Reservoir Competence of Vertebrate Hosts for Anaplasma phagocytophilum

Felicia Keesing

, Michelle H. Hersh, Michael Tibbetts, Diana J. McHenry, Shannon Duerr, Jesse Brunner, Mary Killilea, Kathleen LoGiudice, Kenneth A. Schmidt, and Richard S. Ostfeld

Author affiliations: Bard College, Annandale-on-Hudson, New York, USA (F. Keesing, M.H. Hersh, M. Tibbetts, D.J. McHenry); Cary Institute of Ecosystem Studies, Millbrook, NY, USA (F. Keesing, M.H. Hersh, S. Duerr, R.S. Ostfeld); Washington State University, Pullman, Washington, USA (J. Brunner); New York University, New York, New York, USA (M. Killilea); Union College, Schenectedy, NY, USA (K. LoGiudice); Texas Tech University, Lubbock, TX, USA (K.A. Schmidt)

Abstract

Fourteen vertebrate species (10 mammals and 4 birds) were assessed for their ability to transmit Anaplasma phagocytophilum, the bacterium that causes human granulocytic anaplasmosis, to uninfected feeding ixodid ticks. Small mammals were most likely to infect ticks but all species assessed were capable of transmitting the bacterium, in contrast to previous findings.

Human granulocytic anaplasmosis (HGA), formerly known as human granulocytic ehrlichiosis, is an emerging infectious disease in the United States, Europe, and Asia (1,2). In the United States, most reported cases are concentrated in north-central and northeastern states. Patients with HGA typically have nonspecific febrile symptoms, including fever, chills, headache, and myalgia (1). Most patients with HGA respond well to antimicrobial drug treatment, but complications are not uncommon and some cases are fatal (2). Because of difficulties in diagnosis and lack of awareness of HGA by physicians and the public, many cases are misdiagnosed, and national statistics likely dramatically underreport this disease (1).
HGA is caused by a rickettsial bacterium, Anaplasma phagocytophilum (1), groups of which form dense aggregations in granulocytes (3). The bacterium is passed from host to host through the bite of an infected ixodid tick: Ixodes scapularis in the eastern and central United States and Ix. pacificus in the western United States (4–6). Serosurveys and molecular diagnostics within disease-endemic zones show that many ground-dwelling vertebrate species are exposed to or infected with A. phagocytophilum (2). These data indicate that tick-to-host transmission rates are high and that infection is widespread in host communities.
However, few studies have examined rates of transmission from infected hosts to uninfected ticks, a trait known as the reservoir competence of these hosts. Quantification of host species–specific reservoir competence can identify animals most responsible for producing infected ticks and therefore increasing risk for human exposure. Overall, robust quantitative information on reservoir competence is scarce and key hosts remain unstudied. We determined the reservoir competence for A. phagocytophilum of 14 species (10 mammals and 4 birds) in a disease-endemic region of the eastern United States.