Pseudomonas Otitis Infection in Cats & Dogs

Profile

Definition

• 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.

• P aeruginosa grows within moist environments (eg, soil, vegetation, feces).

• This predisposition makes infectious otitis caused by P aeruginosa more common in tropical climates.

Signalment

Breed Predilection

• Pseudomonas otitis occurs in dogs and cats.

• Dogs with hairy, narrow canals and pendulous pinnae (eg, cocker spaniel) are predisposed to P aeruginosa infection.

• No breed predilection is recognized in cats.

Age

• Pseudomonas spp infections occur at any age.

• In younger patients, infection is commonly associated with allergy or ectoparasites.

• Young cats may have nasopharyngeal polyps as underlying triggers.

• In older patients, immunosuppression (from systemic or endocrine disease) or neoplasms of the external ear canal can increase risk.

Causes

• Pseudomonas spp rarely infect “normal” tissue, so causes should be identified.

• 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.

Risk Factors

• At-risk factors include those that disrupt the normal environment within the ear canal and allow bacterial invasion:

• Disruption to the physical barrier.

• Changes affecting ventilation of the ear canal (eg, humidity, cerumen composition, temperature).

• In dogs, conformation of the ear canal and pinna (eg, spaniels).

• In cats, changes to otic environment; infection with nasopharyngeal polyps.

• Immune system dysfunction also is a common factor. 

• Long-term use of weak antiseptics, to which Pseudomonas spp are inherently resistant, or chronic antibiotic use (systemic/topical) without addressing underlying disease.

• Endocrine disease (eg, hypothyroidism, hyperadrenocorticism).

• Systemic disease (eg, renal, hepatic, pancreatic disease).

• Cats with FeLV or FIV.

Pathophysiology

• In acute otitis, dermal edema can lead to altered barrier function and changes in cerumen composition.

• In the canal, changes lead to increased gram-positive bacteria.

• As the lumen narrows, ventilation is reduced, environment becomes more anaerobic, and humidity and temperature increase.

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

• P aeruginosa has mechanisms for evading the host’s immune response:

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

• Glycocalyx “slime” helps protect P aeruginosa against influences from immune system and topical medication.

Signs

• Otic infection with Pseudomonas spp is usually a unilateral disease with acute onset.

• Animals typically present with head shaking or ear scratching.

• Infected ear pinna is commonly inflamed and often ulcerated.

• Otic discharge, which can extend onto the concave aspect of the ear pinna, is usually mucoid, malodorous, and yellow/green.

History

• The first sign owners usually notice is malodorous discharge.

• Owners may also report signs of neurologic damage (eg, Horner syndrome, facial nerve paralysis):

• Motor nerve damage appears as facial asymmetry.

• Parasympathetic nerve damage may appear as keratoconjunctivitis sicca.

• Animals may have problems chewing hard food, barking, or carrying objects.

• With otitis interna, owners may report hearing loss, head tilt, or nystagmus.

Physical Examination

• Patients usually resent ear examination.

• Their ears are painful rather than pruritic.

• The ear canal is swollen, ulcerated (Figure 2), and uncomfortable with malodorous, often hemorrhagic yellow/green mucopurulent discharge.

FIGURE 2

Video otoscope images of an ulcerated, erythematous, inflamed ear in a dog with Pseudomonas infection.

• The tympanum may or may not be intact. 

• It can remain intact while the middle ear is infected but will appear abnormal (eg, bulging, hemorrhagic, dark [brown/gray] with visible exudate behind it). 

• When infection involves the middle ear, animals may show signs of otitis media or interna.

• Otitis media (with/without tympanic rupture) is reported in ~83% of chronic otitis externa cases.⁷

Pain Index

• The density of nerve endings increases with ear depth.

• Although inflammation and ulceration commonly extend from pinna into middle ear, involvement of horizontal canal and middle ear contributes to severe pain.

• This makes adjunct analgesia important.

Diagnosis

Definitive Diagnosis

• Infected pinnae are typically erythematous, ulcerated, and covered in thick yellow/green mucoid discharge.

• Otoscopic examination (may require sedation/anesthesia) reveals further inflammation and ulceration of the canal, which is often swollen and partially occluded.

• Discharge in the canal is similar to that on the pinna but often is hemorrhagic.  

Differential Diagnosis

• Underlying causes can vary and trigger factors should be thoroughly investigated.

• Factors may include concurrent systemic disease, inappropriate therapy, or changes to the ear canal’s microclimate.

Laboratory Findings

Cytology

• Samples of otic exudates should be stained with Gram or modified Wright’s-Giemsa stain.

• P aeruginosa appears as rod-shaped organisms, typically accompanied by degenerate neutrophils (Figure 3).

FIGURE 3

Cytology of a sample obtained from the external ear canal showing Pseudomonas spp infection with numerous rodshaped organisms (400x original magnification).

• Rod bacteria on cytology samples of otic exudates should undergo culture and antibiotic sensitivity testing.  

Cultures

• Samples for cytology should be obtained from the junction of vertical and horizontal canals.

• In cases of suspected otitis media, samples from the middle ear are needed.

• Samples of bacteria can be obtained with guarded technique by passing a sterile swab or syringe attached to a 4-5 French gauge catheter (through which 0.5 mL of sterile saline can be instilled and suctioned) down a clean, handheld otoscope cone.

• Myringotomy should be performed if the tympanic membrane is intact but abnormal.

• Myringotomy involves a catheter penetrating the pars tensa in the caudoventral quadrant of the tympanum catheter, accessing the middle ear. 

• When the tympanum is intact, it is important to obtain 2 samples (1 each from horizontal canal and middle ear), as bacterial isolate type and antibiotic sensitivity may differ.

Imaging

• Imaging is useful when otitis media is present.

• Open-mouth and lateral oblique radiographic views are most informative.

• However, radiography is a poor modality for viewing mild bony changes. 

• CT and MRI are superior for assessing changes within the tympanic bulla.

• If the tympanic bulla has excessive bony change or contains large amounts of granulation tissue, prognosis for resolution by medical therapy alone is poor.

Treatment

Inpatient or Outpatient

• All Pseudomonas otitis cases benefit from thorough cleansing.

• When cleansing the canal and middle ear, pain is best minimized with the patient under general anesthetic in a hospital environment.

• After cleansing, treatment can be on outpatient basis.

Medical

• Otitis externa is treated using topical flushes and antibiotics.

• Benefits and use of systemic antibiotics for otitis externa (without otitis media) are controversial.

• In cases of otitis media, licensed topical medications may not be appropriate because of risks for ototoxicity; off-license topical therapy may be indicated (with client consent).  

Surgical

• When examination and imaging demonstrate irreversible ear damage, surgical intervention is indicated.

• When both canal and middle ear are involved, total ear canal ablation and bulla osteotomy may be necessary.

• When damage is confined to the middle ear, a ventral bulla osteotomy may be more appropriate.

Medications

Glucocorticoids

• Systemic and topical glucocorticoids reduce formation of exudate and swelling (and pain associated with inflammation).

• An antiinflammatory dose of dexamethasone is invaluable at initial flush.

• Follow-up with topical glucocorticoids incorporated into medication (eg, dexamethasone, betamethasone, mometasone) and/or antiinflammatory doses of systemic prednisolone.

Cleaners & Flushes

• Ear cleaners and flushes (see Flush Solutions & Antibiotic Options) can be used to remove infected debris and inflammatory mediators.

• They break up mucus produced by mucoperiosteum of inflamed middle ear and Pseudomonas spp.

• Adequate flushing provides pain relief and allows penetration of topical antibiotics.

• Acetic acid at 2% solution is lethal to Pseudomonas spp within a minute of contact.^3,4 

• Tris-EDTA is also a useful flush.

• May be prepared as solution or concurrently with other topicals (eg, chlorhexidine). 

• Tris-EDTA increases permeability of Pseudomonas spp cell membranes by binding calcium and magnesium ions.⁵

• Presoaking the ear canal with tris-EDTA helps potentiate aminoglycoside and fluoroquinolone antibiotics.^5,6

Flush Solutions & Antibiotic Options

Flush Solutions

• Acetic acid (Malacetic, dechra-us.com)

• Tris-EDTA (TrizEDTA, dechra-us.com)

Topical Antibiotics—First Line

• Enrofloxacin–silver sulfadiazine (Baytril Otic, bayerdvm.com)

• Gentamicin (Easotic, virbacvet.com; Otomax and Mometamax, merck-animal-health-usa.com)

• Marbofloxacin (Aurizon, vetoquinolusa.com)

• Polymyxin (Surolan, vetoquinolusa.com)

Topical Antibiotics—Second Line

• Amikacin injectable (dilute 250 mg/mL–50 mg/mL) 4–8 drops of 50 mg/mL q12h

• Ticarcillin or ticarcillin and clavulanic acid (Timentin, us.gsk.com)

• Silver sulfadiazine (Silvadene cream [diluted with water] or powder for 1% solution)

• Ceftazidime

Topical Antibiotics—Off-License Use

• Dilute topical antibiotics in 12 mL tris-EDTA; instill 0.5 mL into ear q12h after flushing

• Gentamicin 1 mL (40 mg/mL) (roche.com)

• Enrofloxacin 4 mL (2% solution) (bayerdvm.com)

• Marbofloxacin 4 mL (1% solution) (vetoquinolusa.com)

Antibiotics

• Topical antibiotics, such as polymyxin, gentamicin, and fluoroquinolones (ie, enrofloxacin, marbofloxacin), are useful. 

• Ear drops with these agents are only appropriate with intact tympanum.

• Aqueous solutions of gentamicin, marbofloxacin, and enrofloxacin are safe for ruptured ear drums.

• Potential second-line drugs include amikacin, tobramycin, ceftazidime, silver sulfadiazine, and ticarcillin as off-licensed preparations.^7-9

• Systemic antibiotics should be used when otitis media is present1; however, levels of antibiotics within the bulla will be far lower than when topically instilled.

• Systemic medication may be the only possible therapy when the canal is swollen and hyperplastic, the owner cannot treat topically, or topical medication incited an adverse reaction.

• Systemic therapy includes oral fluoroquinolones and injectable amikacin, based on results of culture and sensitivity testing.

Pain Management

• Opioids (eg, buprenorphine, butorphanol, tramadol) provide pain relief.

• NSAIDs are generally less effective and should not be used concurrently with glucocorticoids.  

Contraindications

• If the ear drum is ruptured or cannot be evaluated, proprietary ear drops are an ototoxicity risk.

Precautions

• If the ear canal is swollen, ulcerated, and painful, cleaners and flushes containing acid, detergent, potent ceruminolytic agent, astringent, or alcohol should be avoided.

Interactions

• Antibiotics, such as aminoglycosides (eg, gentamicin, neomycin) and fluoroquinolones (eg, enrofloxacin, marbofloxacin), are inactivated by acidic solutions.

• To prevent transport of drugs into the inner ear, dimethyl sulfoxide (DMSO)-containing products should not be used in combination with aminoglycosides.

Follow-Up

Patient Monitoring

• The patient should be checked 10–14 days after starting therapy.

• The ear should be reexamined to assess improvement and patient status.

• Cytology should be performed to gauge effectiveness of topical therapy.

• Adjust therapy if response is inadequate.

• Rechecks may be performed q2wk until the canal has healed and cytology results are negative for bacteria and inflammatory infiltrate.

Prevention

• Owners should be taught effective ear cleansing techniques and how to recognize early signs of recurrence.  

• Suitable maintenance flushes include those containing acetic acid or tris-EDTA.

• Identification and treatment of predisposing causes are necessary.

• The patient should be reevaluated with otoscopic examination and cytology regularly (ie, every few months).

In General

Relative Cost

• Flush solutions: $

• Topical antibiotics (first line): $

• Topical antibiotics (second line): $–$$

• Topical antibiotics (off-label): $

Take-Home Points

• Pseudomonas otitis can occur in cats and dogs of any age.

• Predisposing factors for Pseudomonas otitis are those that disrupt normal ear canal environment and allow bacterial invasion.

• All Pseudomonas otitis cases can benefit from thorough cleansing.

• Systemic and topical glucocorticoids reduce exudate formation and swelling, cleansers and flushes remove debris and inflammatory mediators, and topical antibiotics are useful and widely available. 

• Owners should be taught effective cleansing techniques and how to recognize signs of recurrence. 

Cytology stain of Pseudomonas otitis (400x original magnification).