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Heartworm: The Silent Killer

By Dr. Hannah Albright, DVM

The day began just like every other work day. I arrived at the clinic, checked the white board for any lab results that had come in overnight and made my way upstairs to my desk to prepare for morning appointments. It was my fourth day working at St. Bernard’s and only my fourth day working as a veterinarian. I was still adjusting to the day-to-day flow of the clinic and getting settled in to my new job.

The first appointment of the day was a preventive care exam – a healthy pet coming in to update their vaccinations and get their annual wellness check. My second appointment however, was a little different. Toby*, an older, intact large breed dog, was coming in to see me for a history of anorexia (not eating) and losing weight. As veterinarians, our brains immediately start to think about what could be going on with any sick pet. Weight loss and anorexia are extremely vague symptoms and can be clinical signs for many different conditions. I mentally made a list of tests I would like to run and went off to my first appointment.

My first appointment went smoothly, and soon it was time to see Toby. At first glance, it was clear that he had indeed lost quite a bit of weight. However, his physical exam was unremarkable, and did not lead me to any specific diagnosis. I spoke with Toby’s owner about various conditions that are common in older animals, including the word that every pet owner is afraid to say, cancer. We decided to run blood work to look at how Toby’s organs were functioning and for any signs of infection or inflammation. We also drew blood for a 4Dx test, an in-hospital screening test for heartworm and three different infections transmitted by ticks (Lyme, Ehrlichia and Anaplasma).  

I returned to my desk to complete notes and record my physical exam findings. Simultaneously, the technicians downstairs were preparing the blood sample for the 4Dx test. Roughly five minutes had passed when I saw Nicole, one of our assistants, gesture to me. She handed me the 4Dx test, and my heart dropped into my stomach. Two tiny blue dots were present in the bottom corner of the test, indicating not one, but two positive results – heartworm AND Lyme disease.

(Lyme disease is caused by Borellia burgdorferi, a bacteria that is transmitted through tick bites. For an overview of Lyme disease, please see “Protecting Your Dog Against Lyme Disease.”)

Heartworm is a parasite transmitted through the bite of a mosquito. The mosquito acquires larvae (the immature life stage of heartworm) from an infected dog. Once ingested, the larvae develop into the infective stage within the mosquito and can then be transmitted to any dog or cat bitten by that mosquito.  The larvae take roughly six months to mature to an adult and produce microfilaria (young larvae). These microfilaria then grow into mature adults and continue reproducing.

Early infection with heartworm may produce limited to no clinical signs. As the disease progresses and the worm burden becomes greater, affected dogs may develop a mild cough, exercise intolerance, and general decline in overall health. Untreated pets can develop heart failure due to caval syndrome (worms present in blood vessels surrounding the heart).

As scary as this disease sounds, there are very effective ways to prevent infection. Monthly prevention with certain prescription-strength dewormers kill any young heartworms present within the animal’s body, never allowing them to become adults.  As always, prevention is much better than treatment.

Fortunately, we have a very reliable way to test for the presence of heartworms using a simple blood sample. But there is always a downfall. These tests only detect ADULT heartworms present within the blood. This means that your dog will have had the infection long enough for the larvae to have grown into adults. Once adult heartworms are present, a monthly dewormer will only prevent more from being acquired, and will NOT kill the adult heartworms already present.

Treatment of adult heartworms is possible, but is a very long and expensive process. It takes months to complete treatment and can cost around $1000 depending on the size of your pet. (medication costs are based on the weight of the pet). After the initial diagnosis, we perform various tests — bloodwork to evaluate for other systemic diseases; chest x-rays to determine the extent of heart and lung involvement — to check the overall health of the pet and to get an idea of what stage of heartworm disease the dog is in. We then start a one month treatment with an antibiotic called doxycycline, which helps begin the process of killing off the worms. We then wait another month before beginning treatment with injections of special (costly) medication to kill off the adult heartworms. 

Heartworm treatment does not come without risk. The dog must be kept under strict exercise restriction for up to two months following the last treatment to prevent any chance of complications that may result in death. The series of injections to kill the adult heartworms is also painful, although pain medications are used to reduce discomfort as much as possible. The entire treatment process can take up to 4 months, with frequent testing for up to 1 year after initial diagnosis. (Treatment protocol is based on guidelines from the American Heartworm Society (AHS). For more information on heartworm disease, please visit the AHS website at https://www.heartwormsociety.org/pet-owner-resources).

In recent years, we have seen a dramatic increase of heartworm positive dogs in Wisconsin. The majority of heartworm positive dogs are rescued animals coming from southern states. Some might ask the question, “Why aren’t these dogs tested before bringing them to Wisconsin?” These dogs ARE required to have a negative heartworm test prior to entering the state. Unfortunately, the go-to test to diagnose heartworm is based on the presence of the adult worm, and therefore can only detect advanced infections present for 6 or more months, once the immature worms have matured into adults. It is not uncommon for “heartworm negative” dogs and puppies to be transported to Wisconsin and adopted out to owners, then eventually testing positive for heartworms months to years, regardless of whether monthly prevention has been used.

How do these heartworm positive rescue dogs play a part in your pet’s life? Unfortunately, these positive dogs serve as a reservoir for infection. Mosquitos that bite these dogs, pick up the worm and can then transmit it to other dogs, dogs that have never left the state of Wisconsin. This was the case for Toby. He never left Wisconsin, let alone Fond du lac county, yet he is the second dog we have diagnosed with heartworm disease who never left the state of Wisconsin.

Again, I can’t emphasize enough the importance of YEAR-ROUND prevention. With our recent mild winters, mosquitoes can be out and about when you least expect them. Just a month ago, during 40 degree weather, I had a mosquito land on my hand. He may have been moving as slow as a slug, but he still carried the possibility of transmitting heartworms to any dog that may have crossed his path.

Unfortunately for Toby, his disease had progressed too far by the time we caught it and he was humanely euthanized due to a poor prognosis. As gut-wrenching as it is for us to see a beloved pet’s life come to an end, we hope his story can be a learning opportunity for all pet owners on the importance of monthly heartworm prevention and yearly testing.

*Name changed to protect privacy.

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