The 2026 US hantavirus season has reached 22 confirmed cases, and the Southwest — particularly Montana, New Mexico, and Colorado — continues to drive domestic case counts. Hantavirus in Montana and other high-risk western states follows a predictable seasonal ecology, but 2026 case volumes are running above recent averages.
All domestic cases are unrelated to the MV Hondius cluster. These are ecologically acquired — Sin Nombre virus from deer mouse contact in endemic rural zones.
2026 US Season Summary (as of May 20)
- Total confirmed HPS cases: 22
- Active high-risk states: Montana, New Mexico, Colorado, Arizona, Utah, California (Sierra Nevada), Washington
- Strain: Sin Nombre virus (overwhelmingly)
- Transmission route: inhalation of contaminated rodent waste in rural/peri-rural settings
- Highest at this date since: 2012
No confirmed link to MV Hondius. No person-to-person transmission documented in domestic cases.
Why Montana Is a Perennial High-Risk State
Montana combines the right ecological ingredients for elevated Sin Nombre virus risk:
- High deer mouse density across eastern and central grasslands
- Rural housing stock with poor exclusion (old cabins, agricultural outbuildings)
- Outdoor recreation patterns (hiking, hunting, camping) bringing people into deer mouse habitat
- Seasonal rodent population surges tied to mast crop years and mild winters
Montana consistently appears in CDC’s HPS case data most years. 2026 follows this pattern.
Why New Mexico Also Registers Consistently
New Mexico — together with Colorado and Arizona — sits in the core of the original Sin Nombre virus discovery area (Four Corners region). The 1993 outbreak that defined HPS as a disease occurred here.
High-risk environments in New Mexico:
- Rural homes and barns in high desert habitat
- Vacation and hunting cabins opened after winter
- Agricultural grain storage areas
- Rodent-rich piñon-juniper woodlands
What Is Different About 2026
Several factors may explain why 2026 US case counts are running ahead of recent years:
Post-winter rodent population dynamics: A mild winter in the Rocky Mountain region supported higher-than-normal deer mouse survival and reproduction heading into spring.
Drought effects: Drought conditions in some areas push rodents into human structures seeking food and shelter — increasing overlap between human activity and virus-carrying rodents.
Increased outdoor recreation: Post-pandemic behavioral shifts have sustained elevated rural and backcountry activity levels, increasing exposure probability.
The Montana and New Mexico Case Profile
Demographically, US Southwest HPS follows patterns documented over decades:
- Male-skewed: roughly 60–65% of cases are male.
- Working-age adults: peak incidence in 20–50 year olds, often occupationally or recreationally exposed.
- Rural-dominant: most exposures occur in or near agricultural structures, cabins, or rural homes.
- Spring and early summer: peak season typically April–June, aligned with deer mouse breeding season and increased human outdoor activity.
Prevention in Endemic States
For residents and visitors in Montana, New Mexico, Colorado, and other endemic states:
At home:
- Seal cracks and gaps in foundations, walls, and rooflines
- Store food in rodent-proof containers
- Remove clutter that creates nesting habitat
- Use traps in active rodent areas
Cleaning old structures:
- Ventilate thoroughly before entering sealed cabins or outbuildings
- Spray contaminated areas with disinfectant — do not dry-sweep
- Wear a properly fitted N95 respirator in high-contamination scenarios
Outdoors:
- Avoid disturbing rodent burrows or nesting material
- Do not sleep directly on the ground in areas with visible rodent activity
- Use rodent-proof food storage in camp settings
How Sin Nombre Compares to Andes (the Hondius Strain)
Both cause HPS, but they differ in one critical respect:
Sin Nombre virus (US): no confirmed person-to-person transmission. All US domestic cases are acquired directly from rodents or their waste.
Andes virus (Argentina, Chile, Hondius cluster): rare confirmed P2P transmission in household close contacts.
This means the US domestic outbreak carries zero risk of human-to-human spread. Contact tracing for household members of US HPS cases focuses on ruling out common environmental exposure, not preventing secondary transmission.
What Clinicians Should Watch For
In any high-risk state patient presenting with:
- Fever + severe myalgia
- Rural or agricultural exposure in the last 60 days
- Cabin, shed, or outbuilding cleaning activity
- Thrombocytopenia + hemoconcentration on labs
…hantavirus should be on the differential. Do not wait for test results before starting respiratory monitoring.
Track latest US and global hantavirus data: Hantavirus Map
Explore HantavirusMap