Proceedings of the Vertebrate Pest Conference

Plague, caused by the bacterium Yersinia pestis, has been in California for over a century, infecting humans, animals, and fleas. The first human case in Southern California was reported in 1908 in the city of Long Beach, Los Angeles County. Since then, sporadic detections of plague have occurred in animals, fleas, and people across the region, including an urban outbreak that resulted in at least 32 confirmed human cases and 31 deaths in 1924 in Los Angeles. In preparation for the 1953 national Boy Scout Jamboree planned in Orange County, Y. pestis-positive fleas were detected in 1946 from a pool of 107 fleas collected from seven California ground squirrels at the planned event site. This discovery prompted extensive rodent and flea control efforts in 1952, with state and county health authorities closely monitoring the site to safeguard attendees. Following these measures, no plague-positive fleas or animals were detected at the location. In 1975, responsibility for plague surveillance and rodent control was transferred from the local health department to the Orange County Mosquito Abatement District, which was subsequently renamed the Orange County Vector Control District (OC Vector). OC Vector’s program has focused on trapping and sampling both wild and commensal rodents in residential, wildland-urban interfaces, and natural habitats, with plague testing conducted by federal, state, and local laboratories. Since the initial discovery of Y. pestis-positive fleas in 1946, Orange County has documented several additional plague detections: two California ground squirrels at a golf course in 1982, a roof rat at a suburban residence in 1998, two bobcats sampled from a foothill community in 2007, and two urban coyotes that tested positive for Y. pestis via PCR in 2018. Although no human cases of plague have been reported in Orange County, these findings highlight the persistence of Y. pestis in local wildlife. This article provides an overview of plague surveillance in Orange County, examining how past outbreaks and routine detections of plague in humans, animals, and fleas across California have shaped surveillance strategies amid changing environmental conditions.