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Heartworm in Cowlitz County dog April 30, 2008 Recently, a dog from Cowlitz County was diagnosed with heartworm; a parasite carried by mosquitoes and common in some parts of the U.S. A report about the case was televised by KING5-TV on April 28. Brief analysis and overview There is no evidence that heartworm is enzootic (meaning it is here all the time and causes disease) in Washington or that there is any increase in new cases. The most common heartworm occurrence in Washington pets has been diagnosed in those that have relocated to the state from known heartworm enzootic regions. Diagnosis has still been very uncommon and occurred most often on or around military installations and communities with a large mobile population. The Washington Animal Disease Diagnostic Laboratory at WSU’s veterinary college reports an incidence of about one confirmed canine case per decade and those animals are usually imported from enzootic regions to the state. There have been no cases reported by the laboratory this year. The Cowlitz County dog’s history indicated it had never left the county and this rightfully raises some new suspicion about the disease. Over the last three decades there have been a handful of heartworm cases like this in the state. Chalk some of them up to incomplete recollection on the part of owners or to owners that have not had the animal its entire life and so missed a bit of its history. The rest? Well, they very well could be the uncommon case that has occurred. “Hurricane Katrina” dogs Some suggest that adopted animals imported from Hurricane Katrina affected areas have transmitted heartworm into a susceptible mosquito population in Washington. From those mosquitoes it is speculated that the naďve local dog population is receiving the parasite and then developing heartworm disease. This has not been confirmed in Washington nor has there been a spike in cases. It is possible this scenario could occur but there are important reasons why it may not, too. 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. Dr. William J. Foreyt, professor of parasitology at Washington State University’s College of Veterinary Medicine and a heartworm authority, has studied the subject for more than 30 years. He says 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). These areas 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. 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. 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 latitude. It is important to note that Washington does not, require reporting, or maintain a database of heartworm cases. 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 the state’s environment. 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 multi-year heartworm incidence survey at WSU of harvested, free-ranging coyotes from primarily eastern Washington and the areas where an occasional case is reported as described above. To date he has examined 535 coyotes after death (necropsy) with a total sample size projected 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. No cases of heartworm have been discovered in coyotes in the study. A word on testing 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: http://www.heartwormsociety.org/article.asp?id=48 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 Cowlitz County 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: http://www.heartwormsociety.org http://www.avma.org/careforanimals/animatedjourneys/pethealth/canine.asp?#2 Best Regards, WSVMA Office Washington State Veterinary Medical Association |