Tick burdens of Peromyscus leucopus and Borrelia infection rates in Ixodid ticks in a fragmented Virginia landscape

Tanner, Cheryl

The emergence of southern tick-associated rash illness (STARI) has thrown into question the validity of reported Lyme disease cases throughout the southern United States. The symptoms of both are identical and a rapid and affordable method of diagnosis is currently unavailable. The etiologic agent of Lyme disease, Borrelia burgdorferi, is transmitted by the black-legged tick, Ixodes scapularis, whereas the bacterium that causes STARI, B. lonestari, is transmitted by the lone star tick, Amblyomma americanum. The American dog tick, Dermacentor variabilis, is also common in the eastern United States and is known to acquire but not transmit B. burgdorferi. Forest patch size has been shown to affect density of the white-footed mouse, Peromyscus leucopus, and is an important predictor of Lyme disease risk to humans because P. leucopus is the most efficient and abundant reservoir host for B. burgdorferi. The mouse is often more abundant in smaller forest patches. One objective of this research was to compare tick burdens of white-footed mice (Peromyscus leucopus), and the minimum infection rates (MIRs) of ticks by B. burgdorferi and B. lonestari, between small (less than 3 ha) and large (greater than 3 ha) forest patches in Fredericksburg and Spotsylvania County Memorial National Military Park, located in Caroline, Orange, Spotsylvania, and Stafford Counties in Virginia. Neither D. variabilis nor I. scapularis have been tested for the ability to acquire B. lonestari, which was another objective of this research. Mice were sampled randomly using Sherman live traps deployed in a grid design in forest patches. All ticks attached to the heads of mice were removed with forceps and preserved in 70% ethanol for laboratory analysis [DNA isolation and amplification, polymerase chain reaction (PCR), and nucleotide sequencing]. Ticks also were sampled by dragging a 1-m2 white cloth through leaf litter and vegetation to determine densities. All adult and nymphal ticks and a sample of larval ticks found on the cloth were collected and preserved in 70% ethanol for laboratory analysis. Mouse density was significantly greater (P less than 0.05) in small patches, but tick burdens showed no significant (P greater than 0.05) relationship with mouse density. No A. americanum were found on mice. I. scapularis and D. variabilis burdens and infestation rates did not differ significantly (P greater than 0.05) with patch size. Inadequate sample sizes of ticks within individual sites precluded the ability to compare infection rates between patch sizes. Juvenile I. scapularis and D. variabilis showed no selection for mice by age, sex, or weight. From drag samples densities of I. scapularis nymphs, nymphs and adults of A. americanum, and adult D. variabilis did not differ significantly (P greater than 0.05) by patch size. No functional dependence (P greater than 0.05) was found between mouse densities and densities of A. americanum larvae, nymphs, or adults, D. variabilis adults, or I. scapularis nymphs. Leaf-litter depth, ambient temperature, relative humidity, soil moisture, deer usage, and volume of coarse woody debris did not differ with size of forest patch and thus may be more important in determining tick burdens and tick densities than canopy cover, ground cover, or soil moisture, which did differ significantly (P less than 0.05) by patch size. Neither patch size nor other habitat characteristics should be considered exclusive determinants of local tick burdens, MIRs, or risk of Lyme disease to humans. No B. lonestari was detected by PCR analysis in larvae, nymphs, or adults of D. variabilis, larvae or nymphs of I. scapularis, or larval A. americanum. PCR results revealed a MIR of B. lonestari in A. americanum of 3.4% (8/237) in nymphs and 5.9% (1/17) in adults. No B. burgdorferi was detected in any tick species of any life stage collected, supporting the hypothesis that most cases of Lyme disease reported in the South are actually cases of STARI.