Health Care for Mice and Rats

General Information

Domestically raised mice (Murine Rodents) and rats are very popular pets in the United States. They are readily available, relatively inexpensive and easy to care for and usually enjoy human handling.

Mice

The mouse, bearing the scientific name, (Mus Musculus), is thought to have originated in Asia. Its tremendous adaptability, its long-time association with man and his dwellings, and it’s unbelievably prolific breeding potential (one reference cites one million descendants from one breeding pair in one and one half years!) has allowed mice to enjoy a world-wide distribution. Mice are timid, social and territorial animals that spend a disproportionate amount of their time in the wild pursuing an omnivorous (animal and plant material) diet. The latter activity is most often carried out at night in order to escape predators. Laboratory and pet mice are not strictly nocturnal (night-active) but tend to exhibit alternating periods of activity and rest throughout both the day and night.

In the wild, mice may exhibit aggression among themselves, although the establishment of a social “pecking order” tends to reduce this potentially injurious behavior. Single males apparently dominate groups of mice using this social pecking order. Females with litters may fight to defend their nests.

The domestication and subsequent intensive breeding of mice has resulted in a tremendous genetic diversity of mouse populations. The Swiss Albino mouse has become one of the most popular strains for pets but many others are commonly used in this capacity.

Rats

The rat, bearing the scientific name (Rattus Norveglicus), apparently also originated in central Asia. Rats were domesticated in 17th century and the process has continued to the present. Rats, like mice, have been used extensively in biomedical research and the majority of the tremendous number of breeds and strains currently in existence have resulted from intensive in-breeding efforts by research laboratories over the years.

Wild rats are found in all kinds of habitats and nearly all land masses of the world, an enduring tribute to the omnivorous (feed on plant and animal material) but exhibit tremendous opportunism in their feeding habits when living in and around man’s dwellings. Wild rats tend to be nocturnal (night-active) animals but they often use daylight hours to forage for food.

Mice and rats are both relatively short-lived animals which can be disconcerting to owners of these pets.

Handling-Restraint

As stated above, both domestic mice and rates generally tolerate gentle handling, although both are capable of biting if startled or handled roughly. Mice are more likely to bit than rats under these circumstances. In fact, mice housed alone are more likely to be aggressive with a handler than those housed in groups. Cage territoriality or possessiveness may be exhibited by some pet rats.

To pick up a mouse or rat it is best to cup the animal in the palm of the had and support the back and rear end. If giving medications or food directly orally, the handler may grasp or pinch as much skin as possible over the neck, just behind the head. The mouse can then be picked up and turned over on its back by rotating the wrist. The tail can be restrained by gently grasping it between the fourth (ring) and fifth (pinkie) fingers of the same hand.

The best method for picking up a pet is to place one hand over the back, just behind the head, and gently grasp it around the rib-cage and lift upward. The rat can then be gently cradled against the handler’s body, using minimal restraint.

Housing

Proper, adequate housing is a major factor in the maintenance of healthy mice and rats. Mice and rats can be housed within enclosures made of wire, stainless steel, durable plastic or glass. The latter three materials are preferred because they resist corrosion. Wood and similar materials must not be included in the construction of enclosures because they are difficult to clean and these materials cannot withstand the destructive gnawing of the rodents. The construction and design of the enclosure must ensure that the resident(s) cannot escape. Furthermore, the enclosure must be free of sharp edges and other potential hazards.

The enclosure must be roomy enough to allow the resident(s) to pursue normal activities and breeding activity, if the latter is desired. Visual security (a place into or under which the resident(s) can retreat for privacy) must be provided as well as exercise wheels for optimum mental and physical health. Rats, in particular, tend to be burrowers and seem to enjoy hiding under things for extended periods.

Enclosures must be easy to clean and be well lighted and adequately ventilated. Bedding must be clean, non-toxic, absorbent, relatively dust-free and easy to replace. Shredded paper, wood shavings and processed corn-cob are preferred bedding materials. Tissue paper or cotton is often supplied to breeding rats for nest-building materials.

Pet mice and rats seem most comfortable when they are spared exposure to excessive noise, needless excitement and confusion, and other similar or perceived stresses. Sudden environmental temperature changes must also be prevented because pet rodents do not tolerate them well.

Mice can be aggressive toward one another so great care must be taken when housing more than one mouse within the same enclosure. Newly assembled male groups and new males entering established territories, in particular, are likely to fight, so it is wise to always house male mice separately. Domestic female mice seldom fight unless they are defending their nests.

Rats are more communal and, in contrast to mice, several males and females may be housed with in the same enclosure, provided that it is roomy enough. In fact, young rats are raised by the group and nursing responsibilities are shared between females. These nursing females may fight among themselves and males may occasionally bother the young but aggression between rates is generally infrequent.

Every effort to prevent escapes of pet rodents must be made because they can be a tremendous nuisance when allowed the “run of the house.” Escaped rats tend to eventually return to their enclosures.

Hygiene

The frequency with which the enclosure is cleaned will depend on its design, the materials out of which it is made and the number of rodents that reside within it. As a general rule of thumb, however, the enclosure and all cage “furniture” should be cleaned and disinfected weekly. The food and water containers should receive cleansing and disinfection daily. We suggest that more than one set of containers be maintained and the “used” set be washed in a dishwasher, if possible.

A vigorous scrubbing of the enclosure and “furniture” with hot water and soap and a thorough rinse must be followed by the use of a disinfectant. Vinegar is often required to remove the scale deposited by the rodent urine.

Food and water

Good quality food and fresh, clean water should be readily available at all times. Laboratory rodent food (milled pellets or blocks) are preferred. These foods are readily available from feed stores, pet shops and from suppliers or users of such commercial diets. Kibble-type kitten food can be substituted. The rodent diets containing seeds and nuts are not recommended because they contain too many fats and oils, provide inadequate protein levels, and are not necessarily balanced.

Obesity is a common problem with pet rodents (especially rats). Consequently, oil rich foods and those high in fat must be avoided. The healthy maintenance of small pet rodents depends upon their receiving foods possessing relatively high protein levels (16% or more). Seed or nut-based diets generally fail to meet this requirement.

Table scraps and alternative foods can be offered to pet mice and rats but these should be limited to healthy items (whole protein sources such as tuna, chicken) and must not exceed 15% of what the pet consumes on a daily basis. If the above feeding recommendations are followed, malnutrition and malnutrition-related problems will be very unlikely among pet rodents. Vitamin or vitamin and mineral preparations and salt blocks (licks) are generally unnecessary.

Water is most easily made available and kept free from contamination by providing it in water bottles equipped with “slipper” tubes. The tubes themselves can become clogged with food debris so they must be checked daily. The dispensing end of the tube must be accessible to the smallest rodent with in the enclosure. It is important to point out that juveniles, before they are fully weaned, will begin drinking water and eating pelleted foods, so these essentials should be accessible to them at this time. Many deaths involving very young rodents of this age are due to starvation and dehydration.

Food Consumption will vary with the quality of the food(s) offered, the age, health and breeding status of the individual, the environmental temperature and the time of day. Both mice and rats tend to feed at night, although day-time feeding among both is quite common. Mice are voracious eaters and will consume proportionately more food per day than rats. This is because of their small body size and their relatively rapid metabolic rate. Rats tend to be more reserved in their feeding habits. In fact, rats show great caution and selectivity while eating and tend to avoid unfamiliar foods.

Breeding

Sexually mature male mice and rats usually exhibit a prominent scrotum. Sexually mature female mice and rats will usually exhibit prominent double row nipples. Furthermore, the distance between the rectal opening and the penis of the male is greater than the distance between the rectal opening and the urinary of the female in both mice and rats.

Female mice must not be bred before 50 days of age. They are continuously “polyestrous,” which means that they come into heat at fairly regular intervals (every for to five days) throughout the entire year unless they are bred. The actual period during which the female is receptive to the male and will allow breeding is approximately 12 hours and usually occurs at night. Female mice are capable of coming back into heat within 14 to 28 hours after giving birth to a litter. This is called a “postpartum estrus” which means that they can be nursing a litter and pregnant at the same time.

Pregnancy lasts an average of three weeks but can be extended as much as 10 days longer if the pregnant female is suckling a previous litter. Litter sizes average 10 to 12 pups, although it is not unusual for a female’s very first litter to be smaller in number. Litter sizes will also be decreased as breeding females age. Although mutilation and cannibalism of the young are rare occurrences, it is wise not to disturb mice for the first two to three days after giving birth. Pups are usually weaned when they are approximately three weeks old. The female will resume her breeding cycle between two and five days after her pups have been weaned (unless she was bred during her postpartum estrus).

Rats

Female rats must not be bred before 65 days of age. They are continuously “polyestrous” which means that they come into heat fairly regular intervals (every four to five days) throughout the entire year unless they are bred. The actual period during which the female is receptive to the male and will allow breeding is approximately 12 hours and usually occurs at night. Female rats are capable of coming back into heat 48 hours after giving birth to a litter. This is called a “postpartum estrus”.

This period of receptivity is not used when breeding rats because the breeding male is removed from the enclosure just before the female delivers her litter because of the strong probability of injury to the new pups by the male.

Pregnancy lasts an average of three weeks. Litter sizes average 6 to 12 pups, although it is not unusual for a female’s very first litter to be smaller in number. Litter sizes will also be decreased as breeding females age.

Female rats must not be disturbed for the first few days after delivery because stressed females may destroy their pups. Excessive handling, loud noises and even insufficient nesting material have all been implicated with this destructive behavior.

Pups are usually weaned when they are approximately three weeks old. The female will resume her breeding cycle between 2 to 5 days after her pups have been weaned.

Disease Prevention

Strict quarantine or isolation of all new acquisitions for at least 4 weeks contributes greatly to the prevention of disease among pet mice and rats. This recommendation is especially important for pets rodents because of the severity of certain diseases that they may harbor without showing signs of illness.

Rodents and mice are especially sensitive to the irritating effects from exposure to ammonia. This chemical builds up quickly in the bedding from the relatively large volume of urine excreted by mice and rats. Bedding changes must be made two to three times a week or much more often if necessary. Furthermore, ventilation must be adequate in order to reduce or eliminate the irritating effects of ammonia on the respiratory lining of pet rodents.

Obesity

The tendency to become overweight (often grossly overweight) is more often a problem of pet rats than mice. Owners of pet rats need to resist the temptation to feed “junk food” such as French fries, doughnuts, cookies, and candy. Commercial diets specifically designed for rats are always preferred and can be supplemented with whole wheat bread, dry cereal, pasta, fruits, vegetables and non-fat yogurt.

Overgrown Incisors

The incisor (front, gnawing) teeth of all rodents and rabbits grow continuously for the life of the individual. The constant use of these teeth by these animals and the continual wear between the uppers and lowers usually prevents overgrowth of the teeth. Hereditary abnormalities of the jaw bones or teeth, in malocclusion (improper meeting of the upper and lower incisors). Malocclusion, in turn, results in overgrowth of one or more of the incisors with subsequent injury to the mouth. Mice and rats with this problem must have their overgrown incisors trimmed periodically by an experienced person.

Tumors

Both mice and rats are very susceptible to the formation of tumors. Rats over two years of age reported to have an 87% of developing one or more types of tumors.

Mice frequently develop tumors representing a wide variety of tissue types. The tumors may be external or internal.

Both male and female rats develop benign mammary (breast) tumors and females develop benign tumors of the uterine and vaginal linings. These are the most common tumors of pet rats. Because rats have mammary tissue in locations beneath the skin other than the usual (along the underside of the belly), it is not uncommon to find lumps and bumps representing tumors over the shoulders, flanks and around the base of the tail.

The most significant and serious bacterial infection of mice and rats is caused by the rather unusual bacterium, Mycoplasma pulmonis, and is known as Chronic Murine Pneumonia (C.M.P.) or Murine Mycoplasmosis.

Signs and symptoms of C.M.P. include sniffling, sneezing, squinting, red-brown tears, rough hair coat, labored and audible respirations.

If the inner ear becomes involved, a severe, often incapacitating, head tilt usually develops.

Because this disease tends to have a very chronic (long-lasting) course, afflicted individual should receive antibiotic treatment as soon as the symptoms are noticed.

The bacteria responsible for C.M.P. is highly contagious one. It may be transmitted between mother and offspring and by direct contact after birth. Transmission can be through the air.

Mice and rats may carry the Mycoplasma pulmonis organism without showing obvious signs of illness.

Tyzzer’s Disease most often infects gerbils and mice, although rats are known to be susceptible. It is caused by the bacterium, Bacillus piliformis, which is usually transmitted by the ingestion of contaminated food or water. The bacterium may survive in spore form for extremely long periods in soil, bedding and feeding, and is, therefore highly resistant.

Signs of infection are often unapparent but may include lethargy, rough hair coat and sudden death. Another form of the disease results in chronic wasting and death. Diarrhea may or may not be seen.

Antibiotics must be used early in the course of the disease in order to successfully treat it. There is some available evidence that this disease can be transmitted to pregnant women. Therefore, all necessary precautions must be taken to prevent this possibility.

Wounds (from fighting and other forms of trauma) are commonly infected with bacteria that already exist within the living quarters. Abscesses commonly result from wounds when they have gone unnoticed and untreated.

There are large numbers of viruses that create disease in mice and rats. Only a few of the most economically important viral infections will be discussed.

Sendal Virus Infection

In many mouse colonies, Sendal Virus Infection is the most significant and serious respiratory disease. It often complicates Chronic Murine Pneumonia infections, increasing the death rate. This virus would be very unlikely to infect pet mice unless they were acquired from a colony with this infection already established within its members.

Sialodacryoadenitis

This is a highly contagious viral disease of rats and recently weaned mice. Initial symptoms include squinting, blinking and rubbing of the eyes. Later sneezing and swelling in the neck region are noted. As the disease progresses, swelling below or around one or both eyes, bulging of the eyes, red-brown tears and self trauma to the eyes are noted.

There is no specific treatment for this viral disease. This virus would be very unlikely to infect pet rats and mice unless they were acquired from a colony with this infection already established within its members.

External Parasite Problems

Pet mice and rats may be the hosts for a variety of external parasites. Mites, nearly microscopic, spider-like organisms, live on the skin surface and feed primarily on skin debris. They are transmitted by direct contact between infested and uninfected rodents. Signs of infestation range from mild scratching to severe scratching with hair loss, and ulceration of the skin.

Intestinal Parasite Problems

Tapeworms and pinworms are the most common intestinal parasites of pet mice and rats. Their presence often goes undetected unless the parasites are present in large numbers. Symptoms of infection may include weight loss, less active, constipation and excessive licking and chewing of the rectal area and base of the tail.

Stool examinations can be done in order to diagnose these parasitism. An appropriate treatment will be recommended by the veterinarian. Pinworms are especially difficult (sometimes impossible) to eliminate from mice and rats.

Transmission of these parasites to man is possible, but unlikely. It is, therefore, advisable that great care be taken when handling and disposing of rodent feces. Everyone, especially children should wash their hands after handling any animal.

Red-Brown Tears of Rats

Rat owners, at some point in time, will surely notice red-brown tears staining the eye lids, nose and sometimes the front paws. This substance is always mistaken as blood. It is actually a normal secretion from a large gland behind the eyes. Red-Brown tears are noted most often in response to stressful situations (restraint, fright, illness).

Cannibalism

Female rats (mice much less often), disturbed shortly after giving birth to a litter, may destroy the pups and eat them. Male rats also engage in the same behavior. For these reasons, it is important not to disturb female rodents for two or three days after they have given birth and male rats must be removed from enclosures just before females deliver their litters.

This health care information was compiled by Marilyn Lieb, D.V.M.