New RMSF

VECTOR-BORNE AND ZOONOTIC DISEASES

Volume 8, Number 1, 2008

Rickettsia amblyommii Infecting Amblyomma americanum Larvae

ELLEN Y. STROMDAHL, 1MARY A. VINCE,1PEGGY M. BILLINGSLEY,2

NICOLE A. DOBBS,3 and PHILLIP C. WILLIAMSON2

ABSTRACT

Polymerase chain reaction analysis of Amblyomma americanumadults, nymphs, and larvae from Aberdeen Proving Ground, MD (APG), revealed a very high prevalence of a spotted fever group (SFG) rickettsia. Restriction fragment length polymorphism (RFLP) and sequence analysis identified “Rickettsia amblyommii.”

This organism is not yet described or well studied, and its pathogenicity is unknown; however, investigations of the organism are warranted because of its high prevalence in A. americanum. This tick is extremely abundant at military training facilities in the south, central, and Mid-Atlantic United States, and many soldiers experience multiple concurrent tick bites.

Bites by R. amblyommii-infected A. americanummay account for rates of SFG rickettsia seropositivity that are higher than reported rates of Rocky Mountain spotted fever (RMSF) cases from the same location. Seroconversion to SFG rickettsia following bites of A. americanummay suggest that R. amblyommii is infectious in humans. Subclinical infection in the numerous A. americanumtick bite victims could contaminate donated blood and compromise immunodeficient recipients.

Detection of R. amblyommiiin questing A. americanumlarvae suggests transovarial transmission. The absence of R. rickettsii, the agent of RMSF, in A. americanummay be due to transovarial interference by R. amblyommii. The likelihood of pathogen transmission by larvae is magnified by their habit of mass attack.

The very small size of the larvae is also a risk factor for pathogen transmission. High R. amblyommii prevalence in populations of A. americanumpresage co-infection with other A. americanum-borne pathogens.

A. americanumnymphs and adults from APG were found to be co-infected withR. amblyommii and Borrelia lonestari, Ehrlichia chaffeensisand Ehrlichia ewingii, respectively, and larval pools were infected with both R. amblyommiiand B. lonestari.

Co-infections can compound effects and complicate diagnosis of tick-borne disease.

Key words: Rickettsia amblyommii—Amblyomma americanum—Prevalence—Pathogenicity—Co-infec-

tion.

Address reprint requests to:

Ellen Stromdahl

Entomological Science Program

U.S. Army Center for Health Promotion &

Preventive Medicine

5158 Blackhawk Rd.

BLDG E-5800

Aberdeen Proving Ground, MD 21010-5403

E-mail: ellen.stromdahl@us.army.mil