Etiology and pathogenesis
The number of gram negative bacteria proven or implicated to cause pinkeye (IBK) in cattle has expanded over the years. Additionally,
many of the organisms have changed names over the years as bacteriologic nomenclature has changed. The nominal cause of IBK
is Moraxella bovis; however, the mechanism of damage to the eye may be the most important consideration as will discuss later. In addition to
M. bovis, other organisms that have been implicated as a cause of IBK include Neisseria (Brahmanella) catarrhalis, N. ovis, and M. ovis. Recently, hemolytic gram negative cocci isolated from IBK cases have been characterized and the name Moraxella bovoculi has been proposed.
Two microbial factors are important for the development of IBK. The first are the pilin proteins which allow the organisms
to adhere to the corneal epithelium and to colonize the surface of the cornea. The second are the cytotoxins produced by the
hemolytic strains of M. bovis and perhaps the other pathogens. The M. bovis cytotoxin (cytolysin/hemolysin) is a pore-forming protein that appears to "punch" holes in the surface of the corneal epithelium.
This cytotoxin is a 110 Kd protein antigenically related to the RTX toxin of uropathogenic E. coli. Other cattle pathogens such as Mannheimia hemolytica, also contain RTX toxins.
Predisposing factors for IBK
There are a host of environmental and management factors that can exacerbate the occurrence of IBK cases or that help precipitate
outbreaks. An important factor is UV radiation—sunlight—IBK occurs more often the summer and UV radiation damages the corneal
epithelium. Another factor is mechanical irritation and damage of the eye by dust, plant pollen, or weed seeds. These materials
can also irritate the eyes of "carrier cows' in the herd causing M. bovis or other potential pathogens to be shed in the lacrimal or nasal secretions of these animals. Plant awns or foxtails also
irritate the eyes of cattle and can cause lesions similar to IBK. The handling of these cattle—if they are concurrently harboring
pathogens—can aid in the spread of these pathogens iatrogenically to other cattle. Face flies not only irritate the eyes but
they also can easily spread the bacteria to susceptible animals in the herd. Therefore, fly control is central to IBK prevention
in most instances. Any factor that irritates the eyes, causing cattle to have excess lacrimal and/or nasal discharges, or
transfers potentially infectious material from one animal to another will be an important factor in predisposition to IBK.
Clinical diagnosis is straightforward and based on the appearance of a central corneal ulcer and any additional sequelae—corneal
clouding, conjunctivitis, uveitis. The lesions caused by plant awns lodged in the eyes are typically eccentric and the plant
awn can usually be visualized. The eye can be cultured for bacterial isolation and identification and antimicrobial sensitivity
testing. In most cases this is not done, but may yield important information in severe or prolonged outbreaks.
Antimicrobial therapy is indicated and will be the basis of outbreak management along with the elimination of any predisposing
factors. The most productive treatment regimens based on research in beef cattle are outlined below.
1. Long-acting Oxytetracycline (200 mg/ml—Biomycin 200 or LA-200). Dosed at 9 mg/lb (20 mg/kg) SQ or IM at 48 to 72 hour intervals.
2. Florfenicol (Nuflor). Dosed at 9 mg/lb (20 mg/kg) IM repeated in 24 to 48 hours.
3. Florfenicol (Nuflor). Dosed at 18.2 mg/lb (40 mg/kg) SQ as a single treatment.
4. Ceftiofur (Excede). Dosed at 3 mg/lb (6.6 mg/kg) SQ as a single treatment.
5. Tulathromycin (Draxxin). Dosed at 1.1 mg/lb (2.5 mg/kg) SQ as a single treatment.
In addition to these treatment options, procaine penicillin G (300 IU) given subconjunctivally in the bulbar conjunctiva for
3 days has been shown to be efficacious. The use of dexamethasone (1 mg) also given subconjunctivally has been shown to cause
no measurable advantages or disadvantages in IBK management. Neither florfenicol or ceftiofur are currently labeled for IBK
therapy and a veterinarian's prescription and appropriate withdrawal times are necessary for use.