Rod bacteria on cytology samples of otic exudates should undergo culture and antibiotic sensitivity testing.P aeruginosa appears as rod-shaped organisms, typically accompanied by degenerate neutrophils ( Figure 3, Cytology of a sample obtained from the external ear canal showing Pseudomonas spp infection with numerous rod-shaped organisms (400ВҐ original magnification).Samples of otic exudates should be stained with Gram or modified WrightвЂ™s-Giemsa stain.Factors may include concurrent systemic disease, inappropriate therapy, or changes to the ear canalвЂ™s microclimate.Underlying causes can vary and trigger factors should be thoroughly investigated.Discharge in the canal is similar to that on the pinna but often is hemorrhagic.Otoscopic examination (may require sedation/anesthesia) reveals further inflammation and ulceration of the canal, which is often swollen and partially occluded.Infected pinnae are typically erythematous, ulcerated, and covered in thick yellow/green mucoid discharge.This makes adjunct analgesia important.Although inflammation and ulceration commonly extend from pinna into middle ear, involvement of horizontal canal and middle ear contributes to severe pain.The density of nerve endings increases with ear depth.Otitis media (with/without tympanic rupture) is reported in ~83% of chronic otitis externa cases.When infection involves the middle ear, animals may show signs of otitis media or interna.It can remain intact while the middle ear is infected but will appear abnormal (eg, bulging, hemorrhagic, dark with visible exudate behind it).The ear canal is swollen, ulcerated ( Figure 2, Video otoscope images of an ulcerated, erythematous, inflamed ear in a dog with Pseudomonas infection.), and uncomfortable with malodorous, often hemorrhagic yellow/green mucopurulent discharge.Their ears are painful rather than pruritic.Patients usually resent ear examination.Otic discharge, which can extend onto the concave aspect of the ear pinna, is usually mucoid, malodorous, and yellow/green.Infected ear pinna is commonly inflamed and often ulcerated.Animals typically present with head shaking or ear scratching.Otic infection with Pseudomonas spp is usually a unilateral disease with acute onset.

Glycocalyx вЂњslimeвЂќ helps protect P aeruginosa against influences from immune system and topical medication.

Toxins and proteases (eg, exotoxin A, lecithinase).

P aeruginosa has mechanisms for evading the hostвЂ™s immune response:.

These events lead to bacterial population of predominantly gram-negative flora, especially Pseudomonas spp.

As the lumen narrows, ventilation is reduced, environment becomes more anaerobic, and humidity and temperature increase.In the canal, changes lead to increased gram-positive bacteria.In acute otitis, dermal edema can lead to altered barrier function and changes in cerumen composition.Systemic disease (eg, renal, hepatic, pancreatic disease).Endocrine disease (eg, hypothyroidism, hyperadrenocorticism).Long-term use of weak antiseptics, to which Pseudomonas spp are inherently resistant, or chronic antibiotic use (systemic/topical) without addressing underlying disease.Immune system dysfunction also is a common factor.In cats, changes to otic environment infection with nasopharyngeal polyps.In dogs, conformation of the ear canal and pinna (eg, spaniels).Changes affecting ventilation of the ear canal (eg, humidity, cerumen composition, temperature).At-risk factors include those that disrupt the normal environment within the ear canal and allow bacterial invasion:.Two studies in dogs 1,2 have suggested that primary triggers for Pseudomonas spp are not always evident, but infection can be associated with foreign bodies, grooming, and bathing.Pseudomonas spp rarely infect вЂњnormalвЂќ tissue, so causes should be identified.In older patients, immunosuppression (from systemic or endocrine disease) or neoplasms of the external ear canal can increase risk.Young cats may have nasopharyngeal polyps as underlying triggers.In younger patients, infection is commonly associated with allergy or ectoparasites.Pseudomonas spp infections occur at any age.No breed predilection is recognized in cats.Dogs with hairy, narrow canals and pendulous pinnae (eg, cocker spaniel) are predisposed to P aeruginosa infection.Pseudomonas otitis occurs in dogs and cats.This predisposition makes infectious otitis caused by P aeruginosa more common in tropical climates.P aeruginosa grows within moist environments (eg, soil, vegetation, feces).Pseudomonas otitis is an infection of the ear ( Figure 1, see above), most commonly with P aeruginosa, a motile gram-negative bacillus ubiquitous in the environment but uncommon in the canine or feline ear.

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