Status of the "East Side hypothesis" (transovarial interference) 25 years later.

Rocky Mountain spotted fever (RMSF) cases in the notorious Bitterroot Valley outbreak of the early 20th century were peculiarly distributed, with virtually all reported from the west side of the valley. Such a distribution remained unexplained until Burgdorfer and colleagues (1981) reported that endosymbiotic rickettsiae were prevalent in wood ticks on the east side of the Bitterroot River valley but not on the west side. The "East Side agent" was said to prevent the transovarial transmission of Rickettsia rickettsii, thereby severely limiting the prevalence of the latter. This hypothesis has been considered one of the most innovative explanations for an epidemiological conundrum and, indeed, has generally been accepted as a fact in the medical entomology literature. I review the evidence for the interference hypothesis, and suggest that the distribution of the Bitterroot Valley RMSF outbreak might actually have its basis in habitat or microclimate-related factors, as opposed to reflecting interspecific competition by closely related rickettsiae.