What is heartworm disease and why do I need to give my dog heartworm preventative?

Everything you need to know about heartworm disease but never thought to ask!

There is a common misconception that we don’t have heartworm disease here in Oregon. Unfortunately, this is not true, and we’ve seen a steady increase in heartworm diagnoses in Washinton and Multnomah counties in the last few years.  Heartworm is a serious disease, but the good news? It’s completely and easily preventable! Call or email us to get your pet started on heartworm preventative today.

What is Heartworm Disease?

Heartworm disease is a serious and potentially fatal disease of pets in the United States.  It is caused by Dirofilaria immitis worms – and these are really worms in the heart.  In fact, they can grow up to 1 foot long(!).  This then causes physical damage to the heart, lungs, and associated blood vessels.  Having heartworms can also cause damage to other organs because the presence of these parasitic invaders trigger the host’s immune system.  This chronic stimulation calls in antibodies and inflammatory proteins, and these can cause a lot of damage by precipitating into other delicate tissues such as the eye, kidneys, blood vessels, and joints.

What animals get heartworm?

Heartworm disease affects mammals – dogs, cats, ferrets, wolves, coyotes, foxes, sea lions – and very rarely, humans.  However, dogs are the natural hosts for Dirofilaria, meaning that heartworms that live inside the dog mature into adults, mate, and produce offspring.  This means that dogs are the preferential host for heartworms and are at the highest risk of infection (and easiest for heartworms to infect).  If left untreated, a dog can have hundreds of heartworms in his body.

Cats are atypical hosts for heartworms, which means that heartworms in cats do not often survive to the adult stage.  Cats with heartworms typically only have one or two adult worms, but this small number and even the immature worms can still cause a lot of damage to a cat’s respiratory system.

How do pets get heartworm?

Mosquitoes play an essential role in the heartworm life cycle.  Baby heartworms called microfilaria that live in the bloodstream of an infected dog, coyote, fox or wolf get picked up by the mosquito via the bite (us veterinary people call it the “blood meal”. EW. But that’s what it actually is!).  The mosquito then incubates the microfilaria and it develops into an infective larval stage after 10-14 days.  When the infected mosquito bites into another dog, cat, or wild canid the infective larvae are transmitted to the new host.  It takes about 6 months for the larvae to mature into adult heartworms. Once mature, a heartworm can live up to 5-7 years in a dog and 2-3 years in a cat.

What are the symptoms?

In the early stages of the infection, most dogs will not show any symptoms at all.  But as the worm load and the infection progresses, then dogs will start having signs of a mild but persistent cough, reluctance to exercise, fatigue after moderate activity, weight loss, and lethargy.   

How do I know if my dog has heartworm? Is there a test?

A simple blood test can determine if a dog is heartworm positive or not. The test checks for the presence of an adult female heartworm and is very sensitive.  If positive, we follow up with confirmation testing.

My veterinarian said my dog needs to be tested for heartworm before starting a preventative like Trifexis. Is that really necessary?

All dogs are recommended to have a negative heartworm test prior to starting heartworm preventative.  A positive heartworm infected dog can have an adverse reaction to the preventative if already infected with a large worm burden.

Why talk about heartworm now? It’s not summer and I thought heartworm disease was transmitted by mosquitoes?

It takes 6 months after infection to test positive on a heartworm test.  Because of this, the guidelines for when to test your dog for heartworm can seem confusing, but it really boils down to this:

  • Puppies under 7 months of age can be started on a heartworm preventative without testing but should be tested in 6 months and yearly thereafter
  • Currently, in the PNW, if an adult dog has been off of heartworm preventative for less than 7 months, then it is OK to restart the preventative without a heartworm test.  The dog should be tested 6 months then yearly thereafter to establish a negative infection.
  • If over 7 months off of heartworm preventative, a heartworm test is recommended prior to restarting the preventative.

What if my dog gets heartworm? Can it be cured? 

Heartworm disease and treatment in dogs exemplifies the classic saying “an ounce of prevention is worth a pound of cure.”

  • Any infection of heartworms can cause irreversible changes to the heart and lung vessels.
  • The drug Immiticide is the only FDA approved treatment for heartworm disease treatment.  It is an arsenic-based medication, safer than its predecessors but still an ARSENIC based medication so there can still be significant side effects from the worm kill.  And, if that wasn’t scary enough, Immiticide has lately been intermittently unavailable so heartworm positive dogs can’t even be treated.
  • Treatment can take months – first to stabilize the dog, then careful treatment to kill the adult worms without killing the patient.  Afterward, the dog must be on strict limited activity for months to prevent the dying worms from lodging into the lungs and vessels.  Studies have shown that dogs that die after heartworm treatment do so because their owners let them exercise, not because of the drug itself.

Prevention, on the other hand, is an easy, affordable, one-pill-a-month medication (Trifexis, Sentinel) with FDA approved short and long term safety standards.

Wow. I didn’t know it was so important to protect my pet. How do I get my dog on a preventative right now?

Call 503-207-2744 or email us and your veterinarian will get your dog checked, tested and prescribed a safe and effective preventative.

What about my cat? 

As we discussed earlier, cats are not typical hosts for heartworm disease.  Testing for cats is not as straightforward as it is in dogs, so we currently neither recommend routine testing or routine prophylaxis in cats.  If you are concerned that you cat may have been exposed to heartworm, give us a call 503-207-2744 and your veterinarian will talk to you about the best course of action.

What's Next

  • 1

    Call us or schedule an appointment online.

  • 2

    Meet with a doctor for an initial exam.

  • 3

    Put a plan together for your pet.