A Silent Killer: Everything You Need To Know About The Symptoms, Treatment And Prevention Of Heartworm

Heartworm, scientifically known as Dirofilaria immitis, is a potentially deadly parasitic worm. It is important for dog owners to fully realize the severity of this threat. Failure to take action can result in the loss of your beloved dog! This article aims to teach the average dog owner more about this parasite- where it comes from, how it can affect your dog, and treatment options.

The Relationship Between Heartworm And Mosquitoes

Mosquitoes are a crucial part of the heartworm lifecycle. Therefore, heartworm is more prevalent in tropical climates, where mosquitoes thrive. Some of the worst affected areas are locations that have warm to hot temperatures year-round, such as much of the United States and Australia.

Some owners feel that keeping a dog indoors is enough to protect him from the threat of mosquitoes. Unfortunately, all it takes is one infected mosquito to enter your home and your dog’s life is in danger.

When the mosquito contracts microfilariae (heartworm young) by sucking blood from an infected animal, it becomes the parasite’s vector. The parasite will progress through three larval stages within the mosquito over 10 to 17 days. During this time, the parasite’s growth is dependent on temperature. If temperatures drop below 57 degrees Fahrenheit (14 degrees Celsius), the parasite cannot develop. This is why heartworm is unlikely to be a problem in very cold climates or during chilling winter temperatures.

Once the microfilaria progress through three life stages within the mosquito, becoming larvae, they travel to the mosquito’s mouthparts. Here, they wait to be transmitted to a host when the mosquito next feeds.

When the infected mosquito bites a dog, the heartworm larvae enter the wound and burrow under the dog’s skin. They remain here for another two months, developing into immature worms. It is at this point that they enter the dog’s bloodstream and make their way to the dog’s heart.

Home Is Where The Heart Is

When heartworms are developed, they reside in the right ventricle of the heart and the pulmonary arteries (from which blood is carried to the lungs). Here, the heartworm grows to its fully mature size. Mature heartworms can resemble very long, thin earthworms. Females can grow as long as 12 inches (approximately 30cm), with males being slightly shorter with coiled tails.

Consider this for a moment. People are regularly told about the risks of “clogging your arteries” by eating fatty foods. In the case of a heartworm infestation, not only is your dog’s artery physically clogged with living parasites but the heart itself can be affected too. If the heart becomes infested enough, it can fail to function. Heartworms residing in the pulmonary arteries can obstruct blood flow, causing blood clots to develop in the lungs.

Smaller dogs, who have proportionally smaller circulatory systems, will suffer a greater risk from a light heartworm infection than larger dogs will.

Finishing The Lifecycle

By approximately 7 months after infection, male and female heartworms will mate, introducing new microfilariae into the bloodstream, thus beginning the cycle again. Microfilariae may not be present (referred to as an “occult” heartworm infection) in dogs that are infected with only one sex of heartworm. So, even though the dog is still at significant risk himself, the threat of heartworm spreading from that particular dog is nullified.

Others At Risk Of Heartworm

The definitive heartworm host is the dog- this is where the best survive and thrive. They have also been known to infect wild canines such as foxes, wolves, and coyotes. The infection of other kinds of animals is reportedly uncommon.

Cats can become infected with heartworm, though the parasite affects cats differently. Heartworm larvae and microfilariae have trouble surviving in cats, though the presence of an infection can still pose a significant health risk.

There have been very rare reports of heartworm infections in humans, and any infections are usually of little or no consequence. Unlike our canine friends, the human body is remarkably successful at fighting and killing off heartworm infestation.

Symptoms Of Heartworm In Dogs

Trying to determine whether your dog has heartworm from outward symptoms can be difficult. This is because symptoms often don’t start to show until the heartworm infestation is very advanced. Fortunately, there are tests your veterinarian can perform to locate any internal symptoms, such as the presence of microfilariae or other heartworm antigens.

Microfilariae, or heartworm offspring, are present when there are adult male and female heartworms in your dog’s body. A test will show any microfilariae that may be present, thus confirming the presence of an adult heartworm infestation. Unfortunately, this test isn’t 100% accurate, as microfilariae may not always be present when there are adult heartworms. This could be the case if there is only one set of adult heartworm present, or if they use heartworm medication or the dog’s immune system has killed the microfilariae.

Other heartworm antigens can be detected using an ELISA (Enzyme-Linked Immunosorbent Assay) test. This quick blood test can help locate antigens produced by the female heartworm. The only drawback is that a certain amount of this antigen is needed to be detected using the test, which may not be the case if there is only a small heartworm infestation. The ELISA test, however, is considered more accurate than the microfilariae test.

An ECG (Electrocardiography) may show a disturbance of heart rhythm if adult worms are present. Likewise, a chest X-ray may show an enlarged right ventricle and pulmonary arteries, indicating infestation.

As mentioned earlier, outward symptoms of heartworm disease often don’t occur until the infestation is advanced. At this stage, depending on which bodily functions have been affected by the infestation, the following heartworm symptoms may be apparent:

  • Intolerance to exercise
  • Soft coughing
  • Weight Loss
  • Visible pulsation of the veins in the neck
  • Fainting or collapsing
  • Fluid retention in the abdominal cavity
  • Weakness
  • Severe lethargy
  • Blood-tinged phlegm
  • Jaundice
  • Anaemia
  • Death

Of course, these outward symptoms may also be caused by other conditions. If your dog displays any of the above symptoms, it is important to take your dog to the vet as soon as possible.

Because it is so difficult to accurately diagnose and treat a heartworm infestation, it is important to give your dog regular heartworm preventative medication.

Preventative Heartworm Medicine For Dogs

Now that we’ve learned about heartworm and the associated heartworm symptoms, it is important to consider how we might prevent such a parasite from infesting your dog. Preventing heartworm is easy and affordable. Additionally, most heartworm medicine for dogs also helps protect against other parasites such as roundworm or hookworm. Bonus!

The first thing you should consider before administering heartworm medication is to check whether your dog is already infected. This will serve a dual purpose:

  • You will avoid unnecessary delay in the detection of heartworm infection if it is already present.
  • The effectiveness of the heartworm medication is thrown into question if the parasite is only detected after you begin use of preventatives.

Veterinarians should always test for the presence of parasites before prescribing heartworm medicine. If your dog has an established heartworm infestation when a heartworm prevention program is started, the risk of side effects and complications dramatically increases.

Many heartworm medicines are directed to be administered regularly for year-round protection. In the past, heartworm medication was only used leading up to “mosquito season”. However, due to climate changemicro-climates caused by human development, and increased animal mobility, heartworm transmission has become a year-long threat in many countries. Places in America that were previously thought to be unaffected by heartworm are now seeing increasing instances of infestation. It is no longer good enough to only treat your dog sporadically. Consistent, regular preventative action is the only way to ensure your dog’s protection against heartworm. To understand more about heartworm preventative products, it is important to learn more about the key ingredients used.

Common Ingredients Used In Heartworm Medicine For Dogs


Ivermectin is a broad-spectrum antiparasitic agent that is widely used in heartworm medicines. It is also considered one of the safest preventatives for heartworms.
There has been concern about using Ivermectin in certain dog breeds such as Collies or Sheepdogs. This is due to the genetic predisposition in these breeds that allows the drug to cross the barrier separating the bloodstream from the brain, potentially leading to death. Please note however that the quantities of Ivermectin in FDA-approved heartworm medicine are so low that even these highly susceptible breeds do not experience any negative side effects.

Ivermectin has also been used “off-label” by veterinarians to treat certain skin parasites such as scabies or mange. The use of Ivermectin in this way is not approved by the FDA, as the significantly higher dosages required to treat these parasites result in a greater risk of negative side effects. Any “off-label” use of the drug should be strictly carried out according to veterinarian direction.

Some dog owners have sought to find ways to save money on heartworm treatment by buying Ivermectin in bulk quantities online without a veterinarian prescription, instead of specifically formulated Ivermectin for dogs. People have also bought Ivomec (a highly concentrated Ivermectin medicine used in treating livestock) in an attempt to save money. Unfortunately, treating your dog this way is extremely risky as even a slight miscalculation in dosage can result in negative side effects and even death.

One of the most highly recommended heartworm medicines is called Heartgard Plus. Heartgard Plus comes in the form of a beef chewable that is fed to your dog monthly for year-round heartworm protection. It is FDA approved and comes in three dosage strengths according to the weight of your dog, so the guesswork regarding dosage is eliminated. Heartgard Plus also has the added benefit of protection against hookworm and roundworm. Talk to your veterinarian for a prescription.


Milbemycin is another antiparasitic drug that works against roundworms, hookworms, and whipworms as well as heartworms. It is considered to be another very safe heartworm medicine for dogs, though it isn’t considered to be quite as potent against heartworms as Ivermectin.

Milbemycin is the key ingredient in the monthly oral treatment Interceptor. Other products like Program Plus and Sentinel team Milbemycin with the drug Lufenuron offer additional protection against fleas, flea larvae, and eggs, making these products very comprehensive and safe antiparasitic medications.

Some side-effects have been noted regarding the use of this drug, however, these side-effects were usually the result of extreme overdosing or use of Milbemycin after heartworm infection was already present.
Sentinal monthly tablets are highly recommended by veterinarians as a safe treatment against heartworm and numerous other parasites. Sentinel is available in four tablet sizes according to the weight of your dog.


Moxidectin is another common ingredient in heartworm medication, and also has the added benefit of protection from other internal and external parasites. There are some concerns about the use of this drug, as there is a much greater risk of negative side effects and death from its usage. Even though Moxidectin-based products are readily available online, it is strongly recommended that you consult with your vet before treatment, due to the risks involved.

Proheart is one of the most common moxidectin-based medicines. Proheart is available in the form of a monthly tablet (available in 4 different dosage sizes, according to your dog’s weight), a six-month injection, or a twelve-month injection (administered by your veterinarian). Because your vet is required to give the injections, this can be a good way to avoid accidental misuse of the drug, while also being more convenient. Your vet will contact you before your dog’s Proheart booster shot is due.

While the convenience of a once or twice yearly shot is undeniable, many dog owners prefer not to use this higher-risk drug when there are so many safer drugs on the market.


  • Before you start your dog on any kind of heartworm prevention program, ensure that your dog is thoroughly tested by a veterinarian for an existing heartworm infestation. Using preventatives when there are already established heartworms can lead to complications and negative side effects in your dog.
  • Only use heartworm medications that are recommended and approved for your dog. Using bulk heartworm drugs or concentrated drugs for livestock can lead to accidental overdose and could potentially kill your beloved pooch.
  • Heartgard Plus and Sentinel are considered to be effective and safe treatments against several parasites and are highly recommended by veterinarians.

Heartworm Treatment For Dogs

The unthinkable has happened. Your veterinarian has discovered that your dog has a heartworm infection. Whether your dog was exhibiting heartworm symptoms or he was simply due for his annual check-up, now the parasite has been located, something needs to be done. But just what is involved in heartworm treatment for dogs? What options are available, and what are the risks?

The most effective initial treatment for heartworms is to simply stop your dog’s exercise routine. A sedentary dog is less likely to suffer complications caused by the presence of heartworms. This, of course, won’t get rid of heartworms.

One of the most common ways to treat heartworm is to kill off adult worms using an arsenic-based drug. The usual drug given is Melarsomine (brand name Immiticide), as this has the widest margin of safety, is FDA approved, and is very effective in killing the worms. Of course, the use of an arsenic-based product is very dangerous, and should only be administered by a skilled veterinarian. Immiticide is injected into the dog’s muscle. Dogs treated with this antiparasitic drug must have 4-6 weeks of cage rest to avoid complications that occur from the dead worms.

Dead worms can cause blood clots in the dog’s lungs and several other issues. Inflammation of body tissues can often occur, in which case the veterinarian may prescribe steroidal medications to treat the inflamed area. The prescription of aspirin may also help inhibit blood clots from occurring.

The use of Melarsomine can be particularly dangerous for elderly dogs or dogs with health issues. Your vet may discuss the pros and cons of such treatment with you if this is the case. High-risk patients may simply be treated with daily use of low-dose aspirin (to help avoid clots) and reduced exercise (to reduce the risk of complications).

Once the adult worms have been destroyed, your veterinarian will begin treatment to kill off any microfilariae circulating in your dog’s system. Once all traces of heartworm have been removed, a prevention program can begin, to stop the infection from reoccurring.

In the instance of caval syndrome (Dirofilaria Hemoglobinuria), surgery may be required to physically remove the worms. Treatment using Melarsomine may follow surgery to ensure any remaining worms are eliminated. Dogs with severe heartworm infestations do not survive long, and surgery is their best chance at beating the parasite.

Dog owners may be hesitant to put their dogs through such risky treatments. Please be aware that failing to treat a heartworm infection is far more likely to result in your dog’s death than the treatment itself.
As a campaign by the American Heartworm Society says: “Preventing heartworm disease is easy. Treating it, sadly, is anything but.” The best way to ensure your dog’s health and safety is to ensure heartworm prevention through the use of heartworm medicine for dogs.

Andy Erikson

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