Ehrlichia ewingii in Missouri: A New Human Pathogen

Source: Buller RS, et al. N Engl J Med 1999;341:148-155.

A causative agent of canine gran-ulocytic ehrlichiosis, never previously identified in humans, has been identified in four patients presenting with fever in Missouri. The organisms were identified as an ehrlichia by "broad-range" PCR primers, but negative results were obtained when species-specific probes for the agents of human monocytic ehrlichiosis (due to E. chafeensis) and human granulocytic ehrlichiosis were used. Much to everyone’s surprise, sequencing of 16 S-ribosomal RNA matched that of E. ewingii—a canine pathogen.

Interestingly, three of the four patients were receiving immunosuppressive therapy (including prednisone in two, and methotrexate and azathioprine in one each). All four responded to doxycycline.

Morulae were observed in neutrophils from two of the patients (originally misidentified as gram-negative coccobacilli), suggesting a diagnosis of human granulocytic ehrlichiosis, but serologies for this agent were negative. Western blots subsequently performed on specimens obtained from three of the patients during convalescence demonstrated cross-reactivity with one but not other major antigens of E. chafeensis and E. canis. In addition, serologies from one patient’s dog were also consistent with recent infection due to E. ewingii.

Whether these cases represent accidental zoonotic transmission or a common vector remains unclear. Ehrlichiosis should be considered in any patient with an acute febrile illness and a culture-negative sepsislike syndrome who resides in an endemic area such as Minnesota, Wisconsin, Missouri, and the southeastern United States. As a former microbiology instructor of mine, Dr. George Sarosi, used to say, "No patient should die without the benefit of steroids." Perhaps this axiom should be updated to, "No patient should die without the benefit of steroids and doxycycline."