Long-Term Rodent Surveillance After Outbreak of Hantavirus Infection, Yosemite National Park, California, USA, 2012

Mary E. Danforth; Sharon Messenger; Danielle Buttke; Matthew Weinburke; George Carroll; Gregory Hacker; Michael Niemela; Elizabeth S. Andrews; Bryan T. Jackson; Vicki Kramer; Mark Novak


Emerging Infectious Diseases. 2020;26(3):560-567. 

In This Article

Abstract and Introduction


In 2012, a total of 9 cases of hantavirus infection occurred in overnight visitors to Yosemite Valley, Yosemite National Park, California, USA. In the 6 years after the initial outbreak investigation, the California Department of Public Health conducted 11 rodent trapping events in developed areas of Yosemite Valley and 6 in Tuolumne Meadows to monitor the relative abundance of deer mice (Peromyscus maniculatus) and seroprevalence of Sin Nombre orthohantavirus, the causative agent of hantavirus pulmonary syndrome. Deer mouse trap success in Yosemite Valley remained lower than that observed during the 2012 outbreak investigation. Seroprevalence of Sin Nombre orthohantavirus in deer mice during 2013–2018 was also lower than during the outbreak, but the difference was not statistically significant (p = 0.02). The decreased relative abundance of Peromyscus spp. mice in developed areas of Yosemite Valley after the outbreak is probably associated with increased rodent exclusion efforts and decreased peridomestic habitat.


A disease outbreak in North America caused by a hantavirus occurred in 1993 in the Four Corners area of the southwestern United States.[1] Deer mice (Peromyscus maniculatus) were identified as the primary reservoir of Sin Nombre virus (SNV),[2] an orthohantavirus and the etiologic agent of hantavirus pulmonary syndrome (HPS).[3] That outbreak might have been associated with an El Niño weather event the preceding winter, which could have led to increases in deer mouse infestations in buildings.[4] Investigations into the outbreak and subsequent HPS cases found most cases had probable indoor exposures[5,6] and almost one fourth of all human case exposures were associated with a recreational setting.[7]

During the summer of 2012, a total of 10 persons subsequently given a diagnosis of hantavirus infection visited Yosemite National Park in California, USA.[8,9] SNV exposure for 9 case-patients was associated with staying overnight in a signature tent cabin, a canvas tent structure with interior insulated walls, located in Curry Village in Yosemite Valley;[8,9] the tenth infection was associated with lodging in regular tent cabins in the Tuolumne Meadows area. The subsequent environmental investigation found that most of the signature tent cabins had rodent infestations in the insulated walls. A high overnight trap success rate (51%) for Peromyscus spp. mice and a 14% (10/73) SNV seroprevalence in deer mice were observed in Curry Village during the initial trapping event in August 2012.[8] The park responded by closing and subsequently removing the signature tent cabins, increasing staff and visitor education for HPS prevention, enhancing mouse control measures in and around human-made structures,[8,9] and applying rodent exclusion measures to other buildings.[8] In September 2012, the Peromyscus spp. trap success rate in Curry Village was substantially lower (14%), and no (0/10) deer mice were positive for SNV.[8]

We summarize rodent trappings and SNV serosurveys for Peromyscus mice in Yosemite Valley and Tuolumne Meadows after the outbreak of infection with hantavirus during 2012. These activities were conducted to monitor relative abundance of deer mice, help assess the peridomestic rodent control efforts in the park, and reduce HPS risk in this heavily used recreational area. We compared Peromyscus spp. mouse overnight trap success rates and captured Peromyscus mouse species composition and SNV seroprevalence in deer mice from peridomestic sites in Yosemite Valley during 2013–2018 with findings from the initial outbreak investigation in August–September 2012 and with findings of similar trapping events conducted in developed areas of Tuolumne Meadows. We also evaluated whether location or climatic factors influenced relative rodent abundance and SNV seroprevalence. Finally, we sought to identify demographic characteristics of SNV-positive deer mice captured in Yosemite National Park.