House Mouse: Identification and Signs of Mice

The house mouse (Mus musculus), also called the domestic mouse, is the mouse most often found inside London homes, flats, shops and restaurants. It lives almost entirely alongside people, breeds quickly and needs very little space to get in. This guide shows you how to recognise one, the signs it leaves and what to do about it.

What Does a House Mouse Look Like?

• Colour: grey-brown on top with a slightly lighter grey belly.
• Features: a pointed snout, large rounded ears, small black eyes and a long, thin, almost hairless tail.
• Abilities: house mice are excellent climbers and jumpers and can run along pipes, cables and the inside of wall cavities.

House mice are mostly active at night and nest close to food, using shredded paper, insulation and fabric. They prefer grains and seeds but will try almost anything in a kitchen. A female can have five to ten litters a year, each with around five or six young, and the young can breed at about two months old. That is why a couple of mice in autumn can become a real infestation by winter.

How Do Mice Get Into Your Home?

A mouse can squeeze through a gap of about 6 mm, and it will gnaw a smaller gap until it fits. Mice come indoors for warmth, shelter and food, and in most London homes they arrive by one of four routes:

1. Wall cavities: the gap between the inner and outer walls is a hidden corridor. Once mice are in the cavity they can climb to every floor and into the loft without being seen.
2. Neighbouring properties: in terraces, semi-detached houses and blocks of flats, mice move from one home to the next through shared walls, floor voids, lofts and service risers. Treating one property on its own often is not enough.
3. Defects in external walls: damaged or missing air bricks, cracks in brickwork, gaps under door thresholds and holes left around pipes and cables.
4. Pipes and conduits under the floor: mice follow water, gas, waste and cable runs beneath the floor and come up where the pipes rise, usually under the kitchen sink, behind appliances and at the boiler.

Once inside, they head for quiet, warm spaces: lofts, basements, wall cavities, kitchen plinths and the back of cupboards. This is why a proper inspection looks at the whole building, not only the room where the droppings were found.

Hidden Entry Points Inside Your Home

Many mouse problems have nothing to do with the outside of the building. Mice come up through gaps left inside the home by builders, plumbers and kitchen fitters, in places nobody looks at once the work is finished. The most common are:

1. Behind the kitchen cupboards: where the central heating pipes, the water inlet and the gas pipe pass through the floorboards or the wall. The holes are usually cut much bigger than the pipes and are hidden once the units are fitted.
2. Soil stack boxing in the kitchen: boxing around the soil stack that was left unfinished at the bottom or the back, leaving an open shaft straight down to the floor void.
3. Toilet pipe boxing in the bathroom: the same problem behind the toilet, where the waste pipe passes through the floor or wall inside its boxing.
4. Under the stairs: around the cables at the electric meter and the gas pipe at the gas meter, where they enter the floor or the walls.

Each of these gaps joins your living space to the floor void and the wall cavities, which is where mice travel. Finding and sealing them, known as proofing, is what stops the problem from coming back after the treatment.

Signs of Mice in Your Home

Seeing a live or dead mouse is the obvious sign, but most people notice something else first:

• Droppings: small, dark and pointed, often along walls, in drawers or under the sink
• Noises: scratching or scurrying in walls, ceilings and under floors, especially at night
• Gnaw marks on food packaging, skirting boards, wiring or plastic
• Nests of shredded paper, fabric or insulation in hidden corners
• A stale, ammonia-like smell in cupboards and enclosed spaces
• Grease marks along the routes they run every night

Mouse or Rat?

A young rat can look like a large mouse, so check the details:

Learn more in our brown rat guide.

Are Mice Dangerous?

House mice contaminate food and surfaces with their droppings, urine and saliva, and they urinate constantly as they move around. They can spread bacteria such as Salmonella and Listeria, which cause food poisoning.

They also cause damage. Mice gnaw all the time to keep their teeth worn down, and they will chew through wood, plastic, insulation and electrical cables. Chewed wiring in walls and lofts is a fire risk.

Cleaning Up After Mice Safely

Do not sweep or vacuum dry droppings or nests, because that sends dust into the air. Instead:

1. Open the windows and wear rubber or disposable gloves.
2. Spray droppings and nesting material with disinfectant and leave it to soak for a few minutes.
3. Wipe it up with paper towels, bag it and put it in the outside bin.
4. Disinfect the area, then wash your hands thoroughly.

For heavy contamination in lofts, basements or commercial kitchens, our pest sanitation service removes droppings and disinfects the area for you.

How to Keep Mice Out

• Seal gaps larger than 5 mm with wire wool and sealant, cement or metal plates. Expanding foam alone will not stop mice, because they chew straight through it.
• Fit brush strips to the bottom of doors and mesh over air bricks and vents, without blocking the airflow.
• Store food in glass, metal or hard plastic containers, including pet food and bird seed.
• Clean up crumbs and spills, and do not leave pet food out overnight.
• Keep bins closed and clear clutter and overgrown plants from around the walls of your home.

How to Get Rid of Mice

A mouse problem is solved in two parts: removing the mice that are inside and stopping new ones from getting in.

• Trapping: snap traps work well. Place them against walls, at right angles, with the trigger end towards the wall. Use more traps than you think you need.
• Proofing: traps alone will not solve the problem if the entry points stay open. Find them and seal them.
• Be careful with poison: always follow the label, use tamper-resistant bait boxes and keep bait away from children, pets and wildlife. Mice that eat poison can die inside walls and cause smells.

If mice keep coming back, or you would rather not deal with them yourself, our certified technicians can inspect your property, find the entry points, carry out the treatment and quote for proofing.

Frequently Asked Questions

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