Expansion of the Range of Vector-borne Disease in Minnesota
This article was published in the March/April 2006 Disease Control Newsletter.
In recent years, the Minnesota Department of Health (MDH) has documented an expansion of the areas in which Minnesota residents are exposed to several vector-borne diseases. This expansion may be due to changes in human behavior, vector or pathogen distribution, and/or vector density and infection rates. As part of the follow-up on reported cases of mosquito- and tick-transmitted disease, MDH staff contacted case-patients to ascertain likely areas of exposure to vector species. Then, during the summer of 2005, MDH staff began an effort to clarify the distribution areas in Minnesota of two vector species: Ixodes scapularis (deer tick or blacklegged tick, the vector of Lyme disease, human anaplasmosis, and babesiosis) and Ochlerotatus triseriatus (Eastern Tree Hole mosquito, the vector of La Crosse encephalitis). This article reports on those findings. Tick-borne Disease Pathogens transmitted by I. scapularis ticks in Minnesota include Borrelia burgdorferi (Lyme disease agent), Anaplasma phagocytophilum (human anaplasmosis agent), and Babesia microti (babesiosis agent). Most tick-borne disease case-patients in Minnesota historically have been exposed to infected ticks in forested portions of east central counties or in the Saint Croix and Mississippi River valleys of eastern and southeastern Minnesota. Recent data, however, suggest that exposures are occurring in areas of northern, west central, central, and southeastern counties on the periphery of the historical endemic range, indicating a northward and westward expansion of disease risk. A recent Disease Control Newsletter article elaborated on these trends in human disease incidence and exposure. (See “Dramatic Increase in Lyme Disease and Other Tick-borne Diseases, 2004” in the May/ June 2005 issue of the Disease Control Newsletter [vol. 33, no. 3].)
The expanding tick-borne disease risk in Minnesota may be associated with the recent establishment of disease-carrying tick populations in new areas of the state. Despite the lack of systematically collected historical data on tick abundance in these areas, anecdotal reports from land managers, residents, and visitors support this hypothesis. Also, MDH has noted an increasing number of case-patients in new areas. The deer tick distribution study implemented by MDH in the summer of 2005 had two components: 1) field sampling in emerging areas of the state to confirm and compare tick presence and abundance; and 2) polymerase chain reaction (PCR) testing of these ticks for tick-borne disease agents. MDH field staff surveyed
16 forested sites at the edge of Minnesota’s known tick-borne disease risk zone (Figure 1). Field staff collected ticks by dragging a 1 m2 white canvas cloth along specific sampling transects and periodically removing any attached ticks. I. scapularis ticks were identified at 13 of the 16 study sites (Figure 1). Nearly 250 ticks (approximately 160 adults and 90 nymphs) were collected and tested for B. burgdorferi, A. phagocytophilum, and B. microti. Overall, 34% of ticks from nine study sites (range, 20-50) were PCR-positive for at least one disease agent (although tick abundance was too low at some sites to determine infection rates with any degree of precision). A smaller percentage of nymph ticks (16%) than adult ticks (44%) were positive; however, nymphs pose a greater disease risk because they are smaller and more difficult to detect. Evidence of all three pathogens was detected. At a site in Hubbard County, for example, 21 of 36 (58%) adult ticks were positive for B. burgdorferi, 8 (22%) for A. phagocytophilum, and 4 (11%) for B. microti. Simultaneous transmission of these disease agents by ticks with multiple infections can result in coinfections. Further MDH research goals include greater delineation of I. scapularis distribution and determination of infection rates. Medical providers who see patients who live, work, or recreate in these newly determined endemic areas of the state need to be aware that the areas pose an exposure risk. Human exposure risk is greatest in the late spring, early summer, and autumn, when nymph and adult I. scapularis ticks quest for blood meals in the leaf litter or herb/shrub layers of hardwood forests. People visiting these areas should be advised to use personal protection measures such as tick repellents, protective clothing, and prompt inspection and removal of attached ticks. La Crosse Encephalitis La Crosse encephalitis (LAC) is endemic in southeastern Minnesota along the Mississippi River valley, from the Iowa border north through the western Twin Cities metropolitan area. However, in recent years most LAC case-patients likely were exposed just west of the traditional endemic area, including the farming regions of Brown and Faribault counties (Figure 2). The primary LAC vector, Oc. triseriatus, is a woodland mosquito species that is found exclusively in shaded areas and rarely travels more than 200 meters from the water-holding containers (e.g., waste tires, buckets) or wet tree holes where it hatched. The landscape of southwestern Minnesota, formerly dominated by tallgrass prairie, is primarily open agricultural land, but includes many small wooded areas between farm fields and around homes. The only large forested areas in the region are found along its rivers and its few lakes. The apparent westward spread of LAC cases, combined with reported exposures from marginal habitat for Oc. triseriatus, raises the question of whether the virus could continue to move westward and become
established in parts of the state where it has never been previously detected. To evaluate Oc. triseriatus distribution in southwestern Minnesota, MDH conducted mosquito sampling in seven counties during July and August of 2005 (Figure 2). The collections took place over 6 weeks on 11 wooded sites (six on farms, three in state parks, and two in municipal parks). Sampling involved weekly mosquito egg collections using ovitraps and adult mosquito collections using a battery-powered aspirator. Adult mosquitoes found at the sites or reared from eggs found at the sites were tested for LAC virus by PCR. Oc. triseriatus eggs and/or adult mosquitoes were found at all 11 sites. The highest adult mosquito counts were from farms. On three of the farms, high numbers of mosquitoes were collected from numerous man-made containers, especially waste tires. Mosquito numbers at these sites rivaled numbers from high-risk sites in southeastern Minnesota. Egg numbers, however, were low at most of the 11 sites, but were higher at locations that lacked man-made containers. (Containers often compete with ovitraps as mosquito breeding habitat, thus artificially lowering egg counts in the ovitraps.) The lowest adult Oc. triseriatus counts were in the three state parks that were tested; no adult mosquitoes were collected in two of those parks. No mosquitoes at any of the 11 sites were PCR-positive for LAC virus. These data suggest that low-level Oc. triseriatus populations are widespread throughout southwestern Minnesota. On farms, which often lack natural habitat, mosquito populations are greatly aided by the presence of artificial containers that hold water and support Oc. triseriatus reproduction. If LAC virus were introduced to southwestern Minnesota through the importation of infected eggs in artificial containers, sufficient breeding habitat could allow the virus to become established in localized areas. The recent LAC case reported in Faribault County was on a farm that had several water-holding buckets and other containers brought there from a wooded location in LAC-endemic Houston County. Medical providers who have pediatric patients (LAC occurs almost exclusively in people 16 years of age or younger) from southwestern Minnesota presenting with viral encephalitis from July through September should consider testing for LAC during the diagnostic process.