Heartworm Disease – What is it and why does it suck so much?
One of the most frequently asked about subjects in our lobby is heartworm preventatives:
Should my pet be on heartworm prevention year-round or just in the summer?
What’s the treatment if they have heartworm disease?
What’s the heartworm test for? Why should I test my pet? How often?
Why is all this so important?
Basically, there are a lot of questions about the subject, and this makes sense; most people will never see or interact with a heartworm in their life. Human’s aren’t a natural host for heartworms so the larvae typically die in us before reaching adulthood. Therefore it’s one of those diseases that we come across occasionally if we’re researching pet-related topics, but otherwise might never dig into any further. Imagine for a moment that there was a disease carried by mosquitoes that would kill you painfully by blocking blood flow to your heart and that the only cure was to inject arsenic into your back repeatedly. Well, if you’re a dog, that’s pretty much how heartworm disease works. This post will hopefully provide a thorough understanding of what these heartworm things are, why they’re so important in your pet’s life, and what you should be doing about them.
First, what are heartworms and why do they matter?
Heartworms are literally worms that live inside the heart and associated arteries of infected animals. In a dog, these worms can reach up to 12 inches in length and live for 5-7 years, with the average infected dog carrying at least 15 of them. They have a long multiphasic lifecycle that makes getting rid of them extremely difficult, since different stages of the worm’s development are either susceptible or immune to different treatments. Adult worms are immune to almost everything except an extremely dangerous and toxic drug called Melarsomine, which contains arsenic. A heartworm infestation is a very serious, oftentimes fatal disease which can cause exercise intolerance, fainting, respiratory difficulty, lung disease, and ultimately heart failure. Once a dog has become infested with these parasites, the only proven effective method for eliminating them is both extremely expensive and life threatening for the pooch.
Despite highly effective preventatives having been available for decades, the average number of heartworm diagnoses per clinic was up nearly 22% in 2016 vs 2013.
So only dogs can get heartworms?
Nope, while the dog is a natural host for this parasite, cats are considered resistant hosts. This means that cats usually have fewer worms that do not multiply as rapidly or live as long (usually around 2-4 years), and a purportedly lower rate of infection from exposure. An infected dog may harbor 50 to 100 worms, but a recent study on feline heartworm disease from Auburn University found only an average of four worms per cat. However, the feline body’s response is so severe that even these few worms can cause significant damage. Heartworm prevention, testing, and infection works a little bit differently in cats than dogs but the basic principle is the same: during the younger stages in their life the worms are susceptible to a low-risk, low-cost treatment option but if they’re left untreated for too long they rapidly become resistant to these and start requiring more toxic and dangerous treatments.
Well darn, so how does a pet get heartworms?
Heartworms are spread by mosquitoes. They cannot be transferred directly from one dog or cat to another.
In an infected dog, adult female heartworms release their offspring, called microfilariae, into the dog’s bloodstream. When a mosquito bites the infected dog, the mosquito becomes infected with the microfilariae. Over the next 10 to 14 days and under the right environmental conditions, the microfilariae become infective larvae while living inside the mosquito. Microfilariae cannot become infective larvae without first passing through a mosquito. When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite wound. In the newly infected dog, it takes about 6 to 7 months for the infective larvae to mature into adult heartworms. The adult heartworms mate and the females release their offspring into the dog’s bloodstream, completing the lifecycle.
Heartworm larvae are susceptible to a routine, non-toxic treatment during the blue section. Once they reach the red section the cost and risk skyrockets.
Ok, so I only need preventatives when there are mosquitoes out?
Not exactly, this becomes a risk-analysis question very quickly. While it is true, heartworms can only be spread through mosquito bites and it is also true that in Wisconsin the incidence of mosquitoes drops dramatically in the winter when the entire state becomes inhospitable to most forms of carbon-based lifeforms, the important thing overlooked here is that it only takes one. A little bad luck and a mosquito trapped in the house or another warm space that happens to be carrying infectious larvae can easily cost you hundreds if not thousands of dollars or your dog’s life. Heartworm preventatives, by contrast, cost $8 per month.
Think of it like wearing a seatbelt: 99.9% of the time your day would be the same whether you wore it or not; it’s a minor inconvenience and it occasionally chafes a bit, but that’s about all. Then, every once in a great while, wearing it keeps you from dying. So, much like wearing a seatbelt, every sane expert anywhere (including Precision Veterinary) is going to recommend you just keep up with heartworm preventatives year-round. Wisconsin winter lasts about 4 months, so giving up preventatives when it’s cold out saves at most $32. For reference, giving up takeout would save the average American approximately $36.75/week. So, if it would help protect dogs, we’d be happy to share recipes in this blog as well. (Disclaimer: Precision Veterinary does not consider culinary acumen when hiring, recipe quality may vary)
Monthly heartworm preventative medications do not stay in your dog’s bloodstream for 30 days. The active ingredients work to kill any larvae that have been in the system for the past 30 days, clearing the body each month. The medication is only needed once a month because it takes longer than a month for the larvae to develop to a stage where they reach the body tissues. However, it takes less than 2 months, which is why it’s so important not to skip months
Dramatization
Sure, but if I restart heartworm preventatives in the Spring, wouldn’t that kill the larvae?
Only if you’re really lucky. This goes back to the comment at the very beginning about heartworms having a long and varied lifecycle. For up to 51 days after infection the worms are still susceptible to a low dose of macrocyclic lactone, but towards the end of the larval stage, they’ve started to become progressively more resistant to it and you’re going to need to break out the big guns. The sort of big guns that will incapacitate your dog for at least 4 months. So, given that Wisconsin winter lasts 4 months, your odds of catching a latent infection before it’s progressed beyond the cheap and safely treated larval stage isn’t great.
Ok, so what does heartworm treatment actually entail?
Heartworm treatment is lengthy, expensive, and hard on an animal’s body. In most cases, it will be more expensive to treat for heartworm disease than if the dog had been on monthly preventatives for its entire life. Even after the heartworms have been killed off and treated, they often still leave behind damage to the heart and lungs. Therefore, the best way to deal with heartworms is to prevent them from infecting your dog in the first place by using a monthly preventative year-round and testing annually.
Step 1: once we’ve confirmed there’s an infection and that your dog’s condition is stable, we’ll typically start trying to kill off the microfilariae and commensal bacteria (for a fun tangent into the more nuanced biology of the worm google: Wolbachia). As soon as you start attacking any stage of the heartworm’s lifecycle, you’re going to start causing inflammation in the infected tissue, usually the heart and nearby arteries. This means that you’ll want to immediately start serious activity restrictions to avoid a potential embolism.
Step 2: a series of Melarsomine injections spaced out over a month. These are intensely painful and toxic; we sedate the dog through the most excruciating part and send home pain meds and a steroid to help keep the dog as comfortable as possible and control the inflammation that occurs as a direct result of the dying worms.
Step 3: Two more months of bed rest to allow the worms to finish dying off and your dog to recover from the inflammation and damage caused by the worms. Your dog will, by the end of this, have been unable to play or do anything else that might risk elevating their heart rate for a minimum of 4 months and may still have long-lasting damage to their vital organs as a result of this treatment.
Sampson finishing up his last Melarsomine treatment and on the road to recovery. This was not fun for a high-energy active dog.
Why on earth would I do something like that to my dog?!
As awful as that sounds, it’s still the best treatment out there. Pretty much everything else falls into the ‘wait and pray’ category of medicine; basically just hoping that your dog won the infection lottery and their worm burden is low enough that waiting for the worms to grow and die on their own in 5-7 years won’t cause serious long-lasting side-effects or death.
There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms.
Class 1: No symptoms or mild symptoms such as an occasional cough.
Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
Class 3: General loss of body condition, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die.
Heartworm Testing: What is it and why should I do it?
Heartworm testing in dogs typically checks for antigens (for those of you that haven’t taken biology in a while: antigens are typically proteins released by something foreign and antibodies are what your body produces to combat these). Antigen testing will reliably tell you whether or not there are currently live adult heartworms in a dog.
The reason for testing annually, even if your dog is on preventatives is because just about nothing in the real world is 100% effective (except abstinence! Thanks for the awkward memories middle school sex-ed class!!). Preventatives are extremely effective, just like seatbelts. You still shouldn’t drive around ramming people and it’s still possible that a pet encounters a resistant strain of larvae, or misses a monthly dose. So, the goal of annual testing is to catch any infection that might have slipped through the cracks early enough that the prognosis from treatment is still extremely positive.
Jackson – treated, recovered and adopted!
Parting Thoughts
This post is a lot more heavy-handed than our usual. It’s scary, the language is scary and there aren’t a lot of flexible options in it, but there’s a reason for that. We see a lot of heartworm dogs, and it’s not fun. Your dog’s heart is not meant to host worms and it doesn’t do well when it gets stuck with that job. Pretty much all of treatment focuses on minimizing the probability that it fails and heart failure is a terrible way to die (actually, more accurately, most dogs that die from heartworm disease actually drown as their lungs fill with fluid – but that’s another pretty long explanation). There is a simple and low-risk way to reduce the probability of ever finding yourself in this situation to nearly 0% that costs less than the average bill for takeout. I don’t care who you buy heartworm preventatives from or what brand you choose, but please, keep your dog up to date on them.
Resources
The American Heartworm Society – A group whose entire purpose is to learn everything there is to know about heartworms so you don’t have to. But, if you wanted to, they’d be good people to ask…
Heartworm Guidelines – Essentially the textbook for heartworm disease. This is where the information found on almost any site you find out there probably originated
PetMD – Really nice wording regarding the activity of heartworm preventatives
FDA Heartworm Facts – An extremely thorough overview of how the disease works and what to do about it
Paws, Whiskers, and Claws – Heartworm Disease in Cats
Simple Dollar – Provides reference prices for the average cost of takeout vs. cooking at home. This is the only statistic in this post that I can’t fact-check back to the research study it originated from.