Mice in the House: The Signs, Where They Get In, and How to Get Rid of Them Safely

Droppings in a drawer, a chewed cereal box, or scratching in a wall after dark usually means mice have moved in. If it started this fall, that fits: UC IPM, the University of California's pest management program, says that where house mice live outdoors, they frequently enter homes in autumn when nighttime temperatures become colder. The CDC's approach has three parts: seal up the holes, trap the mice inside, and clean up without stirring droppings into the air.

For a musty smell with no droppings or gnawing, see musty smell in the basement. For a bang in the wall the moment a faucet shuts off, see banging pipes.

Signs You Have Mice

The EPA notes you may never see a mouse unless an infestation is severe, and NPIC says they're mostly active at night. So you go by what they leave:

• Droppings. Penn State Extension describes mouse droppings as pointed and about the size of a grain of rice; rat droppings are blunt and about the size of a small raisin. The CDC says they're common in cabinets, drawers, and other spots out of plain sight.
• Gnaw marks on food packaging, and holes chewed through walls and floors (EPA).
• Nests of shredded paper, fabric, or dried plants (EPA). Cornell's IPM program says house mice favor dark, warm spots such as under cabinets and near appliance motors.
• Sounds. University of Illinois Extension says scratching, scurrying, or chewing in walls or ceilings, especially shortly after dark or early in the morning, is a major telltale sign.
• Smudges and smells. NPIC says brown or gray smudges on walls mark mouse paths and entrances. The EPA lists stale smells from hidden areas.

The CDC says droppings alone can't easily tell you whether mice are still active. Its test: clean the area safely (below); droppings that appear afterward mean mice are. Act early either way. UC IPM says a female may have 5 to 10 litters a year, and the CDC says rodents are easier to control before they become established.

Where They Get In

The CDC says to start by checking inside and outside to see how they got in. Mice can fit through a hole the width of a pencil (1/4 inch or 6 millimeters in diameter), the CDC says; Penn State puts it at "about the size of a dime (or a pencil)." UC IPM and NPIC both say to seal openings larger than ¼ inch.

Where to Look

Inside, the CDC lists under and behind kitchen cabinets, closet corners, behind appliances, around pipes under sinks and to the water heater and furnace, around dryer and floor vents, around every utility line, floor drains, the joint between floor and wall, and the attic, basement, and crawl space. Outside: where the foundation meets the ground, doors and windows without weatherstripping, attic and crawl space vents, utility entries, and the eaves and soffits. Penn State says openings mice use may have a dirty or oily mark around them.

What to Seal Them With

• Small holes: the CDC says to fill them with steel wool, then hold it in place with caulk or spray foam. UC IPM notes steel wool may rust, and says gnawable materials such as expanding foam, rubber, vinyl, wood, and plastic screening are ineffective for plugging mouse holes.
• Larger holes: the CDC lists lath screen or metal, cement, hardware cloth, or metal sheeting, cut to fit around pipes.
• Doors: NPIC recommends door sweeps and weatherstripping, and Penn State notes many mice come in through the doorway. If mice gnaw a door's edge, UC IPM says it may need a metal cover. See our weatherstripping guide, which also lists the furnace and water heater openings never to seal.
• Vents: seal around a vent pipe, never across it; the CPSC warns that blocking an appliance's combustion air flow can produce carbon monoxide (its example is foil lining a gas oven). At the dryer's outside hood, the U.S. Fire Administration says not to use wire screen or cloth, which collect lint and clog the vent; our dryer vent guide covers the hood.

Trap Them

The CDC recommends traditional snap traps and says to use poison or bait stations only for infestations that persist. UC IPM calls trapping the preferred method where only a few mice are present: you can see your results and dispose of the mice, avoiding the dead-mouse odors that can follow poisoning indoors.

How to Set Snap Traps

1. Use plenty of mouse traps. The CDC says mouse and rat traps differ. UC IPM says house mice seldom go more than 30 feet from their nest, so space traps no more than about 10 feet apart where you see activity. Penn State says traps set by latching or squeezing the back are safest for your fingers.
2. Bait lightly. The CDC suggests a small amount of chunky peanut butter. UC IPM warns that with too much, a mouse can take it without setting off the trap.
3. Make a "T" with the wall. The CDC says to set the trap on the floor with the baited end against the wall, since rodents run along walls. UC IPM adds that pairs set 1 to 2 inches apart catch mice that jump.
4. Put them where the mice are: by droppings or gnaw marks, behind the stove and refrigerator, at the backs of cabinets and drawers, and in the attic, basement, and crawl space (CDC). Keep traps out of reach of children and pets; Penn State suggests putting them away during the day.
5. Check daily. The CDC says to check traps every day, dispose of dead mice right away, and reset until a week passes with no captures and no new signs. If traps catch nothing while signs continue, Cornell's IPM program suggests removing them for a few days, then setting them in new spots.

Skip Glue Traps and Live Traps

The CDC says not to use glue traps or live traps: they can scare rodents into urinating, which can raise your chance of getting sick. Penn State adds that glue boards don't kill the mouse, and you may be bitten dealing with it.

If You Use Bait

NPIC says to consider prevention, sanitation, and exclusion before a rodenticide, then try a lower-toxicity product first. The EPA says consumer rodenticides are ready-to-use bait stations, labeled either for indoors or for indoors and outdoors within 50 feet of buildings; read the label before you buy to be sure it covers where you'll use it. Use only a tamper-resistant station, out of reach of children and pets, exactly as labeled. Penn State advises avoiding rodenticides inside homes, since poisoned mice die in the walls and smell; Cornell says that smell can last for days.

Clean Up Safely

The CDC explains that when fresh urine, droppings, or nesting materials of an infected rodent are stirred up, hantavirus can get into the air and be breathed in. UC IPM adds that house mouse urine is a major source of allergens. The CDC's steps:

1. Air it out. Open all doors and windows for 30 minutes before cleaning, and leave the area meanwhile.
2. Put on rubber or plastic gloves.
3. Soak it. Spray urine and droppings until very wet with an EPA-registered disinfectant or a fresh bleach solution: 1.5 cups of household bleach in 1 gallon of water (1 part bleach to 9 parts water). Let it soak 5 minutes, or as long as the disinfectant's label says. Washington's Department of Health warns never to mix bleach with ammonia, acids, or other cleaners.
4. Wipe it up with paper towels and put them in a covered garbage can. Never sweep or vacuum.
5. Disinfect floors, countertops, cabinets, and drawers.
6. Wash up. Wash gloved hands with soap and water or disinfectant, remove the gloves, then wash your hands with soap and warm water.

Dead Mice and Nests

The CDC says to spray a dead mouse or nest with disinfectant, let it soak 5 minutes, bag it with any used traps, tie the bag shut, and seal it in a second bag for a covered garbage can. To reuse a snap trap, submerge it, mouse and all, in a bucket of disinfectant for 5 minutes while wearing gloves, then bag the mouse and rinse the trap. If you think a recent illness came from mice, the CDC says to see a healthcare provider and mention the exposure.

When to Call a Pro

Call a licensed pest management professional when:

• Trapping hasn't worked. The CDC says you could then consider a bait station or professional help, and NPIC notes, "There's an art to managing mice."
• Mice are where you can't clean. The CDC suggests a pest control specialist for areas that can't be reached and a professional for rodents in heating and cooling ducts.
• The infestation is heavy. The CDC calls for special precautions, including a respirator with a HEPA filter.
• Droppings are in vermiculite. If attic insulation is loose and pebble-like, the EPA says to assume it contains asbestos and not to disturb it; see our vermiculite guide.
• Wires are chewed. UC IPM says many fires of unknown origin are believed to be caused by rodents gnawing on wires. Have a licensed electrician check chewed wiring.

Before hiring, the EPA says to ask to see the company's license and to call your state pesticide regulatory agency with concerns.

Prevent It

• Food: the CDC says to store food in thick plastic, metal, or glass containers with tight lids, put pet food away, and not leave food or water bowls out overnight.
• Water: Penn State says to fix dripping faucets and leaking pipes; see our dripping faucet guide.
• Clutter: NPIC says to remove cardboard boxes and newspapers mice could nest in.
• The yard: the CDC recommends keeping woodpiles 100 feet or more from the house, raised at least a foot off the ground, and compost bins at least 100 feet away. The EPA suggests removing leaf piles and deep mulch.
• A fall check: add an entry-point walk-around to the caulk and door-sweep work on our fall checklist; the summer guide covers pest-proofing in warm weather.

Frequently Asked Questions

How do I know if mice are in my walls?

Listen after dark: Illinois Extension says scratching, scurrying, or chewing in walls or ceilings, especially shortly after dark or early morning, is a major sign. Look for droppings and smudges where those walls meet the floor and around pipes, and set snap traps there.

Can I vacuum up mouse droppings?

No. The CDC says vacuuming or sweeping rodent urine, droppings, or nests can put tiny droplets containing viruses into the air. Spray them with disinfectant or a fresh 1-to-9 bleach solution, let it soak 5 minutes (or as the disinfectant's label says), and wipe up with paper towels.

Do ultrasonic repellers or a cat keep mice away?

Not reliably. UC IPM says there is little evidence that sound, magnetic, or vibrational devices drive established mice out or keep them away, and that cats and dogs seldom control a mouse population.

How long does it take to get rid of mice?

Plan on a week or more. The CDC says to keep resetting traps until a week passes with no captures and no new signs. Seal first: the EPA notes that even rodents released outside may find their way back in unless the entry points are sealed.

The Bottom Line

Seal every gap bigger than ¼ inch with steel wool and caulk, or metal. Set plenty of snap traps against the walls and check them daily until a week goes by with nothing new. Clean up the CDC way: air out, gloves on, soak with disinfectant or 1-to-9 bleach for 5 minutes, wipe, and never sweep or vacuum. Bait goes only in tamper-resistant stations, by the label. Call a pest management professional for mice you can't reach, a heavy infestation, or ducts, and a licensed electrician for chewed wiring.

Sources