BaxterBoo Blog
August 22, 2014

A Pet Lover's Guide to Heartworm

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Heartworm disease is a terrible-sounding disease, and it is. Imagine foot-long worms living in your pet's heart and lungs!

BaxterBoo cares about pets, so we want you to get the information you need to be sure your furry friends are safe. We all need to do our part to prevent the cycle and spread of this serious and potentially fatal disease. We'll also walk you through what can be done should your pet become infected.

Animals that can be affected

Heartworm is spread through the bite of a mosquito carrier. Although heartworm is most common in dogs, it can also be found in cats and ferrets. Wild animals may also be carriers including coyotes, foxes, wolves, sea lions. On very rare occasions humans can be infected as well. Because many of these wild animals live in close proximity to cities and suburbs, they can contribute to the spread of the disease in pet populations. 

For those of us who have strictly indoor cats, we may think they are safe. But mosquitos have a way of finding their way indoors because of their attraction to the higher levels of carbon dioxide in homes. Remember, it only takes one bite from an infected mosquito for heartworms to spread to your pet. But yes, indoor cats have less of a risk for being exposed. 

The good news is that heartworm is preventable. Yet only 59% of pet owners use heartworm prevention medications. 

The prevalence of heartworm

It is estimated that a million dogs have heartworm in this country, and heartworm can be found in many parts of the world. Areas that are more moist and hot tend to have higher concentrations of affected mosquitos, but heartworm has been diagnosed in all 50 states.

Even desert climates and areas with freezing temps are seeing more cases. This is thought to be due to the fact that our society is increasingly mobile and dogs from high-risk areas are moving with their humans. 

Also, there has been an influx of dogs transported from areas of higher heartworm density through adoption programs. This is because there are more adoptable dogs in the Southeast and rescue organizations are alleviating these overwhelmed shelters by sending homeless dogs throughout the country to get them off of kill lists.

There was a bump in heartworm infections after hurricanes Katrina and Rita devastated those higher heartworm risk areas. Dogs saved from those natural disasters were also transferred throughout the country. While this was a good thing, it did spread the incidence of heartworm cases throughout the U.S.

The cycle and spread of heartworm disease

Heartworm disease is spread through a fairly complicated life cycle. As stated above, the mosquito plays an integral part.

An infected dog or one of its wild cousins will have a female heartworm that is producing microscopic worm offspring that are called microfilaria. These worms spread throughout the bloodstream. When a mosquito takes a blood meal from an infected animal, it ingests these baby worms.

In 10-14 days, these worms develop into larvae which are now at an "infective stage" that will transfer from the mosquito to the new host's skin. When the infected mosquito bites another animal, the larvae that get deposited on the dog, cat or wild dog-like creature's skin. These larvae find their way into the animal's bloodstream through the mosquito's bite wound.

Once the larvae have burrowed their way into the body of their new host, it takes about six months for the parasites to mature into adult heartworms. Adult heartworms can live up to seven years in dogs and two to three years on cats.

It's easy to see that with the worms thriving in the warm protection of your pet's body, the heartworm offspring are ready and waiting for mosquitos to facilitate a new cycle. This means more and more pets and wild animals have the opportunity to be infected from just one dog... for years. 

Dogs are the ideal host for heartworms. Because of this cozy environment, the adult heartworms can mate and produce offspring that mature into adults. Dogs have been known to harbor hundreds of worms in their heart, lungs and arteries. 

Signs and symptoms that your pet may be infected

Dogs: During the early stages of infection, there will be very few, if any symptoms. As the disease progresses, the heart and lungs are essentially being gradually taken over by the heartworms to the point that normal cardiovascular functions are impossible.

The first sign of a heartworm infestation is often a mild yet regular cough. The dog will begin to tire easily during exercises previously tolerated. The appetite may decrease and weight loss is not uncommon.

Dogs in advanced stages may show signs of heart failure which include a swollen abdomen due to excess fluids. Blockages in the heart and vascular system can happen from the sheer volume of worms. Difficulty in breathing, pale gums and dark-colored urine are a sign that your dog is in critical condition. Without immediate veterinary attention and surgical removal of the heartworm blockage, few dogs will survive. 

Cats: The symptoms of ringworm in cats is markedly different than in dogs. This is due to the fact that cats' bodies are not nearly as accommodating to the heartworm as dogs are. Some cats show very few symptoms so as to be undetectable. These mild symptoms may include coughing, asthma-like symptoms and decreased energy and appetite.

Rarely, some cats will show marked symptoms such as decreased motor skills, fainting spells or even seizures. There may also be a distended abdomen due to fluids. Unfortunately, in some cats, the first sign of infection is sudden death.

Preventative medications

Given the fact that heartworm disease causes serious health problems, we pet owners must be proactive in preventing the infection. Additionally, treatment for heartworm disease is up to 15 times more expensive than the heartworm prevention medicines. These meds come in a topical form and a pill form. There are formulations made specifically for dogs and for cats. Both oral and topical preventatives must be administered monthly. 

How heartworm medicine works

Preventative medicines will not prevent the heartworm larvae from entering your pet's body. Instead, the medicine works by killing the heartworm larvae in the bloodstream before they have a chance to mature into adults that will take residence in the heart. A clean bloodstream means a biting mosquito will not be ingesting the microworms that will start the heartworm lifecycle in other animals.

There is an added bonus to treating pets with heartworm preventatives. Heartworm medicines often treat other parasites which may include roundworms, tapeworms, whipworms and hookworms. Some even treat or prevent fleas, ticks and ear mites. 

Some pet owners believe that they only need to give their pets heartworm medicine for six months out of the year, when mosquitos are active. But remember - it can take six months for the immature heartworms to grow into reproducing adults. You'll want your pet protected throughout the year to ensure no juveniles have the opportunity to mature into reproducing adults.

Also, the other maladies that the heartworm medicine can prevent are best treated all year long. 

Heartworm medication is not full proof

Although the preventative medicines work well, there are still factors that can leave your pet at risk. Your pet may discreetly cough up their oral medication without your knowledge. Or they may vomit. Topical medications may rub off or be washed away. Talk to your vet to see if there is a need for an additional treatment.

These possible administration complications are yet another reason to be sure to administer monthly treatments to be sure that there is a smaller window of possible infection. 

Screening for heartworm

Because symptoms of heartworm infections are initially mild, it is important to get yearly screenings for the disease. The earlier the disease is detected, the easier it will be to treat. 

The heartworm test is done by taking a small sample of blood from your pet. The blood will then be screened for a specific protein (antigen) that the female heartworm releases. If your dog tests positive, your vet will guide you through the process of treatment.

For cats, it is much more difficult to determine if they are infected than in dogs. Two tests will be done including the antigen screen as well as an antibody test that will show if a cat's immune system is reacting to the presence of the heartworm larvae. Vets may need to use an ultrasound or x-ray of the heart to see how the disease has progressed. Confirming the disease in cats is imperative as using heartworm prevention meds in infected cats could be fatal. 

Preventative meds in already-infected pets?

As stated above, heartworm infection has to be ruled out in cats before they can receive the preventative medicine. This is because the cat will react to the toxins released when the larvae die from the treatment. 

As far as heartworm-positive dogs, your veterinarian may use preventative medicines to kill the microscopic worms and larvae living in the bloodstream. This has to be done with careful veterinary supervision as the toxins released from the dying organisms can also affect dogs, but the risk is not as extreme as it is in cats.

By using the prevention medications in already-infected dogs, a new heartworm infection will not occur while being treated for the existing infection. This makes the treatment for the original infection more effective.

Also, by eliminating the tiny pathogens from your dog's bloodstream, your dog will not be facilitating the further spread of the disease, should they be bitten by another mosquito.

Treatment for heartworm disease


For cats, there is no treatment available to eradicate the disease. Palliative care of the secondary symptoms may help. As cats are not very good hosts of the heartworms, they will usually only harbor about six adult worms. Hopefully, by giving the cat good medical care, the kitty will outlive the heartworms (which is only two to three years in felines.)

It is still a good idea to keep heartworm-positive cats indoors on the rare chance an adult heartworm is successful in reproducing and releasing microscopic worms into the bloodstream. Another mosquito bite will start the heartworm lifecycle in another animal.


In spite of the gloomy information about heartworm, the good news is that 95% of heartworm-infected dogs can be successfully treated. Here is what you can expect should your dog be diagnosed with heartworm:

  • Exercise restrictions. Keeping an active dog still can be a challenge, but ordinary physical activities have to be curtailed as soon as a heartworm diagnosis is confirmed. This is due to the fact that exercise and movement will increase the rate of damage the heartworms will cause in the heart and lungs. If your dog is exhibiting severe symptoms, his or her activities will have to be curtailed to an even greater degree.
  • Assess your dog's ability to tolerate heartworm treatments. Because heartworm treatments have to be potent enough to kill the heartworms, they can also be tough on your dog's body. In fact, the only available drugs to treat heartworm infections have arsenic in them. Therefore, dogs must be physically stable to tolerate these injections. Antibiotics and other treatments are usually prescribed before the heartworm regimen is administered. This stabilization process may take a few months.
  • Administer treatment. Based on the needs of your pet, a treatment plan will be carried out. Dogs with mild symptoms such as cough or exercise intolerance, have a higher success rate with treatment. Dogs with more severe symptoms can also be cured, but there is a higher chance for complications. These may include life-threatening blood clots to the lungs.
  • Recheck to confirm your pet is heartworm-free. Six months after the treatment protocol is completed, your dog will be screened for heartworm again. This will ensure that the heartworm infestation is completely cured.
  • Prevention.Monthly heartworm prevention medications will be required for the remainder of your pet's life.

Take action!

Now that you understand the risks, prevention and treatment of heartworm, what can you do to help?

Obviously, you'll want to be vigilant about giving your pets their monthly heartworm prevention medications. You'll also want to tell your pet-loving friends and family about the importance of prevention. 

Want to do more? Consider supporting the Wags 4 Hope organization which seeks to educate people about heartworm on a larger scale. 

For more information on heartworm, visit the American Heartworm Society, which also provided the graphics for this article. 

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This entry was posted by Mary.

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