AHS Board Speaks Out

Managing 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.

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.

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.

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.

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.

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.

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.

STEPHEN JONES, DVM

While pet numbers in the U.S. are increasing, the number of annual heartworm preventive doses sold in the U.S. is declining. I believe this is a frightening trend, especially considering that the average number of heartworm cases per veterinary clinic rose by 21% between 2013 and 2016.¹

I recently researched medication adherence in human patients² in order to better understand why people don’t take their own medications. In the process, I learned that the reasons my clients don’t give pets heartworm preventives as directed are similar to the reasons they skip their own cholesterol, diabetes and blood pressure medications.

by Brian A Digangi, DVM, MS, DABVP

Q. The American heartworm Society (AHS) and the Association of Shelter Veterinarians (ASV) recently released a set of best practices for transporting dogs. Why?

A. The AHS and ASV saw a need for heartworm-specific transport recommendations to guide veterinarians and animal shelters. Heartworm disease has become more widespread in the U.S. over the past several decades, due in part to the increased movement of heartworm-positive dogs to regions where heartworm disease was once uncommon. When mosquitoes feed on a microfilaremic dog, they become heartworm vectors capable of transmitting heartworms to unprotected pets.

by Brian A Digangi, DVM, MS, DABVP

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Veterinarians today are fortunate to have several point-of-care heartworm tests. In the majority of cases, these D. immitis antigen tests meet practitioners’ needs for speed and accuracy. However, when test results conflict with clinical expectations, the added step of heat pretreatment (HPT) of the sample should be considered.

Q. What can cause a “no antigen detected” result when the patient is infected?

A. Patient-side diagnostic tests rely on the presence of soluble heartworm antigen for the detection of heartworm infection. In cases where soluble antigen is bound to endogenous antibodies, forming an insoluble unit in the bloodstream known as an “immune complex,” the antigen is essentially blocked from detection. This blocking has been linked to several factors: