The 'No Name Virus' in the Land of 'Those Who Kill': Hantavirus in Yosemite
By Stan Deresinski, MD, FACP, FIDSA, Clinical Professor of Medicine, Stanford University, Hospital Epidemiologist, Sequoia Hospital, Redwood City, CA, is Editor for Infectious Disease Alert.
Prior to 2012, only 2 cases of hantavirus infection had been identified in visitors to California's Yosemite National Park – one in 2000 and another in 2010. Both infections were acquired in the Tuolomne Meadows area, miles distant from the iconic Yosemite Valley. Things changed this summer with 9 new cases identified as of September 13, only one of which was associated with visiting the High Sierra Camps of Tuolomne. The other 8 had stayed in the Curry Village area of the Yosemite Valley, all in "Signature Tent Cabins". These 91 relatively new cabins set off to one side of Curry Village have double-walls, which provides an element of insulation – but also presumably provides a site for rodent excreta to accumulate, while also possibly reducing ambient air exchange. Hantavirus, which may survive for as long as 13 days in the environment, would be protected from inactivating UV rays at this site.
The first case occurred in a visitor who stayed in Curry Village from June 10-12 and became ill on June 26. The Signature Tent Cabins were closed on August 28. Seven of the patients were residents of California, while one each was from Pennsylvania and West Virginia. Three of the 9 have died. Initial notifications were made in August to 30,000 visitors and on September 12th email notifications were sent to 230,000 overnight visitors. Approximately one-tenth of visitors are international and public health officials of 39 countries have also been notified of the outbreak and its consequences.
The hantaviruses, members of the Bunyaviridae, are a large family of >200 species of ssRNA negative strand viruses. All members of the Bunyaviridae such as LaCrosse virus, Rift Valley Fever virus, and Congo-Crimean Hemorrhagic Fever virus, are arboviruses – i.e., transmitted by arthropods. In contrast, those of the genus Hantavirus are primarily transmitted by inhalation of droplets with virus aerosolized from rodent feces and urine.
Sin Nombre
There are at least 11 hantaviruses associated with human disease manifested by two distinct syndromes. Species present in the Old World (e.g., Hantaan, Seoul, Puumula) typically cause hemorrhagic fever and renal syndrome (HFRS), while those in the New World cause hantavirus pulmonary syndrome (HPS) which, because of the frequent presence of myocardial depression, should more precisely probably be called hantavirus cardiopulmonary syndrome. HPS was first identified in the U.S. after an outbreak of illness in the region where the states of Utah, New Mexico, Arizona, and Colorado meet – the "four corners" which gave the virus its initial informal name. The official name first given to it was Muerto Canyon virus, but the local inhabitants strongly objected, fearing the besmirching of their region's reputation. As a consequence, the virus was renamed Sin Nombre – the "no name virus." Other hantaviruses that cause human infection in the U.S. include the New York, Bayou, and Black Creek Canal viruses. Each has its own rodent reservoir and geographic area.
The reservoir of the Sin Nombre virus is the deer mouse, a rodent that is present in most of the U.S.(excepting the eastern and southeastern coastal areas), as well as in the Alaskan panhandle, northern and western Canada, Baja and parts of Oaxaca, Mexico.
The usual incubation period is 10-17 days, but it has been reported to extend to as long as 6 weeks. The initial symptoms are non-specific, consisting of fever, chills, headache, nausea and vomiting. After several days, patients develop a non-productive cough. The development of breathlessness and tachypnea signal the onset of a rapidly progressive course with the development of acute respiratory distress syndrome (ARDS) and respiratory failure, as well as hypotension. The hypotension may be due to loss of intravascular volume due to capillary leak caused by effects of the virus on endothelial cells, as evidenced by low albumin and hemoconcentration, but is exacerbated by the frequent development of myocardial depression and cardiogenic shock.
Laboratory abnormalities that may be detected during the prodromal phase include thrombocytopenia, neutrophilia together with the presence of early forms including myelocytes, and immunoblasts. During the cardiopulmonary phase, chest x-ray is consistent with ARDS and there is evidence of impaired gas exchange that may become severe enough to necessitate mechanical ventilation. Renal function may be mildly depressed. Disseminated intravascular coagulation is uncommon. Echocardiography reveals depressed left ventricular function. In contrast to the usual findings in sepsis, hemodynamics are dominated by the cardiac abnormalities resulting in a low cardiac output and high systemic vascular resistance. Treatment is supportive since no antiviral agent, including ribavirin, has been demonstrated to be effective once HPS has developed. The diagnosis is made using antibody testing.
A total of 510 cases of HPS in the U.S. were reported from 30 mostly western states to the CDC from 1993 through 2009 and 35% of these were fatal.1 Only 36% of cases occurred in females and only 7% in children <16 years of age. Until the current event, 60 cases of HPS had been reported in California since 1993 with 16 of these, 4 fatal, reported from 2001 through 2008. Eleven of these 16 infections were acquired in the Sierra Nevadas.
Ongoing investigations will determine the cause of this localized outbreak. While increased viral virulence as the result of mutation is possible, it is not likely to be the explanation. Instead, the outbreak may have result from a confluence of events, including an increase in the population of deer mice and the two-walled construction of the Signature cabins which, I speculate, may be more airtight than older cabins.
Hantavirus infection of deer mice in the Sierras is highly prevalent (14% are seropositive) and 13.7% trapped (using peanut butter as the lure) in Curry Village in August had antibody to Sin Nombre virus. While the localized nature of this summer's outbreak indicate a significant risk to visitors staying in Signature Cabins in Curry Village, identified human infections have been rare events when one considers that there were 4,098,648 visitors to Yosemite in 2011. It may nonetheless be considered ironic that the word Yosemite is derived from a Miwok word meaning "those who kill", referring to a group living in the valley composed of renegades from multiple tribes, including the Paiute who were traditional enemies of the Miwok.
Reference
- MacNeil A, et al. Hantavirus pulmonary syndrome, United States, 1993–2009. Emerg Infect Dis July 2011; 17: http://dx.doi.org/10.3201/eid1707.101306 http://www.cdph.ca.gov/data/statistics/Documents/hantavirus-episummary.pdf
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