Mouse in the house: why it gets in, how to get rid of it and keep it out

Seeing one mouse does not always mean a large infestation, but it is a good reason to look for other signs and for the openings it uses. Food, water, shelter and open gaps draw mice into buildings.

Trapping alone is often not enough: if food stays accessible and holes stay open, new mice can get in. That is why integrated pest management (IPM) starts with prevention, exclusion and non-chemical tools. In practice: confirm the activity, trap the mouse, clean contaminated spots safely, seal entry points and keep watching.

In this guide, "mouse" means a small rodent that has got into a home. The exact species usually cannot be identified from everyday signs, so the advice focuses on practical steps rather than on a particular species.

Mice enter buildings in search of food, water and shelter. In kitchens, pantries and storage boxes they find both food and nesting material. They often choose nest sites near a source of heat, such as a refrigerator compressor or a water heater.

Mice are active all year, but they are most likely to move into buildings in autumn as temperatures drop. So cold is not the only reason: if food and an open hole are available, a mouse can turn up in any season.

A mouse needs only a very small opening: according to CDC, a hole the width of a pencil, about 6 mm across, is enough. Entry points can be under a door, in a window gap, in the foundation, or where pipes and cables pass through a wall.

Mice are active mostly at night, so you will often notice their signs before you see the animal itself.

• Droppings: small, pointed, about the size of a grain of rice, often along walls, near food or near shelter. Fresh droppings are shiny and soft; old ones are dry, lighter in colour and crumble easily.
• Gnaw marks: chewed food packaging, wood shavings, small holes, shredded fabric or chewed wires.
• Nests: made of finely shredded paper or other fibrous material, in sheltered places.
• Tracks: small footprints on dusty surfaces.
• Smell: a characteristic musky odour, typical of large or long-term infestations.
• Sounds: scratching, gnawing or scurrying at night inside walls, above ceilings or under cabinets. Other animals can make similar sounds, so judge noises together with other signs.

Mice prefer sheltered, dark and rarely disturbed places: behind the stove and refrigerator, at the back of cabinets and drawers, behind boxes, and in storage rooms and garages. Nests are often close to a heat source.

Mice can also move inside wall voids and in the space between ceilings and floors. They usually stay close to the nest: according to ICWDM, one mouse's daily range averages about 3–9 metres across. So the signs often show where to look for the nest and where to set traps.

Seeing a mouse is not a medical emergency, but the risk is real and has two sides. The first is damage: mice spoil food and gnaw electrical wiring, which damages appliances and the building.

The second is health. According to CDC, rodents can spread diseases, including hantavirus, leptospirosis and salmonellosis. People can be infected by breathing in air contaminated with dust from urine, droppings or nesting material, by touching contaminated surfaces and then their eyes, nose or mouth, through a bite, or by eating contaminated food.

Diseases pass from infected rodents, and you cannot tell from a mouse's appearance whether it is infected. A laboratory-confirmed hantavirus infection has been described in Georgia, but the rodent hosts of the virus in the Caucasus are not well studied. That is why safe cleanup matters most. If you develop fever, muscle aches or other symptoms after contact with rodent droppings or nests, see a doctor and mention the rodent contact.

The main rule: never sweep or vacuum mouse droppings, urine or nesting material. According to CDC, this can put tiny particles containing viruses into the air. Wet contaminated material first and only then pick it up.

1. Before cleaning, open doors and windows for at least 30 minutes and leave the room while it airs out.
2. Put on rubber or plastic gloves.
3. Spray droppings and urine until very wet with a disinfectant or a bleach solution, and leave it for 5 minutes or as long as the product label says. Prepare the solution exactly as the label instructs.
4. Never mix bleach with ammonia or any other cleaner: this produces a toxic gas.
5. Pick up the wet material with paper towels and throw it in the rubbish.
6. Then clean the nearby floor, countertops and other surfaces that may be contaminated with the same product.
7. After taking off the gloves, wash your hands with soap and water.

The most reliable result comes from a sequence of steps, not from a single product.

CDC does not recommend glue traps or live traps: a frightened mouse can urinate, which increases the risk of exposure.

1. Confirm the activity: find droppings, gnaw marks and nests so you know where the mouse moves.
2. Cut off food and water: keep food, including pet food, in tightly closed thick plastic, metal or glass containers, clean up spills and crumbs right away, and keep rubbish in a bin with a tight lid.
3. Trap the mouse: CDC recommends snap traps. A small amount of peanut butter works as bait.
4. Check traps every day. Remove a caught mouse wearing gloves: spray it and the trap with disinfectant, leave it for 5 minutes, put it in a plastic bag together with the used trap, tie the bag, place it in a second bag, tie that too and put it in a covered bin. Then wash your hands.
5. Clean contaminated spots safely, as described above.
6. Seal the entry points and keep watching for new droppings or gnaw marks.

Mice prefer to run along walls and objects for safety, so the trap should stand against the wall.

• Put traps where you have seen a mouse, droppings, gnaw marks or nesting material.
• Set the trap at a right angle to the wall with the baited end against the wall, so that it forms a "T" with the wall.
• Use closed, dark places too: behind the stove and refrigerator, at the back of cabinets and drawers, and behind objects.
• If activity is widespread, one trap may not be enough: use enough traps to catch the mice in good time.
• Place traps where children and pets cannot reach them.

Poison (rodenticide) should not be the first choice indoors. A mouse poisoned inside a building often dies in a wall or another inaccessible place and leaves an unpleasant smell there; with a trap, you can see the catch and remove it.

Poison is also dangerous for children, pets and wildlife: an animal that eats a poisoned rodent can be poisoned too. If poison must be used, use only a registered product, exactly as the label says, in a closed, tamper-resistant bait station out of reach of children and pets, remove dead rodents promptly, and never store poison next to food, including pet food.

Do not make homemade poison, and do not use agricultural or unregistered chemicals. If poison is really needed, it is better to leave it to a professional.

For a lasting result, seal entry points and limit access to food. Check the house inside and outside, for both large and small openings.

• Fill small holes with steel wool and secure it with caulk around the edges, or use spray foam.
• Close larger holes with metal mesh, cement, sheet metal or similar solid material.
• Fit door sweeps on exterior doors, and seal gaps around windows, doors, the foundation and pipes.
• Keep food and pet food in tightly closed containers, clean up spilled food right away, and keep rubbish outside in a tightly closed bin.
• Reduce clutter where mice can shelter, and keep the house clean and dry.

If new signs appear after a mouse has been caught, the usual reasons are that an entry point is still open, food or shelter is still available, there were other mice in the house, or new ones are coming in from nearby buildings or the yard.

Seeing a second mouse does not by itself prove there is a nest in the house, but it does mean the job is not finished: check the openings and food access again and keep monitoring with traps.

A single mouse sighting does not require a professional; traps, cleanup and sealing holes are often enough. Professional pest control makes sense if:

• Mice keep appearing after trapping, cleaning and sealing.
• There are many droppings and contamination is widespread.
• Activity is inside walls or ceilings you cannot reach.
• Entry points on the roof or in the foundation cannot be sealed safely.
• Poison is being considered.