Diseases of the Rat (Rattus norvegicus)
Skin Conditions
Staphylococcus aureus - ulcerative dermatitis over nape of neck, shoulders. Prominent colonies of Staphylococcus on surface. Ulcerative dermatitis probably associated with epidermolytic strains of bacteria, but Koch's postulates not fullfilled.
Trichophyton and Microsporum rare in pet rats, can be common in wild rats
Diagnosis: skin scrapings/KOH of lesions, histology with methenamine silver stain
Disease: subclinical to florid ringworm on neck, back and base of tail. Patchy hair loss, hyperkeratosis,
erythema. Arthrospores investing keratin of hair follicles.
Treat with griseofulvin @ 25 mg/kg PO for 30-60 days.
Comment: zoonotic
Ectoparasites
FLEAS Leptopsylla spp., Nosopsyllus spp. Xenopsylla spp.
LICE Polyplax spinulosa (spined rat louse), Hoplopleura pacifica (tropical rat louse)(Zoonotic!)
MITES Demodex spp. Laelaps echidninus (spiny rat mite), Notoedres muris, Ornithonyssus bacoti (tropical rat mite), Radfordia ensifera
Rat arthropods
Arthropods are rare in rats, except Radfordia fur mites. Fleas can serve as intermediate hosts for
Hymenolepis and transmit other agents such as Rickettsia typhy and Yersinia pestis (plague!!)e. Lice can also serve as vectors of disease, including Hemobartonella (Polyplax) and trypanosomes. Mites are rare, except Radfordia, which can be common in some colonies. Non-pathogenic fur mite. Demodex found in as an incidental finding, Laelaps transmits Hepatozoon, but requires absolutely filthy conditions to survive. Notoedres muris is ear mange mite. Burrows in cornified epithelium, causing pruritis and self-trauma. Extremely rare in rats. Ornithonyssus lives off host and is non-selective in host range, including humans. Can cause anemia, debilitation and death in rats. Treat with ivermectin @ 200mcg/kg, or use light dusting of pyrethrin-based flea powder.
Diseases of the Respiratory System
Respiratory disease is one of the commonest reasons for the presentation of pet rats to the vet. The commonest aetiology is a combination of M. pulmonis, CAR and paramyxovirus (Sendai virus). SDAV is also very common. All respiratory diseases are exacerbated by high environmental NH3 levels. Rats possess retrobulbar Harderian glands which secrete tears rich in porphyrins. These tears are often mistaken for blood-staining and should be differentited from such natural staining.
Cowpox Virus
1. Prevalence: rare
2. Diagnosis: lesions
3. Disease: subclinical. Necrotizing rhinitis.
4. Transmission: unknown
5. Duration: unknown
Rat Coronaviruses (Sialodacryoadenitis virus SDAV)
1. Prevalence: v.common
2. Diagnosis: serology, lesions
3. Disease: cervical edema, nasal/ocular discharge including pinkish staining of tears from porphyrin rich Harderian gland production, photophobia, keratitis, megaloglobus, anestrus, fetal resorption. Virus replicates in upper respiratory mucosa (necrotizing rhinitis, tracheitis, bronchitis), with secondary involvement of salivary, lacrimal and lower respiratory tissues (sialodacryoadenitis and interstitial pneumonia). Sublingual salivary gland usually spared. Lacrimal gland dysfunction results in secondary keratitis sicca, ulcers, uveitis, megaloglobus, etc. Chronic wasting disease in athymic rats due to progressive pneumonia and salivary gland lesions. Olfactory mucosal dysfunction in sucklings may lead to failure to nurse, mortality.
Pneumonia is common in infant rats.
4. Transmission: respiratory
5. Duration: acute (except in athymic rats)
Pneumonia Virus of Mice (PVM)
1. Prevalence: common
2. Diagnosis: serology, lesions
3. Disease: subclinical. Lungs develop perivascular and focal interstitial lymphocytic infiltrates in seropositive (recovered) rats. Lesions persist after virus is cleared. Athymic rats likely to have persistent infections with chronic lung disease, but never reported.
4. Transmission: respiratory
5. Duration: acute
6. Comment: PVM more apt to produce lung lesions in naturally infected rats than mice.
Paramyxovirus (Sendai Virus)
1. Prevalence: common
2. Diagnosis: serology, lesions
3. Disease: In enzootic populations, manifests as subclinical or mild disease in rats of one to two months old (when maternal immunity subsides). In naive populations, severe respiratory disease, poor growth and infant mortality. Chronic wasting syndrome/pneumonitis in rnu/rnu rats.
4. Transmission: respiratory
5. Duration: acute
Bordetella bronchiseptica
1. Prevalence: rare, depending on population
2. Diagnosis: culture
3. Disease: may be associated with respiratory disease, but probably in association with primary pathogen, such as Mycoplasma. Rhinitis, otitis, lower respiratory disease.
4. Transmission: contact
5. Duration: unknown
6. Comment: minor primary significance in rats
Cilia-Associated Respiratory (CAR) Bacillus
1. Prevalence: common
2. Diagnosis: silver stain, lesions, serology developed but not used
3. Disease: subclinical to chronic respiratory disease similar to mycoplasmosis. Organisms on apical
membrane among cilia of respiratory epithelium. Often co-infection with Mycoplasma.
4. Transmission: respiratory, presumed
5. Duration: chronic
6. Comment: does not grow in cell-free medium. Found in multiple species. Experimental infection of SPF rats with CAR bacillus will induce chronic respiratory disease and natural infections manifesting as chronic respiratory disease have CAR bacillus in the absence of Mycoplasma, but natural infections are usually mixed.
Hemophilus spp.
1. Prevalence: rare
2. Diagnosis: culture
3. Disease: mild inflammation of lower respiratory tract. Isolation from nose, trachea, lung and female genital
tract.
4. Transmission: unknown
5. Duration: unknown
6. Comment: unclassified species. Single report, but a high prevalence of infection within the colony.
Klebsiella pneumoniae
1. Prevalence: infection common, disease rare
2. Diagnosis: culture
3. Disease: opportunistic gut microflora, but can be associated with respiratory disease (mild rhinitis) and
abscesses.
4. Transmission: orofecal, contact
5. Duration: unknown, probably chronic microflora
6. Comment: seldom significant
Mycoplasma spp. (chronic respiratory disease, murine respiratory mycoplasmosis)
M. pulmonis
M. arthritidis
M. neurolyticum
M. collis
1. Prevalence: common
2. Diagnosis: serology, culture, lesions. Infected rats may be seronegative for months.
3. Disease: often subclinical. Sneezing and snorting characteristic. M. pulmonis is only significan natural pathogen, causing rhinitis, otitis, laryngitis, tracheitis, bronchiolitis, bronchopneumonia with pronounced bronchiolectasis, bronchiolar lymphoid hyperplasia (strong mitogen) and abscessation. M. pulmonis can also cause perioophoritis, salpingitis and torticollis secondary to otitis media. Chronic respiratory disease often associated with CAR bacillus and exacerbated by ammonia, Sendai virus, SDAV, etc. Other mycoplasmas inhabit respiratory or genital tract without disease. M. arthritidis infection can result in seroconversion to M. pulmonis.
4. Transmission: respiratory, contact
5. Duration: chronic
6. Comment: important rat pathogen
Pasteurella pneumotropica
1. Prevalence: common
2. Diagnosis: culture
3. Disease: opportunistic organism. Associated with respiratory, ear, reproductive, mammary gland,
conjunctival and skin lesions, abortion. Co-pathogen with respiratory pathogens (Sendai virus, mycoplasma)
4. Transmission: orofecal
5. Duration: chronic
6. Comment: inhabits respiratory, intestinal tracts as normal microflora
Streptococcus pneumoniae (Diplococcus, Pneumococcus)
1. Prevalence: low
2. Diagnosis: culture lesions, Gram stain exudate
3. Disease: subclinical carriers common (80%). Rhinitis, fibrinopurulent pleuritis, pericarditis, peritonitis, periorchitis, meningitis, otitis and pneumonia. Abscesses.
4. Transmission: respiratory, contact
5. Duration: chronic carriers/acute disease
6. Comment: pathogenicity varies with serotype. Although mice are susceptible to infection, natural infection and disease is rare. Outbreaks of disease can occur among rats.
Pneumocystis carinii
1. Prevalence: infection frequent, disease rare
2. Diagnosis: lesions, silver stain for cysts
3. Disease: pulmonary disease can be induced in naturally exposed young rats by low protein diet combined with steroids. Difficult to induce in older rats. Spontaneous disease not seen in immunocompetent rats and not reported as a problem in athymic rats.
4. Transmission: unknown
5. Duration: chronic
6. Comment: carriers very common in conventional, "dirty" rats, especially rats from colonies infected with multiple viruses (probably a reflection of overall dirtiness)
Diseases of the GI Tract
Rat Adenovirus
Prevalence: common
Diagnosis: serology, inclusions. Cross-reacts antigenically with MAd-2
Disease: subclinical. Intranuclear inclusions in enterocytes of small intestine found incidentally. Inclusions more common in young rats.
Transmission: orofecal
Duration: unknown
Comment: high prevalence of seroconversion with K87 antigen. Rats not susceptible to MAd-1 or -2, so probably separate, but related viruses. Enteric inclusions found only occasionally, but unknown if this is the only adenovirus of rats. Isolation attempts have failed.
Rat Cytomegalovirus (RCMV)
Prevalence: rare or non-existent in laboratory rats, common in wild rats
Diagnosis: histology. Serology, but seldom used
Disease: cytomegaly, inclusions in acinar and duct epithelium of salivary and lacrimal glands, especially
submandibular gland. Mild nonsuppurative interstitial inflammation. Do not confuse with normal exorbital gland morphology.
Transmission: unknown (salivary?)
Duration: chronic
Infectious Diarrhea of Rats (IDIR) Virus
Prevalence: rare
Diagnosis: lesions. Serology available, but not used.
Disease: suckling rats (less than 12 days of age) develop diarrhea, erythema of anus, runting. Malabsorption with watery digesta. Villus attenuation, necrosis, enterocytic syncytia and intracytoplasmic inclusions, especially distal small intestine.
Transmission: orofecal
Duration: acute
Comment: antigenically distinct rotavirus (separate from typical or type 1 rotaviruses). Pathogenesis similar to EDIM in mice, with age-dependent susceptibility to disease but not infection. Infectious to humans and umans probable source of infection in laboratory rats.
Clostridium piliformis (Tyzzer's disease)
Prevalence: rare
Diagnosis: lesions, silver stains, serology developed but not used
Disease: ileitis with dissemination to liver and heart, especially in post-weaning rats. Adynamic ileus can
occur, resulting in megaloileitis, but not a constant feature. Silver stains reveal characteristic intracellular
bacteria.
Transmission: spores, orofecal. In utero transmission possible in immunosuppressed rats, but not likely to be
significant means of transmission.
Campylobacter spp.
Prevalence: variable
Diagnosis: culture, silver stains of lesions
Disease: there is a single report of intracytoplasmic organisms in apical cytoplasm of hyperplastic/neoplastic enterocytes with colonic adenocarcinoma. Agent not characterized, but morphologically reminiscent of Campylobacter-like organisms in hyperplastic enteritides of a variety of species (rabbit, guinea pig, hamster, ferret, lamb, calf, piglet, etc.). Campylobacter jejuni can also be isolated from rats with soft feces. This is probably common and agent can be cultured.
Transmission: orofecal
Duration: unknown
Salmonella enteritidis
Prevalence: low
Diagnosis: culture (especially mesenteric lymph nodes)
Disease: similar to mouse. Often subclinical. Starry coat, soft to fluid feces. Cecal ulcers can persist.
Transmission: orofecal
Duration: acute to chronic. Approximately 60% of rats remain carriers with intermittent shedding.
Comment: infection in rats is currently more common than in mice. Depopulate.
Spirillum minus - see Streptobacillus moniliformis
Streptococcus spp.
Prevalence: organism(s) frequently isolated, but specific disease(s) rare
Diagnosis: culture, lesions
Disease: high morbitity epizootics with 50% mortality among suckling rats. Runting, distended abdomen and fecal staining of perineum. Large numbers of Gram + cocci on surface of normal appearing villi with minimal inflammation. Enterococcus faecium-durans-2 and Enterococcus hirae identified in 2 outbreaks. Streptococcus spp. also an opportunistic pathogen.
Transmission: orofecal
Duration: unknown
Comment: likely to be encountered, but sporadic outbreaks
Cryptosporidium parvum
Prevalence: unknown
Diagnosis: organisms on brush border of villi
Disease: probably opportunistic or marginally pathogenic. Outbreaks of diarrhea with high mortality among infant rats has been reported. Surviving pups runted. Hyperplastic enteritis, especially jejunum, with organisms toward villus tips.
Transmission: orofecal, cysts
Duration: unknown
Comment: Experimental infection in rats is mild and transient, except in immunosuppressed or athymic rats. May have zoonotic potwential. Also may be source of cryptosporidial infections in snakes, although it may be a specific snake pathogen.
Eimeria spp.
Prevalence: common in wild rats, nonexistent in laboratory rats
Diagnosis: intestinal lesions, oocysts in feces
Disease: hyperplastic enteritis, especially in young rats.
Transmission: orofecal, oocysts
Treat with sulphonamides.
Giardia muris
Prevalence: infrequent
Diagnosis: histology, cysts in feces
Disease: subclinical or diarrhea. Low-grade chronic enteritis with trophozoites aligned along brush border of villi.
Transmission: orofecal
Duration: chronic
Spironucleus muris
Prevalence: high
Diagnosis: microscopy of intestine, wet mounts of small intestinal digesta, cysts in feces
Disease: none or rare in rats. If disease present, probably opportunistic.
Transmission: orofecal, cysts
Duration: chronic
Comment: probably not pathogenic in rats
HELMINTHS
NEMATODES - Aspicularis tetraptera, Syphacia muris, Trichosomoides crassicauda
CESTODES - Cysticercus fasciolaris (Taenia taeniaeformis) , Hymenolepis nana and diminuta.
Aspicularis tetraptera, Syphacia muris (pinworms)
Prevalence: common - Syphacia obvelata and Syphacia muris may be same species.
Diagnosis: ova in feces. Scotch tape test for Syphacia, adults in lumen
Disease: none. Syphacia is only species that deposits ova on anus.
Transmission: orofecal. Ova resistant.
Treat with levamisol or fenbendazole.
Cysticercus fasciolaris (Taenia taeniaeformis)
Prevalence: rare
Diagnosis: cysts in liver
Disease: incidental finding of strobilocerci, usually in liver. Rats respond with fibroplasia. Sarcomas can
develop from reactive zone. Studied as a model for parasite-induced neoplasia.
Transmission: contamination of feed and bedding with cat feces
Duration: chronic
Comment: intermediate state of cat tapeworm.
Hymenolepis nana, diminuta - see mouse
Diseases of Uro-Genital System
Leptospira icterohemorrhagiae
Prevalence: rare in pet rats, common in wild rats (<70% of wild rats excreting at any given time).
Diagnosis: culture
Disease: subclinical
Transmission: urinary, contact, copulation, skin wounds
Duration: chronic
Comment: most common L. icterohemorrhagiae in rats, but other species too. Human infection (Wiels Disease) often associated with rat origin.
Trichosomoides crassicauda (bladder thread worm)
Prevalence: infrequent
Diagnosis: bipolar ova in urine, worms in bladder, ureters and renal pelves
Disease: eggs hatch, penetrate stomach wall, migrate through lungs and other viscera and seek urinary tract epithelium. Migrating larvae incite eosinophilia and granulomata, especially in lungs. Adult females burrow in transitional epithelium, smaller males live in lumen or inside female uterus. Reaction minimal, but dead worms may serve as nidus for calculus formation.
Transmission: urine. Egg masses stick to environmental surfaces, resistant to dessication
Duration: chronic
Comment: drug treatment can be effective - try ivermectin @ 200mcg/kg
Chronic renal Disease is common in older rats. By 12-14 months old, up to 50% of rats will have a proteinuria >20mg/dl (normal <5mg/dl). Glomerulosclerosis develops with an interstitial fibrosis. Clinical signs may include polydipsia, fibrous osteodystrophy, hydrothorax and ascites. Biochemistry findings may show elevated BUN (normal 6-23mg/dl) and creatinine (normal 0.2-0.8 mg/dl).
Mycoplasma pulmonis endometritis and pyometra
Preputial Gland Abcess - usually due to Staphylococcus aureus and/or Pasteurella pneumotropica
Musculoskeletal Disorders
Ringtail - if the humidity is too low (<40%) rats can develop annular constrictions around the tail which can cause a distal necrosis and sloughing. Trauma may also give rise to a degloving of the distal tail. Treatment involves surgical removal of the affected area. Then drawing back the skin to exposed the most distal coccygeal verterae, these are remvoed. The skin is then allowed to spring back to cover the stump and the skin is sealed by tissue glue - no sutures to chew!
Disorders of the Sensory Organs and Nervous System
Encephalitozoon may occasionally act as a cause of CNS signs.
Systemic Diseases
Rat Parvoviruses
Rat Virus
H-1 Virus
Rat Orphan Parvovirus
Prevalence: common (all 3)
Diagnosis: serology, lesions (RV). As with mouse OPV, rat OPV does not share virus structural antigens
with RV, H-1. Rat and mouse OPVs are different viruses.
Disease: RV associated with natural disease. Others subclinical. Most RV infections are subclinical, but
can cause fetal resorption, neonatal cerebellar hypoplasia with ataxia, hepatitis, jaundice, steatorrhea.
Hemorrhagic disease in adults, especially when stressed or immunosuppressed. Hemorrhage in CNS and
peritesticular reported.
Transmission: various routes of excretion, in utero possible but not likely
Duration: acute to chronic (latent?)
Comment: RV and H-1 antigenically cross-reactive, but serologically distinct. OPV not cross-reactive, but shares non-structural antigens. IFA serology, using infected cells therefore most useful seroassay.
Corynebacterium kutscheri (pseudotuberculosis)
Prevalence: variable
Diagnosis: culture of lesions, oral washes, cervical lymph nodes. Lesions with characteristic Gram +
colonies
Disease: rats may carry organisms in oral cavity and cervical lymph nodes without disease. Infected cervical lymph nodes may be slightly enlarged (reactive) without abscessation. Stress, immunosuppression, other unknown factors may precipitate bacteremic dissemination to lungs, kidneys, liver, etc. with formation of abscesses. Active lesions possess prominent colonies of Gram + bacilli at periphery, but eventually scar over and resolve.
Transmission: direct contact
Duration: chronic
Comment: pathogenesis may differ from mice. Different organ distribution. Depopulate.
Erysipelas rusiopathiae
Prevalence: rare
Diagnosis: culture lesions
Disease: a single report from Scandinavia with chronic fibrinopurulent polyarthritis, myocarditis and
endocarditis. Organism isolated from lesions.
Transmission: unknown
Duration: unknown
Metabolic Disorders
Neoplasia
Mammary-fibroadenomas are common in adult female rats. Although considered benign these growths can become very large. The mammary tissue in rats and mice can be found at quite a distance from the standard mammary line so such growths may be found throughout the perineal or scapular region and high up on the flanks. Surgery is indicated.
Diseases of the Haematopoetic System
Rat Leukemia Virus (RLV)
Prevalence: 100%
Diagnosis: not done
Disease: none
Transmission: germ line, endogenous provirus
Duration: chronic, latent
Comment: minimal significance in rat. RLV sequences have been combined with MuLV or other RLVs to form experimental rat sarcoma viruses (Kirsten, Harvey, etc.)
Hemobartonella muris
Prevalence: rare
Diagnosis: organisms on surface of erythrocytes; splenectomy of carriers
Disease: usually subclinical. Transient parasitemia, anemia, splenomegaly/
Transmission: Polyplax spinulosa
Duration: chronic, but recovery in some
Comment: severe anemia, parasitemia within 2 weeks of splenectomy. Steroids do not activate subclinical infection.
Miscellaneous
Hemorrhagic Fever with Renal Syndrome (HFRS) Virus
Prevalence: widespread in wild Norway rats. Rare in laboratory rats.
Diagnosis: serology
Disease: none reported in rats, but not thoroughly evaluated
Transmission: urine, saliva, respiratory
Duration: chronic
Comment: Zoonotic. Synonyms: Korean hemorrhagic fever, muroid virus nephropathy, Hantaan virus
(single virus), Hanta virus. Proteinuria, azotemia, petechiae, hemoconcentration, hypotension, renal failure, etc. in humans. Recovery usual, but several fatal cases recently reported in Southwest associated with Peromyscus mouse reservoir.
Mouse Encephalomyelitis Virus (MEV)
Prevalence: seroconversion rare to moderate
Diagnosis: serology
Disease: none
Transmission: orofecal
Duration: unknown
Comment: MHG virus isolated from rats. Antigenically cross-reactive with MEV of mice. Probably mouse origin.
Streptobacillus moniliformis
Prevalence: common in wild rats, rare in pet rats
Diagnosis: culture
Disease: commensal in rats. Disease minimal or absent. Inhabits oropharynx. Can be transmitted by bite, causing abscesses in rats, septicemia in mice (see mice) and rat-bite fever (Haverill fever) in humans.
Transmission: rat bites, milk, contact
Duration: chronic carriers
Comment: Spirillum minus also a cause of rat-bite fever, but not found in laboratory or pet rats.