Deer Mice and Hantavirus: How Rodent Behaviour Shapes Risk

Peromyscus maniculatus - the deer mouse - is one of North America's most successful small mammals. It thrives from sea level to alpine meadows, from the Yukon to central Mexico. It is also the principal reservoir of Sin Nombre virus, the dominant hantavirus in North America, and routinely turns up in surveys of cabins, garages, hay barns and grain stores.

What deer mice want from your house

Two things: food and a thermally stable place to nest. A deer mouse can squeeze through any gap larger than a quarter of an inch (about six millimetres). They are particularly attracted to:

• Stored bird seed, pet food, grain and dried pet treats
• Insulation in attics, behind appliances, in seldom-used vehicles
• Woodpiles, cardboard storage, soft materials they can shred for nesting
• Spaces under sinks and behind fridges where pipes enter the structure

How infected mice make people sick

Infected deer mice excrete virus in urine, droppings and saliva. The virus is fragile in sunlight and on dry surfaces, but in still air it can remain infectious for hours to a few days. Human exposure happens when someone disturbs contaminated material - sweeping out a shed, shaking a tarpaulin, pulling insulation from an attic - and inhales the resulting aerosol.

Notably, deer mice carrying hantavirus appear healthy. There is no way to tell by looking whether a mouse you trap is infected.

The 15% rule, and why it varies

On average about 15% of deer mice in surveyed populations carry hantavirus, but local rates can be much higher or lower. Factors that push the rate up:

• High population density, which increases mouse-to-mouse contact
• ‘Mouse year’ ecological conditions - mild winters, abundant seed
• Older populations: prevalence rises with mouse age, because mice acquire and keep the virus for life
• Disturbed habitats around human structures, which favour Peromyscus over other rodents

Practical implications

If you see one mouse, assume more. If you see signs of infestation in a building that has been closed for months, do not start cleaning - read our cleanup guide first. And remember that deer mice are not the only reservoir; cotton rats, rice rats and white-footed mice carry related hantaviruses across the Americas, and Apodemus species do so across Europe and Asia.