Canine heartworm and Dirofilaria dollar discourse

Ideally, heartworm prevention should be discussed and begun at the first puppy exam

When puppies come in for their first exams, tell clients about different heartworm prevention options.
When puppies come in for their first exams, tell clients about different heartworm prevention options.

Canine heartworm prevention is a common client conversation topic. In the contiguous U.S., the nematode parasite causing heartworm, Dirofilaria immitis, is endemic, and elsewhere (globally) it is an increasing concern due to climate change, subsequent vector (mosquito) abundance, and range expansion.1,2 Similarly, canine importation can prompt heartworm (HW) disease concern by way of infected dogs imported to non-D. immitis endemic countries.

Dogs infected with Dirofilaria immitis, and its common bacterial co-pathogen (Wolbachia spp.), are frequently subclinical on presentation. However, if they do not receive treatment, dogs progress to apparent cardiovascular and respiratory signs, including cough, dyspnea, exercise intolerance, and in severe cases with high worm burdens, right-sided heart failure or caval syndrome.1,2

Despite the dog’s high susceptibility to heartworm, disease is entirely preventable, provided puppies in endemic regions (or at high risk due to travel) are begun on macrocyclic lactones prior to eight weeks old.1,2

Pet owners may challenge the need for preventive care, especially if indicated for lifelong use or in regions not considered heartworm-endemic, for varied reasons. Even without the strain of the continued COVID pandemic, communication in day-to-day practice can be tough. The consideration of individual pet owner needs is evermore essential in the current delivery of veterinary care. Incorporation of spectrum of care, defined as evidence-based veterinary medicine (EBVM) options along the socioeconomic spectrum, within these dialogues can be helpful for clients, patients, and veterinarians alike.3,4

Spectrum of care and EBVM

Evidence-based veterinary medicine has been defined as, “the use of the best relevant evidence in conjunction with clinical expertise to make the best possible decision about a veterinary patient.”3,5,6

This, along with spectrum of care, will be used in an all-too-common example of a canine heartworm preventive care conversation. Further information about this process can be found in an earlier “Canine parvovirus care on a budget” article (Veterinary Practice News, November 2021).

Dirofilaria discourse for Thornton

Let’s meet our patient. Thornton is an adorable eight-week-old, velvet-soft, long-eared, “pandemic pup” with huge chocolate brown eyes.

His owners have not had him very long and let you know at the first appointment they are dedicated to his having a natural lifestyle and want to avoid “products” as much as possible. He eats raw and organic food as his main diet, along with natural treats. Thornton is a healthy pup on your exam, and his owners are quick to relay their health care priorities for him, not necessarily in order, as they are both speaking at once and occasionally disagreeing.

These are 1) a natural lifestyle and diet; 2) reduced cost of veterinary care; 3) they love their long-eared guy and want to do what they can to keep him around for a very long time; 4) while they are fine with vaccines, as they know they need some of them to travel, they prefer natural (i.e. non-chemical containing) methods of parasite control; and 5) if your recommendation is a preventive “product,” they say, “You better prove to me he really needs it and it is not just propaganda, Doc.”

The evidence-based veterinary medicine process

Thornton’s care priorities have now been voiced. Let’s implement EBVM and spectrum of care for this little one as well as his besotted people.

Step 1 is taking time to develop the clinical question, which is critical for efficient literature searches: In Thornton’s case, the question is around heartworm preventive care (i.e. evidence-based prevention vs. heartworm disease therapy) and his owner’s major outcomes of interest: reduced illness, increased lifespan.

Our clinical PICO question,5,6 where P (patient or population), I (intervention), C (control or compare), and O (outcome), might be, “Are dogs living in (or traveling to) heartworm endemic areas (P) who have had prophylactic heartworm medication (I) compared with those without prophylactic heartworm medication (C) at decreased risk for heartworm disease (O) over their lifespan (T).”

Step 2 is the scientific evidence: Table 1 details is a summary of a brief (10 to 15 minutes) veterinary literature search (AKA “prove it to me, Doc”) and critical appraisal (e.g. strength of evidence and study limitations) using the PICO question above and a freely available online search engine (such as

As summarized, the scientific literature1,7 supports appropriate and consistent use of heartworm preventives to reduce illness and increase lifespan as related to preventing heartworm disease. Our level of evidence is categorized as moderate, as it contains one randomized controlled clinical trial and an evidence-based and expert opinion-based consensus guideline.1,7 We can also feel comfortable letting Thornton’s owners know herbal or “natural” prevention therapies have not been shown to be effective based on this literature.1

Additionally, due to reports of resistance to macrocytic lactones, and as a warning that if there are breaks in owner medication application and consistency, it may be indicated to add ectoparasiticide-mosquito repellent to conventional preventives for Dirofilaria.

These products may lower infection risk further and “ramp up” coverage to assist if (or when) a dose of medication is missed.1,2 Environmental control of mosquitoes as part of multi-modal prevention is not being discussed in this article and the reader is referred to the American Heartworm guidelines for review, along with information pertaining to disease diagnosis, travel- and import-related exposure, and therapy.1

Thornton’s spectrum of care cost options

Listed below are examples of spectrum of care cost ranges (<$100, $100-$500, > $500) and summarized client communication. Please note these are estimates and will vary based on region.

Less than $100 for HW prevention. (This does not account for HW therapy.)

Communication with clients: Regular heartworm prevention should be given to all dogs living in endemic areas. If the dog is Dirofilaria-free at the start of treatment, this should prevent infection provided no treatments are missed or late.

Ideally testing for Dirofilaria is performed if a puppy is older than six months of age, as preventive medications are not effective against late larval or worm life stages.

Consultation: $70

Preventive medication (1 tablet): $10-$25

Total: $80-$95 for this visit, additional $10-$25/month ongoing

Between $100-$500 for HW prevention. (This does not account for HW therapy.)

Communication with clients: Dogs in high-risk areas should receive regular medication dependent on region and product specifications.

You emphasize to Thornton’s owners missed or late doses can allow for infection, that resistance to common HW drugs is rare, and preventive medication with traditional drugs should be sufficient.1 However, if there is particular concern for infection, these drugs can be used in combination with mosquito repellents and ectoparasiticides.1,7

Additionally, heartworm screening should be performed annually with tests that detect adult worms and the immature larvae to ensure there has been no infection. In anticipation of questions, you explain both tests are advised, as while the test for adult heartworms is the most effective, a small number of infected dogs may test negative despite infection. Also, if there is concern the dog might have been infected, due to a missed dose, then the dog should stay on preventive medications and testing should be done seven months after the break in coverage.1

Consultation: $70

Blood sample collection: $21

Heartworm antigen test: $50

Microfilaria test: $45

Preventive medication (1 tablet): $10-$25

Total: $196-$211, additional $10-$25/month ongoing

Total: $300-$485 annually

$500+ available if HW disease treatment is needed (i.e. no prevention)

Heartworm (Dirofilaria immitis) exposed in internal organs of a dog during necropsy.

Communication with clients: In the scenario where preventive heartworm care is declined and Thornton is infected, you notify his owners he must be treated and the treatment can be extensive, expensive, and not without risk. Therapy goals for HW disease consist of 1) improved clinical status of the dog; 2) eradication of all heartworm life stages; and 3) minimization of adverse effects and complications of treatment.1

Unfortunately, the longer a dog with heartworm is infected, the more damage these worms cause, and the more likely a dog will die of the infection. Treatment risks include as the worms are treated, they can move into the dog’s blood vessels and occlude or block them with potentially severe outcomes, e.g., bleeding into the lungs, heart failure.

Severely affected dogs in heart failure will die without immediate treatment and require surgical removal of worms. This will greatly increase the cost of treatment.

Further, treatment involves exercise restriction, i.e., keeping Thornton in a kennel at home for most of the day and night for several months with no exercise other than strictly on leash to go out to the bathroom a few times a day.

Therapy also consists of “pre-treatment” for a couple of months to ensure all the immature larvae are dead and only adult worms are left. Finally, it is possible Thornton may still have some life stages that are not treated by either medication and may need a second round of treatment a few months after the first.

Note that at this time, the American Heartworm Society and Companion Animal Parasite Council do not advise use of “slow or trickle-kill” therapy in dogs with confirmed heartworm infection.1 Further literature appraisal on these methods of treatment and long-term exercise restriction is beyond the scope of this article, and curated references are provided.8,9

The treatment protocol below is an approximate of that recommended by the American Heartworm Society:1

Consultation: $70

Blood sample collection: $21

Heartworm antigen test: $50

Microfilaria test: $45

Three-view chest radiographs: $250

Recheck consultation: $50

Preventive medication (12 tablets): $10-$25; $120-$300 annually

Antibiotics: $150 (Wolbachia spp.)

Glucocorticoids: $50

Hospitalization (1/2 day): $45

Recheck consultation: $50 +Adulticide injection (plus injection fee): $100

Recheck consultation: $50 +Adulticide injection (plus injection fee): $100

Recheck consultation: $50 +Adulticide injection (plus injection fee): $100

Recheck consultation: $50 + Sample collection: $21.00 + Microfilaria test: $45.00

Recheck consultation: $50 + Sample collection: $21.00 + Heartworm antigen test: $50

Microfilaria test: $45

+/ Stabilisation for a severely affected dog $400-$800

IV set up/maintenance: $200

Fluid therapy (1 additional day): $70

Additional medications: $150

Hospitalization (2 days): $200

Total: $2,250-$3,000

An estimate for HW treatment can be a helpful discussion point for preventive care conversations, client budgeting, and to illustrate “an ounce of prevention is worth a pound of cure.” Note this estimated fee may be increased or decreased dependent on clinic and severity of clinical signs, and does not include surgical intervention if needed. Cost may be lower with slow or trickle-kill methods, but is still estimated to be > $500.

What happened next for this pandemic pup?

Fortunately, in our fictional case setting (or is it?), Thornton presented at eight weeks of age. As such, provided preventive therapy is begun immediately, our first optimum (and lowest cost) option can be implemented.

As this is a first puppy visit for Thornton and his owners, additional spectrum of care plans for the longer-term are next initiated based on his risk profile to ensure he is dewormed, vaccinated, and receives other patient age- and region-appropriate preventive care.

Ideally, heartworm prevention should be discussed and begun at the first puppy exam. This is another infectious disease, that while horrific and frequently sad (particularly in the severe stages), is entirely preventable with regular and consistent product compliance based on risk, region, and Dirofilaria immitis endemicity.1,2

An EBVM approach can assist with client communication, allow for a spectrum of care approach (with practical cost ranges when indicated), and sometimes provide help in the veterinary trenches for those “what if” and “prove it to me” client conversations.

Michelle Evason, BSc, DVM, DACVIM (SAIM), has worked in general practice, academia, specialty clinical practice, and in the animal health industry. She serves as a small animal internal medicine and nutrition consultant in North America and consults for IDEXX UK. Dr. Evason has published on numerous infectious diseases, antimicrobial stewardship, nutrition, and pet-owner education-related topics. She enjoys ferrying her children to various activities and fulfilling most “Canuck” stereotypes.


  1. Nelson CT, McCall JW, et al. Current canine guidelines for the prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in dogs (revised 2018 and copyright 2020). Accessed January 2022.
  2. Weese JS, Evason M. Infectious Diseases of the Dog and Cat. CRC Press. July 2019.
  3. Stull JW, Shelby JA, et al. Barriers and next steps to providing a spectrum of effective health care to companion animals. J Am Vet Med Assoc. 2018. 253: 1386-1389.
  4. Fingland RB, Stone LR, et al. Preparing veterinary students for excellence in general practise: building confidence and competence by focusing on spectrum of care. J Am Vet Med Assoc. 2021. 259: 463-471.
  5. Launching an evidence-based veterinary medicine manifesto to drive better practise. Vet Rec. 2020: Sep;187 (5): 174-177.
  6. Evidence-based veterinary toolkit, RCVS. Accessed August 2021.
  7. McCall JW, Varloud M, et al. Shifting the paradigm in Dirofilaria immitis prevention: Blocking transmission from mosquitoes to dogs using repellents/insecticides and macrocyclic lactone prevention as part of a multimodal approach. Parasites and Vectors. 2017. 10:75-85.
  8. Ames MK, van Vranken P, et al. Non-Arsenical heartworm adulticidal therapy using topical moxidectin-imidacloprid and doxycycline: A prospective case series. Vet Parasitol. 2020. May;282(0):109099.
  9. 9.Savadelis MD, Coleman AE, et al. Clinical assessment of heartworm-infected Beagles treated with a combination of imidacloprid/moxidectin and doxycycline, or untreated. J Vet Intern Med. 2020;34(5):1734-1745.image/123459829?adppopup=true

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