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New vector: Rocky Mountain spotted fever cases reported in Arizona.

ATLANTA -- Four cases of Rocky Mountain spotted fever identified in Arizona between 2001 and 2003 represent the first documented outbreak of the infection in which the widely distributed brown dog tick, Rhipicephalus sanguineus, was the primary vector, Linda J. Demma, Ph.D., said at the International Conference on Emerging Infectious Diseases.

In the eastern United States, the usual vector of Rocky Mountain spotted fever (RMSF) is the American dog tick, Dermacentor variabilis, while in the West the Rocky Mountain wood tick, Dermacentor andersoni, predominates.

Arizona lies outside of the expected range of these primary tick vectors, so when a fatal case of RMSF was reported from a community on an Apache reservation in August 2003, the Centers for Disease Control and Prevention initiated an outbreak investigation.

Surveillance revealed that in June 2002 an additional case had occurred in the same community, said Dr. Demma of the CDC in Atlanta.

"The appearance of two cases in a small community with a population of approximately 1,500 was disturbing, so we conducted chart reviews of all suspect cases in the local Indian Health Services hospital," she said.

Two additional probable cases were identified, including a second fatality. All four cases were in children.

"We then investigated case households to look for potential risk factors and conducted an environmental investigation to look for possible vectors," she said. Among 13 siblings of the cases, 2 had high titers of antibodies to Rickettsia rickettsii, the causative agent of RMSF.

Eight free-roaming dogs in the community were found to be heavily infested with ticks, and trash and debris in yards and open areas created an ideal microenvironment for ticks. Six of the dogs had high titers of antibodies to R. rickettsii, Dr. Demma said.

A total of 19 ticks were collected from the dogs, and an additional 22 ticks were collected from areas in the environment frequented by the cases. All were R. sanguincus.

"Because this tick is widely distributed in the United States, the close association between infected ticks, dogs, and humans raises concerns for disease associated with this vector," she said at the conference, sponsored by the American Society for Microbiology.

During the outbreak investigation, Dr. Demma and her colleagues also initiated a prospective serosurvey among communities on the reservation to determine the underlying exposure levels to Rocky Mountain spotted fever. Of 92 children who had blood drawn at the Indian Health Services hospital for other reasons, 7 (7.6%) were antibody positive, a level of seroprevalence matching that in endemic areas, she said.

"We hope to conduct further investigations to determine if R. sanguineus is a primary vector throughout the region and to elucidate the role of dogs in disease transmission," she said.

An audience member asked if the outbreak was more likely to be a surveillance phenomenon or an actual upswing in the local vector population.

"There may be some ecologic answers," Dr. Demma replied. The affected community was very close to a major fire in 2002. Mammals--and their resident ticks--may have shifted toward the community, she said.


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Title Annotation:Infectious Diseases
Author:Walsh, Nancy
Publication:Internal Medicine News
Date:May 1, 2004
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