Mice

What it is

The house mouse (Mus musculus) is the most common rodent pest in homes. UC IPM's pest note describes it as a small, adaptable mammal that lives close to people, nests in warm sheltered places, and breeds rapidly. In the Mid-Atlantic you may also meet the deer mouse and white-footed mouse, mainly in wooded areas, garages and outbuildings. Deer mice are linked to hantavirus in parts of the country, which is one reason accurate identification matters.

Habits

House mice are mostly nocturnal. They are curious, good climbers, and can squeeze through very small gaps. They don't need much water, getting most of what they need from food. A mouse typically stays within a small area, often a few dozen feet from its nest, which is why droppings tend to be found close to where the mice are living.

Breeding

A female house mouse can have several litters a year, with around five or six young per litter, and the young can breed within a couple of months. Under good conditions with food and shelter, a small number of mice can become a large number quickly. This is why early action matters.

What they eat

Mice eat almost anything but prefer cereals, seeds and grains. They also eat pet food, crumbs, birdseed, and whatever people leave out. They nibble at many items rather than eating a lot of one, which means they can contaminate far more food than they consume.

How to recognize it

The animal

• House mouse: about 2.5 to 3.5 inches of body, with a tail about the same length. Fur is light brown to gray, with a lighter belly. It has large ears and small black eyes and a pointed nose.
• Deer mouse: bicolored, with a white belly and feet and a brown back, and a tail that is also two-toned. Often has large eyes and ears.

Signs

• Droppings: dark, rod-shaped pellets about an eighth to a quarter inch long, with pointed ends. Fresh droppings are dark and soft; old ones are gray and crumbly.
• Gnawing: damaged packaging, chewed wires and insulation, and holes in cardboard and food boxes.
• Nests: shredded paper, fabric, insulation or similar material in hidden spaces.
• Tracks and rub marks: greasy smudges along walls where mice routinely travel.
• Noises: scratching, scurrying or squeaking in walls, ceilings and under floors, especially at night.
• Odor: a stale, musky smell in enclosed spaces.
• Pet behavior: a dog or cat staring at a wall or appliance.

Look-alikes

• Rats are larger, with thicker droppings and a heavier build. See the rats page.
• Voles and shrews are sometimes found outdoors or in basements. Shrews have a long pointed snout and are insect-eaters.
• Bat or cockroach droppings can resemble mouse droppings.

Where you will find it

Entry points

Mice can get through gaps of about a quarter inch. Common entry points include gaps around pipes and utility lines, damaged vents, gaps under doors, cracks in the foundation, and openings in garage doors and sills. They also enter in boxes, stored items and firewood.

Where they nest indoors

Mice prefer warm, quiet, undisturbed areas near food:

• Wall voids, behind and under appliances
• Kitchens, pantries and cabinets
• Attics, basements and crawl spaces
• Garages, sheds and storage areas
• Inside stored boxes, furniture and stuffed upholstery

Seasonality

Mice are active all year but invade homes most in the fall and winter when outdoor food and shelter decline. Heating and a warm home make a strong attraction.

Why it matters

Disease and contamination

Mice can contaminate food and surfaces with urine, droppings and hair, and are associated with several pathogens, including salmonella. Deer mice and some other rodents can carry hantavirus. According to CDC, people can become infected by breathing in air contaminated with the virus from rodent urine, droppings or saliva, most often when disturbed in enclosed spaces. The illness, hantavirus pulmonary syndrome, is rare but serious.

Property damage

Mice gnaw continuously. They chew insulation, wiring, boxes and stored goods. Chewed wires are a fire risk in walls and vehicles.

Allergies and asthma

Rodent urine, droppings and dander can trigger allergies and asthma in sensitive people.

Stress and food loss

Even a small infestation can mean discarded food, noise at night and worry about contamination.

What to do

Safe cleanup practices

CDC advises that when cleaning areas with rodent droppings or nests, you should avoid stirring up dust:

• Do not sweep or vacuum dry droppings and nests.
• Ventilate the area first by opening doors and windows for a period before cleaning.
• Wear gloves, and consider a respirator for heavy contamination or confined spaces.
• Wet the area with a disinfectant or a bleach solution as directed on the label, let it sit, then wipe up with paper towels and dispose of them in a sealed bag.
• Wash hands thoroughly afterward.

For significant contamination, such as attics or crawl spaces, call a professional.

Remove food and shelter

• Store pantry food, pet food and birdseed in sealed metal or hard plastic containers.
• Clean crumbs and spills, and don't leave dirty dishes or pet food out overnight.
• Keep trash in lidded containers and take it out regularly.
• Reduce clutter in basements, garages and storage areas so there are fewer places to nest.
• Store boxes off the floor, away from the walls.
• Keep firewood and debris away from the house, and trim vegetation touching the structure.

Look for entry points

Walk the outside of your home and inspect the foundation, utility penetrations, vents, doors and garage. Note gaps, and use the list to guide a professional or contractor. Repairs to the structure should be done correctly so the mice are not trapped inside, which is why the order of steps matters.

What not to do

• Don't put out poison. Rodenticides can poison pets, children and wildlife, and a poisoned mouse may die inside a wall, causing odor and attracting other pests.
• Don't seal holes before the mice are gone. You can trap them inside, where they may gnaw new exits or die in the walls.
• Don't handle live or dead mice with bare hands.
• Don't rely on ultrasonic devices or home remedies. Evidence that these work well is limited.

When to call a professional

A single mouse sighting can mean more than one mouse. A professional can find where they are getting in, tell house mice from deer mice, and choose the right methods safely.

Call a pest professional if

• You see droppings in more than one area, or a lot of them.
• You hear noises in walls or ceilings, or find nests or gnawing.
• You find mice in the kitchen or in the room where food is prepared.
• You have tried traps and are still seeing signs.
• You have children, pets or someone with a weakened immune system or asthma in the home.
• You find droppings in an attic, crawl space or other enclosed area. Cleanup needs the right protection.

Call a doctor if

You develop fever, muscle aches, fatigue, cough or shortness of breath after cleaning up or being exposed to rodent droppings. Tell the doctor about the exposure. CDC notes that early symptoms of hantavirus infection can look like the flu, and the illness can progress to breathing difficulty.