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, 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 its 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 predation. Laboratory and pet mice are not strictly nocturnal (nigh-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 will 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. Some, however, feel that having their children experience the relatively short period of companionship from and subsequent death of pet mice and rats is a desirable meaningful way to expose children to the "ups and downs" of life.

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 e aggressive with a handler than those housed in groups. Cage territoriality or possessiveness may be exhibited by some pet rats. The authors have encountered a large number of pet rats, normally docile in nature, that will attack the fingers or hands of the handler upon opening and entering its enclosure. This aggressive behavior is not noted when attempts to pick up these rats are made outside of their enclosures.

It is customary to pick up a pet mouse by gently lifting it up by the tail and placing it into a cupped hand. If a more secure hold is necessary, that is, giving medications or foods directly orally, the handler may grasp or pinch as much skin as possible over the heck, 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 rat is to place one hand over the back, just behind the head, and gently grasp it around the rib-cage and lift it 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. The psycho-social well-being of the animals must be a primary consideration. 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 int he 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 (see Appendix for preferred temperature and relative humidity ranges). 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 are 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 within 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 escape 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" must be cleaned and disinfected weekly. The food and water containers must 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 must 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 percent 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 must be limited to healthful items (whole protein sources such as tuna, chicken) and must not exceed 15 percent 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 "sipper" 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 within 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 must 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 very 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 feeders and will consume proportionately more food per day than rats. This is because of their smaller 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 Considerations

Sexually mature mice and rats must be properly paired in order to enjoy success in breeding them. A single male mouse may be included in an enclosure with one or more female mice without difficulty. Including more than one male mouse in this situation invites fighting between them. By contrast, more than one male and female rat may be house together for breeding purposes within the same enclosure with aggressive displays.

Sexually mature male mice and rats usually exhibit a prominent scrotum. Sexually mature female mice and rats will usually exhibit a prominent double row of 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 opening of the female in both mice and rats.

Breeding-Rearing Cycle

Mice

Female mice must not be bred before 50 days of age. They are continuously "polyestrus," which means that they come into heat at 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 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 "polyestrus" which means that they come into heat at 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 six 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 rates must not be disturbed for the first few says 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 two and five days after her pups have been weaned.

Disease Prevention

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

Mice and rats must be purchased from reputable sources. The prospective pet owner must never purchase an obviously or even suspiciously ill rodent. Furthermore, it is never wise to purchase an animal that has been in contact with one appearing ill, even though the intended purchase appears perfectly healthy. These risky purchases never have happy endings and sometimes unnecessarily expose healthy pet rodents to serious and even life-threatening disease.

Rats 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 pet mice and rats. Bedding changes must be made two to three times each week, or 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.

Medical Conditions of Pet Mice and Rats
Requiring Veterinary Attention

Non-Infectious Diseases

Obesity

The tendency to become overweight (often grossly overweight) is more often a problem of pet rats than mice. Overindulgent pet owners and the feeding of diets rich in seeds and nuts are most often responsible for this condition. Owners or pet rats must 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 veterinarian or veterinary technician.

Tumors

Both mice and rats are very susceptible to the formation of tumors. Rats over two years of age are reported to have an 87 percent chance 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. Leukemia (cancer involving the white blood cells) is quite common in mice as well.

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. These tumors are relatively easy to surgically remove under general anesthesia.

Owners of pet mice and rats must seek veterinary attention at once after discovery of an external lump or bump or unusual tissue protruding from a body opening. If it is mutually agreed upon, the mass can be surgically removed by the veterinarian and biopsied in order to determine its exact identity (tissue type, benign vs. malignant) which, in turn, helps to determine the long term outlook for the patient. Tumors tend to grow continuously larger and may ulcerate and become infected if they reach very large sizes. For this reason, it is always preferable to remove them when they are small in size.

Infectious Diseases

Bacterial Infections

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. This organism is relatively difficult to isolate because it cannot be grown in the laboratory using ordinary culture methods. This makes the definitive diagnosis of C.M.P. established in the case of individual pet rodents based on signs and symptoms without attempts to isolate the causative bacterium.

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. In colony situations, this disease can seriously and adversely affect the reproductive capacity of female rodents resulting in infertility and reduced litter sizes.

Because this disease tends to have a very chronic (long-lasting) course, afflicted individuals must receive antibiotic treatment as soon as the mildest of symptoms are recognized and conservative methods (antibiotics in the drinking water) may be extended for long periods. Individuals exhibiting serious, life-threatening symptoms must be treated aggressively with injectable antibiotics if there is any hope of helping them. Frequently, other harmful bacteria complicate C.M.P. This often necessitates the use of multiple antibiotics in the individual patient.

Elimination of the Mycoplasma pulmonis organism from infected individuals is regarded by most experts as a practical impossibility. The recommendations made above, however, have been successful in reducing the severity of symptoms and in improving the lives of scores of pet rodents.

The outcome of treatment is always unpredictable because there are so many factors that can have an influence on it: individual susceptibility and resistance to the causative agent; age, physical condition and nutritional status of the individual; and the presence of complicating factors (concurrent bacterial or viral infections, high levels of ammonia within the enclosure).

The bacterium responsible for C.M.P. Mycoplasma pulmonis, is a highly contagious one. It may be transmitted between mother and offspring in the womb during embryonic life and by direct contact after birth. Transmission between infected and uninfected older individual rodents results from the exchange of respiratory aerosols and sexual activity.

Rabbits, guinea pigs and other rodents may carry the causative agent but do not manifest signs of disease. Caution must, therefore, be exercised when allowing contact between murine rodents and these potential "carriers." Mice and rats may carry the Mycoplasma pulmonis organism without showing obvious signs of illness.

The severity of C.M.P. can be increased substantially by any agent that causes compromise or harm to the respiratory linings. Concurrent bacterial or viral infections and exposure to the irritating chemical effects of ammonia resulting from the build-up or urine within poorly maintained enclosures are all capable of complicating C.M.P. and making the disease far more deadly.

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

Signs of infection are often inapparent 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 noted.

The disease is difficult to diagnose in individuals before death. It is considerably easier to diagnose during an autopsy. Sacrificing one or two individuals of a large group and performing autopsies on them are recommended in order to successfully treat and spare the majority of the group.

Specific 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.

Miscellaneous Bacterial Infections

A wide variety of other bacteria are capable of creating illness in pet mice and rats. Your veterinarian is best equipped to diagnose and prescribe medications for these diseases.

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. The successful treatment of certain wounds (especially long and deep cuts) and abscesses requires veterinary intervention. Abscesses usually must be surgically excised because the relatively solid nature of rodent pus precludes lancing and draining them.

Viral Infections

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.

Nursing mice and those in the process of weaning are the most commonly and seriously infected. Adult mice may become infected but rarely show symptoms. Signs of the infection include labored breathing, rough hair coat, weight loss and death. Concurrent bacterial infections complicate the picture and usually increase mortality. No specific treatment for this disease exists. A commercial vaccine is available but it is only of practical use with large colonies of susceptible mice.

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, swellings below or around one or both eyes, bulging of the eyes, red-brown tears and self trauma to the eyes are noted. Respiratory symptoms are not uncommon.

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.

Mousepox (Ectromella)

This is a highly contagious viral disease of mice that was only fairly recently recognized in the United States. The mouse is the only natural host of the virus.

The sudden onset form of the disease affects the entire body. Clinical signs include lethargy, hunched posture, rough hair coat, diarrhea, inflammation of the ocular (eye) membranes, swelling of the face and legs, and death. Another form of the disease results in a body-wide skin rash. Soon, the skin becomes swollen and ulcerated. Because of the resulting pain and discomfort, afflicted mice begin to chew on themselves. This behavior often becomes obsessive, resulting in the amputation of appendages.

There is no specific treatment for this viral disease. 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.

Parasitic Infections

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.

A veterinarian must be consulted if a mite infestation is suspected. A scraping of the skin and a microscopic examination of it will be necessary in order to confirm the diagnosis. Treatment will be dictated by the veterinarian and may include a new injectable drug that has proven very effective in treating mange in a wide range of animals.

Lice may also parasitize the hair coats of pet mice and rats. They are flattened, wing-less insects that suck tissue fluids (including blood) from the skin of the host. Lice are larger than mites and can usually be visualized with or without a magnifying lens. Lice are most often transmitted by direct contact with infested bedding and between infested and uninfected individuals.

A veterinarian must be consulted if a louse infestation is suspected. An appropriate treatment will be recommended.

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, inactivity, inappetence, constipation and excessive licking and chewing of the rectal area and base of the tail.

Stool examinations must be conducted by a veterinarian 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. Furthermore, contact between pet mice and rats, their feces, and young children must be limited and always supervised by adults.

Miscellaneous Conditions

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 to three days after they have given birth and male rats must be removed from enclosures just before female deliver their litters.

Skin Disease

A myriad of causes exist for skin disease of pet mice and rats. A number of them are discussed in some detail within specific sections of this handout. Numerous infectious agents including bacteria, viruses, fungi and parasites may be implicated. Cage mates may be responsible for hair loss or wounds to the skin of an individual. Allergies are also suspected to be a cause of skin disease of pet rodents. In these cases, it is wise to replace the bedding being used with plan white, unscented paper toweling.

A veterinarian must be consulted when pet mice and rats exhibit signs of skin disease. The doctor will need to conduct one or more diagnostic tests and an appropriate treatment will e administered based on the results of these tests.

Special Comment Regarding Human Allergies to Pet Mice and Rats

Human allergies to rodents are common and the symptoms can be serious, even life-threatening. This problem is most frequently reported among laboratory personnel but individual pet mouse and rat owners are equally vulnerable.

The offending allergens may be skin dander and hair or the proteins in aerosolized urine. Symptoms among allergic individuals may include runny eyes and nose, sneezing, congestion, coughing, asthma symptoms including shortness of breath and anaphylactic shock. Certain individuals may also manifest a rash or hives, especially in areas contacted by the claws or hair of a rodent.

This information is included in this handout because the possibility of an allergy to a pet mouse or rat must be considered if someone in a household develops any of the symptoms mentioned above. The medical doctor consulted about the problem must be made aware of the existence of a pet rodent or rodents within the home.

APPENDIX

ITEM	MOUSE	RAT
Scientific Name	Mus musculus	Rattus norvegicus
Life Span	2-3 years	3-4 years
Potential Life Span	4 years	7 years
Desirable Environmental Temperature Range	65-80 degrees F (68-72 optimum)
Desirable Relative Humidity Range	30 - 70%
Age at Onset of Puberty	28 - 40 days	50 - 60 days
Estrous (heat) Cycle Length	4 - 5 days	4 - 5 days
Estrus Length (Period during which female is receptive to male for copulation)	12 hours	12 hours
Gestation (Pregnancy) Period Days	19 - 21 days	21 - 23 days
Average Litter Size	10 - 12	6 - 12 (1st litter usually smaller)
Weaning Age	21 - 28 days	21 days