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What are Heartworms and Why Should I Worry?

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Most of us know that we should have our dogs and cats on monthly heartworm prevention, and many of us probably do! That’s good because heartworms are an easily preventable disease that can be deadly if left untreated, so prevention is very important.

Photo courtesy of Dr. Joe Hostetler and Bayer:

heartworms in tissue


What makes heartworms so dangerous, though? How do dogs and cats get heartworms? What is the real risk if my dog or cat becomes infected? Read on to find out what you need to know about heartworms.

A Few Facts About Heartworms:

  • Just one bite from one infected mosquito is all it takes to put heartworm larvae in your dog or cat’s body.
  • When a female mosquito bites and takes blood from a heartworm positive animal, she also takes in the microfilaria of the heartworm that resides in their bloodstream.
  • It takes 10-14 days for this microfilaria to turn into infectious larvae, and after that time when the infected mosquito bites a healthy animal, those larvae will be transmitted through the bite wound to that animal.
  • Heartworms (Dirofilaria immitis) can only be transmitted by the bite of an infected mosquito. They cannot be transmitted from one infected dog to a healthy dog through the air or water bowls.
  • Heartworms do not affect humans except in very rare cases, but many other mammals including dogs and cats are frequent victims.
  • Heartworm disease is potentially fatal without treatment and it is much easier to prevent heartworms than to treat them.
  • Heartworm disease is different for dogs and cats. Dogs are more likely to get heartworms, and in cats, the heartworms rarely live to full maturity.
  • Once a dog is bitten by an infected mosquito, it takes around 6-7 months for the larvae that were transmitted to mature into a fully grown heartworm.
  • A heartworm test will only show positive results after this 6-7 month maturation period in your pet’s body.
  • Mature heartworms can grow up to 12 inches in length and will clog up blood vessels, the heart and the lungs of infected animals and reproduce rapidly.
  • This causes lung and heart disease, as well as damage and failure to other organs in the body. This damage can last even after heartworm treatment has occurred and an animal is declared heartworm free.
  • One heartworm can live 5-7 years, and dogs have been known to have as many as 200-300 heartworms in their system at one time.
  • Cats rarely have fully mature heartworms, and when they do, there are usually only 1-3 worms. This is still dangerous, though, as there is a respiratory disorder that can be caused by only a few heartworms or by immature larvae.
  • A dog or cat without heartworm prevention can be infected by multiple mosquitos and have the heartworms inside their bodies reproduce over and over, leading to massive clogging by handfuls of worms.

What is My Pet’s Risk?

Heartworm disease has been recorded in all 50 states. New irrigation and building in formerly heartworm-free areas are allowing mosquitos to live and reproduce.

Even if there are few reported cases of heartworms in your area, you must be careful when traveling with your pet to areas with a lot of mosquitos.
Infected mosquitos are found to be more and more widespread every year.

Indoor and outdoor pets are both at risk for heartworm disease since mosquitos can easily sneak inside our homes.

Signs and Symptoms

So, how do we know if our dog or cat has heartworms? What should we be looking for so we know when we need to seek treatment immediately?

Besides getting each of our pets their yearly heartworm test, here are the signs and symptoms for which we should also watch:

As heartworms grow and clog the heart and lungs, dogs will start to cough. They will start to tire more quickly and may experience nosebleeds. Many dogs will lose their appetite, start vomiting and lose weight. Some dogs will lose consciousness due to lack of blood passing to the brain. Fluids may start to accumulate in the abdomen, causing a distended belly in both dogs and cats. Cats may also start having trouble walking, start to have seizures or lose consciousness.

Photo courtesy of Dr. Joe Hostetler and Bayer:
internal parasite, heartworms
Eventually, death will occur if the heartworms are left untreated. Death is often attributed to heart failure or complete blockage of all blood flow in the body due to clogging by worms. Many times, dogs and cats may show very few or no symptoms at all before sudden death. This is one reason why yearly heartworm testing is so important.


If you suspect your dog or cat has heartworms, you should get in to see the vet right away. There are two very effective blood tests commonly used to detect the presence of heartworms. Your veterinarian will probably also complete a physical exam, chest x-ray, urinalysis, and possibly other blood tests. Ultrasounds are sometimes used on cats.

Some veterinarians run blood tests in-house, and others will send them off to a laboratory. Either way, you will probably have results in 24 hours or so. After your veterinarian gets the results of the bloodwork back, she will determine at what stage your dog or cat presents with his current symptoms, which will help determine treatment. There are 4 stages of heartworm disease:

Stage 1:

Young, healthy dogs
Beginning signs of heartworm disease present in x-ray
Few or no physical symptoms

Stage 2:

Fairly healthy dogs
Early symptoms of heartworm disease such as coughing and tiring easily
Moderate symptoms on bloodwork, x-ray and urinalysis results

Stage 3:

Progressing symptoms such as weight loss and trouble breathing
Increased damage to bodily organs and functioning

Stage 4:

Severe symptoms such as collapsing regularly, seizures, and severely swollen abdomen
May need immediate surgical intervention to physically pull worms from the body
A dog at Stage 4 may not survive, with or without treatment


The best way to treat heartworms is to prevent them! Heartworm prevention is only available through a veterinary prescription. There are both oral and topical medications to choose from, so discuss what will work best for your dog or cat with your vet. I prefer the meaty chewable pills, and my animals have never given me a problem with taking them.

Check out how to use Advantage Multi here:

At each yearly check-up with your veterinarian, have a heartworm test completed so that any problems can be caught as early as possible. No form of heartworm prevention is 100% effective all the time. Sometimes pets spit out or vomit up a pill that we never see. Sometimes we give the prevention a week late by accident.

We should also keep in mind that while there are treatment options available for dogs who are diagnosed with heartworms, there are NO TREATMENT OPTIONS available for cats, so prevention is essential.

If you have taken a new dog or cat in off the street and been unable to locate an owner, then a heartworm test should be administered before starting heartworm prevention so you can find out your new pet’s current status.


So, your dog or cat ended up with heartworms or you have adopted a dog with heartworms….now what?

Photo courtesy of Dr. Joe Hostetler and Bayer:



There are treatment options available for dogs, and treatment is most effective when started early, at the first signs of heartworms. As I said before, there are no treatment options available for cats, but sometimes a cat’s immune system will resolve the issue on its own since cats are not as ideal a host for heartworms as dogs. However, even if the problem is resolved on its own, lasting damage could still have occurred.

Start treatment as recommended by your veterinarian. Dogs who are in Stage 1 and Stage 2 are usually able to have successful treatment. Dogs in Stage 3 can often be treated with success as well, but may show more lasting effects due to more severe damage in their bodies. Some dogs in Stage 4 may still recover, but the process will be more difficult and they may never be in the same shape physically as they were before contracting heartworms.

A popular treatment option is an injectable drug called Immiticide (or Melarsomine dihydrochloride). Arsenic is the active ingredient in this drug and after two or three injections, the adult heartworms in the dog’s body should die.
Heartworm treatment is meant to kill the larvae and mature worms in your dog’s body. Since the treatment can be stressful and strenuous on a dog’s body, restricting activity during treatment is beneficial. If an infected dog continues to exercise, it can increase the damage done to his body, which may be irreversible. This is because the medication starts to break down the bodies of the heartworms, which can further clog blood vessels and lodge in arteries when the blood gets pumping.

Diagnosis and treatment can run $300 to $1500 depending on your veterinarian’s prices, the area of the country you live in, the diagnostic tests performed, and the severity of your dog’s condition. Treatment will vary from dog to dog since symptoms, age, health before heartworms, and tolerance is different for everyone.

After treatment is complete, you will want to come back in 6 months for another heartworm test just to be sure all heartworms were killed, even the tiny microfilaria.

A dog can get heartworms a second time, so after treatment is complete, you will immediately want to start him on prevention as soon as your veterinarian says it is OK.

Heartworms are pretty scary creatures that can cause a lot of damage to our pets, including death. It is worth it to make sure our dogs are on a form of prevention and to get yearly heartworm tests. The cost of keeping up with monthly prevention and yearly veterinary appointments is much less than the cost (physical and monetary) of dealing with heartworms in our dogs and cats.

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