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Heartworm Diagnostics: Antigen Tests Alone Aren’t Enough

Lindsay Starkey, DVM, PhD, DACVM - Parasitology
Assistant Professor, Department of Pathobiology
Augurn University College of Veterinary Medicine 

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Heartworm Diagnostics: Antigen Tests Alone Aren’t EnoughQuarterly Update 8 2020

The American Heartworm Society (AHS) recommends heartworm antigen testing to screen dogs without clinical signs and verify suspected heartworm infections. While patient-side antigen tests provide a vital tool for the practice, a full understanding of their role in heartworm diagnosis is essential.

Q. Today’s commercial antigen tests have great utility in heartworm screening. Nevertheless, these tests also have certain limitations, don’t they?

A. Antigen tests detect circulating heartworm antigen from adult worms and are nearly 100% specific. While highly accurate in the vast majority of cases, even the most sensitive heartworm test can fail to detect the presence of antigen when levels of circulating antigen are low due to the presence of immature worms, male-only infection or few female worms. In other cases, sufficient antigen is present but bound by the antibodies produced as part of the body’s response to infection. This is known as an immune complex formation.

The States of Heartworm Incidence

Chris Duke, DVM, President, AHS
Bienville Animal Medical Center
Ocean Springs, Mississippi

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Spring 2020 Quarterly UpdateSince 2002, the American Heartworm Society has conducted a triennial nationwide survey of veterinary practices and animal shelters that tracks heartworm incidence. Nearly 6,000 practices and shelters participated in the latest survey, which reflected data from the 2019 calendar year. These data were also used to produce the 2019 AHS heartworm incidence map.

Q. What did the latest AHS survey reveal? What stands out?
A. There’s no question that heartworm disease is more common in some areas than others, and the states with the highest percentages of positive cases continue to be in the Southeastern and Gulf Coast regions, where the climate creates ideal conditions for heartworm transmission. Sadly, my home state of Mississippi has been the #1 state in heartworm incidence for the past six years. The remaining states in the top 10 were Louisiana, South Carolina, Arkansas, Alabama, Texas, Tennessee, Georgia, North Carolina and Oklahoma.

Hit the “Reset” Button on your Client Education Program

Chris Duke, DVM, President, AHS
Bienville Animal Medical Center
Ocean Springs, Mississippi

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It’s spring—the season when many clients come to your clinic for heartworm checks, medication efills and their annual dose of heartworm education. As you think through how to once againSpring 2020 Quarterly Update
make a compelling case for heartworm prevention, consider the following: 

1. Clients learn best when there is a perceived need.

Pet owners must see a payoff if they’re going to make the effort to learn about heartworms. I emphasize that heartworm disease is preventable and position preventives as “insurance policies” against it. Using a life cycle diagram, I explain that heartworms undergo three life stages inside the pet and two stages within the mosquito vector—then show clients how heartworm preventives interrupt this cycle.

I admit that I’m often tempted to tell my clients everything I know about heartworms. However, I’ve learned it is better to limit my talking points to essential information and to save the “nice to know” scientific facts for clients who demonstrate genuine interest.

2. Use visuals to facilitate understanding.

Most clients learn better if I combine visuals with my heartworm talk. That’s why I keep copies of the AHS heartworm incidence map in every exam room. Clients are always drawn to the color of our locale on the map and it helps them understand the implications for their pets. I’ve also had clients point to parts of the country where they didn’t expect heartworms to be endemic and say, “Wow. I’m going to tell my friend Mary that she’d better be giving her pets heartworm prevention!” 

Like thousands of other veterinary practices, my clinic leverages social media to educate clients. Facebook, the most popular platform, is a highly visual and shareable medium that is used by seven in 10 U.S. adults, with three-quarters of those adults visiting the site once a day. I recommend you ask the administrator of your social pages to like/follow organizations such as the American Heartworm Society, so they can see and share engaging posts with infographics, videos, fact sheets and slide shows dedicated to heartworm facts.

3. Building client trust and rapport pays off.

Clients who trust veterinarians and staff members are more willing to comply with our recommendations. I remind clients that, like them, I’m a busy pet owner who needs help remembering my own pets’ heartworm preventives. Rather than lecture them about medication reminders, I pull out my phone and show clients the reminders I have set up to ensure I don’t miss doses.
Trust is also critical if I’m to expect disclosure on the part of pet owners who have had lapses in preventive administration. With compliance as the ultimate goal, I can turn an honest admission of noncompliance into a teachable moment. Keeping my clinic a no-judgment zone can be critical to the health of my patients going forward.

What’s the Protocol?

bsoManaging Dogs with Severe Heartworm Disease

Marisa Ames,DVM, DACVIM (cardiology)
Associate Professor
Colorado State University,
College Of Veterinary Medicine

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Managing Dogs with Severe Heartworm Disease

Fortunately, most infected dogs have no clinical signs or only mild signs of heartworm disease (HWD) and tend to undergo adulticide therapy with few major complications. The likelihood of complications associated with HWD increases with the chronicity of infection. Prognosis is dependent on stabilization, the ability to administer subsequent adulticide therapy and the ability of the family to commit to treatment of chronic conditions. Following is a brief overview of these complications, including clinical signs, diagnosis and treatment. For information on drug dosages, please visit www.heartwormsociety/treating-severe-HWdisease.

Heartworm Disease Causes Lifelong Damage

Making the Case for Heartworm Prevention in Cats

Stephen Jones, DVM
Immediate Past President, American Heartworm Society
Lakeside Animal Hospital
Moncks Corner, South Carolina

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Q. You have studied the short- and long-term effects of heartworm disease in dogs in order to better understand the pathology it causes. What have you learned?

A. By conducting necropsies of experimentally infected dogs and documenting my findings, I’ve learned that heartworm disease actually begins long before clinical signs are evident—in fact, the damage to the pulmonary vasculature begins before heartworms can be diagnosed with standard antigen tests.

Once the worms reach their adult length of 10 to 12 inches, the larger pulmonary arteries are affected and the disease progresses, with the number of worms present, the duration of the infection, and the activity level of the dog all affecting disease severity. Given time, heartworm infection leads to a significant thickening of the pulmonary arteries, obstructive disease, perivascular inflammation and fibrosis, while natural, random worm death can cause dramatic embolic and inflammatory disease with significant acute and long-term consequences.

Q. While necessary, treatment of adult heartworms can present complications of its own. Can these complications be mitigated?

A. The American Heartworm Society (AHS) protocol recommends concurrent use of a a macrocyclic lactone, doxycycline, and a glucocorticoid to reduce the degree of embolic and inflammatory disease that typify complications of worm death. Restriction of a pet’s activity is also a critical factor in treatment success. Exercise, overheating and excitement during treatment increase blood
flow to blocked vessels, causing capillary delamination, rupture and fibrosis.

While adulticide treatment can result in complications, it is important to eliminate adult heartworm infections as quickly as possible. The AHS protocol was designed to improve the condition of the patient, curtail disease progression and eliminate all stages of heartworms with minimal post-treatment complications.

Q. What should veterinarians take away from these findings?

A. The goal of heartworm treatment is to halt the progression of disease and to substantially resolve the acute disease over time. However, in my necropsy work, I have never seen a dog with a
heartworm infection that didn’t have heartworm disease—and I have never necropsied a dog treated for heartworms that did not have permanent damage, even when the infection occurred
many years before. While many of these dogs remained clinically normal throughout their natural lives, evidence of mummified heartworms remnants along with vascular and pulmonary fibrosis on necropsy offer proof that at least some heartworm disease is lifelong.

It’s important to understand that these observed permanent changes are not the result of adulticide treatment but, rather, a consequence of having had heartworms in the first place. The takeaway should be that year-round, lifelong prevention is one of the most important recommendations veterinarians can give to protect the lifelong health of their patients.


Making the Case for Heartworm Prevention in Cats

Making the Case for Heartworm Prevention in Cats

While they are harder to diagnose than dogs,While they are harder to diagnose than dogs,cats get heartworm disease, too

Elizabeth Clyde Druin, DVM
Owner, Clyde's Animal Clinic, Matton, Illinois

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Q. You’re a big believer in heartworm prevention in cats. What has driven that belief?

A. I saw my first feline heartworm case more than 25 years ago. The cat had died suddenly, so I performed a necropsy and found two adult heartworms in the heart. Back then, there were no preventives for cats, and veterinarians and owners were largely unaware of the problem. Today, awareness is higher and we have several products to choose from, which makes compliance easier. But we still have a long way to go.

Tackling Tough Conversations

AHS 18 Winter Bulletin PDF

About Heartworm Treatment Costs

Christopher Rehm, DVM
President, AHS; Owner, Rehm Animal Clinics of Mobile and Baldwin Counties Mobile, Alabama

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Q. Breaking the news to an owner that a dog has heartworms and needs treatment isn’t easy, and explaining the costs can compound the difficulty. How do you justify the cost of treatment to a client who has trouble paying for it?

A. No question, heartworm treatment is expensive. In my southern Alabama practice, I work with many clients and rescue groups with limited resources, so I understand how difficult it can be to discuss these costs with clients. My approach is to explain that I’m recommending what’s best for the pet’s long-term health. Beyond eliminating heartworm infection, the disease caused by damage to the cardiopulmonary system can be long-term. The treatment protocol we use can make a difference in minimizing disease over time.

The Wrath and Aftermath of Hurricanes

AHS 18 Winter Bulletin PDF

What are the Implications for Heartworm Infection?

by Clarke Atkins,DVM, DACVIM

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Q. Historically, what do we know about the connection between devastating storms and heartworm incidence?
A. While high winds associated with hurricanes may reduce mosquito populations for a short period of time, resultant standing water and non-functioning mosquito abatement programs allow extensive mosquito blooms. Hurricanes also cause destruction, evacuation and financial misery in affected areas. Because of reduced care or abandonment (an estimated 250,000 dogs were abandoned in New Orleans alone after Hurricane Katrina), massive numbers of dogs miss doses of heartworm preventive and suffer exposure to mosquitoes 24 hours a day. In heavily endemic heartworm areas, where many dogs are already infected with heartworms, the result can be a heartworm epidemic.

The AHS Guidelines: 2018 Updates

AHS Jul 18 Summer Bulletin PDF

Guidance on Prevention, Testing and Treatment

by Tom Nelson, DVM

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While heartworms are a tough opponent, knowledge is a powerful weapon. To arm veterinarians with the most up-to-date and comprehensive information about heartworm management, the American Heartworm Society (AHS) created guidelines on heartworm prevention, testing and treatment, as well as heartworm epidemiology and biology. Guideline revisions are published as needed, based on sound principles of heartworm management. The 2018 guidelines include the following recommendations.

The AHS Protocol vs. “Slow Kill”

AHS Jul 18 Summer Bulletin PDF

by Andrew Moorhead, DVM, MS, PHD, DIP

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Heartworm treatment can be time consuming and expensive, causing some clients to balk when presented with a treatment plan. Because heartworm is both serious and progressive, I advise clients that treatment is no place to cut corners.

Q. What is the rationale for the steps involved in the American Heartworm Society (AHS) heartworm treatment protocol?

A. The AHS protocol starts with administration of a macrocyclic lactone (ML) drug on Day 1 and Day 30 of treatment, along with oral doxycycline at 10 mg/kg BID on days 1–30.

  • Giving the ML eliminates any juvenile stages of worms that might be present. Melarsomine is not labelled for treatment of worms less than 4 months or age
  • Administering doxycycline eliminates Wolbachia bacteria. This bacteria plays a key role in heartworm pathogenesis, and eliminating Wolbachia signifi- cantly reduces inflammation and worm mass, thus decreas- ing the severity of pulmonary thromboembolisms (PTE).
  • Glucocorticoids should also be given as needed to control clinical signs of PTE.

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