Mice and Rats as Pets

Male rats are sexually mature by 6–10 wk; female rats are sexually mature by 8–12 wk. The breeding life of both male and female rats is 9–12 mo. Estrous cycle length in female rats is 4–5 days, and estrus lasts 10–20 hr. Female rats ovulate ~10–20 eggs. Gestation lasts 21–23 days; pseudopregnancy from sterile matings lasts 12 days. Rats have an average litter size of 8–18 pups. Weaning takes place at ~21 days.

Both male and female mice are sexually mature by 6–8 wk and have a breeding life of 9 mo. Estrous cycle length in female mice is 4–5 days, and estrus lasts 10–20 hr. Female mice ovulate ~6–10 eggs. Gestation lasts 19–21 days; pseudopregnancy from sterile matings lasts 12 days. Mice have an average litter size of 5–12 pups. Weaning takes place at ~21 days.

Male mice and rats produce a small amount of sperm daily at puberty (eg, 40–50 days in a rat). It is not until 75–100 days (10–14 wk) in rats that optimal sperm production and reserve occur. Male rodents show a constant libido after sexual maturity; in contrast, females are receptive to copulation only during estrus. Males cannot fertilize females until 6–8 wk after reaching puberty.

The average life span of mice is 18–24 mo and of rats 18–36 mo. Restricting dietary calories without compromising overall nutrition results in increased life span. Obesity in pet rats and mice is common, and calorie-restricted pets live significantly longer lives.

The best cages are made of a material that is easy to clean and deodorize and is indestructible to rodent chewing or digging in the corners. The cage floor can be solid but should be waterproof and easy to clean. Wire mesh floors should be avoided, because rats and mice can trap their feet and especially hindlimbs in the openings, resulting in fractures and injuries.

Animal cage bedding is described based on its use (eg, contact, noncontact, and enrichment bedding) or the material from which it is made (eg, wood-based [chips, shavings, peelings, wood-wool and sawdust], paper-based [cotton and pulp fiber, recycled paper], corn [husks and cobs], cellulose, and vermiculite). The purpose of bedding is to keep animals dry and clean. Pet owners generally choose bedding based on cost and availability, whereas laboratory veterinarians choose bedding based on cost and water-holding capacity. Some pet bedding contains lemon and chlorophyll to give it a pleasant scent. This type of bedding irritates pet rodents, and the coloring agents can stain the coat of white rats or mice. Traditionally, owners prefer paper and softwood chips, such as pine or aspen, to straw because it requires fewer bedding changes. Cedar and other wood chip shavings reduce ectoparasite problems and have a pleasant scent. However, such bedding is not recommended, because it emits toxic aromatic hydrocarbons that increase the incidence of cancer in animals and cause mouse and rat pup mortality.

Owners must combine frequent bedding changes with good husbandry such as regular cage cleaning, low animal density, and low environmental temperature and humidity. This will reduce toxic or odor-causing gases such as ammonia from building up, because urease-positive bacteria in the feces act to break down urea in the urine. Aquariums are not suitable cages for rats and mice because of inadequate air circulation and subsequent buildup of ammonia. Environmental temperature and relative humidity can depend on husbandry and housing design and can differ considerably between the cage and room. Factors that contribute to variation in temperature and humidity include cage material and construction, number of animals per cage, frequency of bedding changes, and bedding type.

Size and manipulability of bedding material are the main determinants of mice and rats’ choices. Mice and rats avoid bedding consisting of small particles, whereas they prefer bedding consisting of large, fibrous particles. When exposed to different types of nesting materials such as paper strips, cornhusks, sawdust, and wood materials (shavings, peelings, and chips), rats choose long strips of soft paper. Rats also select opaque or semiopaque nest boxes to transparent nest boxes. Mice show no preference between paper and wood-derived materials but show a clear preference for materials that they can manipulate such as paper tissues, string, and wood materials (shavings, peelings, and chips). Many mice will combine two preferred nesting materials to make complex nests. Mice typically dislike wire-bottomed cages; however, both male and female mice will spend more time in a wire-bottomed cage with nesting material, despite the grid floor that they usually avoid.

In environmentally enriched cages with hollow tubes, mice have no overwhelming group preference for shape, opacity, or openness of tubes and prefer to sleep in sawdust when it is available. Mice will sleep in hollow tubes only after the sawdust is removed; they use short, wide tubes more frequently than long or short narrow tubes.

Mice and rats are optimally maintained at temperatures of 64°–79°F (18°–26°C). Relative humidity should also be controlled but not nearly as narrowly as temperature. An acceptable range of relative humidity is 30%–70%. Both temperature and humidity regulation are important to prevent ringtail and exacerbation of respiratory disease. Avascular necrosis of the tail, or ringtail, occurs primarily in young rats in low-humidity environments. Excess heat and humidity cause heat stroke and indirectly cause decompensation of chronic respiratory disease, resulting in death.

Mouse and rat owners should check water bottles daily. If mice are deprived of water for only a short time and experience dramatic fluctuations in the surrounding temperature, especially >99°F (37°C), they die. In contrast, healthy rats tolerate water deprivation and temperature fluctuations better. They can live for 7 days without water, in temperatures between 64°–79°F (18°–26°C), although they may lose up to 65% of their body weight.

Minimal space allocations of 23 sq in. (58 sq cm) are recommended for individual rats weighing ≤200 g, and 60 sq in. (152 sq cm) for rats weighing ≥ 500 g. For mice, minimal space allocations of 15 sq in (38 sq cm) for individual mice weighing >25 g are recommended. These are small surface areas, and veterinarians should advocate that owners provide larger space for their pets.

Most rodent enrichment studies focus on modifying the area inside the enclosure rather than the size of an enclosure. However, rats prefer larger cages, and experimental studies suggest rats should have a larger space that allows sharing of the environment with up to four other rats.

The minimum recommended height of cages should take into account typical postures of rats and mice. This includes the animal standing fully erect on its hindlegs, and vertical movements such as stretching upward and possibly climbing. Consequently, minimal cage heights of 12–15 in. (30–38 cm) for rats and 7–8 in. (18–20 cm) for mice are recommended.

Environmental enrichment is important for both mice and rats. For example, suspended cloth hammocks are popular with rats, and suspended (plastic or stainless steel) shower hooks fitted into one another can make a swinging chain. Rats will use more enrichment devices than mice, but they usually stop using the devices after 3–4 days. Rotation of enrichment toys and introduction of novel devices excite their curiosity. Food treats are also valuable enrichment items. These can range from simple, inexpensive treats such as a daily piece of a breakfast cereal to formulated nutritious or calorie-free treats. Rats also love chocolate, and it is not toxic when fed in small amounts. Pet rodents accustomed to handling will eat food treats out of the owner’s hand. This daily routine can allow owners to detect subtle changes in the pet’s behavior. Sick rodents effectively hide signs of disease. Sick rats do not show the same interest in their daily treat, and this can alert the owner early to disease when it is still treatable and/or reversible.

Housing male and female rodents together will result in mating and subsequent litters. Mice and rats experience postparturient estrus, and fertilization can occur. However, implantation of the resulting blastocysts is delayed during lactation and occurs at weaning, ensuring that the next litter is not born until the earlier one has been weaned. Female mice become sexually mature at 6–8 wk of age, and female rats are sexually mature by 10 wk of age. Unless opposite-sex rodents housed together are separated or neutered, having a new litter every 3–5 wk is possible.

Rats and mice are omnivores and will eat food of both plant and animal origin. In the wild, rats and mice will eat a wide variety of seeds, grains, and other plant material as well as invertebrates, small vertebrates, and carrion. Their ability to scavenge partly accounts for their successful colonization of diverse geographic regions.

Formulated pelleted diets for laboratory rodents are convenient and nutritionally balanced diets for early life and reproduction. However, laboratory rodent diets are relatively high in fat and low in fiber, and when provided ad libitum, they cause obesity. Consequently, the amount of pelleted diet owners provide daily should be limited. Diets formulated specifically for pet rodents are now commercially available. Owners should supplement their pet’s diet with feeds high in fiber such as vegetables, limited amounts of fruit, and occasional treats.

Coprophagy is a normal behavior in rats and mice. It is an inherent behavior, because it is seen in germ-free rodents purposely bred for research. Unlike rabbits, which eat cecal feces from their anus, mice and rats eat fecal pellets on the floor of their caging. The amount of feces eaten varies between rodents, their age and physiologic status (eg, coprophagy increases in pregnancy), and the diet fed. On a nutritionally complete diet, rats will eat ~10% of their feces. When rats are housed together, they ingest each other’s feces. The unique odor of a colony is due in part to ingestion of feces, and the group scent enables distinction between members and nonmembers. Mice engage in coprophagy ~6 times/day. Growing mice show vigorous coprophagous activity, eating 13 pellets/day. However, it gradually decreases to 1½ pellets at 2 yr of age.

Owners should house different species of rodents separately to prevent interspecies disease transmission. For example, rats carry Streptobacillus moniliformis (see Global Zoonoses ^a), a cause of fatal septicemia in mice, as part of the normal nasopharyngeal bacterial flora. Rodents of the same species should be housed in such a way to protect vulnerable animals from more aggressive members of their group. This includes separating young animals from older ones. Female rodents are generally compatible when housed in the same cage, unless one female has lived much of its life alone. Male rats are generally compatible, especially if raised together. However, owners should never house strange male rats in the same cage together because they will fight. Male mice generally fight if housed together, unless they are littermates raised together without females present. Male mice are best housed singly or with female mice.

Rats are social animals, and grooming is a socially affiliative behavior. Singly housed rats may develop isolated-rat stress syndrome if left alone without human contact and environmental enrichment. Isolation-reared rats will experience fighting, physical injuries, and weight loss when placed into a colony of socially experienced rats. Young rats should be housed together to develop social affiliation. Singly housed rats should not be put together. Owners should be advised to encourage enrichment and human contact with pet rats.

Observing the condition of the rodent’s living quarters provides useful information. Information obtained from a physical examination is often limited because of the mouse or rat’s size. Activity of the rat, condition of grooming, and the presence of a head tilt or discharges in the cage should be noted. If dyspnea or depression is seen, extreme care should be used when handling the animal, because it is probably very sick and could die from the stress of a physical examination.

Pet rodents that have been frequently and gently handled usually require only minimal restraint. Less cooperative animals need to be more firmly restrained, and use of a towel or even heavy gloves may be required. Weight measurement is essential to calculate appropriate dosages of medications and provides an opportunity to gauge the rodent’s temperament before beginning the actual physical examination.

The head, ears, eyes, and nose should be examined for discharges, and the oral cavity for dentition. Lymph nodes and glands of the head can be observed for size and palpated for consistency. Assessment of the head is probably the most time-consuming part of the examination. The abdomen can be palpated for consistency and the presence of unusual masses. Animals should not be squeezed too vigorously, because overzealous palpation can result in visceral rupture. The anogenital region should be examined for discharges and staining of the fur or skin. When a rodent is picked up, it generally urinates and defecates. A dipstick should be ready to perform an immediate urinalysis; feces can be caught in a small tube and examined later. The limbs should be palpated for tenderness or fractures, with special attention given to the paws, noting the length of the nails and condition of the footpads.

Respirations and heart rate in mice and rats are rapid and, therefore, difficult to measure. Instead, signs of dyspnea should be noted. Some respiratory infections, such as mycoplasmosis, are clinically silent. These diseases can be better heard than seen; abnormal sounds called “snuffling” in rats and “chattering” in mice are noticeable without a stethoscope.

The most common subcutaneous tumor in rats is fibroadenoma of the mammary glands. The distribution of the mammary tissue is extensive, and the tumors can develop anywhere from the neck to inguinal region. Tumors can reach 8–10 cm in diameter and are seen in both males and females. The surgical technique for tumor removal is straightforward, and survival after mastectomy has been reported to be good if the tumor is benign. The prevalence of mammary tumors, as well as that of pituitary tumors, is significantly lower in ovariectomized rats than in sexually intact rats. However, the recurrence of fibroadenomas is common in uninvolved mammary tissue, and often several surgeries are needed.

In contrast, mammary tumors in mice are nearly always malignant and often are not amenable to surgical removal. The most common spontaneous tumors associated with the skin are mammary adenocarcinomas, followed by fibrosarcomas. The incidence of mammary tumors varies according to the mouse strain and the presence or absence of mouse mammary tumor viruses; the incidence is as high as 70% in some strains. In wild and outbred mice, the incidence of fibrosarcomas is 1%–6%.

Subcutaneous tumors are nearly always malignant and often have ulcerated by the time a diagnosis is made. Tumors can be treated by surgical excision, but the chance of recurrence is high and the prognosis is poor.

As rats have become popular as pets, children now account for >50% of the cases of rat bite fever (RBF) in the USA; 14% of RBF cases in children involved exposure to a rat at school. RBF manifests as a bacteremia and septicemia characterized by fever, chills, myalgia, arthralgia, headache, and vomiting. A petechial rash develops over the extremities, in particular the palms and soles, but sometimes is present all over the body. The incubation period is 3–10 days (average 5 days). Infants and children may experience severe diarrhea resulting in weight loss. Mortality occurs in 7%–13% of untreated patients. After infection, a polyarthritis develops in 50%–70% of patients. Prognosis is good with treatment with parenteral penicillin early in the disease. Fatalities are associated with late reporting or late recognition of the disease. The nonspecific initial presentation combined with difficulties in culturing Streptococcus moniliformis produce a significant risk of delay or failure in diagnosis. Rats are not recommended as pets for children. Adults with pet rats should practice regular hand-washing and avoid hand-to-mouth contact when handling rats or cleaning rat cages.

There are sporadic but persistent reports of cases of Ornithonyssus bacoti dermatitis from pet rodents that manifests as an erythematous papular rash. It often excoriates because of intense pruritus. Occasionally, vesicles, urticarial plaques, diffuse erythema, or hemorrhage are seen. Affected areas of the skin are those usually covered by clothing (such as arms and trunk), with the face and webs of fingers usually spared. There is considerable evidence that O bacoti carry and have the potential to transmit several human pathogens. Experiments have shown that O bacoti transmits Rickettsia akari (rickettsialpox), Yersinia pestis (plague), Coxsackie virus, Francisella tularensis (tularemia), and Trypanosoma cruzi (Chagas' disease). Researchers have also documented O bacoti specimens with Coxiella burnetii (Q fever), hantavirus, Borrelia spp (Lyme disease), Bartonella spp, and Rickettsia spp.

Mice are inapparent carriers of dermatophytes, and pet mice are a zoonotic risk for their owners, especially children.