Welcome to the all-new Vetlearn

  • Vetlearn is becoming part of NAVC VetFolio.
    Starting in January 2015, Compendium and
    Veterinary Technician articles will be available on
    NAVC VetFolio. VetFolio subscribers will have
    access to not only the journals, but also:
  • Over 500 hours of CE
  • Community forums to discuss tough cases
    and networking with your peers
  • Three years of select NAVC Conference
  • Free webinars for the entire healthcare team

To access Vetlearn, you must first sign in or register.


  Sign up now for:
Become a Member

Veterinary Therapeutics Winter 2007 (Vol 8, No 4)

Survey of 11 Western States for Heartworm (Dirofilaria immitis) Infection, Heartworm Diagnostic and Prevention Protocols, and Fecal Examination Protocols for Gastrointestinal Parasites

by Dwight D. Bowman, MS, PhD, Christopher Torre, Claire Mannella, DVM, DACVIM, DABVP

    Clinical Relevance

    Heartworm infection in dogs and cats in the western United States is a fairly new phenomenon, and for this reason it is often considered to be of minimal significance. The purpose of this survey was to collect data from 11 western states (Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming) in an effort to assess awareness of heartworm disease and identify areas in which improvement in understanding is needed. To accomplish this goal, veterinary clinics and hospitals in these states were sent a one-page survey in early 2006. The results of the survey demonstrate that cases of heartworm disease have been reported in all 11 states, illustrating the importance of annual testing and the routine use of preventives.


    In 1992, Zimmerman and associates reported that Dirofilaria immitis was present in all states in the northwestern United States and in British Columbia, Canada. They wrote that although infections were reported in the 1970s in southern Oregon, by the time of their survey in 1990, infections had been confirmed in nearly all geographic regions in the state. They documented native infections in Wyoming (a single case near Gillet), Utah (in and north of Salt Lake City and close to Hill Air Force Base, where nearly 30 dogs acquired heartworm infections locally), Montana (three cases east of the Continental Divide), Idaho (a dog from Sandpoint and a dog from Boise that had traveled to Portland, Oregon), Washington (4 dogs that had never left the state), and British Columbia (46 dogs from the Okanagan Valley with locally acquired infections).1

    Frank and colleagues looked at heartworm in Colorado and purposely used microfilarial presence rather than antigenemia as the measure of heartworm infection. These authors chose a microfilarial test because they stated that at the time of the study design, the serologic tests available had been shown to have low accuracy when used as a primary screening tool. Their work showed an overall prevalence in Colorado in 1990 to be 0.77% of 7,818 dogs (about 8 dogs/1,000). The authors excluded dogs for which there was a poorly known travel history from part of the analysis and concluded that autochthonous transmission was occurring in the eastern portion of the state.2

    Terrell and Courtney performed a survey of Alaskan veterinarians that included 515 dogs tested for heartworm and the testing of sera from Alaskan wolves (42) and coyotes (10). Of the 515 canine blood samples tested for heartworm, 13 tested positive. All the positive dogs had a history of recent travel to or residence in a heartworm-endemic area outside of Alaska. The authors suggested that because they found evidence of at least one heartworm-infected dog in Fairbanks and two others in nearby Delta Junction in 1994, heartworm transmission could possibly occur in central Alaska in the future.3

    Despite these findings, many veterinarians in the western United States are not aware that heartworm disease is present or are unwilling to accept that heartworm causes sufficient risk to warrant annual testing and routine preventive use. We therefore decided to examine the prevalence of heartworm in dogs and cats in the western United States using a survey of veterinarians. Hopefully, these findings will provide sufficient evidence to encourage a discussion of the potential for heartworm disease to be present in these communities. This is the only comprehensive survey of veterinarians in the western United States to detail locations of heartworm infections; it differs from surveys performed by the American Heartworm Society (AHS) in that it gives some information on where the dogs may have acquired their infections.


    The survey targeted veterinarians in 11 states in the western part of the country (Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming). The Novartis Animal Health US, Inc., client account database was used as the primary source of contact information. In addition, an attempt was made to contact each state's regulatory agency to request or purchase a mailing list (one-time use only) of licensed veterinarians within the state. Utah and Washington were unable to provide their lists; the Montana State Veterinary Medical Association provided the information for that state. All lists obtained were cross-referenced with the existing list of Novartis accounts. Only veterinary clinic or hospital addresses were used; research facilities, commercial accounts, equine-exclusive practices, and obvious private residences were excluded from the survey.

    Surveys consisting of nine questions (see survey) were faxed to clinics with current fax numbers; the remaining surveys were mailed. Whenever possible, returned or undelivered faxes were retransmitted at a different time of day to attempt delivery. A total of 6,585 clinics in the 11 states were sent surveys (Table 1). All data were gathered and compiled between January and April 2006.

    The accumulated responses to the different questions from responding clinics were summarized by entry into a Microsoft Excel spreadsheet. It is understood that not all cases of heartworm disease within each state were identified and that the meanings of "local" and "travel outside the local area" are likely to have been interpreted differently by the reporting clinics.


    Surveys were sent to 6,585 clinics; 1,101 completed surveys were returned (faxed or mailed) for an overall response rate of 16.7% (Table 1). Montana had the highest percent return (24.1%), and California had the lowest (13.4%).

    Canine Heartworm Infection

    Of the 1,101 responding clinics or hospitals, 481 (43.7%) reported cases of canine heartworm infection diagnosed in 2005 (Table 1). Canine heartworm infections were reported in all 11 states, with a total of 2,224 cases reported. New Mexico had the highest percentage of respondents reporting cases of diagnosed canine heartworm infection (63.2%); Idaho had the lowest percentage (17.9%). California reported the highest number of diagnosed cases of canine heartworm infection (1,290), followed by Colorado (289), Arizona (177), Oregon (161), and New Mexico (122). Wyoming reported the lowest number of cases (6).

    Of the 2,224 reported cases of canine heartworm infection, 1,512 (68.0%) occurred in local dogs (Table 2), 935 of which (42.0% of the total heartworm cases) had no history of travel outside their local area. Every state except Idaho and Wyoming reported cases in local, nontraveling dogs; Nevada reported one case in a local dog with an unknown travel history. The eight remaining states reported that 51.9% to 90.3% of their cases of diagnosed heartworm infection were in local dogs that had not traveled outside their local area.

    Feline Heartworm

    Overall, 58 clinics reported that they had diagnosed (but not necessarily confirmed) 156 cases of feline heartworm infection in 7 of the 11 states (Table 3). There were no cases of feline heartworm infection reported in Idaho, Montana, New Mexico, or Wyoming.

    Of the 156 cases of feline heartworm infection reported, 113 cases were in local cats, 63 of which had no history of travel. California reported the most cases (110), with 94 local cats being infected, 60 of which had no history of travel. Colorado had 10 clinics reporting 19 cases, including 7 confirmed infections in local cats. In Washington, two clinics reported a total of 12 cases; 1 case was confirmed in a local cat, but the travel history for the other 11 cats was not known. Arizona and Oregon each reported 5 cases (two clinics in Arizona and four clinics in Oregon); in both states, 4 of the 5 cases were in cats that were considered local. In Utah, three clinics reported a total of 3 cases, 2 of which were confirmed; all occurred in local cats with no history of travel. In Nevada, 2 cases were reported, both in cats from outside the local area.

    Frequency of Testing

    The frequency of heartworm testing in dogs varied among states (Table 4). The most common categories were testing annually, testing under specific circumstances (e.g., if pet travels, if there has been a lapse in preventive administration, as needed, testing once before starting preventives), or not testing at all. California had the highest percentage (51.0%) of clinics and hospitals recommending annual heartworm testing, and Washington (4.7%) and Idaho (10.3%) had the lowest percentage of clinics that tested dogs annually. Of the clinics reporting cases of canine heartworm infection, the percentage recommending annual heartworm testing was similar to the overall percentage of clinics testing annually and was not necessarily higher.

    Recommendations for Prevention

    Of the 1,101 responding clinics, 810 (73.6%) routinely recommend canine heartworm prevention (Table 5). Of the clinics recommending prevention, the majority (536 [66.2%]) recommend heartworm prevention for 12 months/year. Another 114 clinics (14.1%) recommend 6 months of prevention, and the remaining 160 clinics (19.8%) either reported recommending canine heartworm prevention for varying lengths of time (2 to 10 months or on an "as needed" basis) or did not provide details of their recommendation. Colorado demonstrated the most variability in lengths of time for heartworm prevention recommendations. Of the clinics reporting cases of canine heartworm infection, the percentage recommending heartworm prevention for some part of the year varied from 14.3% in Idaho to 100% in New Mexico (Table 6).

    Fecal Analysis

    Most clinics typically reported routinely using more than one fecal analysis procedure (Table 7). Simple passive flotation was the most frequently reported procedure, and the direct smear technique was second. Centrifugal flotation came in third, followed by samples being sent to outside laboratories for evaluation. There were 28 clinics that reported they did not do routine intestinal parasite evaluations.

    Intestinal Parasite Rankings

    When asked to rank six intestinal parasites (roundworms, hookworms, whipworms, tapeworms, Giardia, coccidia) in decreasing order of frequency encountered in fecal examinations, roundworms were most commonly listed first, tapeworms came in second, and coccidia or Giardia was most frequently listed third (Table 8).


    The work reported here indicates that dogs in the western United States are infected with heartworms and to some extent by heartworms transmitted locally. From the data collected, it is clear that no more than 51% of the responding clinics in every state perform annual canine heartworm testing. It is unclear whether the rate of testing is necessarily appropriate based on the areas where the disease is most prevalent. Thus, increased testing in areas without infections or local transmission might lower the overall prevalence, while increased testing in areas with higher background levels might serve to increase the apparent prevalence in a state.

    The nature of the survey was such that it might underrepresent the number of cases actually occurring. A survey by the AHS estimated the number of canine heartworm cases diagnosed in 2004 to be higher in these 11 states than reported here.4 Apotheker and associates reported on a review of the antigen test results in the medical records of 871,839 dogs seen in the practices of Banfield, the Pet Hospital between January 2002 and December 2005.5 For dogs seen at Banfield clinics, 1.46 of every 100 dogs tested positive for heartworm; the authors go on to speculate that based on these findings, in conjunction with published estimates of the number of pet dogs in the United States and the proportion of dogs likely to be receiving heartworm preventives, it can be estimated that 509,932 dogs had heartworm infection.5 This number calculated by Apotheker and associates is higher than that suggested by our survey.

    On the other hand, the survey reported here could overestimate the actual number of cases of heartworm infection, most likely from a lack of confirmation in dogs testing positive on antigen detection tests. Although the antigen tests are excellent for detecting infections, the results must be considered in light of the prevalence of infection in a given population of dogs (i.e., in light of the positive predictive value of the test).6 False-positive results occur with every antigen testing methodology, and the positive predictive values of these tests (i.e., the probability of having an infection in an animal that has tested positive) decrease markedly in areas of low prevalence. In the states surveyed, the background prevalence is very low and should be considered zero in dogs on preventives if their owners practice good compliance. Thus, in dogs in these states, as with dogs elsewhere receiving monthly prevention, a single positive antigen test should not be sufficient to declare a dog actually infected and initiate arsenical treatment. For a dog testing positive for heartworm infection on an antigen test, confirming the diagnosis requires further verification of infection, such as physical examination, thoracic radiography, repeat antigen testing (using different methodology), ultrasonography, and perhaps microfilarial testing. This survey relied on individual veterinarians defining dogs as positive, and no questions were asked about how the heartworm infection status of the dogs was determined or confirmed. Another reason that this survey might overestimate the number of positive dogs is that clinics with heartworm cases may have been more likely to respond than those without cases. However, this survey indicated that fewer dogs overall had heartworm infections than did either the AHS or Banfield survey, suggesting that overreporting of cases was not a real issue in our survey.

    In every state surveyed except Idaho and Wyoming, local dogs with no history of travel were diagnosed with heartworm infections; more than 50% of the diagnosed cases in these nine states were reported in dogs that had never left their geographic area and hence were likely to have acquired their infections autochthonously. These results are consistent with previous studies from the same geographic regions. In 1986, the prevalence in Ben Lomond, California, was 13%.7 In 1980, the prevalence of heartworm infections in west central Colorado was 5% to 14%.8 As of 1987, heartworm was considered enzootic in Salt Lake City,9 and the prevalence of infections is considered to be increasing.10,11 For Washington, blood samples from 754 dogs that had not traveled out of western Washington included three samples from infected dogs, and for eastern Washington, 6 of 788 dogs were found infected in the areas of Richland, Moses Lake, Okanogan, and Omak; the authors concluded that enzootic transmission of D. immitis is occurring in both eastern and western Washington.12

    These 11 states were selected for this survey because of our perception that there was considerable variability within the local veterinary communities regarding protocols for heartworm testing and prevention. In this survey, clinics that reported cases of canine heartworm infection diagnosed on their premises often reported that they did not routinely test dogs on a regular basis and many did not prescribe preventives or did so inconsistently. The top five states recommending heartworm preventives—New Mexico (94.7% of responding clinics), Arizona (92.5%), Colorado (90.2%), Utah (87.9%), and California (82.5%)—accounted for 1,941 of the total 2,224 diagnosed cases of heartworm infection in this survey. Within this group, even the state with the highest reported number of cases of heartworm infection (California, with 1,290 reported cases) had only 51% of responding clinics testing annually for canine heartworm infection. Only 42.1% of responding clinics tested annually in New Mexico, 47.2% in Arizona, 50.0% in Colorado, and 21.2% in Utah. The 2005 AHS guidelines state that "annual testing is an integral part of ensuring that prophylaxis is achieved and maintained."13 Clinics that diagnosed cases of canine heartworm infection were no more likely to recommend annual heartworm testing than those that had not diagnosed cases in their practice. Oregon reported the fourth highest number of cases of heartworm infection (161), following California (1,290), Colorado (289), and Arizona (177). Yet only 79.5% of the responding clinics recommended heartworm prevention in Oregon, and less than 30% tested for heartworm annually.

    The prevalence of cats with locally acquired heartworm infections in our survey was also similar to results reported by others. Watkins and associates reported on the results of 396 feline serum samples from California tested for heartworm by both antibody and antigen tests by Animal Diagnostics (St. Louis, MO) wherein 22.6% of the samples tested positive using antibody tests and 4.5% tested positive using antigen tests.14 These authors felt the high percentage of positive results might be the result of prescreening of the samples by the reference laboratories or specialty practices that submitted them for examination.14 In a survey of antibody levels in cats from around the United States, Miller and colleagues found that the highest levels of antibody occurred in Auburn, California, with 33% positive.15 In this study, the number of infected cats represented about 7% of the total number of infected dogs.15

    In this survey of fecal diagnostic methods, most of the veterinary clinics (787) used passive flotation, often in combination with a direct smear technique. The direct smear technique was the second most commonly used method (408). Despite the documented better sensitivity of centrifugal flotation,16,17 only 224 of the 1,101 responding clinics reported using centrifugal flotation as their most routinely used methodology. It is suspected that improved parasite egg and cyst detection methods would increase the number and types of parasites seen in fecal examinations; however, because roundworms are the most common intestinal parasite of dogs throughout the United States, they would likely remain the primary parasite detected.18

    The goal of heartworm prevention and internal parasite prevention is to assure clients that they do not have to worry about their pets developing a totally preventable form of heart disease, to minimize the number of dogs annually treated for heartworms, and to reduce the number of intestinal parasitic infections in dogs and cats. Much of the western United States has the advantage that the risk of a pet becoming infected with heartworms is lower than in some other parts of the country, but that simply means that it is should be easier to prevent infection through the year-round use of monthly preventives. It seems that veterinarians in much of the western United States, even with some knowledge of heartworm disease transmission in their community, still have trouble conceding that heartworm is a risk that warrants clients placing their pets on preventives or the performance of annual testing. Risk-benefit analysis and risk management are very complicated subjects, but hopefully, the work presented here gives veterinarians more information on which to base their recommendations as they discuss the risk of heartworm disease and annual heartworm and/or fecal testing with their clients.

    Downloadable PDF

    Funding for this research was provided by Novartis Animal Health US, Inc., Greensboro, North Carolina.

    1. Zimmerman GL, Knapp SE, Foreyt WJ, et al: Heartworm infections in dogs in the northwestern United States and British Columbia, Canada. Proc Heartworm Symp '92:15-19, 1992.

    2. Frank GR, Grieve RB, Mok M, et al: Survey of heartworm (Dirofilaria immitis) infection in Colorado dogs: A model for surveying prevalence in low-endemic areas. Proc Heartworm Symp '922-0, 1992.

    3. Terrell SP, Courtney CH: Heartworm in Alaska. Prevalence in domestic dogs and wild canids. Recent advances in heartworm disease. Proc Heartworm Symp ' 98:83-86, 1998.

    4. Guerrero J, Nelson CT, Carithers DS: Results and realistic implications of the 2004 AHS-Merial heartworm survey. Proc AAVP 51st Ann Meet:62-63, 2006.

    5. Apotheker EN, Glickman NW, Lewis HB, Glickman LT: Prevalence and risk factors for heartworm infection in dogs in the United States, 2002-2005. Proc 2006 Merck/Merial National Vet Scholar Symp:73, 2006.

    6. Peregrine AS: Rational use of diagnostic tests, in Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat, ed 6. St. Louis, Elsevier Saunders, 2005, pp 489-492.

    7. Pensinger RR: Fifteen-year observation of heartworm disease in Monterey Bay area of California. Proc Heartworm Symp '86:135-138, 1986.

    8. Sears BW, McCallister GL, Heideman JC: Dirofilaria immitis in west central Colorado. J Parasitol 66(6):1070, 1980.

    9. Scoles GA, Dickson SL: New foci of canine heartworm associated with introductions of new vector species: Aedes albopictus in New Orleans and Aedes sierrensis in Utah. Proc Heartworm Symp '95:27-35, 1995.

    10. Rogers LE, Erekson N, Dickson SL: Report on the increasing incidence of heartworm in northern Utah. Proc 51st Ann Meet Utah Mosquito Abatement Assoc:18-19, 1998.

    11. Rogers LE: Reported incidence of heartworm in Utah dogs—2000. Proc 53rd Ann Meet Utah Mosquito Abatement Assoc:11, 2000.

    12. Theis JH, Stevens F, Law M: Distribution, prevalence, and relative risk of filariasis in dogs from the state of Washington (1997-1999). JAAHA 37(4):339-347, 2001.

    13. Nelson CT, McCall JW, Rubin SB, et al: Guidelines for the diagnosis, prevention and management of heartworm (Dirofilaria immitis) infection in dogs. Vet Parasitol 133:255-266, 2005.

    14. Watkins BF, Toro M, Toro G: Prevalence of heartworm antibody and antigen-positive sera among submissions to a commercial laboratory in the USA. Proc Heartworm Symp '98:145-152, 1998.

    15. Miller MW, Atkins CE, Stemme K, et al: Prevalence of exposure to Dirofilaria immitis in cats in multiple areas of the United States. Vet Ther 1(3):169-175, 2000.

    16. Dryden MW, Payne PA, Ridley R, Smith V: Comparison of common fecal flotation techniques for the recovery of parasite eggs and oocysts. Vet Ther 6(1):15-28, 2005.

    17. Zajac AM, Johnson J, King SE: Evaluation of the importance of centrifugation as a component of zinc sulfate fecal flotation examinations. JAAHA 38(3):221-224, 2002.

    18. Blagburn BL, Lindsay DS, Vaughan JL, et al: Prevalence of canine parasites based on fecal flotation. Compend Contin Educ Pract Vet 18(5):483-509, 1996.

    References »

    NEXT: Switching NSAIDs in Practice Insights from the Previcox (Firocoxib) Experience Trial


    Did you know... In western North America, the most relevant environmental factor affecting tick survival is summer desiccation.Read More

    These Care Guides are written to help your clients understand common conditions. They are formatted to print and give to your clients for their information.

    Stay on top of all our latest content — sign up for the Vetlearn newsletters.
    • More