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Heartworm Testing: A Yearly Necessity?

Heartworm Testing: A Yearly Necessity?


Clarke Atkins, DVM, Diplomate ACVIM (Internal Medicine & Cardiology)

It is a pleasure to introduce a new series, which will be presenting questions asked by veterinary professionals and clients about heartworm infection. This series is co-sponsored by Today’s Veterinary Practice (todaysveterinarypractice.com) and The American Heartworm Society (heartwormsociety.org). Each article will present a question or questions on a particular area related to heartworm infection, prevention, diagnostics, and/or treatment.

You asked…

I have heard that yearly testing is no longer necessary due to the high efficacy of heartworm preventives. What is your opinion?

It is true that the monthly oral and 6-month injectable preventives are very good. In addition, when inadvertently administered to a dog with microfilaremic heartworm infection, they are safer than the previous preventive (diethylcarbamazine), which produced severe, sometimes fatal, reactions in microfilaremic dogs. Reactions, albeit rare, can occur in microfilaremic dogs that are inadvertently treated, depending on microfilaria numbers and the product used.

The argument for performing yearly testing includes the facts that:

  • Heartworm infection is spreading
  • Heartworm prevalence is increasing
  • The percentage of dogs on preventives continues to decrease.

One important consideration is compliance (Figure 1). Every study to date has shown that compliance, based on practice/owner surveys and product sales, is suboptimal, at best. In fact, a recent survey showed that more than 50% of dogs leave the clinic without their owners purchasing the amount of heartworm preventive recommended; this is true regardless of the geographic region under consideration. This indicates that veterinary practices are failing to effectively communicate with pet owners about the importance of heartworm prevention and testing.

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Figure 1. Approximately 100 veterinarians attending the American Heartworm Society presentations at the NAVC Conference 2012 were surveyed. It is clear that heartworm preventive compliance was an important issue to these veterinarians.

Other factors—based on variable pet owner diligence—that support yearly testing include:

  • The pill/topical may not be administered correctly, in a timely manner, or at all
  • Owners may fail to return for their 6-month injection in a timely manner
  • Dogs may swim immediately after topical medication has been applied
  • Pills, which appear to be eaten, are expectorated later.

Most important, a majority of families do not purchase enough preventive to meet the recommendations of the clinics that care for their pets.

With all these factors supporting the likelihood that preventive administration may be compromised, regular testing provides early knowledge of infection. If a pet is only tested every 2 or 3 years, undiagnosed infections create lung damage, while yearly testing can markedly reduce this damage by allowing earlier detection of heartworm presence. In addition, microfilaremic dogs receiving macrocyclic lactone preventives, which might be the case in undiagnosed infections, are considered at risk for contributing to the development of resistance.

What do I say to the client who asks “Why do I need to test annually for heartworm infection when you tell me the preventives are nearly perfect and I give them to my dog monthly, without fail?”

For this client, my goal is to make sure the client understands that the question is a very good one. I go on to address, however, the following points:

  • Test Limitations: While heartworm tests currently used are excellent, they have limitations due to the fact that:
    • Immature and all-male infections are not detected
    • Infections with small numbers of females are frequently missed.
  • Compliance: Data indicates that client compliance (purchase and administration) is below par; I emphasize that even good clients, including veterinarians and veterinary technicians, are occasionally late in administering preventives or forget altogether (Figure 2).
  • Variables: As mentioned in the previous question, dogs may:
    • Not receive the pill/topical in a timely matter (or at all)
    • Vomit or expectorate the pill
    • Go swimming immediately after topical preventives are applied
    • Not be returned at the appropriate time for the injectable preventive.
  • Infection Damage: Undiagnosed infections cause damage to pulmonary, vascular, renal, and cardiac tissues. Identifying and eliminating the infection as soon as possible lessens the amount of damage to these vital tissues.
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Figure 2. This Window of Infection chart, prepared for Lucky, a dog residing in the Mississippi River Delta, illustrates the following: (1) A very good preventive purchase history, January 2008 to February 2011. (2) A 90-day gap in the purchase pattern, indicating Lucky was unprotected in the dangerous months of June, July, and August. (3) A negative test at the end of 2009. (4) A positive heartworm test in August 2010. If these owners had chosen to test biannually, Lucky would have maintained an unrecognized and untreated heartworm infection for over 2 years. Reprinted from Heartworm: Prevention, Infection, Treatment (heartwormedu.com), courtesy of Merial (merial.com)

In most circumstances, clients understand the reasons behind the recommendation for yearly testing. However, other issues, mainly ones related to the expense of testing, may result in clients declining testing or postponing it. In these cases, reaching a compromise (see Finding a Happy Medium) is in the best interest of the pet and disease prevention.


While the goal of any clinician is to provide the highest, or gold, standard of care for all patients, there are times when reaching a compromise with a client ensures that the pet is receiving the best level of care that this particular client can afford. Without these options, there’s a good chance that the pet will not receive any type of preventive care, which is definitely the worst case scenario.

Staggering Testing

For example, if a client is resistant to yearly testing due to the cost, I offer a compromise, making it clear that my concern is for the client’s and their neighbors’ dogs (not the money made on heartworm testing). Two options, depending on the client’s situation, are:

  • Have the client agree to biannual testing or
  • If the family has multiple dogs, rotating testing to evaluate each dog every 3 years.

I would never advocate testing any less frequently than every 2 to 3 years, regardless of the number of dogs in the family. In addition, this compromise is only offered to those very good clients whose dogs spend most of their time indoors and do not live in hyperendemic areas, such as the tropics, Mississippi Delta, and other states in the deep southern U.S. or Mississippi River Valley.

Buy One, Get One Free

Another possibility is to provide the heartworm test at no cost (please, do not gasp!) to clients that return at the appropriate time and are willing to buy 12 months’ worth of preventive. I have been told that this is cost effective, and an incentive for the client to commit to year-round heartworm prevention.

What a wonderful message to send to clients—if the client commits to the gold standard of care—year-round heartworm prevention—then you reward that commitment by offering the testing for free. This approach has positive results for everyone involved—the client is incentivized to pursue year-round prevention, the pet receives the highest quality of care, and the veterinary practice has improved its bottom line by selling a year’s worth of preventive.


American Animal Hospital Association

(AAHA; aahanet.org):

  • Parasite Counselor Program: AAHA online program offering 4 free, hour-long modules
  • Heartworm Disease in Pets: Brochure answering clients’ common questions

American Heartworm Society

(AHS; heartwormsociety.org):

  • Canine & Feline Heartworm Guidelines: Guidelines that cover all aspects of heartworm infection, organized by species
  • Heartworm University: Interactive continuing education program
  • Think 12 in 2012: Offers fact sheets, articles, “heartworm heroes,” and archived information
  • Heartworm Incidence & Compliance Maps: Maps detailing incidence and compliance of heartworm disease across the U.S.

American Veterinary Medical Association

(AVMA; avma.org)

  • Heartworm Disease: Client brochure that provides up-to-date information on the disease
  • AVMA Collections—Heartworm Disease: Information compiled into a selection of brief, easy-to-read key points

Companion Animal Parasite Council

(CAPC; capcvet.org)

  • Canine & Feline Heartworm Guidelines: Guidelines available on CAPC’s website
  • Prevalence Maps: Maps detailing the spread of heartworm disease across the U.S.
  • Expert Articles & Resource Library: Extensive collection of articles and other resources on heartworm infection, which are available on the website’s homepage

Food & Drug Administration

(FDA; fda.gov)

  • FDA website “heartworm” search: Provides a significant collection of materials related to heartworm, including consumer information, drug information, and updates on federal regulations

Author_C-AtkinsClarke Atkins
, DVM, Diplomate ACVIM (Internal Medicine & Cardiology), is the Jane Lewis Seaks Distinguished Professor of Companion Animal Medicine at North Carolina State University. He is also a member of the Today’s Veterinary Practice Editorial Peer Review Board and American Heartworm Society’s Executive Board. Dr. Atkins received the 2004 Norden Award recipient for excellence in teaching. His research involves canine and feline heartworm disease and pharmacologic therapies for cardiac disease. Dr. Atkins received his DVM from University of California–Davis and completed his internship at Angell Memorial Animal Hospital in Boston, Massachusetts.