Ehrlichia chaffeensis: a Prototypical Emerging Pathogen

Abstract

Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. The moderate to severe disease caused by E. chaffeensis in humans, first identified in 1986 and reported for more than 1,000 patients through 2000, represents a prototypical “emerging infection.” Knowledge of the biology and natural history of E. chaffeensis, and of the epidemiology, clinical features, and laboratory diagnosis of the zoonotic disease it causes (commonly referred to as human monocytic ehrlichiosis [HME]) has expanded considerably in the period since its discovery. In this review, we summarize briefly the current understanding of the microbiology, pathogenesis, and clinical manifestations associated with this pathogen but focus primarily on discussing various ecological factors responsible for the recent recognition of this important and potentially life-threatening tick-borne disease. Perhaps the most pivotal element in the emergence of HME has been the staggering increases in white-tailed deer populations in the eastern United States during the 20th century. This animal serves as a keystone host for all life stages of the principal tick vector (Amblyomma americanum) and is perhaps the most important vertebrate reservoir host for E. chaffeensis. The contributions of other components, including expansion of susceptible human populations, growth and broadening geographical distributions of other potential reservoir species and A. americanum, and improvements in confirmatory diagnostic methods, are also explored

Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis — United States

“…The organisms multiply in cytoplasmic membrane-bound vacuoles, forming tightly packed clusters of bacteria called morulae. In patients with fatal E. chaffeensis ehrlichiosis, systemic, multiorgan involvement has been described with the greatest distribution of bacteria in the spleen, lymph nodes, and bone marrow (128). Unlike in RMSF, direct vasculitis and endothelial injury are rare in ehrlichiosis.…”

“…Case-fatality rates among persons who are immunosuppressed are higher than those among the general population, on the basis of U.S. passive surveillance and some case series (5,13,145); delays in recognition and initiation of appropriate antibacterial treatment in this population might contribute to increased mortality (149). Although older age (≥60 years) and immunosuppression are risk factors for severe ehrlichiosis (5,10,13), many cases of severe or fatal ehrlichiosis have been described in previously healthy children and young adults (128). Pediatric patients frequently have an asymptomatic or a mild infection (51,128,132); however, children aged <10 years have the highest case-fatality rate among passively reported cases (5,13).…”

“…Symptoms of E. chaffeensis ehrlichiosis typically appear a median of 9 days (range: 5-14 days) after the bite of an infected tick (128). Fever (96%), headache (72%), malaise (77%), and myalgia (68%) are common signs and symptoms.…”

“…Approximately one-third of patients develop a skin rash during the course of illness; rash occurs more frequently in children than in adults. Rash patterns vary in character from petechial or maculopapular (128,132) to diffuse erythema (133) and typically occur a median of 5 days after illness onset (10). The rash typically involves the extremities and trunk but can affect the palms, soles, or face (134).…”

“…chaffeensis ehrlichiosis can cause severe disease or death, although at lower rates than have been observed for RMSF. Approximately 3% of patients with symptoms severe enough to seek medical attention die from the infection (51,128). The severity of ehrlichiosis could be related, in part, to host factors such as age and the immune status of the patient.…”