3-year-old female rat, Rattus norvegicus.This pet rat presented for less than 12 hours of neurologic signs and urinary incontinence. On examination the rat had left head tilt with bilateral mydriasis and absent pupillary light reflexes. The body was also tilted to the left. Ventral to the right ear, there was a hyperemic swelling; when compressed, pus exuded from the ear canal. Over the next 12 hours the animal was treated with antibiotics but deteriorated and was euthanized.

Gross Description:  

There was a 1 x 1 x 0.3 cm swelling immediately ventral to the right pinna. On manipulation of the swelling, yellow, creamy exudate mixed with heterogeneous, firm, yellow and brown material oozed from the ear canal. The right tympanic bulla was enlarged and measured 10 x 9 x 4 mm, whereas the left tympanic bulla measured 7 x 3 x 3 mm. The right cerebral hemisphere was slightly swollen and there was thick, creamy, yellow-green, opaque exudate in the leptomeninges covering the rostral cerebellum.

Histopathologic Description:

Skull (multiple levels): Bilaterally associated with ulceration of the external ear canal, extending into and multifocally effacing the tympanic cavities of the middle ears, and extending into the cranium on the right side, are innumerable degenerate neutrophils and fewer macrophages mixed with abundant cellular debris, edema, and fibrin (lytic necrosis). In the right external ear, there is fibropapillomatous epithelial proliferation, with abundant underlying granulation tissue that entraps squamous epithelial cells and debris. The right tympanic bulla is four times normal size and contains large amounts of keratin and multifocal inflammatory cell infiltrate as previously described. The cell infiltrate also fills the left tympanic cavity and bilaterally infiltrates the extensively ulcerated stratified squamous epithelium and the subepithelial tissue of the middle ears, accompanied by multifocal resorption and remodeling of the temporal bones. There is no evidence of the auditory ossicles on either side. Within the submucosal stroma of the middle ears, the infiltrate is predominantly lymphoplasmacytic. There is also goblet cell metaplasia/hyperplasia and multiple glands distended with amphophilic (mucous) secretion. The inflammation extends along and multifocally infiltrates degenerate trigeminal nerves, and results in necrosis of approximately 10% of the right cerebral hemisphere. Adjacent to the necrotic regions in the brain, multifocal areas of neuropil are rarefied (malacia). Multiple neurons are degenerate, with regional gliosis. Within the right lateral and the third ventricles of the brain, there are small numbers of neutrophils, macrophages, lymphocytes, and plasma cells. The midline of the cerebrum is slightly displaced to the left. Multifocally and primarily at the edges of the foci of necrosis in the tympanic cavities and in the brain there are colonies of gram-negative rod-shaped and gram-positive coccoid bacteria. The inflammatory process also involves the laryngeal submucosa and skeletal muscle, resulting in myofiber necrosis characterized by hyalinization and fragmentation. The intracranial bone marrow appears hypercellular, with moderate myeloid hyperplasia.

Morphologic Diagnosis:  

Ears: Otitis externa and media, bilateral, proliferative, ulcerative, and necrosuppurative, chronic, severe, with intralesional mixed bacteria and extension to encephalitis.

Lab Results:  

Postmortem cultures yielded Prevotella melaninogenica from the ear, brain, and an acutely congested and edematous lung. Enterococcus sp. was also isolated from the ear. Mycoplasma culture on the same set of samples was negative.


Otitis media

Contributor Comment:  

Otitis media (OM) is common in laboratory and pet rats. An otoscopic study of 80 Wistar laboratory rats housed in barrier versus non-barrier units showed incidences of spontaneous OM to be 5% and 20%, respectively.(13) The condition is likely underestimated in most laboratory and domestic mammals, with the notable exception of horses, in which OM is usually only associated with temporohyoid osteoarthropathy.(15) In developed countries, 80% of children will have an episode of otitis media by their third birthday, and 40% will have at least six episodes by age seven.(11)

Primary or secondary bacterial infection is virtually ubiquitous in otitis media, and bacterial ascension through the tympanic bulla or via the Eustachian tube are proven routes of entry. Evidence for hematogenous infection remains circumstantial. The progressive pathology of otitis media is well-described.(15) To reflect the clinical progression of OM, otolaryngology provides an instructive classification scheme: 1) Otitis Media with Effusion (OME) corresponds to asymptomatic persistent middle ear effusion, 2) Acute Otitis Media (AOM) is recurrent middle ear effusion with clinical signs, and 3) Chronic Suppurative Otitis Media (CSOM) refers to discharge through a perforated tympanic membrane for greater than 2 weeks.(11) Complications of OM include hearing impairment, vestibulopathy, extension to otitis interna, and meningoencephalitis. Globally 21,000 people die annually due to OM-related complications.(7) The presence of stratified squamous epithelium, goblet cell metaplasia/hyperplasia and mucus-producing submucosal glands in the middle ear is consistent with a chronic suppurative otitis media in this rat. Although not evident in the current case, concurrent otitis interna was strongly suspected due to extension of the inflammatory process into the brain. 

Clinical signs of OM are ear pain, odor, head tilt, vestibular signs, scratching at the ear, and neurologic signs related to meningoencephalitis. Discharge/debris from and below the ear canal may be seen. Otoscopic examination may reveal an inflamed tympanic membrane, pus behind the membrane, or membrane rupture.(5) Clinical diagnosis can be aided by radiography, CT scan, cytology, and bacteriology. 

Organisms commonly implicated in otitis media in rats are Mycoplasma pulmonis, Streptococcus pneumonia, Pasteurella pneumotropica, Staphylococcus sp., Corynebacterium kutscheri, and Klebsiella sp.(5) In this case Mycoplasma culture of the ear, brain, and lung was negative. Prevotella melaninogenica (formerly Bacteroides melaninogenicus) is an anaerobic gram-negative bacillus. Of the anaerobic gram-negative bacilli involved in animal diseases, Fusobacterium is the most important genus, and other genera include Bacteroides, Dichelobacter, and Porphyromonas. P. melaninogenica is also classified as a black pigmented bacterium due to its production of black iron metabolites on blood-containing media; thus, the species name is a misnomer as melanin pigments are tyrosine-based. Virulence factors include hemolysin, neuraminidase, and collagenase.(3,12) P. melaninogenica is a component of the normal flora in the rumen and the human oral cavity, gut and vagina. It is mainly a periodontal pathogen but also considered to be an emerging pathogen in human OM.(1,10) A search of the current veterinary literature identified P. melaninogenica as a common pathogen in bovine footrot and bite wounds from dogs and cats, but no reports of otitis in rats were found.

JPC Diagnosis:  

  1. Brain: Meningoencephalitis, suppurative, focally extensive, with neural degeneration. 
  2. Middle ear: Cholesteatoma.
  3. Middle ear: Otitis media, suppurative, focally extensive, with mucosal ulceration, squamous metaplasia, and bone remodeling. 
  4. Eustachian tube: Eustachitis, suppurative, diffuse, chronic-active. 
  5. Nasopharynx: Nasopharyngitis, suppurative, diffuse, with mucus metaplasia. 
  6. External ear canal: Otitis externa, ulcerative, focally extensive, chronic-active. 

Conference Comment:  

The opportunity to observe an entire pathologic process in a single section makes this a truly unique case. It is likely the initial insult was bacterial colonization within the nasopharynx that spread up the Eustachian tube to the middle ear followed by its extension into the calvarium. Of special importance here is the formation of a cholesteatoma within the middle ear. Cholesteatomas are common in people in this geographic location, usually associated with chronic otitis media. They are non-neoplastic, cystic lesions lined by keratinizing squamous epithelium or metaplastic mucus-secreting epithelium and filled with amorphous debris.(7) The pathogenesis behind their formation is still unclear, but a widely acknowledged theory is that the negative pressure and dysfunction of the Eustachian tube causes a deepening retraction pocket that, when obstructed, desquamated keratin cannot be cleared from the recess.(8) They are known to erode the ossicles, labyrinth, and adjacent bone by their production of cytokines such as RANKL and MMPs,(14) and it is likely this specific lesion permitted invasion of the calvarium in this case. 

With the common occurrence of ear infections in infants and children, there is an extensive amount of published information regarding their pathogenesis and treatment in people, most of which revolves around the formation of biofilms. Biofilms are complex bacterial communities that adhere to the surface of implanted biomaterial or mucosa and play a major role in chronic ear infections.(6) In addition to enhancing adherence, biofilms increase bacterial virulence by protecting the microbes from immune effector mechanisms and increases their resistance to antimicrobial drugs.(9) In the ear, biofilms may contribute to cholesteatoma formation, as well as suppurative and non-suppurative otitis, and their formation can exacerbate the infection. This is why the first choice of treatment with internal ear infections in people is often surgery, and why all tissue with potential to harbor biofilms must be removed at that time or risk recurrence of infection.(6)

Conference participants were afforded the opportunity to review the intricate and minute anatomy of the outer, middle and inner ear, during which no less than thirty specific anatomical terms were described within the cochlea alone. Nomenclature aside, appropriate function of the ear requires vibration of the tympanic membrane by sound waves, which are then conducted into the cochlea via the ossicles of the middle ear (malleus, incus and stapes) by a push on the oval window. This movement incites a fluid wave within the cochlea, which is completely full of lymph fluid called endolymph. Where along the snail-shaped cochlea this wave intersects with the Organ of Corti and subsequently delivers information to the brain depends on its frequency. Of vital importance to this conduction system is the maintenance of a strong positive endocochlear potential within the endolymph, which requires precise control and recycling of potassium. In fact, potassium recycling defects in the cochlea is an important cause of deafness in people.(2)


1. Brook I. The role of anaerobic bacteria in chronic suppurative otitis media in children: Implications for medical therapy. Anaerobe. 2008;14(6):297-300.

2. Chen J, Zhao HB. The role of an inwardly rectifying K(+) channel (Kir4.1) in the inner ear and hearing loss. Neuroscience. 2014;265:137-146.

3. Eley BM, Cox SW. Proteolytic and hydrolytic enzymes from putative periodontal pathogens: characterization, molecular genetics, effects on host defenses and tissues and detection in gingival crevice fluid. Periodontol. 2000. 2003;31:105-24.

4. Ginn PE, Mansell JE, Rakich PM. Skin and appendages. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. 5th ed. Vol. 1. Philadelphia, PA: Elsevier; 2007:677-693.

5. Joint Pathology Center. VSPO S-M02.

6. Kaya E, Dag I, Incesulu A, Gurbuz MK, Acar M, Birdane L. Investigation of the presence of biofilms in chronic suppurative otitis media, nonsuppurative otitis media, and chronic otitis media with cholesteatoma by scanning electron microscopy. Sci World J. 2013:638715. doi: 10.1155/2013/638715. eCollection 2013.

7. Lingen MW. Head and neck. In: Kumar V, Abbas AK, Aster JC, eds. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Philadelphia, PA: Elsevier Saunders; 2015:740.

8. Maniu A, Harabagiu O, Perde Schrepler M, Catana A, Fanuta B, Mogoanta CA. Molecular biology of cholesteatoma. Rom J Morphol Embryol. 2014;55(1):7-13.

9. McAdam AJ, Milner DA, Sharpe AH. Infectious diseases. In: Kumar V, Abbas AK, Aster JC, eds. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Philadelphia, PA: Elsevier Saunders; 2015:349.

10. Monasta L, Ronfani L, Marchetti F, et al. Burden of Disease Caused by Otitis Media: Systematic Review and Global Estimates. PLoS One. 2012;7(4):e36226.

11. Morris PS, Leach AJ. Acute and Chronic Otitis Media. Pediatr Clin North Am. 2009 Dec;56(6):1383-99.

12. Soukos NS, Som S, Abernethy AD, et al. Phototargeting oral black-pigmented bacteria. Antimicrob Agents Chemother. 2005;49(4):1391-6.

13. Verdaguer JM, Trinidad A, Gonz+�-�lez-Garc+�-�a JA, et al. Spontaneous otitis media in Wistar rats: an overlooked pathology in otological research. Lab Anim (NY). 2006;35(10):40-4.

14. Welkoborsky HJ. Current concepts of the pathogenesis of acquired middle ear cholesteatoma. Laryngorhinootologie. 2011;90(1):38-48. 

15. Wilcock, BP. Eye and ear. In: Maxie MG, ed. Jubb, Kennedy, and Palmers Pathology of Domestic Animals. 5th ed. Vol. 1. Philadelphia, PA: Elsevier; 2007:546-551.

Click the slide to view.

1-1. Right external ear canal

1-2. Brain

1-3. Skull

1-4. Left tympanic bulla

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