Ask the Vet – Heartworm Disease by Kristel Weaver DVM, MPVM

Every year at your annual preventive care visit, your veterinarian most likely recommends a heartworm preventative for your dog. So why do veterinarians feel this is so critical? Why is heartworm prevention a key component of maintaining your pet’s health?  If your pet is already on a heartworm prevention program – great job! If not, here are some specifics on heartworms and why it is important to give your dog that monthly dose.

What is a heartworm infection?

Heartworms are a type of roundworm that looks similar to angel hair pasta.  These worms live in the blood vessels around the heart, and inside the heart itself.  The worms cause a blockage of blood flow leading to right-sided heart failure.  When a dog has heart failure from heartworms they will cough, breath faster than normal, have a distended belly and will be unable to exercise.  If left untreated, heartworm infections can be fatal.

How do dogs get heartworms?

Mosquitoes spread heartworms.  Once a carrier mosquito bites a dog, it takes six to seven months before there are adult worms living in the heart.  The adult heartworms then make baby heartworms, called microfilaria, which swim in the blood and can be picked up by a mosquito bite and spread to another animal.  Heartworm cannot be spread without going through a mosquito.

Can it be spread to people or cats?

Humans are rarely infected with heartworms.  The worms cannot complete their lifecycle in humans and become walled off as a round nodule in the lungs.  Cats can get heartworm infections but are much more resistant than dogs.

How is it treated?

The treatment plan for an individual dog is based on the severity of the infection.  Typically, dogs are given a drug to kill the adult worms (Immiticide), antibiotics for secondary infections, and heartworm preventatives to kill the microfilaria.  After getting Immiticide, tiny chunks of dying worms are present in the lungs and can cause difficulty breathing.  Dogs must be kept calm, quiet, and rested for one to two months after getting the treatment to avoid side effects as their body breaks down the dying worms.

How is it prevented?

There are multiple heartworm preventatives on the market.  These are safe and easily administered (usually a chew or a topical ointment).  Collies and other herding breeds can have negative side effects to the drug used in heartworm preventatives at high doses.  The low doses used in heartworm preventatives have been proven safe for all breeds.

How much heartworm disease do we see in the San Francisco East Bay?

Our clinic sees a handful of heartworm infections each year; while it’s not a huge problem it definitely does occur.  In the southern part of the United States heartworm disease is really common.  In a year with heavy mosquitoes we could have an outbreak here in the Bay Area.  If you travel with your dog you could be taking him/her to areas much more infested with heartworms.  Keeping your dog on monthly heartworm prevention is ideal for optimum health.  These preventatives have an added benefit of being a general dewormer.

Talk to your veterinarian about which preventative is best for your dog.  Preventing a heartworm infection is much easier and safer than treating the disease. Like Benjamin Franklin said, “An ounce of prevention is worth a pound of cure.” 

Dr. Kristel Weaver is a graduate of the Veterinary School at the University of California, Davis where she received both a DVM and a Master’s of Preventative Veterinary Medicine (MPVM).  She has been at Bishop Ranch Veterinary Center & Urgent Care in San Ramon since 2007.  She currently lives in Oakland with her husband and their daughter, Hayley. If you have questions you would like Dr. Weaver to answer for future articles, please email info@webvets.com