“Doctor, I missed a dose!”

When the COVID pandemic hit North America in March, the immediate goal of government leaders and public health officials was to preserve personal protective equipment (PPE) and avoid overwhelming human health-care systems. Lines were quickly drawn between “essential” and “nonessential” services, with definitions varying from state to state. In most states, veterinary personnel were considered essential workers and pet owners were allowed continued access to veterinary care; however, for a time, this was limited for many to emergency medical services.

As stay-at-home orders and other restrictions curtailed consumer activity, many veterinary clinics experienced a temporary disruption in the provision of well pet care. Wellness exams and routine disease screenings, including for heartworm, were delayed, requiring veterinarians to devise interim strategies to safeguard patient health. In April 2020, the American Heartworm Society (AHS) issued a series of recommendations to help guide veterinarians through decisions about heartworm diagnosis, prevention, and treatment that were prompted by delays in annual exams, heartworm screening, and treatment visits. The primary goal: To avoid disruptions to heartworm prevention that could increase the potential for disease transmission.

In cases where existing veterinarian-client-patient relationships (VCPRs) and compliant owners assured uninterrupted heartworm prevention, short-term delays in annual screening for the parasite had minimal impact; veterinarians simply extended refills on heartworm preventatives for a limited time (e.g. up to six months). However, in other cases where compliance with year-round prevention lapsed or was unknown, a number of questions have arisen, even as pet wellness care and screening have resumed in veterinary practices.

The following are common questions and answers regarding lapses in administration of heartworm preventives.

Q) While most clients in my practice were faithful in giving their dogs heartworm preventatives, some have admitted lapses of several months when their annual visits were delayed. How do I address the risks of these lapses with clients?
A) Monthly heartworm preventatives work by eliminating the heartworm larvae that have accumulated in the subcutaneous tissues during the previous month. These larvae molt into immature adults as early as 52 days after they have entered the dog via a mosquito bite. By this time, preventatives are less effective. The immature worm then penetrates a blood vessel and is transported in the circulation to the pulmonary arteries, where they continue to grow. Pathology of the pulmonary arteries is present by the third month postinfection.

A dog who has missed just two monthly doses of preventative or is late for a heartworm injection can become infected with heartworms. In such cases, dogs should be screened with antigen and microfilariae tests, and their owners advised that standard heartworm testing cannot detect infection until about the sixth month postinfection. For this reason, dogs who initially test negative following lapsed heartworm prevention should be retested six months later. Waiting an entire year to test a potentially heartworm-positive dog could allow an undiagnosed infection to progress and increase the degree of permanent damage to the pulmonary vessels and lung tissue.

Q) In my practice area, the pandemic led to a flurry of new-pet adoptions. Are there any special considerations related to heartworm management that should be explained to owners of newly adopted pets?
A) The most important step is to get the new dog on a heartworm preventative as soon as possible, with year-round administration recommended. Even though continuous transmission may not occur nationwide, administering broad-spectrum heartworm medications for 12 months a year promotes compliance and can help protect dogs from other pathogenic and/or zoonotic parasitic infections.

The exact recommendations for newly acquired dogs may vary, depending on the patient’s age and origin.

  • A new puppy should be started on heartworm prevention by two months of age.
  • Puppies between three and six months of age should be put on heartworm prevention, and then tested six months later. Again, any potential infection will not be detectable on a heartworm test until six months postinfection. With preventive dosages based on patient body weight, growing dogs should be weighed frequently to ensure they are on the proper dose.
  • Young dogs who are older than six months, as well as adult dogs with unknown prevention history, should be tested before being given heartworm preventatives and, if negative, retested six months later.

With in-person appointments limited in many practices, it is critical that clients be educated about heartworm disease. AHS offers a number of client resources on heartwormsociety.org, including a “heartworm basics” overview.

Q) I recently saw a client who had delayed their annual visit by six months because of the pandemic. Our clinic refilled their prescription in the interim so that heartworm prevention would not be interrupted. Now the dog is testing heartworm-positive and the owner claims they were compliant. What is the next step?
A) The heartworm antigen test should be repeated by sending a new blood sample to a reference lab for confirmation. If the second test is positive and the client purchased an adequate number of doses of heartworm preventative and/or received their preventative injection on schedule—and tests positive—the standard operating procedure should be to report the case to the product’s manufacturer. The pet should then be treated according to the treatment protocol described in the AHS Guidelines.

Q) My practice is in a northern state, but I’ve always recommended year-round use of heartworm preventatives. However, I have a client who wishes to trim household expenses because of job loss and is asking if she could drop preventive use during the winter. What is the appropriate response to her question?
A) To be maximally effective, heartworm preventatives should be given year-round. If seasonal treatment is chosen, the owner must understand that simply giving the preventative during warm-weather months is not enough. Because heartworm preventatives work by eliminating infections acquired during the previous month, prophylaxis should be commenced at least one month prior to the anticipated start of heartworm transmission. In addition, prevention should continue for a specified length of time following the end of the transmission season, with the exact time period depending on the product selected.

Another consideration is whether the client lives in a rural or urban area. While it is true development of heartworm larvae is temperature-dependent, microclimates exist in urban areas due to the urban “heat island” effect. Buildings and parking lots retain heat during the day, creating microenvironments that can support the development of heartworm larvae in mosquito vectors, even during colder months. These microclimates, along with the northern advancement of the Asian tiger mosquito, have extended the heartworm transmission season in cold-weather urban areas.

While pandemic-related disruptions to veterinary care may have raised questions about heartworm management, veterinarians nationwide are reporting resumption of full-service veterinary care, albeit with changes such as curbside drop-off and pickup. For these reasons, it is recommended veterinarians follow guidelines for annual testing and year-round prophylaxis, using their clinical discretion in cases where screening must be delayed.

Tom Nelson, DVM, is research chair for the American Heartworm Society (AHS) and medical director for VCA Animal Medical Center of NE Alabama in Anniston.







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“Doctor, I missed a dose!”