Outbreak of Hantavirus Infection in the Four Corners Region of the United States in the Wake of the 1997–1998 El Nino—Southern Oscillation

Hantavirus cardiopulmonary syndrome (HCPS), a rodent-borne zoonosis, has been endemic in the Americas for at least several decades. It is hypothesized that the 1991–1992 El Nino—southern oscillation (ENSO) caused increased precipitation that allowed an increase in rodent population densities, thereby increasing the possibility of transmission to humans. The result was a 1993–1994 outbreak of the disease in the Four Corners states of the southwestern United States. A second strong ENSO occurred in 1997–1998, after a period of considerable public education about the risks of hantavirus infection that began during the 1993–1994 outbreak. The caseload of HCPS increased 5-fold above baseline in the Four Corners states in 1998–1999. Regions that had received increased rainfall in 1998 were especially affected. A large majority of the 1998–1999 case patients reported indoor exposure to deer mice. Hantavirus outbreaks can occur in response to abiotic events, even in the face of extensive public education and awareness.