Infectious bovine keratoconjunctivitis (IBK), commonly known as pinkeye, is an economically important and often frustrating disease of cattle.  The disease is quite common and highly contagious.  A 1993 survey of Kansas cattle producers found IBK to be the second most common disease.  A Missouri study found 45.4% of cattle herds to be affected by the disease.  Cattle affected by the disease are painful and often become temporarily blind and, therefore, do not consume as much feed.  This leads to economic losses due to decreased daily gain and lower weaning weights.  The cost of treatment adds to economic losses.  A 1984 study estimated that $200 million were lost due to IBK.  This is a significant disease to the cattle industry and prevention and control should be of utmost importance.  However, current treatment and prevention measures can be unrewarding and often do not circumvent the economic losses.

  IBK is a multifactorial disease.  The primary etiologic agent is the bacterium Moraxella bovis.  However, there are many factors that add to the virulence of this microorganism.  In addition, other bacteria have been isolated from eyes with clinical IBK in the absence of M. bovis, indicating that other bacteria may be a primary pathogen in some cases of IBK.

  M. bovis possesses virulence factors that allow it to colonize the eye and result in pathology.  Cell membrane pili are responsible for colonization of the cornea.  Two functionally distinct pili have been identified.  The Q pilus is responsible for attachment and the I pilus allows for local persistence and maintenance of an established infection.  It is thought that bacteria possessing the Q pilus can convert to the I pilus during the course of disease.  Non-piliated strains do not cause clinical IBK.  M. bovis  also possesses  hemolysins.  The hemolysins damage neutrophils that are recruited to the area of infection.  It is the destruction of neutrophils and release of collagenolytic enzymes that ultimately lead to corneal liquefaction and ulceration.  Non-hemolytic strains of M. bovis are non-pathogenic.  Seven serologically distinct groups of M. bovis have been identified based on antigenic differences in membrane pili.  Of these serogroups, groups 3 and 4 (strains Epp63 and Fla64, respectively) are most common in the United States.

  IBK is most often seen in summer and early fall.  This is most likely because ultraviolet light and face flies are the two most important contributing factors.  Ultraviolet light causes nuclear fragmentation and corneal epithelial loosening, which enhances the ability of M. bovis to colonize the cornea.  Flies are the primary vector for M. bovis.  Musca autumnalis is the important specie of fly in transmission. Direct transmission of M. bovis between cattle is rare.  Therefore, face flies are likely the most important factor in outbreaks of IBK.  Wind, dust, tall grasses, hay, and high ammonia levels in the air contribute to eye irritation and initiation of IBK.  A relationship between vaccination with a modified live IBR vaccine and the development of IBK has been found, suggesting the ability of IBR virus to enhance M. bovis infection.

  Several other bacteria have been implicated as a predisposing factor and primary cause of IBK.  Mycoplasma bovoculi and Branhamella ovis have been cultured from normal eyes and eyes displaying clinical signs of IBK.  M. bovoculi  is known to enhance IBK caused by M. bovis by extending its colonization.  M. bovoculi alone can cause a mild catarrhal conjunctivitis.  B. ovis, alone, has been shown to cause a keratoconjunctivitis similar to M. bovis.  However, like M. bovis, factors contributing to ocular irritation are required for clinical disease. B. ovis has also

been cultured concurrently from eyes with M. bovis infection.  This suggests that B. ovis plays a role as a primary pathogen and a contributing factor in IBK.

  Diagnosis of IBK is by clinical signs which include blepharospasm, epiphora, photophobia, chemosis, corneal edema, corneal ulceration and blindness.  Culture and sensitivity is recommended in outbreaks to determine the bacteria involved and the best treatment options.  Conjunctival swabs and lacrimal secretions are the best samples to submit for culture.  M. bovis is extremely susceptible to dessication; therefore, the sample should be transferred to appropriate growth media prior to shipment to a diagnostic laboratory.  For best results, the sample should be transferred to 5% blood agar within 2 hours of collection.  It may be necessary to request Mycoplasma culture in refractory cases or in outbreaks where predisposing factors are unclear.  M. bovis cultures may be serotyped for pili through immunofluorescence or immunogold electron microscopy.  A fluorescent antibody test is available to demonstrate M. bovis in lacrimal secretions.   An ELISA for identifying serum or lacrimal IgG is useful in determining the M. bovis serogroup responsible for the outbreak;  however, these special tests for serotyping are not widely available.

  Treatment of IBK relies on the use of antibiotics and the prevention of further ocular irritation.  Oxytetracycline is the only antibiotic labeled for the treatment of IBK.  Two doses of long-acting oxytetracycline given 72 hours apart is usually effective in accelerating healing time.  Parenteral florfenicol can also shorten healing time.  Daily subconjunctival injections of penicillin have also been used in cases of IBK.  This treatment is economical but requires more handling and labor.  Two doses of benzathine cloxacillin at 375 mg given topically 72 hours apart has been shown to be effective.  However, commercial mastitis preparations currently available are not concentrated enough for use in IBK.  Topical gentamicin and betamethasone spray preparations have also been used topically.  Corticosteroids, such as dexamethasones, are often used to reduce the inflammation and pain associated with IBK although classically contraindicated in cases of corneal ulceration.  The use of 3^rd eyelid flaps, tarsorrhaphy and eye patches can aid in reducing irritation due to environmental factors.  Although treatment reduces healing time, research has not proven the ability of treatment to improve weight gain in affected animals.

  Several commercial vaccines are available, including whole cell monovalent and multivalent M. bovis bacterins.  Vaccination for M. bovis has met with mixed results. It has been shown that lacrimal secretory IgA is required for resistance to reinfection.  The available bacterins produce adequate serum IgG titers which have little correlation with resistance to infection;  therefore, currently available bacterins may offer little protection.  A mucosal vaccine would be much more effective, but none are currently available.  In addition, there is little cross-protection among serotypes.  Monovalent vaccines do not protect against heterologous serotypes.  Studies using multivalent vaccines have reduced the incidence of IBK, but not the severity of disease.  Currently, multivalent bacterins, including the Fla64 and Epp63 strains of M. bovis are available.  While providing cross-protection for other strains within their serogroups, this bacterin cannot protect against strains in the other five serogroups.  These serogroups are less commonly encountered in the United States, but can still cause outbreaks.  Recombinant multivalent vaccines will allow for more serotype inclusion but are not currently available.  Autogenous vaccines may be considered where uncommon serotypes are isolated from a herd.

  Prevention of IBK centers on reducing the predisposing factors of the disease.  Since face flies are the major vector of IBK, prevention should focus on fly control.  The use of insecticide ear tags, back rubs and feed-through insect growth regulators can significantly reduce the incidence of IBK.  In the case of ear tags, both ears must be tagged to effectively reduce the number of face flies.  Affected cattle should be isolated from other cattle if possible.  New additions should be isolated for 30-60 days prior to introduction to the herd.  Mowing tall grasses and noxious weeds from pasture and ensuring adequate head space at the bale feeder can aid in reducing ocular irritation.  In case of an outbreak, mass treatment with parenteral oxytetracycline may be beneficial.  The use of oxytetracycline in the feed may also be considered.  Vaccination with a modified live IBR vaccine should be avoided during an outbreak.  Vaccination with an M. bovis bacterin should be considered in herds with recurring problems with IBK.  A multivalent pili vaccine is recommended.  In addition, culture, serotyping and sensitivity should be performed to determine the bacterial pathogens and serotypes involved in the outbreak.  This information can aid in tailoring treatment and vaccination programs.

-by Jill Franks, Class of 2003

-edited by Dr. Duane Murphy, ADDL  Pathologist

References