College of Veterinary Medicine Home What's New
College of Veterinary Medicine
Graduate programs
Research programs
Veterinary Hospital
Diagnostic Laboratory
Service Units
People in the College of Veterinary Medicine
College of Veterinary Medicine Home
  Recent Spokane Valley Heartworm Reports Discussed    

July 3, 2006
Charlie Powell, Public Information Director, WSVMA
(509) 595-2017 24hrs.

Purpose of this report

This report is written to provide information for Washington State Veterinary Medical Association members, their clients, and the public at large concerning a recent group of three heartworm cases reported in eastern Washington. Newspaper reports of these cases have generated an unusually high call volume from concerned clients of some practices.

This report was prepared and reviewed by both the current WSVMA leadership and faculty of the Washington State University College of Veterinary Medicine. Specific expertise was lent by Dr. William J. Foreyt, professor of parasitology, WSU College of Veterinary Medicine as noted below.

This report can be distributed and copied as necessary by anyone with attribution.

Lack-of-actual-data disclaimer

It is important to note that Washington does not require reporting or maintain a database of heartworm cases. Trends noted below are speculative and are not supported with statistical data except for portions attributed to the American Heartworm Society. This discussion is intended to provide perspective for Washington, based only upon the number of cases that have been diagnosed in both government and commercial diagnostic laboratories.

Review of current situation

Recently, two cats and one dog from Liberty Lake, Wash, were reported to be diagnosed with heartworm in the July 1, 2006, edition of Spokane’s Spokesman-Review newspaper.

All three animals were the patients of a Liberty Lake practice and had no history of travel from the Liberty Lake area. Positive tests were reported by Antech Diagnostics.

Why has this occurred?

Anecdotally, it has been suggested that perhaps adopted animals imported from Hurricane Katrina affected areas have transmitted heartworm into the susceptible mosquito population of eastern Washington. From those mosquitoes it is speculated that the nave local dog population is receiving the parasite and then developing heartworm disease.

This scenario has not been confirmed in Washington. It is possible it could occur but there are reasons why it may not, too. It has been postulated to occur in other communities that adopted Katrina animals.

A recent history of heartworm in Washington

In the last decade, heartworm has been diagnosed rarely in Washington dogs that have had no discoverable travel history. Until now, Dr. Foreyt is unaware of a confirmed case of heartworm occurring in a cat in Washington during his career and reserves some respectful, professional skepticism of the cases reported in Spokane. The disparity in infection between species is consistent with disparate occurrence rates seen in enzootic heartworm areas.

The most common occurrence of heartworm in Washington pets has been diagnosed in those that have relocated to the state from known heartworm enzootic regions elsewhere. Diagnosis has still been uncommon and occurred most often on or around military installations with a large mobile population.

There have also been very sporadic case reports that occurred in some of the warmest local regions of Washington (i.e. Tri-Cities, Othello/Moses Lake, and the Yakima Valley). Some patients had allegedly never traveled from the local region, although histories have also been reported to be less than complete. It should be noted that the areas in question support a large population of agricultural and related industry workers who migrate annually with their pets following seasonal employment. This migration typically moves from heartworm enzootic areas to Washington and beyond.

The Washington Animal Disease Diagnostic Laboratory reports an incidence of about one confirmed canine case per decade and those animals are usually imported from enzootic regions to the state.

Regions of the state are known to support populations of at least three species of mosquitoes that can carry and support the development of the heartworm parasite. There is dispute as to whether or not the majority of Washington experiences the sustained environmental conditions (degree-days, moisture, moderate temperature fluctuations, etc.) to support a threshold population of any of the three species known to carry the microfilariae into the larval stage. Consider the following from the American Heartworm Society:

A climate that provides adequate temperature and humidity to support a viable mosquito population, and also sustain sufficient heat to allow maturation of ingested microfilariae to infective, third-stage larvae (L3) within this intermediate host is a pivotal prerequisite for heartworm transmission to occur. Laboratory studies indicate that development and maturation requires the equivalent of a steady 24-hour daily temperature in excess of 64F (18C) for approximately one month. Intermittent diurnal declines in temperature below the developmental threshold of 57F (14C) for only a few hours retard maturation, even when the average daily temperature supports continued development. At 80 F (27 C), 10 to 14 days are required for development of microfilariae to the infective stage. The length of the heartworm transmission season in the temperate latitudes is critically dependent on the accumulation of sufficient heat to incubate larvae to the infective stage in the mosquito.

The peak months for heartworm transmission in the Northern Hemisphere are July and August. Algorithmic predictions based on analysis of historical temperature records have consistently overestimated actual transmission periods confirmed independently by a variety of field studies and appear to represent conservative guidelines. Under the most favorable conditions, these estimates range from less than four months in southern Canada to potentially all year in the subtropical zones of southern Florida and the Gulf Coast. The model predicts that heartworm transmission in the continental U.S. is limited to six months or less above the 37th parallel, i.e., Virginia-North Carolina State line.

Washington lies within the parallels of 45 degrees 32 minutes N to 49 degrees N latitude.

Is Washington experiencing a heartworm disease increase now?

No data is currently available to state conclusively that the incidence of heartworm in dogs and cats is increasing in Washington. During the last decade, there has not been a documented increase in heartworm diagnoses that would suggest or indicate that the state is becoming enzootic for the parasite and its life cycle.

It can be argued that if heartworm transmission could be sustained by Washington mosquitoes it should have occurred previously and certainly within those areas that historically have had cases imported on a regular basis. Similarly, it could be argued that local regions of the state with the highest mosquito populations should be showing an increased incidence of heartworm in dogs.

Neither of these hypothetical situations has been observed to be occurring.

Assuming for discussion that microfilariae are infecting Washington mosquitoes, there is no epidemiological evidence that the parasite can sustain its life cycle. In communities where a small number of cases have been diagnosed in local animals on an intermittent but recurring basis, the subsequent seasons have not shown increasing or sustained caseloads.

Current studies

Dr. Foreyt is currently conducting a heartworm incidence survey at WSU of harvested, free-ranging coyotes from around the state. To date he has necropsied 457 coyotes with a total sample size to be more than 600 at completion. Coyotes in the study were harvested before, during, and in the months after Hurricane Katrina and subsequent domestic dog importations. Harvest areas include areas of the state that have been known to report heartworm in domestic dogs as described above.

No cases of heartworm have been discovered in coyotes in the study.

What about cats?

Cats more rarely get heartworm and the probability for infection is very low, especially in areas where heartworm does not exist or is very rare, like what has been the experience in Washington. In enzootic areas, heartworm is diagnosed at the rate of one cat per every five to twenty dogs (5 – 20 percent). Dogs are the definitive host. Heartworm in living cats is difficult to diagnose because usually only one to two adult worms are present and production of microfilariae can be variable to non-existent. In the last decade, at least one confirmed case of heartworm (visualized microfilariae) has been reported in a Washington cat by Phoenix Laboratories.

A word on testing

For additional testing information, please contact:
Dr. Bob Lobingier
Director of Pathology Services
Pacific Northwest, Antech Diagnostics

A brief discussion of heartworm testing is also important here, specifically the sensitivity/specificity of the tests and especially in an area with low incidence. Most sources suggest ruling out heartworm with the antibody test in the cat, and rule it in with the antigen test. Even with the antigen test in the dog, diagnostic laboratories report occasionally seeing false positives (i.e., could not prove an active infection in the dog), especially in areas of low incidence. Laboratory testing should always be confirmed with additional diagnostics (such as radiographs) especially in an area with low incidence.

Complete guidelines for diagnosis and treatment of heartworm in dogs and cats can be found at:

Conclusions and recommendations

  • It is theoretically possible but unproven that dogs and cats imported to Washington from enzootic heartworm areas may be responsible for an increased incidence of the parasite.
  • The occurrence of heartworm in Washington dogs and cats remains very rare and has not shown evidence of a sustained presence.
  • Clients who expect to travel with their pets to heartworm enzootic areas should have the pets undergo testing and prophylaxis as indicated within current standards of care.
  • The current situation reported in Spokane does not indicate that it is necessary for immature or asymptomatic dogs and cats living exclusively in Washington be tested or treated for heartworm.
  • Clients inquiring about heartworm in Washington dogs and cats should be given the benefit of client education based upon the latest research and recommendations (see links below) as well as this discussion for purposes of making an informed medical and economic decision guided by the benefit of their veterinarian’s experience and judgment.
  • Clients presenting dogs and cats with signs and symptoms of heartworm or presenting with a history consistent to exposure and infection should be counseled to consider diagnosis and treatment as is medically indicated.

Important information sources:

Revised April 5, 2001     |     Printer Friendly Version

Contact us: 509-335-9515 | Accessibility | Copyright | Policies
College of Veterinary Medicine, PO Box 647010, Washington State University, Pullman, WA 99164-7010 USA
Emergency Preparedness & Safety Links