Ehrlichia and spotted fever group Rickettsiae surveillance in Amblyomma americanum in Virginia through use of a novel six-plex real-time PCR assay

Abstract

The population of the lone star tick Amblyomma americanum has expanded in North America over the last several decades. It is known to be an aggressive and nondiscriminatory biter and is by far the most common human-biting tick encountered in Virginia. Few studies of human pathogen prevalence in ticks have been conducted in our state since the mid-twentieth century. We developed a six-plex real-time PCR assay to detect three Ehrlichia species (E. chaffeensis, E. ewingii, and Panola Mountain Ehrlichia) and three spotted fever group Rickettsiae (SFGR; R. amblyommii, R. parkeri, and R. rickettsii) and used it to test A. americanum from around the state. Our studies revealed a presence of all three Ehrlichia species (0-24.5%) and a high prevalence (50-80%) of R. amblyommii, a presumptively nonpathogenic SFGR, in all regions surveyed. R. parkeri, previously only detected in Virginia's Amblyomma maculatum ticks, was found in A. americanum in several surveyed areas within two regions having established A. maculatum populations. R. rickettsii was not found in any sample tested. Our study provides the first state-wide screening of A. americanum ticks in recent history and indicates that human exposure to R. amblyommii and to Ehrlichiae may be common. The high prevalence of R. amblyommii, serological cross-reactivity of all SFGR members, and the apparent rarity of R. rickettsii in human biting ticks across the eastern United States suggest that clinical cases of tick-borne disease, including ehrlichiosis, may be commonly misdiagnosed as Rocky Mountain spotted fever, and that suspicion of other SFGR as well as Ehrlichia should be increased. These data may be of relevance to other regions where A. americanum is prevalent.

FIG. 1.

Mean number of human cases of ehrlichiosis or spotted fever rickettsiosis in Virginia counties and cities averaged over a 3-year period from 2009–2011.

FIG. 2.

Amblyomma americanum collection sites across Virginia. Richmond City area: 1, Henrico County; 2, Goochland Co.; 3, Chesterfield Co. Charlottesville area: Sites 4–6, Albemarle Co. Southside Virginia area: 7, Brunswick Co.; 8, Mecklenberg Co., and Site 28, Fort Pickett, Nottoway County. Lynchburg area: Sites 9–14, Lynchburg City. Fredericksburg area: 15, Fredericksburg; 16, Stafford Co.; 17, Spotsylvania Co.; 18, King George Co.; Site 27, Fort A.P Hill, Caroline Co.; Fairfax County Sites 19–23 and Site 28, Fort Belvoir (Fairfax Co. and adjacent counties/cities). Hampton Roads Area: 24, Portsmouth City; 25, Cheaspeake City.