Bovine Infectious Bovine Keratoconjunctivitis – NAVLE Study Guide

Overview and Clinical Importance

Infectious bovine keratoconjunctivitis (IBK), commonly known as pinkeye or New Forest eye, is the most common ocular disease affecting cattle worldwide. It is a highly contagious bacterial infection primarily caused by Moraxella bovis, a Gram-negative, ?-hemolytic, aerobic, rod-shaped bacterium. IBK causes significant economic losses estimated at over $150 million annually in the United States due to decreased weight gain, reduced milk production, treatment costs, and decreased market value of affected animals.

The disease is characterized by inflammation of the cornea and conjunctiva, manifesting with blepharospasm, epiphora, photophobia, corneal edema, opacity, vascularization, and ulceration. Severe cases may progress to corneal perforation and permanent blindness. Young cattle, particularly calves, are most susceptible, with affected calves showing weaning weights approximately 8.9 kg (20 lbs) lower than healthy calves.

Etiology

Primary Causative Agent

Moraxella bovis is the primary etiologic agent of IBK. It is an obligate mucosal parasite found in the eyes and nasal cavities of infected cattle. Key characteristics include:

• Gram-negative, rod-shaped bacterium
• ?-hemolytic (produces zones of complete hemolysis on blood agar)
• Aerobic organism
• Multiple serotypes exist (at least 7 serogroups based on pili antigens)
• Carrier animals may harbor the organism year-round without showing clinical signs

Other Associated Organisms

While M. bovis is the primary pathogen, other organisms may contribute to or complicate IBK:

Pathogenesis

The pathogenesis of IBK is a three-step process: corneal damage, microbial adhesion, and cytotoxicity. Understanding this process is crucial for NAVLE success.

Step 1: Corneal Damage (Predisposing Factors)

Initial damage to the corneal epithelium is typically required for M. bovis to establish infection. Predisposing factors include:

• UV radiation (sunlight): Damages corneal epithelium directly; cattle with non-pigmented eyelids are more susceptible
• Face fly feeding (Musca autumnalis): Causes mechanical irritation while feeding on ocular secretions
• Dust and plant material: Grass awns, seed heads, and pollen cause mechanical abrasion
• Feeding from round bales: Young cattle eating from large bales may injure eyes on hay
• Concurrent viral infection (IBR): Compromises local immune defenses

Step 2: Bacterial Adhesion (Pili)

Type IV pili (fimbriae) are essential virulence factors that enable M. bovis to adhere to corneal epithelium:

• Pili are hair-like protein structures on the bacterial surface
• Only piliated strains can cause disease - non-piliated strains are avirulent
• At least 7 different pili serogroups exist
• Phase variation (pilin gene inversion) allows antigenic switching to evade host immunity
• Antibodies against pili are serogroup-specific, limiting cross-protection

Step 3: Cytotoxicity (RTX Toxin)

Once attached, M. bovis produces an RTX cytotoxin (MbxA) that causes corneal damage:

• Mechanism: Pore-forming toxin that creates calcium-dependent transmembrane pores in target cells
• Target cells: Corneal epithelial cells, bovine erythrocytes, neutrophils, lymphocytes
• Result: Cell lysis leading to corneal ulceration
• Only hemolytic strains (RTX+) cause clinical disease
• The RTX operon (mbx operon) defines a pathogenicity island - absent in non-hemolytic strains

Clinical Signs and Stages of Disease

IBK typically progresses through predictable clinical stages over 3-5 weeks. The disease is usually unilateral (one eye affected), though both eyes can be involved. Recovery with appropriate treatment typically occurs in 3-5 weeks.

Clinical Stages of IBK

Risk Factors and Epidemiology

Breed Predisposition

Significant breed differences in IBK susceptibility have been documented:

Key point: Breeds lacking periocular pigmentation (Hereford, Charolais) have increased UV sensitivity and decreased local immune response, making them more susceptible. Heritability of IBK susceptibility is estimated at 0.10-0.25.

Age Susceptibility

• Young cattle (calves) are most susceptible due to underdeveloped immune systems
• Adult cattle develop protective antibodies and are rarely affected more than once
• Bull calves have slightly higher incidence than heifer calves
• First-lactation cows soon after calving may be affected during outbreaks

Environmental Risk Factors

The Face Fly (Musca autumnalis)

The face fly is the primary vector for M. bovis transmission between cattle:

• Biology: Non-biting fly (5-8 mm); feeds on ocular, nasal, and oral secretions
• Life cycle: Eggs laid in fresh cattle manure; development to adult in less than 2 weeks
• Transmission mechanism: Carries M. bovis on legs and mouthparts; can regurgitate bacteria from crop
• Seasonal pattern: Most abundant in summer and early fall when IBK peaks
• Additional role: Intermediate host for Thelazia eyeworms

Diagnosis

Clinical Diagnosis

Diagnosis is typically based on characteristic clinical signs and herd history. Key diagnostic features:

• Pathognomonic finding: Central corneal ulceration
• Herd outbreaks during summer/early fall
• Young animals most commonly affected
• History of predisposing factors (flies, dust, UV exposure)

Fluorescein Staining

Fluorescein dye is used to detect corneal ulcers. The dye is retained by denuded stroma (appears bright green under UV light) but not by intact epithelium. This confirms the presence and extent of ulceration.

Laboratory Confirmation

While not always necessary for clinical cases, laboratory testing can confirm etiology:

• Culture: M. bovis grows on blood agar with ?-hemolysis; swabs from conjunctiva or lacrimal secretions
• PCR: Can differentiate M. bovis from M. bovoculi
• Antimicrobial sensitivity: Recommended when treatment failure occurs

Differential Diagnoses

Treatment

Early treatment is essential to eliminate the organism, reduce pain, prevent transmission, and minimize corneal scarring. Moraxella bovis is typically sensitive to oxytetracycline and penicillin.

Systemic Antibiotic Therapy

Local/Topical Therapy

• Subconjunctival penicillin: 300,000 IU procaine penicillin G injected under the bulbar conjunctiva; effective but requires good restraint; ELDU requiring prescription
• Topical oxytetracycline: Ophthalmic formulation (oxytetracycline HCl with polymyxin B); requires frequent application
• Intramammary cloxacillin: Used topically (ELDU); similar efficacy to subconjunctival penicillin in some studies

Supportive Care

• Eye patches: Protect from UV light and flies; note that eye cannot be monitored while covered
• Third eyelid flap: For deep ulcers at risk of perforation; suture nictitating membrane to upper eyelid
• Tarsorrhaphy: Suturing eyelids closed for severe cases
• Shade: Provide access to reduce UV exposure
• Isolation: Separate affected animals to reduce transmission
• NSAIDs: For pain relief (ELDU; prescription required)

Prevention and Control

Risk Factor Management

Since vaccines are inconsistently effective, mitigating risk factors is crucial:

• Fly control: Insecticide ear tags (pyrethroids or organophosphates - rotate annually to prevent resistance), pour-ons, dust bags, fly traps
• Pasture management: Mow pastures with mature seed heads before turning cattle out; reduce dust
• Shade provision: Reduce UV exposure, especially for white-faced breeds
• Nutrition: Maintain adequate vitamin A, copper, and selenium status
• Early detection and isolation: Remove affected animals to reduce transmission
• Biosecurity: Use gloves, disinfect equipment between animals

Vaccination

Important: Vaccine efficacy is controversial and inconsistent. Multiple commercial vaccines exist (Piliguard, Alpha 7/MB-1, I-Site XP, Pinkeye Shield, etc.), but controlled trials have not consistently demonstrated efficacy in preventing naturally occurring IBK.

Reasons for variable vaccine efficacy:

• Multiple pili serogroups - immunity is serogroup-specific
• Antigenic phase variation in M. bovis
• M. bovoculi may also cause IBK and is not covered by M. bovis vaccines
• Multifactorial nature of disease requires addressing other risk factors
• Systemic IgG response may not provide adequate local ocular immunity

Vaccination recommendations: If used, administer at least 4 weeks before expected IBK season (summer). Some protocols recommend both M. bovis AND M. bovoculi vaccines given separately. Autogenous vaccines may be more effective in some herds.

Prognosis and Economic Impact

Prognosis

• Mild to moderate cases: Excellent prognosis with treatment; complete recovery in 3-5 weeks
• Untreated cases: May recover spontaneously in 3-5 days (mild) to several weeks, but with increased scarring risk
• Severe cases: Corneal perforation leads to permanent blindness; enucleation may be necessary
• Residual effects: Central scar (macula or leukoma) may persist; minimal impact on vision in most cases

Economic Impact

• Annual losses exceed $150 million in the U.S.
• Affected calves have weaning weights 8-9 kg (18-20 lbs) lower than healthy calves
• Bilateral infections result in 16-30 kg (35-65 lbs) lower weights
• Animals with scarred or "blue" eyes receive significant market discounts
• Additional costs: treatment, labor, fly control programs

Exam Focus - Memory Aid "PINKEYE":

P - Pili for attachment (essential virulence factor)

I - Infectious (highly contagious, transmitted by face flies)

N - Non-pigmented eyelids increase risk (Herefords)

K - Keratoconjunctivitis (cornea + conjunctiva affected)

E - Eye ulcer (central corneal ulcer is pathognomonic)

Y - Young animals most susceptible (calves)

E - Eliminate with oxytetracycline or tulathromycin (labeled treatments)