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Care of Pet Mice and Rats

November/December 1994 Vol. F, No. 6 Veterinary Practice STAFF

This information leaflet is designed as a method of communication between veterinarians and clients who own pet mice or rats. The environment, handling, and diet of these rodents are discussed, and common health problems specific to each species are explored. If you have any questions, be sure to ask your veterinarian to answer them for you.

Mice and rats make excellent pets when they are properly cared for (Table 1). These timid, social rodents are fun to watch while they perform their natural behaviors of burrowing, searching for food, and playing. Pet rats and mice have periods of activity during both day and night. They seldom bite when handled with care.

Diet

Good quality food and clean, fresh water should be available at all times for your pet mouse or rat. pelleted rodent rations which are processed as dry blocks or pellets are recommended. Typical maintenance diets should contain about 14% protein and 4 to 5% fat, while diets for growth and reproduction should contain 17 to 19% protein and 7 to 11% fat. While many rodents prefer sunflower-based diets to pellets, seeds are low in calcium and high in fat and cholesterol. When fed exclusively, seed-based diets can lead to obesity and nutritional deficiencies. It is best to give your pet mouse or rat seeds only as a supplement to a pelleted diet, or as a treat.

Many factors can affect your pet rodent's food intake. Such factors may include environment temperature and humidity, food quality, breeding status, and health status. On average, an adult mouse will consume about 15 g of feed and 15 ml of water per 100 g body weight daily. Comparatively, an adult rat will consume approximately 5 g of feed and 10 ml of water per 100 g body weight daily. Both rats and mice typically eat at night. Water should be provided in bottles equipped with sipper tubes. Tubes should be positioned low enough to allow the pet easy access. Inadequate water consumption can result in dehydration, lower body weight, infertility, and, even death. Empty, clean, and refill your rodent's water bottle with fresh water each day.

Handling

Pet mice and rats will become tame and seldom bite if they are properly restrained and accustomed to handling. Always exercise caution when approaching a nervous or frightened pet. It is best not to disturb sleeping rodents as they can be quite cranky when awakened . Some rats in particular, are very protective of their environments. Coax the rat to walk into your hand rather than just picking it up. Mice housed individually tend to be more aggressive and apprehensive.

Both rats and mice can be easily picked up by scooping them into a can or cupped hands in order to move them out of their territory and into a neutral area. Another way to move these rodents is by grasping the base of their tails between your fingers. Never pull at the tip of the tail because the skin may tear and strip off the tail.

  TABLE I

Facts About Mice and Rats

 

Mice

Rats

Lifespan

1.5-3 years

2.5-3.5 years

Cage temperature range

65-80°F

65-80F

Relative humidity range

40-70%

40-70%

Breeding age

50 days (male)

65-110 days

 

50-60 days (female)

 

Estrous cycle

4-5 days

4-5 days

Gestation period

19-21 days

21-23 days

Litter size

10-12 young

6-12 young

Weaning age

21-28 days

21 days

 

Housing

Several types of cages are suitable for housing pet mice and rats. (The two different types of rodents should never be housed together.) Many rodent cages come equipped with exercise wheels, tunnels, and nest boxes. These accessories contribute to the pet's psychological well being as well as its physical need for activity The most preferred type of cage has rounded corners to discourage chewing. Because rodents easily chew through wood and thin plastic, recommended caging materials are wire, stainless steel, durable plastic, and glass. Care should be taken when using glass and plastic enclosures because they can restrict air circulation and may lead to temperature or humidity problems. If using these materials, at least one side of the enclosure should have air holes drilled into it to ensure proper ventilation.

Both pet mice and rats thrive in solid-bottom cages with deep bedding and ample nesting materials. Bedding should be clean, nontoxic, absorbent, and relatively dust-free. Pelleted paper, shredded paper, or organic pelieted products (nontoxic and digestible) are all acceptable beddings. Bedding should also be free of mold, mildew, or other contamination. Avoid using cedar chips, pine shavings, or chlorophyll-scented shavings; these products have been linked to both respiratory and liver disease. Cotton and shredded tissue paper make excellent nesting materials. At least one inch of nesting material should be provided to allow for normal burrowing behavior.

Adult mice require a minimum floor area of 15 square inches and a cage height of 5 inches. Adult rats need at least 40 square inches of floor space and a minimum of 7 inches cage height. If you are planning to breed your pet mice or rats, a much larger square-inch area per rodent is required. The optimal temperature range for all pet rodents is between 65°to 80°F with a relative humidity of 40 to 70%. Twelve-hour light cycles are preferred, although most rodents are more active during the night.

Pet rats and mice can be housed singly or in groups, although rodents are colony oriented by nature. Occasionally, an overly aggressive mouse or rat may have to be caged by itself. Territorial disputes also develop when cages are overcrowded or when a rodent is deprived of food or water by its cage mates. To avoid this scenario, equip a group cage with multiple food and water sources.

Cages and accessories should be thoroughly cleaned at least once a week. An exception to this schedule would be when newborn babies are present. In this case, you should wait at least 10 days following birth before thoroughly cleaning the cage. Cages should be sanitized with hot water and nontoxic disinfectant or detergent, then thoroughly rinsed. Water bottles and food dishes should be cleaned and disinfected daily.

Breeding

It is relatively easy to distinguish mate rodents from female rodents. Neonatal male rodents have a one-and-half to two times greater distance between the anus and urogenital opening than their female counterparts. Sexually mature male rodents exhibit a prominent scrotum and large testicles, while females can be identified by their more prominent bilateral rows of nipples.

Sexually mature mice and rats need to be properly paired in order to avoid fighting and permit successful mating. Adult male mice often fight when caged together, especially in the presence of females. Rats, on the other hand, can usually be caged in mixed groups of males and females without aggression.

Female mice become sexually mature at about 50 days of age. They have an estrous cycle of about every 4 to 5 days throughout the year unless they are bred. Female mice are usually receptive to males for about 12 hours of this cycle, typically at night. They also have a fertile postpartum estrus; which means that they can be bred within 24 hours after giving birth.

Gestation in mice lasts approximately 3 weeks, but can be as much as 10 days longer if the pregnant female is also nursing a litter. Litter size averages 10 to 12 young. A smaller litter size is common for a first litter or in older females. New litters should not be disturbed by owners for the first few days to minimize injury of the young or abandonment by the mother. Baby mice are weaned at about 3 weeks of age.

Female rats become sexually mature at about 65 days of age. They have an estrous cycle of approximately every 4 to 5 days throughout the year. Breeding usually occurs at night. Rats also have a fertile postpartum estrus and can be bred within 48 hours after giving birth; however, unlike mice, this usually does not occur because the male should be removed from the cage prior to the birthing process. Adult male rats are prone to injure their young.

Pregnancy lasts about 3 weeks in rats. Litter size averages 6 to 12 pups, but smaller litters are common under the same circumstances as described for mice. The female and her litter should not be disturbed for the first few days after birth. A stressed female rat may injure or destroy her young.

Medical Conditions

CHRONIC MURINE PNEUMONIA (MURINE MYCOPLASMOSIS)

Mycoplasma pulmonis is a very elusive bacterium which causes one of the most common and serious infections in rats and mice. The organism is difficult to isolate by standard laboratory culture procedures. As a result, a presumptive diagnosis is typically made based on the rodent's signs which can include sniffling, sneezing, labored breathing, squinting, red-brown tearing, nasal discharge, and a rough haircoat. If the inner ear becomes infected, a head tilt and neurological signs may develop. In addition to respiratory signs, a genital infection may occur. Manifestations of the genital form of this disease include infertility, miscarriage, and small litter size. Compromise to the respiratory tract by other bacterial or viral infections or exposure to inhaled irritants can increase the severity of mycoplasmosis. The disease runs a chronic course, which may result in death if not treated early.

Antibiotic therapy is generally initiated at the first signs of infection. Due to the chronic nature of this condition, long-term treatment with antibiotics may be necessary. Severely affected rodents may need injectable medications and extensive supportive care. In addition, secondary infections are common and sometimes lead to the use of multiple medications. While the goal of the veterinarian is to reduce the severity of signs of this disease, complete elimination of the organism is difficult.

Mycoplasma pulmonis is highly contagious. The infection is spread from rodent to rodent through ordinary daily contact. A female can pass this disease to her unborn young. Rabbits, guinea pigs, and other rodents can serve as carriers of this disease without exhibiting any clinical signs. Other mice and rats may also be carriers. It is extremely important, therefore, to restrict contact between new rodents of unknown health status until a quarantine period of 4 to 6 weeks has elapsed. Any animal exhibiting even the slightest signs of respiratory illness should remain isolated.

TYZZER'S DISEASE

Tyzzer's disease is a common infectious disease caused by a bacterium (Bacillus piliformis) which infects living cells. The disease causes high mortality in young, stressed rodents, particularly mice and gerbils. Clinical signs are nonspecific but include ruffled fur, lethargy, hunched posture, and poor appetite. Diarrhea may also be present. The disease causes pathologic changes in the heart, liver, lymph nodes, and digestive tract which can only be observed on necropsy.

Tyzzer's disease can be prevented if owners take special precautions. Strict sanitation procedures and limiting stressors greatly reduce its occurrence in colony situations. Tyzzer's disease is often carried by healthy-looking rodents. It usually affects rodents that are stressed by weaning, shipping, or adjusting to a new environment, which is why proper sanitation and reduction of stressors can prevent outbreaks.

SENDAI VIRUS

The Sendai virus causes one of the most significant and severe respiratory infections in rodents. Suckling and weanling mice are most commonly affected; however, it is unlikely for a pet mouse to become infected unless it was acquired from an infected colony. Rats, hamsters, guinea pigs, and pigs can also be infected with this virus and are often carriers.

Signs of infection of the Sendai virus are most commonly seen in nursing mice. Adult mice rarely show signs. These signs include labored breathing, chattering, rough haircoat, and weight loss. Secondary bacterial infections often cause this virus to take on a more severe form, resulting in a higher death rate. There is no specific treatment for Sendai virus. Supportive care and treatment of secondary bacterial infections may lessen the severity of signs. A vaccine is available, but it is only practical for use with large colonies of infected mice. Prevention involves selecting pet mice from a Sendai virus-free source and keeping them isolated from mice of unknown backgrounds and other susceptible rodents that may carry the disease.

SIALODACRYOADENITIS

Rats are a natural host for this highly contagious viral disease. Sialodacryoadenitis is usually self-limiting in young rats. Recently weaned mice may also be affected. The disease is spread from infected rodents through.- the air or by direct contact with respiratory secretions. Infected rodents carry and secrete the virus for about 7 days.

Signs are variable depending on the age and immune status of the infected rodent. The most serious signs are seen in rats between 2 to 4 weeks old with no maternal antibody protection. Initial symptoms include squinting, blinking, and rubbing of the eyes. Sneezing and swelling in the neck area may occur later. A final indication of this disease is swelling below or around the eyes, bulging of the eyes, production of red-brown tears, and self trauma to the eyes. Respiratory signs may be present, especially if complicated by Sendai virus or murine mycoplasmosis. The rat affected with sialodacryoadenitis usually remains active with no loss of appetite during the course of the disease.

TUMORS

Rats and mice are both very susceptible to the development of tumors. Rats over 2 years of age have an 87% chance of developing tumors. Breast tumors in both male and female rats are the most common. Because female rats have such widely distributed mammary tissue beneath the skin, it is not unusual to find tumorous lumps behind their front legs, along the sides, in the flanks, and along the underside of the body. Tumors of both the breast and mammary glands can be

removed surgically, but often recur. If not surgically removed, these masses continue to enlarge, ulcerate, and become infected. Early surgical removal allows for the best outcome with the least chance of complications or recurrence.

RED-BROWN TEARS

Rats are prone to a condition in which they secrete red tears from a gland behind their eyes. This is a normal secretion of porphyrin pigments produced by the Harderian gland. These tears are often mistaken for blood. They usually appear during stressful situations such as an illness. The eyelids, nose, and forepaws may be smeared with the pigment. When present, the underlying cause of stress should be sought and relieved.

RING TAIL

Low humidity and high temperatures may result in ring tail in young rats. Ring tail presents as constrictive bands along the tail. Other factors that have been implicated in this condition include the vascular stricture of the tail, the presence of endotoxins, and a diet high in fat. Treatment involves correcting the environmental conditions that may lead to this condition.

Glossary

Antigen - Allergy-causing substance

Endotoxin - A toxic product of bacteria which is associated with the structure of the cell, and can only be obtained by destruction of the cell

Estrus - Period in which the female rodent is fertile and acceptable to breeding

Porphyrin - Colored organic compounds which form the basis of respiratory pigments

Ulcerate - Break in the skin or mucous membrane that is often accompanied by pus

Urogenital - Pertaining to the urinary and genital organs

Vascular - Relating to or containing blood vessels

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