Tick-Borne Diseases in North Carolina: Is“Rickettsia amblyommii”a Possible Cause of Rickettsiosis Reported as Rocky Mountain Spotted Fever?

Abstract

Cases of Rocky Mountain spotted fever (RMSF) in North Carolina have escalated markedly since 2000. In 2005, we identified a county in the Piedmont region with high case numbers of RMSF. We collected ticks and examined them for bacterial pathogens using molecular methods to determine if a novel tick vector or spotted fever group rickettsiae (SFGR) might be emerging. Amblyomma americanum, the lone star tick, comprised 99.6% of 6,502 specimens collected in suburban landscapes. In contrast, Dermacentor variabilis, the American dog tick, a principal vector of Rickettsia rickettsii, comprised < 1% of the ticks collected. Eleven of 25 lone star tick pools tested were infected with "Rickettsia amblyommii," an informally named SFGR. Sera from patients from the same county who were presumptively diagnosed by local physicians with a tick-borne illness were tested by an indirect immunofluorescence antibody (IFA) assay to confirm clinical diagnoses. Three of six patients classified as probable RMSF cases demonstrated a fourfold or greater rise in IgG class antibody titers between paired acute and convalescent sera to "R. amblyommii" antigens, but not to R. rickettsii antigens. White-tailed deer, Odocoileus virginianus, are preferred hosts of lone star ticks. Blood samples collected from hunter-killed deer from the same county were tested by IFA test for antibodies to Ehrlichia chaffeensis and "R. amblyommii." Twenty-eight (87%) of 32 deer were positive for antibodies to E. chaffeensis, but only 1 (3%) of the deer exhibited antibodies to "R. amblyommii," suggesting that deer are not the source of "R. amblyommii" infection for lone star ticks. We propose that some cases of rickettsiosis reported as RMSF may have been caused by "R. amblyommii" transmitted through the bite of A. americanum.

Absence of Rickettsia rickettsii and Occurrence of Other Spotted Fever Group Rickettsiae in Ticks from Tennessee

“…30 More recently, a group in North Carolina obtained serum samples from patients considered probable cases of RMSF and found that some of the patients had higher end-point titers against R. amblyommii antigen than R. rickettsii . 11 In our study, the most common rickettsial species in all sexes and stages of ticks was R. amblyommii . The finding of R. amblyommii in the A. americanum larval pools indicates that transovarial transmission of R. amblyommii may be occurring.…”

“…8,9 Rickettsia amblyommii , in particular, has been suggested as a potential cause of high SFGR seroprevalence. 10,11 Little is known about the diversity and prevalence of SFGR in ticks in Tennessee because of spatial and technical limitations of previous studies. In 1974 Burgdorfer and others 12 identified R. rickettsii in D. variabilis in the Land Between the Lakes area bordering Kentucky by examining hemolymph, which is not as reliable for species identification as modern molecular techniques.…”

Spotted fever group Rickettsia infecting ticks (Acari: Ixodidae) in the state of Santa Catarina, Brazil

“…Although different strains of R. amblyommii have been reported to infect different tick species of the New World (Labruna 2009, Castellaw et al 2010, Ogrzewalska et al 2010, Bermúdez et al 2011, the pathogenicity of R. amblyommii in humans remains to be demonstrated. Interestingly, there is serological evidence suggesting that it is a human pathogen in the United States (Apperson et al 2008).…”

Retrospective Case Reports of Two Central North Carolina Residents

“…Lone star tick pathogens include the long-implicated Rickettsia rickettsii, the agent of RMSF [25,26]; Francisella tularensis, the agent of tularemia; Ehrlichia chaffeensis, Ehrlichia ewingii, Panola Mountain Ehrlichia; [3] and other rickettsial organisms including R. parkeri and R. amblyommii [3]. The latter is now suspected of being a mild pathogen and may contribute to cross-reactivity when testing for R. rickettsii [3,27]. A newly identified lone star tick-vectored virus commonly called Heartland virus has caused 2 deaths and 5 other cases in the Midwest and South [28].…”