When house mice live in or around structures, they almost always cause some degree of economic damage. In homes and commercial buildings, they may feed on various stored food items or pet foods. In addition, they usually contaminate foodstuffs with their urine, droppings, and hair. On farms, they may cause damage to feed storage structures and feed transporting equipment. A single mouse eats only about 3 grams of food per day (8 pounds [3.6 kg] per year) but destroys considerably more food than it consumes because of its habit of nibbling on many foods and discarding partially eaten items.
House mice living in fields may dig up and feed on newly planted grain, or may cause some damage to crops before harvest. But losses in stored foods are considerably greater. Mice destroy packaging materials in warehouses where food and feeds are stored. Much of this loss is due to contamination with droppings and urine, making food unfit for human consumption.
House mice cause structural damage to buildings by their gnawing and nest-building activities. In livestock confinement facilities and similar structures, they may quickly cause extensive damage to insulation inside walls and attics. Such damage also occurs in homes, apartments, offices, and commercial buildings but usually at a slower rate because mouse populations in such structures are smaller. House mice often make homes in large electrical appliances, and here they may chew up wiring as well as insulation, resulting in short circuits which create fire hazards or other malfunctions that are expensive to repair. Mice may also damage stored items in attics, basements, garages, or museums. Damaged family heirlooms, paintings, books, documents, and other such items may be impossible to replace.
Among the diseases mice or their parasites may transmit to humans are salmonellosis (food poisoning), rickettsial pox, and lymphocytic choriomeningitis. Mice may also carry leptospirosis, rat bite fever, tapeworms, and organisms that can cause ringworm (a fungal skin disease) in humans. They have also been found to act as reservoirs or transmitters of diseases of veterinary importance, such as swine dysentery, a serious bacterial disease of swine often called “bloody scours.”
The presence of house mice can be determined by a number of signs described below. Droppings may be found along runways, in feeding areas, and near shelter. Differentiating between mouse droppings and those of certain insects may be difficult. Mouse droppings are about 1/4 inch (0.6 cm) long, whereas those of cockroaches are usually 1/8 to 1/4 inch (0.3 to 0.6 cm) long and under a magnifying glass show distinct longitudinal ridges and squared-off ends. In comparison, droppings of bats contain insect fragments and are more easily crushed between the fingers.
Tracks, including footprints or tail marks, may be seen on dusty surfaces or in mud (Fig. 2). A tracking patch made of flour, rolled smooth with a cylindrical object, can be placed in pathways overnight to determine if rodents are present. Urine, both wet and dry, will fluoresce under ultraviolet light, although so will some other materials. Urine stains may occur along travelways or in feeding areas.
Smudge marks (rub marks) may occur on beams, rafters, pipes, walls, and other parts of structures. They are the result of oil and dirt rubbing off mice’s fur along frequently traveled routes (Fig. 3). They may be less apparent than rub marks left by rats.
Gnawing may be visible on doors, ledges, in corners, in wall material, on stored materials, or on other surfaces wherever mice are present. Fresh accumulations of wood shavings, insulation, and other gnawed material indicate active infestations. Size of entry holes (often 1 1/2 inches [3.8 cm] in diameter or less for mice, 2 inches [5 cm] or larger for rat) or tooth marks can be used to distinguish rat gnawing from mouse gnawing. Mice keep their paired incisor teeth, which grow continuously, worn down by gnawing on hard surfaces and by working them against each other.
Sounds such as gnawing, climbing in walls, running across the upper surface of ceilings, and squeaks are common where mice are present. Visual sightings of mice may be possible during daylight hours, and mice also can be seen after dark with the aid of a flashlight or spotlight. Nests frequently are found when cleaning garages, closets, attics, basements, and outbuildings where mice are present. They consist of fine, shredded fibrous materials.
Odors may indicate the presence of house mice. A characteristic musky odor is a positive indication that house mice are present, and this odor can be used to differentiate their presence from that of rats.
Estimating Mouse Numbers
Mouse sign and visual sightings are of limited value in accurately estimating mouse numbers, but they are the simplest and often the only practical method available. Search premises thoroughly when looking for mice. In structures, searches should include attics, basements, around foundations, crawl spaces, and behind and under stored materials.
One method to detect the presence of mice is to make nontoxic tracking-dust patches of flour or talc at 20- to 30-foot (6- to 9-m) intervals throughout a structure. The number of patches showing tracks after 24 hours, and the abundance of tracks in each patch, indicate the size of the population. Because house mice, unlike rats, do not travel far from their nests or shelter, the percentage of patches showing tracks is a good indicator of the relative size and distribution of the mouse population. Snap trapping is also an excellent way to determine the presence of mice. A relative index of mouse abundance can be calculated from the number of mice trapped for a certain number of traps set during 1 or more nights (for example, 35 mice caught per 100 trap nights).
House mice are not protected by law. They may be controlled using any pesticide registered by federal or state authorities for this purpose, or they may be controlled by use of mechanical methods such as traps.
Accurate data on mouse damage, control, and their cost are difficult to obtain. Estimates of losses of food-stuffs, structural damage, and the amount of labor and materials expended to control mice are usually only educated guesses.
In one survey of corn in a Midwestern state, 76% of about 1,000 grain samples were contaminated with rodent droppings. Mouse droppings outnumbered rat droppings twelve to one. A house mouse produces about 36,000 droppings in a year’s time. Mouse infestations are so widespread that droppings and hairs often end up in many types of food commodities intended for human use. Certain levels of rodent contamination are grounds for condemning food commodities. Structural damage caused by rodents can be expensive. In recent years, the trend toward use of insulated confinement facilities to raise swine in the northern Great Plains has led to an increased amount of rodent damage. Mice, in particular, are very destructive to rigid foam, fiberglass batt, and other types of insulation in walls and attics of such facilities. In one small swine finishing building near Lincoln, Nebraska, rodent damage required the producer to spend $5,000 in repairs to the facility only 3 years after initial construction.