Rats are more commonly kept as pets than mice. Both species are used extensively in research. Although a great deal of information has been accumulated on wild and laboratory rats and mice, very little of this information pertains to these rodents as pets.
The prevalence and type of diseases in mice and rats seen in clinical practice are quite different from those seen in a research setting. The diagnosis and treatment of pet mice and rats involves evaluation and care of an individual animal from a household, not the health management of rodents from a research colony. Most problems in mice and rats are dermatopathies, respiratory infections, and neoplasia.
Biology
Male rats are sexually mature by 6–10 weeks; female rats are sexually mature by 8–12 weeks. The breeding life of both male and female rats is 9–12 months. Estrous cycle length in female rats is 4–5 days, and estrus lasts 10–20 hours. 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 weeks and have a breeding life of 9 months. Estrous cycle length in female mice is 4–5 days, and estrus lasts 10–20 hours. 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 weeks) 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 weeks after reaching puberty.
The average life span of mice is 18–24 months and of rats 18–36 months. 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.
Husbandry
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 semi-opaque 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 inches (58 sq cm) are recommended for individual rats weighing ≤200 g, and 60 sq inches (152 sq cm) for rats weighing ≥ 500 g. For mice, minimal space allocations of 15 sq inches (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 inches (30–38 cm) for rats and 7–8 inches (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, which can be fed in small amounts without toxicity. 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. Unless opposite-sex rodents housed together are separated or neutered, having a new litter every 3–5 weeks 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 years of age.
Owners should house different species of rodents separately to prevent interspecies disease transmission. For example, rats carry Streptobacillus moniliformis, 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.
Physical Examination
Observing the condition of the living quarters of the mouse or rat provides useful information. Information obtained from a physical examination is often limited because of the mouse or rat’s size. Activity level, coat condition, general posture at rest, and any blood or bodily discharges in the cage should be noted. If dyspnea or depression is seen, extreme care should be used when handling the animal, because the stress of a physical examination could result in death of a severely debilitated animal.
Dr. Jennifer Frohlich
Pet rats and mice that have been frequently and gently handled usually require only minimal restraint. Less cooperative animals need to be more firmly restrained. A common method for restraint of mice is scruffing, in which the restrainer grasps the scruff of the mouse between the thumb and forefinger while maintaining a grip on the base of the tail to secure the animal. A useful restraint method for handling rats for physical examination is a technique sometimes known as the "baseball hold" or the "claw hold" (see image). 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 before handling. 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.
Courtesy of Dr. Jennifer Frohlich.
Blood collection can be difficult in small rodents, especially in mice because of their small body and blood vessel size. In both rats and mice, the saphenous vein or lateral tail vein can be used to collect small amounts of blood for blood smears, blood glucose measurement, etc. To collect large amounts of blood, the cranial vena cava can be used; however, animals must be sedated or anesthetized to accomplish this safely. Other methods to collect larger samples of blood in mice, such as the facial vein puncture or the retrobulbar, are useful but aesthetically unappealing, making them difficult for use in pet mice.
Courtesy of Dr. Jennifer Frohlich.
Courtesy of Dr. Jennifer Frohlich.
Infectious Diseases
Bacterial Infections in Rats and Mice
Rats are the natural host of the bacterium Streptobacillus moniliformis. It is a commensal organism of the nasopharyngeal flora and does not cause disease in rats. However, people bitten by rats may become infected with S moniliformis and develop rat bite fever, a potentially fatal disease. Mice housed with rats will become infected with S moniliformis and develop fatal septicemia characterized by polyarthritis, cervical lymphadenitis, pneumonia, and splenomegaly.
Respiratory disease caused by infectious agents is the most common health problem in rats. Three major respiratory pathogens cause overt clinical disease: Mycoplasma pulmonis, Streptococcus pneumoniae, and Corynebacterium kutscheri. Other organisms such as Sendai virus (a paramyxovirus), pneumonia virus of mice (a paramyxovirus), rat respiratory virus (a hantavirus), cilia-associated respiratory (CAR) bacillus, and Haemophilus species are minor respiratory pathogens that rarely cause overt clinical disease by themselves. However, the minor respiratory pathogens interact synergistically as copathogens with the major respiratory pathogens to produce two major clinical syndromes: chronic respiratory disease (CRD) and bacterial pneumonia.
CRD is the best-understood multifactorial respiratory infection in rats. M pulmonis is the major component of CRD, and the disease is also known as murine respiratory mycoplasmosis. Rats with CRD rarely live >2 years. Clinical signs are highly variable. Initial infection commonly occurs without any clinical signs; early signs involve both the upper and lower respiratory tracts and may include snuffling, nasal discharge, polypnea, weight loss, hunched posture, ruffled coat, head tilt, and red tears. Dyspnea, the primary presenting complaint, is caused by ciliostasis, subsequent buildup of lysozyme-rich inflammatory exudate in airways, bronchiectasis, and bronchiolectasis from inflammatory damage to bronchiolar membranes. Chronic disease often includes middle ear infection (via the eustachian tube).
Treatment with antibiotics may alleviate clinical signs but does not eliminate the infection. Enrofloxacin (10 mg/kg) and doxycycline (5–10 mg/kg) given orally twice a day for 14 days often alleviates severe clinical signs. Bronchodilators are the primary treatment for the inflammatory aspect of CRD and can either be provided orally (theophylline 10 mg/kg, twice a day) or as an aerosol (albuterol, salmeterol). Additional treatments, such as daily nebulization therapy with 7% hypertonic saline to break down the mucus biofilm in respiratory passages, and reducing ammonia levels in cages by daily removal of dirty bedding, will ameliorate the disease. For chronic cases, additional supportive therapy with analgesics or supplemental nutrition may also be required. Chronic low dose doxycycline (5–10 mg/kg/day, PO) helps to prevent acute relapses. The doxycycline is both bactericidal for mycoplasma organisms and has a marked immunomodulatory effect on the chronic airway inflammation.
The most important aspect of CRD for clinicians is that respiratory mycoplasmosis varies greatly in disease expression because of environmental, host, and organism factors that influence the host-pathogen relationship. Examples of such factors include intracage ammonia levels; concurrent infection with Sendai virus, coronavirus (sialodacryoadenitis virus), pneumonia virus of mice, rat respiratory virus, and/or CAR bacillus; the genetic susceptibility of the host; the virulence of the Mycoplasma strain; and vitamin A or E deficiency. Although CRD is rarely seen in laboratory rats, most pet rats have CRD to some degree. One survey of pet ratteries in the northwest USA showed virtually all (95%) were positive for CAR bacillus and M pulmonis, and approximately half were positive for other viral respiratory agents.
Bacterial pneumonia is nearly always caused by S pneumoniae, but seldom in the absence of some combination involving M pulmonis, Sendai virus, or CAR bacillus. Infection with C kutscheri also results in pneumonia but only in conjunction with debilitation or immunosuppression. In pet rats, immunosuppression can result from diabetes, neoplasia, or dietary deficiencies. C kutscheri pneumonia is rare in pet rats. Pneumonia caused by S pneumoniae can be of sudden onset. Young rats are more severely affected than older ones, and often the only sign they exhibit is sudden death. Mature rats may demonstrate dyspnea, snuffling, and abdominal breathing. A purulent exudate may be seen around the nares and on the front paws from wiping of the nostrils. A tentative diagnosis is based on identification of numerous gram-positive diplococci on a Gram stain of the exudate or in a sample submitted for cytologic examination. Severe bacteremia is an important consequence of advanced disease and results in multiorgan abscesses and infarction. Treatment is amoxicillin/clavulanic acid (13.75 mg/kg, PO, twice a day) or beta-lactamase–resistant penicillins such as cloxacillin, oxacillin, and dicloxacillin, which can all be administered orally or parenterally.
Ulcerative dermatitis caused by Staphylococcus aureus infection results from self-trauma associated with fur mite infestation or, more commonly, from scratching of the skin over an inflamed salivary gland. Rats have a remarkable ability to resist infection with S aureus. Treatment consists of clipping the toenails of the hindpaws, cleaning the ulcerated skin, and applying a topical antibiotic. Systemic treatment is rarely necessary.
The two most common causes of clinical respiratory disease in mice are Sendai virus and M pulmonis infection. Sendai virus is associated with an acute respiratory infection in which mice display chattering and mild respiratory distress. Neonates and weanlings may die. Adults generally recover within 2 months. When the disease expression exceeds this pattern, the cause is most likely concurrent mycoplasmal infection. M pulmonis is the cause of chronic pneumonia, suppurative rhinitis, and occasionally otitis media. Chattering and dyspnea are caused by accumulations of purulent exudate in inflamed and thickened nasal passages. Survivors often develop chronic bronchopneumonia, bronchiectasis, and pulmonary abscesses. Antibiotic therapy may alleviate clinical signs but does not eliminate the infection.
Viral Infections in Pet Rats and Mice
Viral diseases of mice and rats are common. However, most diseases are subclinical and important only in laboratory animals in which they have the potential to have a significant effect on research.
Courtesy of Dr. Dean H. Percy.
Courtesy of Dr. Dean H. Percy.
Sialodacryoadenitis virus, a coronavirus, causes inflammation and edema of the cervical salivary glands in rats. Owners of infected rats often describe their pets as having mumps. Sialodacryoadenitis virus infection is highly contagious. It initially causes rhinitis followed by epithelial necrosis and inflammatory swelling of the salivary and lacrimal glands. Cervical lymph nodes also become enlarged. There is no treatment for this disease. Glandular healing follows within 7–10 days, and clinical signs subside within 30 days, with minimal residual lesions remaining. During acute inflammation, affected rats are at high risk of anesthesia-related mortality because of the decreased diameter of the upper respiratory tract lumen. Ocular lesions such as conjunctivitis, keratitis, corneal ulcers, synechia, and hyphema can develop secondary to lacrimal dysfunction. The eye lesions usually resolve but occasionally progress to chronic keratitis and megaglobus.
Parasitic Infections in Rats and Mice
Protozoa
Endoparasites are relatively common in mice. However, only two parasites regularly encountered in the digestive tract, the protozoan parasites Spironucleus muris and Giardia muris, are considered pathogenic, even though they are not associated with clinical signs in immunocompetent hosts. Diagnosis is based on demonstration of characteristic trophozoites in wet mounts of fresh intestinal contents or feces. Treatment is metronidazole (10–40 mg/kg/day, PO, or 0.04%–0.1% drinking water solution for 14 days), but it does not completely eliminate the infection.
Nematodes
Courtesy of RADIL, University of Missouri.
Pinworms are ubiquitous and considered nonpathogenic in mice. Two are commonly encountered in mice: Syphacia obvelata and Aspicularis tetraptera. Rats may become infested with these two species, as well as the rat pinworm Syphacia muris. Often, the only indication of pinworm infestation is rectal prolapse due to straining. To establish a diagnosis of S obvelata or S muris infestation, a clear cellophane tape impression of the perianal skin can be made. Adult Syphacia females deposit ova around the anus. A tetraptera does not deposit its ova in this area, and fecal smear or flotation is required to confirm a diagnosis. Ivermectin (2 mg/kg, PO, given twice at a 10-day interval) or fenbendazole (20 mg/kg, PO, twice a day for 5 days) eliminates pinworms from mice.
Mites
Courtesy of Dr. Louise Bauck.
Most infectious causes of alopecia and dermatitis in mice are associated with fur mites. Generalized thinning of the hair, especially on difficult-to-groom areas such as the head and trunk, is seen. The coat often has a greasy appearance and, in cases of heavy infestation, noticeable pruritus and self-inflicted dermal ulceration may occur. Three mites are commonly seen: Myobia musculi, Myocoptes musculinus, and Radfordia affinis. M musculi is the most clinically significant mouse mite. Infestations are usually caused by more than one species. Mites are spread by direct contact with infected mice or infested bedding. Diagnosis is based on identification of adult mites, nymphs, or eggs on hair shafts with the use of a hand lens or stereoscopic microscope. Adults and nymphs appear pearly white and elongate; eggs are oval and can be seen attached to the base of hairs or inside mature females. Mite infestations are treated with ivermectin (0.2 mg/kg, SC or PO, twice at 10-day intervals). Alternatively, a few drops of ivermectin solution (diluted to 1:100 in equal parts of water and propylene glycol for three treatments) can be placed on the mouse’s head to allow spread by grooming and ingestion.
Ectoparasitic infestation is less common in rats than in mice. Occasionally, the fur mite Radfordia ensifera is seen. Although R ensifera infestation produces few ill effects, heavy infestation may lead to self-trauma and ulcerative dermatitis.
Ornithonyssus bacoti, the tropical rat mite or red mite, is a bloodsucking parasitic mite primarily found in wild rats such as the brown rat (Rattus norvegicus) or the black roof rat (Rattus rattus). It has a wide host range and occasionally infects pet hamsters, gerbils, rats, and mice living in very old buildings or when building construction or renovations disturb colonies of wild rodents that had been on the premises and acting as hosts. O. bacoti can survive for long periods in the environment and travel considerable distances in search of new hosts. If it does not find a suitable rodent host, it will feed on people. Its common name is a misnomer, because it is found worldwide in tropical and temperate climates.
Courtesy of Dr. Jennifer Frohlich.
Mites appear white before feeding and become red-brown after engorgement. Heavy infestations on pet rodents resemble fine sawdust within the fur. O bacoti typically does not cause clinical signs in pet rodents, but heavy infestations cause anemia, debility, weakness, pruritus, and death in small rodents. As well as causing discomfort, the mite is a vector for the rodent filarial nematode Litomosoides carinii and a potential vector of several human pathogens. Most infestations occur in late spring or early summer when fledglings are leaving the nest and the mites are searching for another food source. Treatment of the environment as well as the animal is essential because O bacoti lives primarily in the bedding and only attaches to the host for feeding. Host animals should be treated with bathing and/or topical selamectin (15 mg/kg). The pet's cage can be treated with insecticides that are safe for rodents such as permethrin-impregnated cotton balls (7.4% placed inside the cage weekly for 6 weeks), fipronil spray, or synthetic pyrethroids. The last step for eradication of O bacoti infestation is to eliminate the source of O bacoti by using appropriate pest management in the home.
Fungal Infections in Rats and Mice
Dermatophytosis is uncommon in pet mice and rats. It is caused by Trichophyton mentagrophytes. Lesions, when present, are most common on the face, head, neck, and tail. The lesions have a scurfy appearance, with patchy areas of alopecia and variable degrees of erythema and crusting. Pruritus is usually minimal to absent, and the lesions do not fluoresce under a Wood's lamp. T mentagrophytes can be isolated from the fur of clinically normal mice but is rare in rats.
Neoplasia
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.
Neoplasia of lymphoid and hematopoietic tissues is estimated to have a prevalence of 1%–2% in mice. These tumors can present as lymphoma or leukemia; the most common presentation is lymphocytic leukemia originating in the thymus. Mice are also unusual in that pulmonary tumors occur with relatively high frequency and are mostly primary pulmonary tumors, not metastatic from another location.
Miscellaneous and Iatrogenic Disorders
Dental Overgrowth in Rats and Mice
Dental problems are commonly seen in pet mice and rats because of their continually erupting teeth. Overgrown incisors are seen most frequently in rats and mice, in contrast with cheek teeth malocclusion seen in guinea pigs and chinchillas. Overgrowth is easily treated with a high-speed drill that cuts through the overgrown incisors without splitting or splintering them, leaving a clean, smooth surface. Cutting the teeth with rongeurs does not produce good longterm results, and problems may arise. The incisor may fracture longitudinally; the fracture may reach the apex and cause the animal discomfort. Bacteria can enter the fractured tooth, track down to the apex, and cause an apical abscess. Extraction of the incisors is an alternative to trimming; however, this procedure is difficult because of the incisors' long roots.
Age-related Disorders in Rats and Mice
Courtesy of Dr. Louise Bauck.
Chronic progressive nephrosis is a common age-related disease in rats. The kidneys are enlarged, pale, and have a pitted, mottled surface that often contains pinpoint cysts. Lesions consist of a progressive glomerulosclerosis and widespread tubulointerstitial disease, primarily involving the proximal convoluted tubule. Proteinuria often is >10 mg/day. The disease occurs earlier and is of greater severity in males than in females. Dietary factors appear to have an important role in the progression of kidney disease. Restricting calories, feeding low-protein diets (4%–7%), and limiting the source of dietary protein reduce the incidence and severity of the disease. Treatment is supportive.
Avascular necrosis of the tail, or ringtail, is seen primarily in young rats, and occasionally in young mice, kept in low-humidity environments. If ringtail is diagnosed, treatment involves amputation of the tail below the necrotic annular constriction.
Skin Disease in Mice
Most of the diseases seen in pet mice are associated with the skin and represent >25% of all cases. Behavioral disorders, husbandry-related problems, and microbiologic/parasitic infections are relatively straightforward to diagnose and treat. However, many skin diseases characterized by chronic or ulcerated skin (often secondarily colonized by bacteria) are diagnosed as idiopathic. This group is commonly unresponsive to treatment, topical or systemic, and affected mice are often euthanized.
In mice, idiopathic skin disease is characterized by ulcerative dermatitis with pruritus that is negative for primary ectoparasitic, bacterial, or mycotic infections. Histopathologic examination and immunofluorescent microscopy of selected inbred strains of mice have revealed an underlying vasculitis attributed to immune complex deposition on dermal vessels. Dietary factors and dysregulated fatty acid metabolism have been implicated in the development of the ulcerative dermatitis in these mice. This common disease of mice of a C57BL6 background is caused by an underlying immune-mediated vasculitis, and the severity appears to be modulated by dietary fat and vitamin E content.
Various topical treatment regimens have been tried: 0.2% cyclosporine in 2% lidocaine gel, neutral buffered 0.015% hypochlorous acid, triple antibiotic ointment, combination antibiotic and steroid ointment, and dilute 0.005% sodium hypochlorite solution, to list a few. Additional systemic therapies include dietary vitamin E supplementation, ibuprofen, and maropitant citrate. None of the aforementioned treatments have resulted in a 100% success rate. Topical treatment should be combined with regular trimming of the toenails, especially those of the hindfeet, to prevent further damage to the ulcerated skin.
Barbering and fighting are manifestations of social dominance, a form of behavior relating to the social rank and dominance status of an individual mouse in a group. Barbering is a unique condition seen in group-housed mice in which the dominant mouse nibbles off the whiskers and hair around the muzzle, eyes, or other body parts of cagemates. There are no other lesions, and only one mouse, the dominant individual, retains all of its fur. Removal of the dominant mouse stops barbering; however, another mouse may assume the dominant role. Barbering is often seen in female mice caged together. Male mice, except littermates raised together from birth, are more likely to fight, often very savagely, and inflict severe bite wounds on one another, especially over the rump, tail, and shoulders.
Mechanical abrasion resulting from self-trauma on cage equipment is a form of husbandry-related alopecia. Small patches of alopecia appear on the lateral surfaces of the muzzle. They result from chafing on metal feeders, poorly constructed watering device openings, and metal cage tops. Unlike barbering, dermatitis may also be associated with alopecia. Treatment consists of replacing the poorly constructed equipment. Individually housed mice can display aberrant stereotypic behavior such as polydipsia and bar chewing that results in mechanical abrasion and alopecia. In these cases with one mouse, replacing the cage equipment does not help. Instead, environmental enrichment such as running wheels or hollow tubes should be provided. Nursing mice often have ventral abdominal and thoracic alopecia; this is normal and is nearly always associated with extensive distribution of mammary glands.
Sometimes a pet mouse presents with clinical signs of mite infestation but no evidence of mites or known history of recent exposure to other animals. Biopsy samples may be useful in these cases to distinguish active acariasis from dermal hypersensitivity to mites or other allergens such as timber chip bedding. Dermal hypersensitivity is well described in certain inbred strains of mice and is characterized by severe pruritus, the presence of fine dandruff all over the body, and occasionally ulcerative dermatitis.
Skin swellings in mice are usually tumors or abscesses. Needle biopsy often reveals the nature of the contents and allows diagnosis. Three opportunistic pathogens, Staphylococcus aureus, Pasteurella pneumotropica, and Streptococcus pyogenes, are isolated frequently and can cause abscesses in other organs. Antibiotic therapy with penicillins or cephalosporins, concurrent with drainage and debridement of the abscess, is effective.
Chromodacryorrhea in Rats
Courtesy of Dr. Dean H. Percy.
The Harderian glands of rats are located behind the eyes and secrete porphyrins that are increased in response to stress and disease, coloring the tears red. When porphyrin-enriched tear fluid dries around the eyes and external nares, it resembles crusts of blood. Owners often describe their rats as hemorrhaging from the eyes and nose. The porphyrins can be readily differentiated from blood with a Wood’s lamp, because they fluoresce under ultraviolet light. Although chromodacryorrhea is not pathologic, it is a consequence of acute-onset stress such as that caused by pain, illness, or restraint. It usually indicates a chronic underlying disease.
Zoonotic Risk
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 can result in 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.
