Opossums, Fleas and Human Disease: A Collaborative Approach for Mitigating an Outbreak of Flea-Borne Typhus in a Mobile Home Community, Los Angeles County, California 2015

BACKGROUND:  Flea-borne typhus (FBT) is an acute febrile illness caused by Rickettsia typhi or R. felis bacteria, transmitted to humans by fleas. Cases have been documented in Texas, Hawaii, and California, but outbreaks are rare. In June 2015, three FBT cases residing in a 90-unit mobile home community (MHC) were reported to Los Angeles County Department of Public Health (LACDPH) by an infection preventionist. LACDPH coordinated a multi-agency investigation in order to determine the extent of the outbreak and prevent further illness.

METHODS:  LACDPH initiated active case finding for outbreak-associated cases occurring from March 1^st through July 31^st. An outbreak-associated case was defined as a MHC resident with a positive laboratory result for FBT and fever during the outbreak period. LACDPH arranged a community meeting for MHC residents, at which LACDPH provided FBT risk reduction education and FBT testing for individuals reporting fever. Multiple onsite visits were conducted with other regulatory agencies to identify and mitigate environmental risk factors. San Gabriel Valley Mosquito and Vector Control District performed trapping of feral animals, from which fleas were collected and submitted for FBT testing.

RESULTS:  Five outbreak-cases of FBT with symptom onsets from April 9^th to June 5^th were identified. All five cases had R. typhi antibody titers >1:64 via indirect fluorescent-antibody technique; however, this method cannot distinguish R. typhi from R. felis. Three hospitalized cases had IgM titers and two cases identified through on-site testing had only IgG titers. Fleas from two opossums tested positive for R. felis and negative for R. typhi via polymerase chain reaction techniques, suggesting that the etiology of this outbreak may be R. felis. Multiple disease risk factors were identified at this MHC: 1) numerous opossums and outdoor cats, 2) a large flea population, 3) places of animal harborage, along with many outdoor pet food sources. Follow-up by regulatory agencies ensured that MHC operators adhered to permitting requirements regarding number of pets and other identified risk factors. No additional outbreak-cases have been reported five months post-outbreak period, indicating mitigation efforts were successful.

CONCLUSIONS:  An outbreak of FBT occurred in a MHC; the specific rickettsial serogroup of the causative agent remains unclear. Risk factors observed included an overabundance of fleas, associated with opossums and free-roaming feral cats supported by an ample domestic pet food supply. Collaboration from multiple county, city, and state regulatory agencies, as well as private organizations, was required to mediate the risk factors identified.