Expanding Range of Amblyomma americanum and Simultaneous Changes in the Epidemiology of Spotted Fever Group Rickettsiosis in the United States

Abstract

Abstract. Spotted fever group (SFG) Rickettsia species are etiologic agents of a wide range of human infections from asymptomatic or mild infections to severe, life-threatening disease. In the United States, recent passive surveillance for SFG rickettsiosis shows an increased incidence and decreased severity of reported cases. The reasons for this are not well understood; however, we hypothesize that less pathogenic rickettsiae are causing more human infections, while the incidence of disease caused by more pathogenic rickettsiae, particularly Rickettsia rickettsii, is relatively stable. During the same period, the range of Amblyomma americanum has expanded. Amblyomma americanum is frequently infected with "Candidatus Rickettsia amblyommii", a SFG Rickettsia of unknown pathogenicity. We tested our hypothesis by modeling incidence rates from 1993 to 2013, hospitalization rates from 1981 to 2013, and case fatality rates from 1981 to 2013 regressed against the presence of A. americanum, the decade of onset of symptoms, and the county of residence. Our results support the hypothesis, and we show that the expanding range of A. americanum is associated with changes in epidemiology reported through passive surveillance. We believe epidemiological and acarological data collected on individual cases from enhanced surveillance may further elucidate the reasons for the changing epidemiology of SFG rickettsiosis.

Involvement of invasive eastern red cedar (Juniperus virginiana) in the expansion ofAmblyomma americanumin Oklahoma

“…The distribution of Dermacentor variabilis , the main vector for R. rickettsii (Rocky Mountain spotted fever), has been relatively static since 1940 (Mitcham et al ) but populations of Amblyomma americanum , the main vector for R. amblyommii, E. chaffeensis , and E. ewingii , have shifted dramatically westward since the 1970s (Barrett et al ). The potential involvement of A. americanum in the geographical increase of these important tick‐borne diseases (Paddock and Yabsley , Dahlgren et al ) generates important ecological questions as to how a tick species, normally associated with more humid environments, has been able to establish viable populations in dry, arid western Oklahoma.…”

Comparative vertical transmission of Rickettsia by Dermacentor variabilis and Amblyomma maculatum

“…Rickettsial pathogenicity is posited as a key factor mediating transmission, as vertical maintenance favors non-pathogenic SFG Rickettsia , while horizontal transmission events are required for pathogenic SFG Rickettsia maintenance (Werren, 1997; Macaluso and Paddock, 2014). In the United States, the geographical range of multiple tick species continues to expand, coinciding with a rise in human cases of SFG rickettsiosis (Dumler, 2010; Paddock and Goddard, 2015; Dahlgren et al, 2016; Drexler et al, 2016). Thus, it is important to anticipate tick transmission potential for SFG Rickettsia to further understand the epidemiology of tick-borne rickettsial diseases (TBRDs).…”

Epidemiologic profile and clinical course of four confirmed rickettsiosis cases in Southern Mexico during 2016

“…In Mexico, the Rickettsia species are causal agents of diseases such as murine typhus, Rocky Mountain, and Mediterranean spotted fevers, which cause infections with various clinical manifestations in humans .…”