flaws lie. The adult worms are kept in the heart causing irreversible damage (heart failure, respiratory issues, etc.) while we 'wait' for these 'slow kill' methods to 'work'. Pebbles is living proof that this 'method' isn't always a guarantee.

Disclaimer: Though we are potentially creating resistant heartworms, I do support the work rescuers are doing to keep these dogs from progressing into a larger and heavier heartworm burden.

Update: I have received a few messages  in regards to the 'Slow Kill' Method involving both heartworm prevention AND Doxycycline. Doxy weakens a parasite within the adult heartworm and it is believed it weakens them and they die faster. I did not include this type in this post because I am talking about folks who ONLY use heartworm prevention and call this a 'treatment' for heartworms.