CHAPTER 57. Rodents
Patricia A. Schenck
HUSBANDRY
I. Caging and sanitation
A. Cages need to be made of material that is easily cleaned and resistant to chewing
B. If mesh floors are used, pay attention to the size of the mesh to prevent foot problems
C. Clean cages frequently and maintain good sanitation
D. Provide objects for gnawing and exercise
E. Maintain the room temperature between 70° and 75° F
II. Nutrition
A. Feed a commercial pelleted diet designed for that species
B. Supplement all guinea pigs with vitamin C
C. Sipper bottles are best for providing water. Change the water daily so that it is always fresh
ROUTINE PROCEDURES
I. Blood samples
A. The lateral or medial saphenous veins can be used in larger animals (over 100 g)
B. The cephalic vein can be used in guinea pigs
C. Use tail bleeding only if blood cannot be obtained from another site
II. Injections
A. Give subcutaneous (SC) injections over the shoulder blades
B. Give intramuscular injections in the caudal thigh muscles
III. Euthanasia is performed by induction of inhalant anesthesia followed by an overdose of barbiturates
IV. Hemostasis is very important because of the small blood volume of these animals
DISEASES OF MICE
I. Dermatology
A. Ectoparasites
1. Fleas, lice, and mites affect mice, causing alopecia on the head and dorsal midline
2. Treat fleas or lice with pyrethrin powder. Treat mites with ivermectin
3. Thoroughly clean the cage; the environment may also need to be treated
B. Dermatophytosis also can occur. Treat with griseofulvin
C. Bacterial disease
1. Staphylococcus aureus causes ulcerative dermatitis resulting in pododermatitis, mastitis, and abscesses. Treat with appropriate antibiotics
3. Actinobacillus or Corynebacterium can cause SC abscesses. Treat with ampicillin
4. Males commonly have preputial gland abscesses caused by E. coli or S. aureus. Treat topically
D. Neoplasic
1. Mammary neoplasia is common and is usually malignant. Metastasis to the lungs is common
2. Lymphoma also commonly causes SC masses
E. Otitis
1. Otitis externa is usually caused by ear mites. Treat by cleaning debris and administering ivermectin
2. Otitis media or interna is caused by invasion of bacteria from an abscess. Head tilt is common
F. Miscellaneous
II. Ophthalmology
A. Epiphora is commonly caused by poor sanitation or overgrown incisors. It may also be an early sign of respiratory disease
B. Retinal degeneration, cataracts, and other ophthalmic conditions also occur
III. Respiratory
A. Murine respiratory mycoplasmosis is a common respiratory disease caused by Mycoplasma pulmonis. The disease is activated by stress and can remain dormant for long periods. Signs include dyspnea, mucopurulent discharge, and anorexia. Treat with tylosin and nutritional support. Survivors are carriers and should be quarantined
B. Sendai virus is a common cause of pneumonia that causes acute death in suckling or weanling mice. Treat with antibiotics. A vaccine is available; animals that recover are resistant to future infection
C. Bacterial pneumonia is typically caused by Streptococcus, Corynebacterium, Pasteurella, Pseudomonas, and Klebsiella. Treat with appropriate antibiotics
D. Mammary adenocarcinomas may metastasize to the lungs, causing respiratory signs
IV. Gastroenterology
A. Parasites
1. Hymenolepis nana is a tapeworm that usually does not cause clinical signs. However, it is directly transmissible to humans. Treat with praziquantel and improve sanitation
2. Pinworms cause anal pruritus and rectal prolapse. Treat with piperazine or fenbendazole
3. Spironucleus muris is a protozoa that causes diarrhea in young mice. Treat with oxytetracycline and improve sanitation
4. Giardia spp. can cause diarrhea and is zoonotic. Treat with metronidazole
5. Cysticercus fasciolarus causes hepatic cysts, which are the infective form of Taenia taeniaformis in carnivores
B. Diarrhea
1. Viral diseases
a. Lethal intestinal virus of infant mice causes diarrhea in mice 10 to 20 days old. Treat supportively; those that survive should be quarantined
b. Epizootic diarrhea of infant mice usually occurs in mice less than 2 weeks old, with low mortality. Treat supportively and quarantine survivors
c. Mouse hepatitis virus causes yellow diarrhea and encephalitis in 1- to 2- week-old mice. Older animals show progressive hepatitis. Treat supportively and quarantine survivors
d. Reovirus causes an oily diarrhea in older suckling mice. Most affected mice eventually die
2. Bacterial diseases
a. Transmissible murine colonic hyperplasia caused by Citrobacter freundii results in diarrhea, rectal prolapse, and stunted growth. Treat with appropriate antibiotics
b. Salmonellosis (mouse typhoid) is usually caused by contaminated feed and results in anorexia, conjunctivitis, arthritis, and diarrhea. Treat supportively and quarantine survivors. Sanitation is important because Salmonella is zoonotic
c. Bacillus piliformis causes Tyzzer disease with high mortality in weanlings. Treat with tetracycline and reduce stress
C. Overgrown incisors are common
V. Theriogenology
A. Infertility can be due to poor management, poor nutrition, or crowding
B. Pyometra is also common
VI. Urology
A. Glomerulonephritis is common in old mice and may be secondary to viral infection
B. In aged male mice, urethral obstruction may develop from plugs of inspissated ejaculum. Hematuria may be present before obstruction. Surgical removal is required
C. Mice can be carriers of leptospirosis
VII. Streptobacillus moniliformis causes infectious polyarthritis. Treat with antibiotics and supportive care
VIII. Neurology
A. Head tilt is common, resulting from otitis media. Trauma is also a common cause of neurologic signs
B. Lymphocytic choriomeningitis is caused by an arenavirus. Acute signs include photophobia and seizures and typically occur in 3- to 6-week old mice. Glomerulonephritis develops later. If infected after weaning, there is weight loss, arthritis, and conjunctivitis
C. Theiler disease is a poliomyelitis-encephalomyelitis that causes encephalitis in very young mice. Mice 6 to 10 weeks of age show rear limb weakness progressing to paralysis. Albino mice are predisposed. Treat supportively
D. Seizures may occur from trauma, otitis, renal failure, liver failure, sepsis, meningitis, encephalitis, or neoplasia
IX. Hematology
A. Eperythrozoon coccoides is a rickettsial parasite found in red blood cells (RBCs). Transmission is via the louse, but affected mice generally have no symptoms. Treat with tetracyclines
B. Leukemia is transmitted through mammary tissue or through the placenta and is viral in origin
DISEASES OF RATS
I. Dermatology
A. Mammary tumors are common, developing in 59% to 90% of adult females and 15% of adult males. Most are benign fibroadenomas
B. Fleas, lice, and mites infest rats as in mice
C. Ulcerative dermatitis caused by Staphylococcus aureus also occurs in rats as in mice
D. Fibrous bands of tissue can form around the tail if the environmental humidity is less than 40% and can result in gangrene of the tail. Incise the ring of tissue and apply DMSO, steroid, and antibiotics. Keep humidity above 50% to prevent ringtail
II. Ophthalmology
A. Sialodacryoadenitis virus is a coronavirus that causes keratoconjunctivitis, uveitis, periorbital swelling, and pneumonia. There is typically no loss of activity or appetite. Treatment is usually not necessary in mild cases. If respiratory signs occur, treat with antibiotics
B. Mycoplasma, Streptococcus, Pseudomonas spp., and others may cause mucopurulent ocular discharge
C. Cataracts may develop secondary to diabetes mellitus, or they may be primary
III. Respiratory
A. Murine respiratory mycoplasmosis is common in rats
B. Streptococcus pneumoniae may cause bacteremia with stress
C. Cornebacterium kutscheri, Pasteurella pneumotropica, and Pseudomonas aeruginosa can cause respiratory signs
IV. Cardiovascular system
A. Congestive heart failure is common in older rats. Treat supportively and give furosemide and digitalis
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