Pandemic Watch
Sin Nombre virus vs Andes virus
A factual side-by-side comparison of two hantavirus strains: Sin Nombre virus and Andes virus. Reservoir biology, geographic distribution, clinical course, case-fatality rate, and outbreak history.
Discovery and history
Sin Nombre virus
The name — Spanish for 'no name' — reflects the circumstances of its discovery. In May 1993, the U.S. Indian Health Service began investigating a cluster of acute respiratory deaths among previously healthy young Navajo individuals in the Four Corners region (New Mexico, Arizona, Colorado, Utah). Within weeks, CDC and partners identified a previously unknown hantavirus as the cause. The virus was provisionally called 'Muerto Canyon virus' after the area where the index cases lived, then 'Four Corners virus,' but local communities objected to both for stigmatizing reasons. Researchers settled on the deliberately neutral 'Sin Nombre virus' — the virus with no name. The same investigation established hantavirus pulmonary syndrome (HPS) as a distinct clinical entity for the first time.
Andes virus
Andes virus was isolated in 1995 from cases in El Bolsón, in the Argentine province of Río Negro, and was named for the mountain range running through its endemic region. The following year, an outbreak in the same town produced evidence that fundamentally changed hantavirus epidemiology: case clusters in family members and physicians who had attended the index patients in Buenos Aires showed transmission patterns that could not be explained by shared rodent exposure. Genetic sequencing of the patient isolates confirmed direct human-to-human spread — the first such evidence for any hantavirus. Subsequent outbreaks have repeatedly reproduced the finding.
Reservoir and ecology
Sin Nombre virus
Sin Nombre virus persists in the wild in deer mice (Peromyscus maniculatus), one of the most abundant and widely distributed mammals in North America. Infected deer mice show no symptoms but shed virus in urine, saliva, and droppings throughout their lives. Human risk fluctuates with deer mouse population density, which itself tracks regional food availability — wet winters that boost seed crops produce mouse population booms the following spring and summer, and HPS cases in the Mountain West often follow those cycles by six to twelve months.
Andes virus
The primary reservoir is the long-tailed pygmy rice rat (Oligoryzomys longicaudatus), a small nocturnal rodent native to the Patagonian forests and steppes of Argentina and Chile. Like other hantavirus reservoirs, infected animals are persistently and asymptomatically infected and shed virus throughout life. The rodent prefers brushy forest edges and the bunchgrass habitats of southern South America, putting it in contact with rural and peri-urban human settlements where its populations periodically expand following bamboo (caña colihue) seeding events that release abundant food.
Clinical course
Sin Nombre virus
Sin Nombre virus produces a classic biphasic illness. After an incubation period of one to eight weeks, patients develop the prodromal phase: sudden fever, severe muscle aches (especially in the thighs, hips, and back), headache, fatigue, and sometimes gastrointestinal symptoms. This phase lasts three to seven days and is clinically indistinguishable from influenza or many other viral illnesses. The transition to the cardiopulmonary phase is what defines the disease — patients develop cough, dyspnea, and rapidly progressive pulmonary edema. Lungs fill with protein-rich fluid; cardiac function follows. The case-fatality rate is roughly 36 to 38 percent in survivors of the prodromal phase. The interval between the first respiratory symptom and life-threatening illness can be less than 24 hours.
Andes virus
The clinical course mirrors Sin Nombre virus infection: a one-to-eight-week incubation, then a febrile prodrome with severe myalgia, then a cardiopulmonary phase with pulmonary edema and shock. Case-fatality is similar at 35 to 40 percent. The crucial epidemiological difference is the documented person-to-person transmission — close family contacts, healthcare workers, and acquaintances can be infected through respiratory droplets and possibly fomites during the symptomatic phase of an index case. Documented incubation periods for human-acquired cases extend to about 45 days, the period over which exposed contacts are monitored.
Notable outbreaks
Sin Nombre virus
The 1993 Four Corners outbreak (32 initial cases identified, ultimately tracing back to earlier cryptic cases as far back as 1959) remains the defining Sin Nombre event. The 2012 Yosemite National Park outbreak — ten HPS cases and three deaths among guests who stayed in the Curry Village tent cabins — prompted park-wide cabin remediation and a notification effort that reached thousands of past visitors. Single-case or small-cluster events occur every year across the endemic range, predominantly in spring and summer.
Andes virus
The 1996 El Bolsón outbreak (Argentina) provided the original evidence of person-to-person transmission. The 2018 to 2019 Epuyén outbreak in Chubut province produced 29 cases and 11 deaths across at least three generations of transmission and prompted provincial-level quarantine measures unprecedented for a hantavirus event. The 2025 to 2026 multi-country cluster among passengers and crew of an expedition cruise — currently tracked on this site — appears to involve Andes virus contracted in Tierra del Fuego.
Prevention
Sin Nombre virus
Standard hantavirus precautions apply with particular emphasis on three settings unique to Sin Nombre's ecology: (1) rural cabins and outbuildings that have been closed for months and may have accumulated deer mouse activity — ventilate for 30+ minutes before entering; (2) attics, sheds, and crawlspaces where wet-method cleanup with a 1:10 bleach solution should always replace dry sweeping or vacuuming; and (3) camping in known endemic areas, where raised cots and rodent-proof food storage materially reduce exposure.
Andes virus
In addition to standard rodent-exposure precautions, Andes virus is the one hantavirus where person-to-person transmission protocols matter. Public health response in Argentina and Chile includes respiratory isolation of symptomatic cases, contact tracing for close contacts during the symptomatic period, and 45-day monitoring of contacts for incubation-period symptoms. Travelers to endemic regions during periods of known outbreak should follow local public health guidance.
Full strain profiles: Sin Nombre virus · Andes virus. For the broader disease guide, see hantavirus.
