In the fight against heartworm disease, the greatest weapon veterinarians and pet owners have is knowledge about the parasite, its life cycle, and the damage it causes to the vital organs of its host.  Adult heartworms are thin round worms that live in the heart and surrounding vessels.  They can grow to be up to 26 cm (10 inches) long inside of an animal’s heart.  Their natural host is the canine, but other animals can be infected as well, such as cats and ferrets, and wild animals like coyotes, foxes, and wolves.  They have a predilection for the pulmonary artery (the vessel that connects the heart to the lungs), but they can live inside the chambers of the heart (atria and ventricles) and in the other large vessels of the heart as well, such as the aorta and vena cava.  The usual life span for an adult heartworm is 3-7 years if they go untreated.  Dogs can carry hundreds of worms and sometimes the infection can be so bad that conventional treatments do not work and the only option is surgical removal.

Understanding the life cycle of the heartworm can help you understand how infections work as well as the best ways to prevent infection in your own pets.  The most immature stages of the heartworm are known as microfilariae or L1 (for Larvae 1).  These baby worms come about as most babies do, from the mating of a male and a female adult in the heart of an infected animal.  The microfilariae then float around in the bloodstream.  When a mosquito bites the dog some of the microfilariae are ingested by the mosquito.  Inside the mosquito, the L1s become L2s, and the L3s.  It is these third stage larvae that are infective to our pets.  The L3s live in the mouthparts of the mosquito (think about how tiny they are!), and are transmitted into the skin of an animal when the mosquito bites it.  The L3s become L4s in the tissues of the dog and spend the next 8-12 weeks migrating around and trying to find a blood vessel.  Just before the worm enters the vascular system, there is one final molt and the L4 becomes a young adult.  The adult worm migrates through the wall of a blood vessel and travels to the heart.  The entire process from mosquito to the heart takes about 4 months.  Even though the worms are now in the adult stage, they are still not detectable by the testing methods available at this time.  The worms must be at least 6 months old to be detected via blood test.

These days, the most common method of testing used by veterinarians is a tabletop antigen test.  Tabletop tests are convenient because they can be performed in the clinic and are quick and easy to run.  Just 1-3 drops of blood are needed for the test.  The blood is placed onto the test along with a conjugate and takes about 10 minutes before results are ready.  Antigen testing is very reliable because it detects antigen that can only be produced by an adult female worm.  The specific antigen the test is designed to detect is produced by the female sex organs of the adult heartworm.  If the antigen is detected in the blood stream of a dog, there must be an adult female heartworm living in that dog’s vascular system.  However, due to the specificity of the test, immature stages and adult males cannot be detected.  This is why it is not necessary to test puppies less than 6 months old as any heartworms present would not be mature enough to cause a positive test result.  However, it is important to note that if that puppy has some immature adults living in its vessels when it is started on heartworm prevention (maybe at 4-5 months of age), those worms will still be able to mature and may cause a positive test the next year when the dog is tested.  The reason for this is because current preventives are only effective at killing L3’s and L4’s, the stages present in the tissues, before they enter the bloodstream.  This is why it is so important to get a puppy on heartworm prevention as soon as possible. 

Obviously, worms living in the heart is a health hazard, but let’s talk about exactly what kind of damage these worms can cause.  During the life cycle, the worms must get from the skin into the vessels.  They do this by migrating through the wall of a blood vessel and then travel to the heart.  The vessels are damaged from the worm entering the vascular system, but the endothelium (the inner wall) of the blood vessels are also damaged as the worm moves around inside of them.  This leaves scarring that causes blood flow to be more turbulent, leading to inflammation.  When the inflammation cascade is activated by the presence of worms, the body goes into overdrive to try to fight them off.  White blood cells, platelets, and antibodies are rushed to the sight of the insult.  However, despite the bodies efforts, these forces are not strong enough to combat a live adult worm and only cause more problems for the animal infected.  Also, since the worm never leaves, this inflammatory cascade is in a permanent state of attack and cannot shut itself off.  This overload of white blood cells can lead to a weakened immune system, as stores are depleted in the fight against heartworms.  Additionally, the increased levels of circulating platelets can cause life-threatening thrombosis (stationary blood clots) and emboli (blood clots that travel through the blood stream and lodge elsewhere).

Inside of the heart, the worms can cause more damage to the delicate tissues of the heart especially the valves that separate the chambers and vessels and aid in proper blood flow into and out of the heart.  Once, the valve leaflets are damaged, they become scarred and can develop leaks leading to backflow of blood and a heart murmur.  The damage to the heart can cause the organ to have weaker contractions which decreases the outflow of blood.  The blood can get backed up in the heart causing an increase of blood pressure in the lungs (aka pulmonary hypertension).  This can lead to an enlargement of the right side of the heart, and ultimately heart failure.

The damage that heartworm infections cause is not exclusive to the heart, lungs, and vessels.  Heartworm disease can also affect the kidneys of your pet!  Remember the inflammatory cascade discussed earlier?  Well those antibodies that your body produces have to go somewhere.  The antibodies themselves are easy enough for your body to dispose of, but when they attach themselves to tiny antigens released by the worms, the antigen-antibody complexes become too large for the body to easily excrete.  These large complexes damage the delicate filter the kidney uses to excrete water and waste products, but retain other substances needed for the body such as proteins, red blood cells, and electrolytes.  This damage to the endothelium is known as glomerulonephritis and can eventually lead to full-blown kidney failure.

It is important to note that some of the effects of heartworm disease are due to the LIVE worms in the system and the inflammation that they cause.  Endothelial damage, lung infiltrates, decreased immunity, and kidney damage will continue to worsen as the dog remains untreated and the worms are left living in the vessels.

The good news is, there is an easy way to keep you fur babies safe from these tiny parasites year round!   All you have to do is remember to give your dog a monthly dose of preventive every month forever.  Yes, this can seem like a daunting task.  I often advise pet owners to make it routine by always dosing on the 1st of the month, or setting a repeating reminder on their phone.  All heartworm preventions come with stickers so that you can put them on a paper calendar, and most veterinarians send out reminders when it is time for you to come back in and renew your prescription.  That’s right, you need a prescription for heartworm prevention.  This is because it is important to ensure that your dog is heartworm free before placing him/her on a preventive.  A sudden dose of certain preventives can be very dangerous for a dog with a severe heartworm infestation because it can cause a mass die-off of microfilariae in the blood stream which can result in thrombi, emboli, and even anaphylactic shock.  Keeping this in mind, you must go to your veterinarian and have your dog tested for heartworms once a year.  During this appointment, your dog should also have a physical exam.  Any time a veterinarian writes or fills a prescription for a patient, they are legally required to have what is called a “Veterinary-client-patient-relationship” (or VPCR for short) with you and your pet.  This is established by having sufficient knowledge of the health of your pet (ie., performing yearly physical exams).  These physicals can also help keep you on top of vaccination needs, or developing health problems that may have gone unnoticed over the course of the past year.

If you’re wondering what type of prevention is the “best” there is a simple answer for that.  Whichever one works for you, your pet, and your lifestyle!  Do you need a preventive that also covers fleas? Do you want a chewy treat or is your dog good at taking pills for you?  Would you prefer a topical?  Is cost an issue for you?  All of these questions are best addressed by a veterinarian, but here are some fast facts to get you started:  For cats, Revolution is the only labeled heartworm preventive, and it also covers fleas, ear mites, and some ticks.  There are no preventives that cover everything (intestinal parasites, fleas, all ticks, and heartworms), so using two products together is often the best way to go (like Heartgard for all the worms, and Nexgard for fleas and ticks).  Some of the oral preventives may make your dog vomit, but do not lose hope!   Those products may be using a certain flea preventive that can cause stomach upset, but there are other products that use different ingredients that may not affect your dog’s tummy.  Whatever your preference or needs, your veterinarian can help you find the perfect product! 

Many people feel that preventives are unnecessary for their pets because they are “inside only.”  Unless these dogs are trained to go to the bathroom inside, then they are not truly “inside only”.  If you have trained your dog to go to the bathroom inside, I would suggest you take that baby outside for walks every once in a while for some enrichment activity!  Either way, every dog needs to be placed on prevention.  Have you ever seen a mosquito inside of your house?  I know I have.  A study out of North Carolina State University found that 25% of the cats they found infested with heartworms were indoor only cats.  Even as humans, when we go outside for short periods of time, we are often bitten by mosquitoes.  Additionally, the burden of heartworms in dogs and wild animals is heaviest in south Texas and the Mississippi River Valley, and these areas are rapidly growing!  It is also important to note that once a dog is infected with heartworms, the treatment is incredibly expensive and dangerous for the animal. It is much less expensive, safer, and less stressful for you and your pet to simply use a monthly heartworm preventive.

Treating an animal infected with heartworms can cost anywhere from $2000-$5000.  This includes the drug to kill the heartworms, steroids and antihistamines to prevent anaphylaxis, and hospitalization will be required for at least the days the treatment is administered.  The most dangerous aspect to the treatment is what happens to the worms after they die.  Your pet needs to be healthy enough to break down the dead worms in the blood stream.  Blood clots are a serious threat and your pet must be monitored closely during and after treatment.  The critical period lasts about 4-6 weeks.  Before treatment, chest X-rays will be required to stage the heartworm disease to determine how many treatments your pet will need to undergo.  Additionally, the drug used to treat heartworms in dogs cannot be used in cats, so there is no treatment available for cats.  Once a cat is infected, it must be monitored carefully until the worms die off on their own.   The drug used to treat heartworm infestations is called melarsomine, or Immiticide.  It is basically a derivative of arsenic and is currently the only effective treatment.  In the past, veterinarians would use what is called the “slow-kill” method.  This method uses certain heartworm preventions to “slowly kill” adult worms, and eventually the dog will test negative.  However, more and more research is being published showing that this method could be harming the fight against heartworms, not helping.  First, if we are just going to put a positive dog on prevention, why are we still requiring tests for prescriptions?  We are requiring them because veterinarians are legally obligated to do so, but placing an infected dog on a preventive can be a dangerous practice if done incorrectly.  This practice is also contributing to strains of resistant heartworms showing up in our pets.  When many of the preventives came out, they were 100% effective and that number has dropped to 95% for many products.  Exposing adult worms to preventives that are not designed to kill them is allowing their offspring to develop resistance and those resistant microfilariae can be passed to other dogs.  This is one reason why the “slow kill” method is no longer recommended by the American Heartworm Society or the American Veterinary Medical Association.  Another reason is that it may not actually be working at all.  A recent study out of Oklahoma State University shows that many heartworm positive dogs placed on preventive (for the “slow kill” method) produced false negatives when retested for heartworms.  This is because the antigens that are detected by the test becomes bound to antibodies produced by the dog and are not available for binding to the test.  The final reason that slow kill is not the best option is simply for the health of your pet.  Remember all those health effects discussed earlier?  Those do not stop occurring until all of the heartworms in the dog are killed.  It seems that the “slow kill” method may just be allowing the heartworms to live out their lives while you pay for your dog to be on preventive.  It is also important to note that all the dangers that come from treatment with Melarsomine (or “fast kill”) are still present with “slow kill”, but with fast kill all the worms die at once so it is shortened, albeit intensified.  With slow kill, the worms die on their own, at any time, and your dog cannot be hospitalized, monitored, or placed on cage rest for 3-7 years!   In short, it is cheaper, easier, and safer to keep your pet on a heartworm preventive year-round than to face any sort of heartworm infestation.  Call your veterinarian to set up an appointment to get your pet started on prevention today! At The Shot Spot, you can visit us during our mobile events, where we will be happy to assist you with heartworm testing, preventative, or any questions you may have !