# Signs You May Have a Mouse Infestation | Illinois Pest Control

> The signs of mice in the house (droppings, gnawing, nests, noises), how mice get in, why one mouse usually means more, and when to call for rodent control.

URL: https://illinoispestcontrol.us/guides/signs-of-a-mouse-infestation/

**Short answer:** The clearest signs of mice are droppings, fresh gnawing, and tracks, along with nests made of shredded paper or fabric in sheltered spots, according to the Illinois Department of Public Health. Scratching in walls at night, chewed food packages, and grease marks along baseboards are other clues. Droppings in several rooms usually mean an established population.

House mice are the most common rodent in developed areas, according to the [Illinois Department of Public Health](https://dph.illinois.gov/topics-services/environmental-health-protection/structural-pest-control/house-mouse-prevention-control). They're small, quiet, and mostly active at night, so the evidence they leave is usually how you find out.

## The signs to look for

IDPH lists droppings, fresh gnawing, and tracks as the signs of where mice are active, and notes their nests, made of finely shredded paper or other fibrous material, turn up in sheltered locations. In practice, look for:

- **Droppings:** small, dark, and pointed, often along walls, in drawers, under sinks, and behind appliances.
- **Gnawing:** chewed food packages, cardboard, wires, or wood.
- **Nests:** shredded paper, insulation, or fabric balled up in a hidden spot.
- **Noises:** scratching or scurrying in walls and ceilings, mostly at night.
- **Grease marks:** dark smudges along baseboards and edges where mice run.
- **Odor:** a musky smell in closed spaces.
- **Pets:** a cat or dog suddenly fixated on one spot.

## Why one mouse usually means more

IDPH notes a female house mouse can have five to 10 litters a year, usually five or six young each. A population can grow quickly in a warm house with food. Droppings in more than one room, or on more than one floor, suggest mice are established rather than passing through.

## How mice get in

IDPH says mice can slip through a crack a pencil fits into, slightly larger than 1/4 inch. Common Illinois entry points:

- Gaps where pipes, gas lines, and cables enter the house.
- Dryer vents and exhaust fans without covers.
- Worn garage door seals and gaps at the sill.
- Foundation cracks and basement windows.
- Gaps under exterior doors.

Fall is the usual start. As Illinois nights drop below freezing, from roughly 110 to 136 nights a year depending on the city, mice look for warm shelter.

## What to do

IDPH's approach has three parts: sanitation to remove food and shelter, "mouse-proof construction" to seal openings, and population reduction with traps or baits when an infestation exists.

1. **Store food sealed:** pantry items, pet food, and birdseed in hard containers.
2. **Find and seal entry points** with durable materials like metal mesh, sheet metal, or concrete; foam alone can be chewed through.
3. **Don't sweep droppings dry** or handle them with bare hands.
4. **If you use traps or baits,** keep them away from children and pets and follow the label. Rodenticide baits belong in tamper-resistant stations.

## When to call a professional

Call when droppings show up in several rooms, you hear activity in walls, trapping hasn't stopped new mice, or you manage a rental or business. Professional [rodent control](/rodent-control/) usually combines inspection, trapping or baiting, and exclusion. HomeAdvisor's June 2026 data puts rodent extermination at $189 to $655, with exclusion at $200 to $600.

## Quick answers

**If I see one mouse, are there more?**

Often. IDPH notes a female house mouse can have five to 10 litters a year of usually five or six young each, so one sighting can mean an established population. Check for droppings in several places.

**How do mice get into the house?**

Through small gaps. IDPH says a mouse can slip through a crack a pencil fits into, slightly larger than 1/4 inch. Common entry points are gaps around pipes and utility lines, dryer vents, garage doors, and foundation cracks.

**Is it safe to clean up mouse droppings?**

Don't sweep or vacuum droppings dry, which can stir up dust, and don't handle them with bare hands. Ask a pest control professional about safe cleanup, especially for large amounts.

Referral disclosure: Illinois Pest Control is an independent contractor-referral service. We help connect Illinois homeowners and property owners with participating pest control professionals. Services are performed by independent providers. We may receive compensation for qualified referrals. Phone: (217) 626-5123.