July 3, 2006
Charlie Powell, Public Information Director, WSVMA
(509) 595-2017 24hrs.
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
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
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
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 naïve 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 64°F (18°C) for
approximately one month. Intermittent diurnal declines in temperature below the
developmental threshold of 57°F (14°C) 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
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
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.
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
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
A word on testing
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:
- 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.
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
- 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.
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.
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.