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Heartworms in Dogs: Your Questions Answered

Out of the many dogs thought to be heartworm positive only a handful are ever diagnosed why? Is your dog one of them? Heartworms in dogs can be devastating to not just your dogs, but your family too! I’ve never personally met a dog with heartworms, have you? There are so many questions that are asked about heartworms it makes me think that, even though we’ve all heard about them, heartworm infections still remain quite a mystery to most people. And if that’s the case are we really protecting our animals to the best of our abilities? By that I mean making informed decisions about our petkids heath. I know I never really gave it a second thought. I just gave the preventative and forgot about it. There’s actually much more to it than just giving a pill or slapping on a topical each month.

What are Heartworms?

Exgtracted heartworms

Heartworms are actually a parasite called Dirofilaria Immitis a type of round worm that makes their homes in the heart, lungs, and circulatory system of our furry friends causing sever lung disease, heart failure, organ damage, and death. Found in all 50 states, heartworms are most common along the eastern coastline from the Gulf of Mexico to New Jersey and along the coast of the Mississippi River. Most importantly, heartworms can ONLY be transmitted by mosquitoes there is no dog to dog transmission. Obviously, they can affect dogs, but they also pose a threat to cats and ferrets.

Life Cycle of the Heartworm

AHS New HW Life Cycle CLIENT CLR 2018

Dogs are the natural hosts of the heartworm and as such the worm can mature, mate, and reproduce within the dog’s system.  There is quite the size difference between male and female heart worms. Males can measure 4”-6” long, while the female is considerably larger measuring 10”-12” in length. At the age of 6-7 months the mature female worm will release her offspring (microfilariae) into the host dog’s bloodstream. There they will be picked up by the feeding female mosquitoes where  they will undergo two molts (a matter of 10-14 days depending on temperature) becoming infective larvae, also called L3s. Once in their infective larvae stage the immature worms can be passed from mosquito to dog. L3s are deposited in the dogs skin by the feeding mosquito where within 3 days they will normally molt into fourth-stage larvae (L4s) in the subcutaneous tissues. These now L4s migrate through the tissues for several weeks. Approximately 2 months after infection the L4s undergo a final molt into a sexually immature adult stage. This is the stage where they enter the bloodstream and are carried to the heart and pulmonary arteries. It takes about 6 to 7 months for a heartworm to fully mature coincidently this is about the time microfilaria can be detected by blood test. If left untreated these worms can reside in a poor pooch for 5 to 7 long years.

What does a Heartworm Infection Look Like?

The number of adult worms in the dog’s system is known as worm burden. The worm burden is usually around 15 worms but can climb as high as 250 worms. Severity of symptoms depend on the worm burden and are broken down into classes.

Class 1 the dog shows few or no symptoms. Symptoms may include an occasional cough.

Class 2 The dog shows mild to moderate symptoms such as occasional cough and tiredness after a moderate amount of exercise. Chest x-ray may begin to pick up heart and lung changes.

Class 3 More severe symptoms present including a sickly appearance, persistent cough, and tiredness after mild exercise, trouble breathing, and signs of heart failure are common.

Class 4 This stage is also known as Caval Syndrome. Due to the heavy worm burden the blood flow back to the heart is blocked. Caval Syndrome is life-threatening, and the only treatment is surgical removal of the worms. This surgery is very risky, and many don’t make it through.

Heartworm Testing is SO Important

Quick heartworm test performed by veterinarians on dogs.

In 2013, CAPC announced a preventative resistant strain of Diofilaria Immitis which caused them to amend their previous recommendations. Now it is recommended that every dog above the age of 7 months be tested yearly and those who live in prevalent areas be tested bi-yearly. Year-round prevention is still strongly recommended in both areas. Which is why yearly testing, even if your dog is on prevention is absolutely necessary.

It is recommended by the American Heartworm Society that every dog over the age of 7 months be tested for heartworms yearly even if they are currently on a preventative. Your veterinarian can perform a simple blood test to tell whether or not your dog has heartworm disease. These tests can either detect the female heartworm’s proteins (antigens) or the microfilaria, but neither blood test can detect the presence of heartworms earlier than 5 months after infection. Starting a dog that is heartworm positive on preventatives could end very badly. Preventatives only kill the immature microfilariae. In a positive dog this can cause a mass die off triggering a shock response and possible death.

Treating Heartworm Disease

Unlike other parasites there is no over-the-counter medication that will work to cure heartworm disease. Melarsomine dihydrochloride (known by Immiticide and Diroban) is the only FDA approved treatment effective in killing adult heartworms for stages 1 through 3. Unfortunately, this treatment does contain arsenic. Consequently, treatment is potentially toxic to the dog and must be done under the care of a veterinarian. Injections are given deep in back muscles for about 6 months in some cases longer.

Stage 4 heartworm disease, also known as Caval Syndrome, is a life-threatening condition and is much harder to treat. The worm burden at this stage is so great that blood flow is physically slowed or blocked. Surgical removal of the worms is the only treatment available. Even with treatment most dogs who make it to this stage don’t survive surgery.

How Much Does Heartworm Treatment Cost?

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Prices for treatment vary depending on your location, the clinic itself, and how far the disease has progressed. If you’re lucky and catch the disease in its early stage, it doesn’t break bank averaging around $100. Mid stages price jumps dramatically. Depending on the course your vet decides to take (some vets will keep the dog in clinic until treatment is complete) the mid stage heartworm treatment can run upwards of $1000 depending again on how much the vet keeps your dog hospitalized. Surgical removal for stage 4 can cost as much as $3500.

Heartworm Prevention

There are multiple different  prevention options for heartworms in dogs.
Beef chewable for pet dog to prevent treat heartworms in white background

In this instance an ounce of prevention is worth TEN pounds of cure. There are many FDA approved products on the market that are used to prevent an infection. Available in monthly topicals, pills, and chews or an injection that covers your dog for 6 months they work by eliminating the immature larvae deposited by mosquitoes. These medications do all require a prescription from your vet but are much more affordable than having to treat an infected dog.

What Are the Real Risks of Your Dog Becoming Heartworm Positive?

heartworm incidence map 2016

This is where things get a little fuzzy. There are those who believe less is more or that the companies manufacturing the preventative is just out for money. Here are the facts the best I can find on the risks associated with heartworm infection. It is estimated that over 1 million dogs nationwide are infected with heartworms but only 30% of them are diagnosed. According to the Companion Animal Parasite Council (CAPC) between 2013 and 2016 the number of heartworm positive test rose in lesser common areas 11.4% and in the southeastern US where heartworms are much more common, they rose 17.9%. In those southeastern parts transmissions can occur throughout the year.

Transmission is all based on mosquitoes and climate. Immature larvae need to remain in their mosquito host until they mature into infective larvae which again take approximately 10-14 days.  There have been quite a few studies done on the effects of climate and the ability of the worm to transmit. At the University of Pennsylvania Vet School it was concluded that microfilariae cannot develop in the mosquito unless temperatures of 80°F are sustained for 2 weeks. This was supported by another study done by Washington State University’s School of Veterinary Medicine, which also added that worms could not mature unless day and night temperatures were sustained above 64°F for 1 month otherwise development would be slowed. With the slowed development the female mosquito may not live to transmit the infective larvae.

Why Year-round Prevention of Heartworms Is Recommended

Mosquitoes transmit heartworms to dogs.

I’ve read many other sits that insist that the American Heartworm Society only recommends year-round treatment to make money for the pharmaceutical companies or that they think pet owners are too forgetful to start the medication back up. When health and money come together it does seem to elicit a conflict of interest. While I can’t speak for their intentions, I feel their recommendations have quite a bit of merit.

We are changing the dynamics of how our world functions way this world works. Urban development is creating “heat islands”, buildings and parking garages are retaining heat during the day creating mosquito microenvironments and changing the drainage systems allowing for more mosquito breeding grounds. Couple that with either natural or man-made climate changes, the increasing rate of relocated and traveling dogs all adds to the spread of this parasite into previously low incident areas allowing the potential transmission of heartworms. As a matter of fact, I am sitting in my living room right now, it’s the beginning of February in Western New York and I just got bit by a mosquito on my forehead. Although the likelihood of infection is low the risk is increasing every year.

Heartworms are scary little critters that can turn your world upside down when you least expect it. The real take away here is there is a growing risk to your pooch. Hopefully now you have the tools necessary to make an informed decision on your pet’s health. If you do choose to not put your pup on monthly preventative, make sure that your dog is tested regularly through your vet. Early detection and treatment are crucial for happy outcomes. Dogs already leave us too soon, let’s make sure their stay is as long as it possibly can be.

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Jennifer Norris

Thanks for your informative blog! I will let my dog-people know about it!

marguerite gangloff

very nice keep them commin