How do I know if I have mice or rats?

Size is the fastest test. House mouse droppings average 1/4 inch long, while roof rat droppings are about 1/2 inch and Norway rat droppings can reach 3/4 inch. Location helps too: house mice seldom venture more than 30 feet from their nests, roof rats usually nest aboveground, and Norway rats are mostly found low in the house, in basements or on the ground floor.

How fast do house mice breed?

Females are usually sexually mature 5 to 6 weeks after birth, and a female may have 5 to 10 litters a year of about 5 or 6 pups each. With plentiful, stable food and shelter, house mice can breed year-round.

Where do house mice nest in a house?

ICWDM lists garages, closets, attics, basements and outbuildings among nest sites. Nests are built from fibrous materials and are usually 4 to 6 inches across. House mice seldom venture more than 30 feet from their nest sites.

What attracts house mice to a home?

UC IPM notes house mice frequently enter homes in autumn when nighttime temperatures become colder. With plentiful, stable food and shelter they can breed year-round, so sealed food storage and sealing gaps down to 1/4 inch both help.

Can house mice climb walls?

Yes. House mice are excellent climbers and can run up any rough vertical surface. They can jump up to 12 inches from the floor onto a flat surface and run along wire cables or ropes. Vertical agility is why exclusion must seal openings at every level of the exterior envelope rather than only at ground level.

Do house mice carry hantavirus?

The rodents CDC names as carriers of US hantaviruses are deer mice, white-footed mice, rice rats and cotton rats; the house mouse is not on that list. House mice are linked to other illnesses, including lymphocytic choriomeningitis and salmonella. Because deer mice and other wild mice can be mistaken for house mice and can share a structure, clean up any droppings the CDC way: wear rubber or plastic gloves and soak droppings with disinfectant rather than sweeping or vacuuming. For heavy infestations, CDC adds respiratory protection with a HEPA filter.

Rodent Species Identification

House Mouse: Identification and Behavior

Mus musculus

A field guide to identifying house mice, understanding how they breed and invade homes, and eliminating infestations through trapping and exclusion.

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How to Identify House Mouse

The house mouse is the smallest of the three commensal rodents. Adults are about 5 to 7 inches long including a 3 to 4 inch tail, and they weigh about 1/2 ounce. The ears are relatively large, the eyes small and black, and the tail nearly hairless with obvious scale rings.

Fur is usually light brownish to gray, with a gray or buff belly. Unlike the deer mouse, the house mouse does not show a sharply defined two-tone pattern. An adult mouse also lacks the oversized head and feet of a very young rat.

House mouse droppings average 1/4 inch long and are pointed, about the size of a grain of rice. They collect along runways, near food and close to nest sites.

Size Reference

About 5 to 7 inches, tail included

Compared to a juvenile rat:A young rat has a large head and feet for its body; an adult mouse has proportionately much smaller ones
Compared to a deer mouse:Overall gray coat, without the white underside of a deer mouse
Compared to a vole:Longer tail (common voles have short tails)

Distinguishing From Similar Species

vs Deer Mouse: Deer mice have a white underside with a distinct line where it meets the brown back. House mice have an overall gray coat.
vs Juvenile Norway or Roof Rat: A very young rat has a big head and big feet for its size; an adult mouse does not.
vs Vole: Voles have short tails. House mice have long, nearly hairless tails.

Where House Mice Live

House mice live across the continental US, Hawaii and Alaska, and they are more common than rats in homes and commercial buildings.

Habitat selection centers on proximity to food. ICWDM lists garages, closets, attics, basements and outbuildings among nest sites. House mice seldom venture more than 30 feet from their nests.

House mice can enter openings as small as 1/4 inch. Places worth checking include door sweeps, garage thresholds, utility line penetrations, dryer vents, weep holes in brick veneer and basement window frames.

House Mouse Behavior

House mice are mainly nocturnal, although considerable activity may occur during the day. Seeing mice during the day does not necessarily mean a large population is present. One mouse travels an area averaging 10 to 30 feet in diameter during its daily activities.

In a single year a female may have 5 to 10 litters of about 5 or 6 pups each, and young are born 18 to 21 days after conception. House mice are usually sexually mature 5 to 6 weeks after birth, and with plentiful, stable food and shelter they can breed year-round, which can produce high population densities.

Feeding patterns differ from rats. House mice are nibblers, sampling many food sources in a single night rather than committing to large meals. They eat up to 20 percent of their body weight daily in about 200 small meals each night. They quickly detect new objects but do not fear them.

They can run up any rough vertical surface, jump up to 12 inches from the floor onto a flat surface, and fit through holes about the size of a pencil. This combination of agility and small size means exclusion must address every gap in the structure, not just the largest visible openings. For a drawing of how that climbing works on a house, the guide Can Mice Climb Walls? adds a surface chart and a printable checklist. To see that jump on a ruler next to the other published heights, read How High Can Mice Jump?

Damage House Mice Cause in Homes

A mouse produces about 36,000 droppings a year and contaminates far more food than it eats through its urine and feces. Droppings left in food make it unsafe to eat.

Mice often damage large electrical appliances by chewing wiring and insulation. UC IPM notes that many fires of unknown origin are believed to be caused by rodents gnawing on wires.

Health Risks From House Mice

UC IPM lists leptospirosis, salmonella and lymphocytic choriomeningitis (LCM) among the diseases house mice carry, and CDC ties LCM to the common house mouse.

CDC says the most common hantavirus that causes HPS in the United States is spread by the deer mouse, and the CDC list of hantavirus carriers quoted by ICWDM (deer mice, white-footed mice, rice rats and cotton rats) does not include the house mouse. Because look-alike wild mice can share a structure, treat all rodent droppings with care and clean them up wet.

Asthma and allergy connections are notable. House mouse urine is a major source of allergens, and UC IPM notes it has been linked to early onset of asthma in sensitive children.

How House Mice Enter Homes

House mice can enter through openings as small as 1/4 inch, a hole about the size of a pencil. That size means inspection has to cover small cracks and gaps, not only the obvious openings.

Places worth checking include gaps at door thresholds and garage doors, utility line penetrations (especially where pipes enter through block foundations), dryer vents without rigid covers, weep holes in brick veneer, band joist gaps between floors, unscreened attic louvers, gaps behind window trim, and openings where HVAC lines enter the exterior wall.

Expanding foam, wood and other gnawable materials do not work for plugging mouse holes. Mouse exclusion seals openings down to 1/4 inch, where rat exclusion works to 1/2 inch.

House Mouse Control Considerations

House mouse control pairs rapid population reduction with exclusion of openings down to 1/4 inch. Snap traps go where mice are active, spaced no more than about 6 to 10 feet apart and set perpendicular to walls with the trigger end nearly touching the wall. House mice quickly investigate new objects, and ICWDM notes they almost immediately enter bait stations and sample bait.

Exclusion fills gaps with stainless steel or copper wool, then adds door sweeps, weep hole covers, and 1/4 inch wire mesh over openings such as attic louvers. Ongoing prevention focuses on sealed food containers, pet food lifted overnight, bird seed kept in metal bins, and periodic re-inspection of door sweeps and utility penetrations.

Mouse Control addresses the house mouse's fast breeding with saturation trapping and sealing gaps down to 1/4 inch.

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Where House Mice Are Found

ICWDM records house mice across the continental US, Hawaii and Alaska.

UC IPM describes house mice as very adaptable animals with high reproductive potential. As ICWDM puts it, wherever humans are, mice may be present.

Signs You Have a House Mouse Problem

Small pointed droppings near food
House mouse droppings average 1/4 inch and are pointed. They turn up along runways, in feeding areas and near nests, which can include attics and basements.
Small gnaw marks
ICWDM puts mouse gnaw marks at about 1/32 inch wide, half the width it gives for rat gnaw marks.
Musky odor and urine traces
A characteristic musky odor is a strong sign that mice are present. Mouse urine glows under ultraviolet light, but so do many other things, including some cleaning products.
Shredded nesting material in undisturbed areas
Nests are built from fibrous materials in places such as garages, closets, attics and basements. They are usually 4 to 6 inches across.

Common Questions About House Mice

Size is the fastest test. House mouse droppings average 1/4 inch long, while roof rat droppings are about 1/2 inch and Norway rat droppings can reach 3/4 inch. Location helps too: house mice seldom venture more than 30 feet from their nests, roof rats usually nest aboveground, and Norway rats are mostly found low in the house, in basements or on the ground floor.
The rodents CDC names as carriers of US hantaviruses are deer mice, white-footed mice, rice rats and cotton rats; the house mouse is not on that list. House mice are linked to other illnesses, including lymphocytic choriomeningitis and salmonella. Because deer mice and other wild mice can be mistaken for house mice and can share a structure, clean up any droppings the CDC way: wear rubber or plastic gloves and soak droppings with disinfectant rather than sweeping or vacuuming. For heavy infestations, CDC adds respiratory protection with a HEPA filter.

Sources and Further Reading

Sources for house mouse facts on this page:

House Mouse (UC Statewide IPM Program (UC IPM))
Pest Notes: Rats (UC Statewide IPM Program (UC IPM))
House Mice (Internet Center for Wildlife Damage Management (ICWDM))
House Mouse Biology (Internet Center for Wildlife Damage Management (ICWDM))
House Mouse Damage Identification (Internet Center for Wildlife Damage Management (ICWDM))
House Mouse Damage Prevention and Control Methods (Internet Center for Wildlife Damage Management (ICWDM))
Deer and White-footed Mouse Damage Identification (Internet Center for Wildlife Damage Management (ICWDM))
Is There a Mouse in the House? Eliminate Mice with IPM (Penn State Extension)
Common Urban Pests: Identification, Prevention, and Control (Penn State Extension)
About Hantavirus (Centers for Disease Control and Prevention (CDC))
About Lymphocytic Choriomeningitis (Centers for Disease Control and Prevention (CDC))
How to Clean Up After Rodents (Centers for Disease Control and Prevention (CDC))

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