Most Commonly Asked Vet Heartworm Questions

As a pet owner and veterinarian, heartworm disease sounds especially awful to me.

Who wants to think about their beloved pet suffering from the presence of a tubular parasite living in the heart and blood vessels?

Heartworm disease is common in the U.S. and is one of the most life-threatening infectious disease affecting pets.  Fortunately, it’s a very preventable disease provided pet owners partner with their veterinarians to provide routine diagnostic testing and preventative medications.

Let’s cover the basics of heartworm disease and its diagnosis and prevention.

What is Heartworm Disease?

Heartworm refers to Dirofilaria immitis, a parasitic nematode (roundworm) that definitely does not belong in your dog’s body.

There are four classes of heartworm disease (Class I-IV), which cause clinical signs increasing in severity with progression of class.

With Class I, your pet may show no clinical signs or could just exhibit a mild cough.

As the disease progresses through Class IV, pets will appear more-obviously ill and can show decreased appetite, weight loss, unkempt appearance and eventually breathing problems and heart failure.

Sometimes you don’t know if your pet has heartworm disease until it becomes more advanced, which is why it’s crucial to team with your veterinarian to regularly be aware of your pet’s status (hopefully negative) and to focus on prevention of the spread of heartworm disease.

How is Heartworm Disease Spread?

Heartworm is spread by the bite of a mosquito.  Not all mosquitoes carry heartworm, as the parasite enters mosquitos only after feeding on the blood of a heartworm-positive animal.

Additionally, not all animals carry heartworm disease.  Heartworm positive pets or carriers like coyotes, feral cats, seals, and other wildlife are potential sources for mosquitos to pick up the parasite while taking a blood meal.

Dogs can’t directly spread heartworm disease to each other, but an infected pooch can serve as a carrier and potential source of infection for other dogs in the household.  If a mosquito comes along and bites the positive dog in a household and then bites the other dog in the household, then transmission can occur.

Are Dogs and Cats Both Susceptible to Heartworm Disease?

Yes, both dogs and cats are susceptible to heartworm disease.  It’s more common in dogs, as they tend to spend more time outside in environments where mosquitoes can thrive. Some cats lead to an indoor/outdoor existence, so they are just as prone as their nine counterparts.

Simply because a cat has an indoor existence is not a sufficient reason to not take preventative measures as pertains to heartworm disease.  Mosquitos are able to get inside through open windows, doors, and places were screens are damaged.  So, no animal is really 100% safe from the potential to contract heartworm disease.

The American Heartworm Society’s Feline Heartworm Disease reports:

“Although outdoor cats are at greater risk of being infected, a relatively high percentage of cats considered by their owners to be totally indoor pets also become infected. Overall, the distribution of feline heartworm infection in the United States seems to parallel that of dogs but with lower total numbers.”

Cats infected with heartworm disease can have clinical signs like feline allergic bronchitis (“feline asthma”) and the diagnosis may be missed if the appropriate diagnostics (blood testing, x-rays, trans-tracheal wash for cytologic evaluation, etc.) aren’t pursued during routine wellness or illness appointments.

Are Humans Susceptible to Heartworm Disease?

Fortunately, heartworm does not thrive in the human body like it does in our canine and feline companions. People can still be infected with heartworm through the bite of a infected mosquito, but the parasite is not able to survive in the human blood stream.

Sometimes, heartworms can form a granuloma, which is a nodule-like area composed of the offending substance (heartworm, foreign body, etc.) that’s been surrounded and walled off from the rest of the body by layers of encircling tissue and inflammatory cells.

Are there areas of the U.S. that are more affected by Heartworm Disease?

Yes, some parts of the U.S. are better recognized as places where your pet can be infected by heartworm disease.

It’s common and seen year-round in in warm and humid climates that support the mosquito lifecycle, like on the south East and Gulf Coast areas.  In the north East Coast, and plain states, mosquitoes are prevalent on a seasonal basis and thrive in the spring to fall months.

Even in locations where heartworm disease is less common, it can be spread by positive pets or wildlife carriers.  According to Heartworm Incidence in 2013 (click here to see the trend in your area), the disease has been detected in all 50 states but is reported in low numbers in certain regions due to climate or to lack of veterinary hospitals in that area to perform the test to diagnose heartworm.

Fortunately, where I live and practice in Southern California heartworm disease is relatively uncommon, but it is reported by LA County Public Health to be a growing problem as a result of climate change and the movement of heartworm positive dogs to the Los Angeles area after Hurricane Katrina.

I recently diagnosed the second case of heartworm disease I have seen in 10 years of practicing in Los Angeles. The dog is a cancer survivor and routine blood testing was yielded a positive result despite the lack of any clinical signs. The patient had missed three doses of heartworm preventative, but otherwise had been continuously medicated for years with monthly heartworm preventative since moving from Mexico in 2013.

How is heartworm disease diagnosed?

There are a couple of ways that heartworm can be diagnosed, including:

Blood Testing

The most common test for heartworm disease is a simple blood evaluation for heartworm antigen (toxin or foreign substance that stimulates an immune response) by ELISA (Enzyme-Linked Immunosorbent Assay).

Yet, sometimes infection with a single or a few worms does not produce a significant quantity of antigen.  Plus, the antigen testing is specific to female heartworms, so there’s potential that an infestation with adult male heartworms may go undetected.

Ultimately, pairing the heartworm antigen by ELISA with an antibody (immune protein produced in response to antigens) or other testing (direct blood smear for microfilaria, x-rays, cardiac ultrasound, etc.) provides a more thorough approach which can be tailored by your veterinarian to suit your pet’s clinical presentation.

There can be some other blood testing clues that a pet is infected with heartworm.  The complete blood count (CBC) can show increases in white blood cells (WBC) associated with inflammation or parasitism.  This can appear as a high-normal or high levels in the overall WBC count or the Eosinophils (cells produced in higher numbers to fight parasitism).

Additionally, increases in globulin counts can be seen.  Globulins are immune system proteins (antibodies) that are created to help manage infection and inflammation.  As time goes own, a variety of other blood test abnormalities can be see including kidney and liver damage, protein loss, anemia, and more.

Generally, a full baseline blood panel including chemistry, CBC, thyroid, and other testing should be performed when a dog is diagnosed positive with a heartworm disease.

Radiographs (x-rays)

When an pet is diagnosed as being heartworm positive, it’s important to perform x-rays of the chest to get an assessment on the degree of severity to which the pet is affected.  Heartworm can compromise the lungs in a manner where oxygenation does not properly occur and can cause heart and blood vessel enlargement that reduces cardiac function.

Echocardiogram

Besides radiographs, imaging of the organs to determine the severity of heartworm disease is merited when a diagnosis has been established.  Echocardiogram is an ultrasound of the heart to which permits the veterinarian to look inside the heart and blood vessels to see the degree to which worms may be present.  Typically, the presence of worms and larger numbers present creates more of a cause for concern.

How can pet owners protect our dogs and cats from heartworm disease?

Pet owners should start by using screens and keeping doors and windows closed to so mosquitoes can’t enter the seemingly safe confines of our homes.  Additionally, don’t permit any standing water on your property (ponds, plant dishes, etc.), as such serve as breeding grounds for mosquitoes.

Next, have your pets undergo a physical examination by a veterinarian at least every 12 months and follow recommendations for heartworm testing and prevention.

Cats and dogs having an unknown heartworm disease status or those not being consistently medicated with a veterinary-prescribed preventative must have at blood testing confirmation that their heartworm disease status is negative before starting a monthly preventative.  If a pet is heartworm positive and a preventative is used, large numbers of heartworm larvae (microfilaria) can be killed and a life-threatening toxic response can occur.

Pets testing negative can receive veterinary-prescribed heartworm preventatives (milbemycin, ivermectin, moxidectin, selamectin, etc.) and should be medicated every month despite seasonal climate changes.

For heartworm disease, prevention is truly the best medicine, as heartworm-infected cats and dogs will experience potentially-irreversible and life-threatening health consequences if the infection goes untreated.

Meet the Author: Patrick Mahaney

Dr. Patrick Mahaney VMD, CVA, CVJ is a veterinarian and certified veterinary acupuncturist providing services to Los Angeles-based clients both on a house call and in-clinic basis. Dr. Mahaney’s unique approach integrating eastern and western medical perspectives has evolved into a concierge house call practice, California Pet Acupuncture and Wellness (CPAW), Inc. Additionally, Dr. Mahaney offers holistic treatment for canine and feline cancer patients at the Veterinary Cancer Group (Culver City, CA).

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