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Hantavirus in Yosemite Tourists

By Amesh A. Adalja, MD, FACP, September 7, 2012

During the summer of 2012, 8 cases of hantavirus pulmonary syndrome (HPS)—3 of which were fatal—occurred in tourists exposed to the murine-spread virus at Yosemite National Park.1 Hantaviruses are members of the Bunyaviridae viral family. They are spread via aerosolized rodent urine, primarily that of the white-footed mouse and the deer mouse.2 This case cluster serves as a reminder that this highly lethal virus is endemic in certain parts of the United States and merits consideration in certain clinical scenarios.

Two Distinct Clinical Syndromes

Infection causes 2 distinct clinical syndromes: HPS, which has occurred in the United States, and hemorrhagic fever with renal syndrome (HFRS), which has occurred in Korea, China, and Russia. HPS was thought to have surfaced in the United States with the famous 1993 Four Corners outbreak caused by the Sin Nombre hantavirus,2 but earlier cases have since been discovered, and sporadic cases have occurred in 34 states during the past 2 decades. In all, approximately 580 cases have been reported in the United States to date.3

Clinical Features

HPS presents as a febrile influenza-like illness that progresses over 4 to 5 days and causes changes in vascular permeability that lead to pulmonary edema and shock. As a general clinical rule, unexplained pulmonary edema in a previously healthy young adult should prompt consideration of hantavirus. One laboratory marker associated with HPS is an elevated hematocrit, reflecting a degree of hemoconcentration.2

Diagnosis, Treatment, and Prevention

There are no FDA-cleared diagnostic tests for hantavirus, but serological, PCR, or immunohistochemical testing performed by reference laboratories can confirm the diagnosis.2 There are no antivirals available for treatment, either; thus supportive care, which may include the use of extracorporeal membrane oxygenation (ECMO),4 is the mainstay of treatment. Judicious use of fluids and vasopressor therapy is necessary.2 Prevention depends on avoiding exposure to rodent urine, as no vaccine is available.

With regard to the 8-case cluster in Yosemite, the specific cause is not clear, but it is likely that rodent populations have increased, which, in turn, has increased human contact with rodent urine. The heightened awareness prompted by these cases should provoke healthcare providers throughout the nation to consider the possibility of HPS in appropriate differential diagnoses. The public should be encouraged to take precautions when cleaning barns and cabins and engaging in other activities that could lead to exposure to rodent urine.


  1. CDC. August 2012-Yosemite National Park Outbreak Notice. Accessed September 4, 2012.

  2. California encephalitis, hantavirus pulmonary syndrome, and Bunyavirid hemorrhagic fevers class. In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010.

  3. CDC. Hantavirus. Accessed September 4, 2012.

  4. Dietl CA, Wernly JA, Pett SB, et al: Extracorporeal membrane oxygenation support improves survival of patients with severe Hantavirus cardiopulmonary syndromeJ Thorac Cardiovasc Surg 2008;135:579-584.