Andes Virus vs Other Hantavirus Strains: What Makes It Different

Why Andes virus is treated differently from other hantaviruses

Every hantavirus species causes disease through the same fundamental pathway: humans inhale aerosolized rodent excreta containing infectious viral particles, the virus replicates in the lungs and vascular endothelium, and the resulting immune response damages capillaries throughout the body. The disease is severe. The mortality is high. The reservoir is always a specific rodent species. This is true for Sin Nombre virus, Hantaan virus, Puumala virus, Seoul virus, Dobrava-Belgrade virus, and the other major hantavirus species.

Andes virus follows the same pattern, with one exception. It is the only hantavirus species with documented, repeated, scientifically confirmed person-to-person transmission. Family members caring for symptomatic patients have been infected. Healthcare workers have been infected. Intimate partners have been infected. The transmission is not common, it does not produce sustained chain transmission, and R₀ remains consistently below 1.0. But the transmission is real.

This single difference shapes the entire response framework for Andes virus events. Environmental investigation alone is sufficient for Sin Nombre, Hantaan, or Puumala outbreaks. For Andes virus outbreaks, environmental investigation must be paired with contact tracing of symptomatic patients and monitoring of household contacts through the 42-day maximum incubation window.

The discovery of Andes virus (1995)

Andes virus was first isolated and named in 1995, following an outbreak in the El Bolsón region of Argentine Patagonia. The investigation identified the virus as a novel hantavirus species genetically related to Sin Nombre virus but with distinct genomic characteristics. The reservoir was identified as Oligoryzomys longicaudatus, the long-tailed pygmy rice rat, which is widely distributed throughout the southern South American region.

The clinical syndrome was identified as hantavirus pulmonary syndrome, the same disease previously described for Sin Nombre virus following the 1993 Four Corners outbreak. The case fatality rate during the discovery outbreak was approximately 50%, though subsequent surveillance and improved supportive care has brought the typical CFR into the 35-40% range.

The person-to-person transmission evidence

The first strong evidence for person-to-person transmission of Andes virus came from an outbreak in southwestern Argentina in 1996. Investigators documented a cluster where the epidemiological pattern was inconsistent with shared environmental exposure. Cases occurred in family members and intimate contacts of index cases, with timing intervals that matched the hantavirus incubation period. Subsequent genetic analysis confirmed that the viruses circulating in the cluster were genetically identical, supporting transmission rather than independent environmental acquisition.

Since 1996, additional clusters have provided further evidence. The 2018-2019 Epuyén cluster in Argentina is the largest documented person-to-person Andes virus cluster, with 29 cases and 11 deaths. Genetic and epidemiological analysis established person-to-person transmission as the dominant route within the cluster, beyond the initial environmental exposure.

The transmission requires close, prolonged contact during the symptomatic phase. It is not casual airborne transmission. Healthcare worker infections have occurred in settings where personal protective equipment was inadequate during aerosol-generating procedures. Household transmission requires close cohabitation. R₀ measurements from documented clusters consistently show values below 1.0, meaning each infected person transmits to fewer than one secondary case on average. Sustained outbreaks do not occur. Chains self-extinguish within one or two generations.

Comparison with Sin Nombre virus

Andes virus and Sin Nombre virus are the most clinically similar hantaviruses. Both cause hantavirus pulmonary syndrome. Both have case fatality rates in the 35-40% range. Both spread primarily through aerosolized rodent excreta. Both have rodent reservoirs in the Cricetidae family. Both occur in the Americas.

The differences are geographic and transmissional. Sin Nombre virus is endemic to the western United States, Canada, and Mexico, with the deer mouse (Peromyscus maniculatus) as reservoir. Andes virus is endemic to southern South America with the long-tailed pygmy rice rat as reservoir. Sin Nombre virus has no documented person-to-person transmission despite extensive surveillance over three decades. Andes virus has documented person-to-person transmission in multiple confirmed clusters.

For travelers, this means that travel to the western United States carries a different risk profile than travel to Patagonia. The disease severity is similar, but the secondary transmission risk in Andes virus regions adds a layer of consideration not present for Sin Nombre virus regions.

Comparison with Old World hantaviruses

The Old World hantaviruses (Hantaan, Seoul, Dobrava-Belgrade, Puumala) cause a different clinical syndrome called hemorrhagic fever with renal syndrome (HFRS). Unlike HPS, HFRS primarily affects the kidneys rather than the lungs. The case fatality rates are lower (Hantaan 5-15%, Dobrava 5-12%, Seoul 1-2%, Puumala <1%) but the morbidity is significant.

None of the Old World hantaviruses show documented person-to-person transmission. Hantaan virus, despite causing severe disease and being well-studied since the 1970s, has no confirmed human-to-human transmission. Puumala virus, despite being the most commonly reported hantavirus globally (most cases in northern Europe), has no confirmed person-to-person transmission.

This makes Andes virus epidemiologically unique. It combines the high mortality of New World HPS strains with limited person-to-person transmission that is found in no other hantavirus species. The reasons remain incompletely understood but likely involve specific characteristics of viral glycoproteins and replication kinetics during symptomatic infection.

Geographic distribution and exposure risk

Andes virus is endemic throughout the southern South American region. Within Argentina, the highest incidence is in Patagonia (Rio Negro, Chubut, Neuquén provinces). Chile has documented cases throughout the country with highest incidence in the southern regions. Uruguay, Paraguay, and southern Brazil all have documented Andes virus circulation, though at lower reported incidence than Argentina and Chile.

Exposure risk varies seasonally and with rodent population dynamics. The reservoir species (Oligoryzomys longicaudatus) reproduces rapidly under favorable conditions. Wet years and abundant seed production lead to population booms followed by elevated human case counts approximately 3-9 months later. The 2026 MV Hondius cluster appears to have followed a similar pattern, with environmental exposure during Patagonia portions of the cruise itinerary preceded by a year of elevated rodent population reports.

For travelers, the practical considerations are: stay in well-maintained accommodations rather than rural cabins, avoid disturbing rodent-infested areas, use appropriate PPE during cleaning of long-closed structures, and recognize that the southwestern United States risk profile does not transfer to South America without modification.

Travel medicine considerations

Travel medicine protocols for endemic Andes virus regions differ from protocols for other hantavirus regions. The key difference is the contact tracing implication if exposure is suspected.

For Sin Nombre virus exposure (western United States): the affected individual is monitored for symptoms. No contact tracing required.

For Andes virus exposure (southern South America): the affected individual is monitored for symptoms, and if symptoms develop, close contacts are identified and monitored through the 42-day maximum incubation window.

This distinction matters for travel insurance, employer health policies, and individual planning. A symptomatic traveler returning from Patagonia is treated differently than a symptomatic traveler returning from Yosemite, even though the clinical syndrome is identical.

Andes virus in current events

The 2026 MV Hondius cluster is currently the most significant Andes virus event globally. The cluster involves cases across roughly 20 countries through cruise-related travel exposure and possibly limited person-to-person spread among passengers. WHO Disease Outbreak News, PAHO, and multiple national health ministries are actively monitoring.

The MV Hondius cluster illustrates the operational complexity of Andes virus events. The initial environmental exposure happened in Patagonia. The cases appeared across multiple continents weeks later, as passengers returned home and developed symptoms during the incubation period. Contact tracing extends across roughly twenty national jurisdictions. The 42-day post-last-case window for declaring the outbreak over has not yet been reached.

For real-time monitoring of the MV Hondius cluster and other Andes virus events, the HantaOSINT live dashboard at hantaosint.com provides aggregated surveillance data updated as new content publishes from WHO, CDC, ECDC, PAHO, and national health ministries.

Frequently asked questions

Why is Andes virus the only hantavirus that spreads person-to-person?

The exact biological reason is not fully understood. Andes virus is genetically related to other New World hantaviruses (especially Sin Nombre), but it has specific characteristics in its glycoproteins that may facilitate human-to-human transmission. Some research suggests differences in viral replication kinetics during the symptomatic phase produce higher viral loads in respiratory secretions.

Person-to-person transmission requires close, prolonged contact (typically family members, caregivers, or intimate partners during the symptomatic phase). It is not casual transmission. R₀ remains consistently below 1.0 even in confirmed clusters, meaning the chains are short and self-limiting.

How dangerous is Andes virus compared to Sin Nombre virus?

Clinically, the two are nearly identical. Both cause hantavirus pulmonary syndrome with similar case fatality rates (Andes 35-40%, Sin Nombre ~36%). The clinical phases are the same: prodromal (1-7 days), cardiopulmonary (4-10 days), and (if survived) recovery (1-2 weeks). Treatment is identical: aggressive supportive care including ECMO when available.

Epidemiologically, Andes virus is more concerning because of the person-to-person transmission potential. Outbreak response requires contact tracing in addition to environmental investigation. This makes Andes virus events more operationally complex even when case counts are similar.

Where in the world is Andes virus endemic?

Andes virus is endemic to the southern South American region: Argentina (particularly Patagonia), Chile, Uruguay, Paraguay, and parts of southern Brazil. The reservoir species (Oligoryzomys longicaudatus, the long-tailed pygmy rice rat) is widespread throughout this region.

Travelers to these regions, especially those staying in cabins, rural lodgings, or partaking in outdoor activities in endemic zones, have non-trivial exposure risk. The 2026 MV Hondius cluster appears to involve environmental exposure during Patagonia portions of the cruise itinerary.

Has Andes virus ever spread outside South America?

Andes virus cases outside South America have been documented in returning travelers — "travel-imported" cases. These individuals were infected in endemic regions and developed symptoms after returning home. Documented import cases have occurred in the United States, Spain, Germany, Switzerland, and other countries.

The 2026 MV Hondius cluster is currently the largest known multi-country event with Andes virus, with cases across roughly 20 countries through travel and possibly limited person-to-person spread during the cruise. Travel-imported cases do not establish local transmission because the reservoir species is geographically limited to South America.