Integumentary System and Mammary Glands

Nearly all problems seen in pet mice are associated with the skin. A survey from a large diagnostic laboratory housing research animals indicated that skin disease in mice represents 25% of all diagnostic problem-solving cases (for all species) submitted.⁵⁵ We categorize four groups of skin problems in mice: behavioral disorders, husbandry-related problems, microbiologic and parasitic infections, and idiopathic conditions. Behavioral, husbandry, and infectious causes of skin disease 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 topical or systemic treatment, and affected individuals are often euthanatized. Most damage to the skin is done by toenails as the rodent scratches itself. It is difficult to prevent a rodent from scratching. The nails can be trimmed or filed to remove sharp ends, but attempts to fit the rodent with a bandage or protective wrap are often pointless, as these animals are adept at removing all bandages. Restraint collars can be used, but rodents are not able to eat easily with a collar in place and it will not be effective unless the underlying cause is detected and treated simultaneously.

Mice exhibit well-studied social and sexual behaviors. Social dominance, a form of behavior relating to the social rank and dominance status of an individual mouse in a group, is manifested as barbering and fighting. Barbering is commonly a condition seen in group-housed mice, where the dominant mouse nibbles off the whiskers and hair around the muzzles and eyes of cage mates. No other lesions are present, and only one mouse (the dominant one) retains all of its fur. Removal of the dominant mouse stops barbering; frequently, however, another mouse then assumes the dominant role. Barbering occurs during acts of mutual grooming in which one member of a mouse pair grasps individual whiskers or hairs with its incisors and plucks them out. Although plucking appears painful, recipients are passive in accepting barbering and even pursue conspecifics for further grooming.⁸⁵ Barbering may also be seen associated with sexual overgrooming as a form of stress-evoked behavior, and lactating mice may display “maternal” barbering, produced in the process of suckling pups, in which alopecia is seen from tail to chin.⁴⁷ Although rare in mice, consider infection with Trichophyton mentagrophytes in cases of alopecia that involve face, head, neck, or tail in all or just a few mice.²² Barbering has a genetic-based behavioral background, as aggressive inbred strains of mice do not use barbering in their behavior.⁴⁷ In pet mice, 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, resulting from chaffing on metal feeders, poorly constructed watering device openings, and metal cage tops. Unlike barbering, dermatitis may also be associated with the alopecic area. Treatment consists of replacing the poorly constructed equipment with nonabrading equipment. Individually housed mice can display aberrant stereotypic behaviors such as polydipsia and bar chewing, which result in mechanical abrasion and alopecia. In these cases, environmental enrichment must be provided. Food items such as Lafeber parrot Nutriberries, Avicakes or Nutriforage (www.lafebervet.com) and toys such as running wheels or hollow tubes are helpful.

Most infectious causes of alopecia and dermatitis are associated with fur mites. Ectoparasites are common in mice purchased from pet stores.⁷⁹ In affected animals, the hair is generally thin, especially on difficult-to-groom areas such as the head and trunk. The coat often has a greasy appearance; 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. The most clinically significant mouse mite is M. musculi. 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 the identifying adult mites, nymphs, or eggs on hair shafts with the use of a hand lens or a stereoscopic microscope. Adults and nymphs appear pearly white and elongated (being about twice as long as they are wide); eggs are oval and seen attached to the base of hairs or inside mature females.

Treat mite infestations with avermectins (e.g., ivermectin, selamectin) or milbemycins (moxidectin). Ivermectin (0.2 mg/kg SC or PO) twice at 10-day intervals is effective. Selamectin (10-12.5 mg/kg topical)³⁵ or moxidectin (0.5 mg/kg topical⁷⁸ or 2 mg/kg PO⁷⁷) administered twice at an interval of 10 to 15 days are also effective (see Table 27-2).

Sometimes an owner presents a single pet mouse negative for primary ectoparasitic, bacterial, or mycotic infections with severe pruritus characterized by self-mutilation, dermal ulceration, necrosis, and fibrosis. Idiopathic ulcerative dermatitis is a well-recognized disease in black laboratory mice on a C57BL strain background with a characteristic distribution on the thorax and head. The cause is an underlying vasculitis attributed to immune complex deposition on dermal vessels.⁴⁸ Dietary factors and dysregulated fatty acid metabolism have been implicated in the development of the disease and the severity appears to be modulated by dietary fat and vitamin E content. Gavaging affected mice with 0.1 mL per day of liquid from an essential fatty acid supplement containing omega-3 fatty acids was associated with regressed lesions and resolved pruritus in a small sample of affected mice.⁶⁰ We have obtained good treatment results using a generic omega-3 fatty acid supplement at 0.1 to 0.2 mL PO q24h. The ulcers may heal with fibrosis and resulting skin contracture or progress to a Staphylococcus xylosus secondary bacterial infection.^48,100

Progressive necrosing dermatitis of the pinna in mice, similar to idiopathic ulcerative dermatitis, may also be seen (Fig. 27-1).⁹⁴ It occurs in outbred mice, and there is no strain background association. Initially a lesion on the dorsum of the pinna, resembling an engorged blood vessel or slight erythema, oozes serum and peripheral necrosis begins. Several days later the necrotic area sloughs and the pinna is left notched. In severe cases, the site becomes secondarily infected, the lesion becomes pruritic, and the mouse self-mutilates from the ear to the neck and over the shoulders. Treat these mice topically twice daily with of 0.2% cyclosporine in 2% lidocaine gel supplemented with 50 μg/mL gentamicin.

Fig. 27-1 Progressive necrosing dermatitis of the pinna in a mouse. The intense pruritus often causes the mouse to excoriate the area next to the ear pinna and the lesion then extends from the ear to the neck and over the shoulders. The condition is similar to idiopathic ulcerative dermatitis a well-recognized disease with a characteristic distribution on the thorax and head. Both conditions are characterized by severe pruritus, self-mutilation, dermal ulceration, necrosis, and fibrosis. The cause is an underlying vasculitis attributed to immune complex deposition on dermal vessels, although neurogenic abnormalities have been proposed for progressive necrosing dermatitis of the pinna.

Ringworm, caused by Trichophyton mentagrophytes, is uncommon in pet mice. 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.²² Skin swellings are usually tumors or abscesses. Needle biopsy often reveals the nature of the contents and allows diagnosis. Three opportunistic bacterial pathogens—Staphylococcus aureus, Pasteurella pneumotropica, and S. pyogenes—are often isolated³ and can cause abscesses in other organs (e.g., P. pneumotropica is sometimes associated with conjunctivitis, panophthalmitis, and swollen eye abscesses). Antibiotic therapy with penicillins or cephalosporins, concurrent with drainage and debridement of the abscess, is effective.

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.⁹⁸ In wild and outbred mice, the incidence of fibrosarcomas ranges from 1% to 6%.⁴¹ Subcutaneous tumors are nearly always malignant and have often 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. Attempts to treat skin tumors in pet mice by radiation or chemotherapy have not been reported.

Digestive System

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 demonstrating characteristic trophozoites in wet mounts of fresh intestinal contents or feces. Treatment is metronidazole (two treatments of 10-40 mg/kg PO q5d) (see Table 27-2).

Pinworms are ubiquitous, considered nonpathogenic, and found frequently in mice purchased from a pet store.¹⁶ Two are commonly encountered in mice: Syphacia obvelata and Aspicularis tetraptera. Often the only indication of pinworm infestation is rectal prolapse due to straining. To establish a diagnosis of S. obvelata infestation, make a clear cellophane tape impression of the perianal skin. Adult S. obvelata females deposit ova around the anus, whereas A. tetraptera does not deposit its ova in this area and fecal smear or flotation is required to confirm a diagnosis. Ivermectin (2.0 mg/kg PO given twice at a 10-day interval) eliminates pinworms from mice. Ivermectin 1% is diluted 1:9 in vegetable oil to establish a concentration of 1.0 mg/mL; affected mice are dosed with a volume of 0.2 mL/100 g PO.³¹ The recommended package label dose for mice with ectoparasites (0.2 mg/kg given twice at a 10-day interval) does not eliminate pinworms (see Table 27-2).

Diarrhea is not usually seen in adult mice. Digestive disease in adult mice usually is caused by a varying combination of pathogenic and opportunistic infectious agents. Fecal flotation and fresh wet mounts of feces usually yield positive results and do not necessarily give a definitive diagnosis. However, these techniques are sometimes helpful in identifying heavy endoparasite infestations. Treatment is generally directed at clinical signs and consists of the judicious use of antimicrobials.