Mus musculus

Background

The laboratory mouse in the wild is known as the common mouse. Through introduction by man the common house mouse now frequents most of the world. The wild mouse was originally thought to be native to Asia, India, and western Europe. There are both commensal and wild forms of the house mouse. The commensal forms often move out from buildings into surrounding fields in the spring and summer and return to the shelter of the buildings in the fall. The common house mouse is light brown to black above and whitish below, often with a buffy wash and tail that is lighter below. Commensal forms of Mus musculus tend to have longer tails and to be darker than the wild forms. Commensal forms of the house mouse are active at any hour; the wild forms seem to be active mainly at night. The range of movement in a common house mouse is very limited; it may be an area of only 15 square meters. They are not afraid of water and swim well. Nests are often made of soft shredded material wherever suitable cover and food are present.

Commensal forms feed on any human food that is available and also on paste, glue, soap, and other household articles. They damage much more than they eat. They seem to survive on very little, hence the term "poor as a church mouse". In the wild they eat many kinds of vegetables such as seeds, fleshy roots, and leaves and stems; insects and some meats may be eaten when available. They will also store food at times. They breed throughout the year at least in the warm parts of the range, and may have five or more litters a year. Because of this high fecundity, wild populations become very high at times. Population increases occurred in 1926-1927 and in 1941-1942 in the central valley of California. A population of more than 82,000, per acre was estimated in the first of these ecological explosions. The mouse has worked the sparsely vegetated soil until it appeared to be recently cultivated. Millions of mice were warming around the area until the population trend was reversed. "Sing", "waltzing", and "shaker" genetic abnormalities are common in house mice. "Singing" mice are so called because a fait but audible uttering is emitted by house mice in their shelters and has been reported from various parts of the world. The waltzing or shaker mice are caused by a defect in the equilibrium system; they "waltz" or "shake" instead of moving about like normal mice. Common house mice cause food spoilage and damage to household articles and also transport the hosts of typhus, spotted fever, and possibly other human diseases. The albino strains of M. musculus are used extensively for laboratory work. The genetics of the house mouse have been more thoroughly studied than that of any other mammal.

Handling Techniques

In handling laboratory mice, there are several things that must be kept in mind. You must be firm but gentle and always handle the animal in the same way. A mouse is best handled by picking it up by the base of the tail, then gently grasping a pinch of lose skin over the shoulder area between the thumb and forefinger (Figure 1)

The mouse is restrained in such a way can be easily manipulated. If the mice are excitable and moving around in the cage so that grasping the base of the tail is difficult, cup the hand over the top of the mouse and then grasp the tail gently at the base with a thumb and forefinger. Care must be taken not to attempt to grab the tail other than close to the base because this may result in slippage of the skin and subcutaneous tissue from the bone leading to necrosis, infection, and sloughing of the tail area where the skin has been pulled off.

Experimental Uses

The mouse, although the smallest of the common laboratory animals, is in the greatest demand in terms of numbers as an experimental animal. Up to 80% of all animals used in laboratories are mice. The small size, rapid reproduction, and relatively high position on the evolutionary scale provide numerous characteristics useful in all areas of research. The mouse is used in a wide variety of studies including drug toxicity, microbiology, radiobiology, cancer research, behavior research, nutrition, and genetic studies.

Strains and Sources

There are more inbred strains of mice than any other mammalian species. These inbred strains offer the investigator a wide range of capabilities and are suitable for almost any research protocol. These inbred strains have, for the most part, been genetically defined to enable the investigator to select a model to fit his particular need with ease. A primary source of inbred laboratory mice in the United States is the Jackson Memorial Laboratory in Bar Harbor, Maine. The majority of inbred strains available are maintained at this laboratory and are available for investigator use. Other individual suppliers of laboratory mice may have many of the strains available. Care must be taken to investigate fully the supplier's reputation in the research field before utilization of these mice. It is recommended that once an individual supplier is selected and a study is initiated, the researcher should resist changing suppliers until the individual study is completed. Although several suppliers may offer the same genetic strain, the variability between one supplier and another can be great. It cannot be over emphasized that extreme care and caution must be taken when selecting a source and a strain of animals to be used for specific research protocol. The laboratory mouse is not specifically covered in the rules and regulations under the various public laws administered by the U.S. Department of Agriculture. Therefore no specific permits or licenses are required to produce or handle these animals.

Husbandry

Mice should be kept in rooms with the temperature set at about 70F and humidity at 50%. Lights should not be too bright since most white mice are albinos and too much light hurts their eyes. They are diurnal which means they need about 12 hours of light and 12 hours of darkness each day. Their bedding should not be wood shaving since some wood emits toxic fumes to mice. They should have fresh mouse or rat food and water available at all times. Their bedding should be changed 2 or 3 times a week to prevent the buildup of urea.

Signs of Disease

Signs of disease in the mouse can be broken down into several groups:

Cutaneous Signs - "Barbering" is common vice among mice, particularly among adult breeding mice. The muzzle and other areas of the body are frequently "shaved" by cage mates. Other causes of hair loss or discoloration is generally due to disease. Swellings under the skin should be examined for cause: abscess, tumor, infection, etc. Biting and cage trauma are common causes of limb or tail injury.

Gastrointestinal Signs - Diarrhea in mice is generally caused by disease or diet. Pendulous abdomen can be caused by pregnancy or disease. Rectums occasionally prolapse.

Respiratory Signs - Nasal and ocular discharge and dyspnea are generally due to disease, including lymphocytic choriomeningitis.

Neuromuscular Signs - Trauma and disease can cause a variety of signs from head tilt to seizures to paralysis.

Other Signs - Weight loss can be from a variety of signs including dehydration, starvation, parasitism, disease, neoplasia and malocclusion. Stress from extreme temperatures and rapid changes of temperature, noises, trauma, etc. can cause death. Pesticides and detergents misuse-used can lead to death.

Data is from many sources and is meant only as a quick reference. Figures are from Harkness, "The Biology and Medicine of Rabbits and Rodents" Text is mostly from Uniformed Services University of the Health Sciences.

Mouse Formulary