Heartworm Testing: A Yearly Necessity? (2023)

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

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.

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.

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.

HEARTWORM DISEASE RESOURCES

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

Clarke 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.