Sun, surf, and tick-borne disease: Tularemia on the California Coast

A coastal site in San Mateo County, CA has been linked to human cases of tularemia (Francisella tularensis) since 2004. Field surveillance from 2017-2021 looked into the prevalence of F. tularensis in ticks, the biologic vector, during which time two additional human cases were reported. A total of 3,021 D. occidentalis (Marx) and 1,019 D. variabilis (Say) were collected. Of those, 25 positive pools of F. tularensis were detected (five ticks per pool, overall minimum infection prevalence: 0.62%) the majority of which were from D. occidentalis collected from the western half of the site, nearest to the ocean. We also conducted rodent surveillance in June of 2019 through a collaborative effort with the California Department of Public Health. Twenty-four small mammals were collected. Nineteen mice were tested for hantavirus (Sin Nombre virus); four out of 15 deer mice (Peromyscus maniculatus) tested positive. Blood samples from 14 deer mice, two California mice, and two chipmunks tested negative for tularemia by serology. Multi-agency collaboration and communication with the public are key to tracking down elusive pathogens and for keeping pathogen spillover events insignificant.