Making sense of heartworms: What you need to understand

What I Wish Pet Owners Knew About Heartworm Disease

by Dr. Stephanie A. Malmquist

Most people know that mosquitos can cause illnesses in humans all around the world (malaria, Zika virus, and many others), but people often forget that these pesky biters can cause life-threatening disease for pets, as well: heartworms! For those who live in warmer climates, it’s likely that mosquitos are a year-round threat.  Others may experience only a seasonal risk.  Please have a conversation with your veterinarian about the prevalence of heartworm disease in your area and the associated risks, so you can ensure your pet is protected against this deadly worm.  Also, let your vet know if you plan to travel with your pet to a place where heartworms are prevalent so prevention can be prescribed.

Heartworm disease is a threat to pets and has been diagnosed in all 50 of the United States.  Humanitarian efforts and relocations after natural disasters have aided in distributing heartworms throughout the U.S.  For example, after Hurricane Katrina hit Louisiana in 2005, thousands of animals (many of which were heartworm positive) were relocated throughout the United States.  In addition, many new rescue and adoption programs are taking animals from overcrowded shelters in the south (where heartworms are very prevalent) and transporting them to the northern states to be adopted.  This redistribution of pets across regions makes heartworm disease a problem that everyone needs to be aware of, even if prevalence in your area is low.

Pets that live inside need prevention too!  Mosquitos do sneak inside the home, and it only takes one bite to cause infection! In a clinical study, it was found that more than 25% of heartworm positive cats in the study were indoor-only![1]  In addition, you can make your pet’s environment less inviting to nasty mosquitos by eliminating standing water and spraying for them.

How preventatives work:

When you give monthly heartworm preventative, you are protecting against the mosquito that bit and infected your pet a month or two ago.  This is called “reach-back.”  This is also the reason that you need to continue giving your pet preventatives when it’s so cold that mosquitos may not be as big of a threat… you’re protecting your pet from the bite it got a couple of months ago! When an infected mosquito bites your pet, it deposits little larvae (immature heartworms) that enter the skin through the bite wound and spend several months wiggling through the tissues, trying to get to the bloodstream.  I like to call this the “immature tissue phase.”  Monthly heartworm preventatives work mostly by targeting the worms in this immature phase. 

Once the little larvae make it to the blood stream, they mature into adults and our monthly preventatives are no longer able to kill them.  They eventually make a home in the vessels that lead from the heart to the lungs and from there they reproduce, cause trauma and disrupt blood flow.

Heartworm Treatments:  There are many treatment protocols used by veterinarians, so make sure to ask your vet which they use.  I personally use the guidelines established by the American Heartworm Society.

• Dogs– If allowed to mature into adults (move out of the tissue phase and into the bloodstream), heartworms must be eliminated with an expensive and somewhat risky injection that is hard on both dog and owner. This is the only way to truly “kill’ adult heartworms.  The so-called “slow kill method” is really just attempting to reduce inflammation while preventing further infection and waiting for the worms to die off. Although this “slow-kill” method isn’t as effective, it may be the best option for some pets (those with concurrent medical problems, those who cannot be crated, etc). It’s really important for pet owners to understand that they are NOT “killing off” the adult worms with the “slow-kill method.”
• Cats– There are several preventatives available for cats, but there are NO TREATMENT options for them.  The heartworm treatment available for dogs is toxic to cats.

Testing for heartworms IN DOGS:

Basic heartworm tests conducted by veterinarians detect the presence a specific substance in a drop of the dog’s blood. This substance is a hormone produced by the uterus of a female heartworm, so only female heartworms that are reproductively mature are detected.  As you can imagine, there could be many immature females or males present too, so a positive test does not tell us anything how many worms are there and any single negative test does not necessarily ensure no heartworms are present.

Testing is recommended to start at about 7-8 months of age and should be done once a year after that.  Why?… Because it takes about 7 months for a heartworm to develop into an adult reproductive female that will be detectable in a blood test.

Testing for heartworms IN CATS:

Testing for heartworms in cats is more complicated than the test for dogs.  For cats, bloodwork may need to be sent to a lab and x-rays or an ultrasound of the heart may need to be done.

What happens in a heartworm positive DOG?

An adult heartworm can live 5-7 years and they can do major, permanent damage in that amount of time! Adult worms can easily reproduce and reach large numbers (hundreds)! Although some dogs get lucky and are able to clear this disease, others progress to displaying respiratory signs, such as coughing, lack of energy, weight loss, vomiting and shortness of breath.  Sometimes, in heavy infections, heartworms will block blood flow at the heart and cause sudden death. Even heartworm positive dogs should be on monthly prevention to prevent further infection with more worms.  HOWEVER, talk to your veterinarian about which preventions are safe for a heartworm positive dog.  Some preventives have the potential to cause serious (or fatal) reactions if given to a dog with heartworms.

What happens in a CAT with heartworms?

Cats are called an “atypical host” for heartworms, so many worms do not mature into adults inside them.  Even so, the immature worms can cause a major respiratory illness called Heartworm Associated Respiratory Disease (HARD).[2]  The cats that do have adult worms are at risk of death, from just one worm.  An adult heartworm can live 2-3 years in the cat and is able to do major, permanent damage in that amount of time! In cats, a single adult worm can cause death!  Some cats experience illness that looks like asthma, hairballs or other respiratory disease, with coughing, lack of energy, weight loss, vomiting and shortness of breath.  Unfortunately, there are many cats where the only sign of heartworm disease is sudden death.

Please talk to your veterinarian, browse the VetPet Box Education Library and visit the American Heartworm Society website to learn more about Heartworm Disease in pets! 

[1] Atkins CE, DeFrancesco TC, Coates JR, Sidley JA, Keene BW.  Heartworm Infection in Cats: 50 cases (1985-1997). J Am Vet Med Assoc. 2000; 217(3):355-358.[2] Blagburn BL, Dillon AR. Feline heartworm disease: solving the puzzle. Vet Med. 2007:102(3) (suppl):7-14.