Recent Discoveries About Infectious Bovine Keratoconjunctivitis
ACVIM 2008
D. John Angelos, DVM, PhD, DACVIM
Davis, CA, USA

Introduction

Infectious bovine keratoconjunctivitis (IBK; "pinkeye"), the most common ocular disease of cattle occurs in cattle populations throughout the world. Classic signs of IBK include corneal ulceration, corneal edema, lacrimation, photophobia, and blepharospasm. In most cases animals recover with mild to moderate corneal scarring, however, severely affected animals may develop a ruptured cornea and prolapse of the lens/iris that leads to permanent blindness. All breeds of cattle may be affected, however, some breed predilections are reported with higher numbers of cases in Hereford cattle.1 Lower incidences in Brahman cattle and in cattle with greater pigmentation at the ocular margins have been reported.2,3 Economic losses due to reduced weight gains along with treatment associated expenses accounted for high annual losses in the USA estimated at $150 million in 1976.4 In a study of over 45,000 health records of weaned calves, lighter weaning weights of nearly 20 pounds were observed as compared to healthy calves.1

Etiology

Koch's postulates have only been satisfied for Moraxella bovis (M. bovis) and IBK.5 Despite the association of M. bovis with IBK, other potentially pathogenic bacteria have been reported from IBK-affected cattle. These bacteria have included hemolytic gram-negative (G-) cocci from calves with severe keratitis and corneal ulceration,6 Neisseria spp in 24 of 25 outbreaks of IBK,7 and Neisseria (Branhamella) catarrhalis.8 Moraxella bovis and Neisseria spp have also been isolated from healthy bovine eyes.9,10 An organism identified as Neisseria ovis was experimentally inoculated into the eyes calves, however, IBK did not develop.11

In summer 2002, 138 IBK-affected corneas in beef and dairy calves from northern California were cultured and hemolytic gram-negative cocci (HGNC) were isolated 68 times, and M. bovis was isolated 29 times; biochemical and molecular differences between the HGNC and other species of Moraxella warranted classification of the HGNC as a novel species of Moraxella which we named Moraxella bovoculi (M. bovoculi).12

Pathogenesis

In Moraxella bovis, the expression of two proteins are necessary for pathogenesis: cytotoxin and pilin. Pili of M. bovis are the N-methyl phenylalanine type (type 4 pili 13-15) and these proteins allow M. bovis to adhere to the corneal epithelium and colonize the surface of the cornea.16-18 The M. bovis cytotoxin is an RTX (Repeats in the structural ToXin) toxin encoded by a classical RTX operon that is encoded in a pathogenicity island from M. bovis.19-21 Broth supernatants of hemolytic but not nonhemolytic strains of M. bovis will lyse bovine erythrocytes, neutrophils, lymphoma cells and corneal epithelial cells in vitro.22-25 The lytic activity of M. bovis cytotoxin is via calcium-dependent formation of transmembrane pores in target cell membranes.26 Ocular lesions induced by a purified hemolytic and cytolytic fraction of M. bovis are identical to the ocular lesions observed in naturally occurring IBK; extracts from nonhemolytic M. bovis do not result in corneal lesions.27

The most well characterized RTX toxin is HlyA of uropathogenic E. coli. The gene encoding HlyA, hlyA, is contained within a 4 gene operon organized 5'-C-A-B-D-3'. The product of the RTX A gene is a structural toxin that requires activation by the RTX C gene product to become hemolytic.28-30 This activation occurs via fatty acylation of specific lysines in the protein.31,32 Following activation, HlyA is secreted by membrane transport proteins encoded by the B and D genes and a third protein, TolC.33,34 The regulation of transcription through the RTX operon in E.coli involves the protein RfaH and JUMPStart DNA sequences.35 Similar to the E. coli hemolysin, the M. bovis cytotoxin gene (mbxA) is contained within an operon that encodes activation, and export proteins; this operon is called the mbx operon of M. bovis, and it is absent in nonhemolytic M. bovis.20,21 The mbx operon defines a pathogenicity island and acquisition/loss of mbx genes may explain the ability of M. bovis to change from a hemolytic to nonhemolytic phenotype.21

Prevention

Prevention of IBK hinges on minimizing risk factors for disease, reducing infection of the ocular surface with M. bovis via antimicrobial use, and vaccination. For reduction of fly populations, ear tags that are impregnated with insecticide, and topically administered insecticides administered by way of back and face rubbers can be used. Clipping mature grasses before turn out may also help to reduce the risks of mechanical corneal injury associated with exposure to plant awns such as foxtails.

Moraxella bovis is susceptible to antimicrobials including penicillin administered subconjunctivally,36 oxytetracycline administered parenterally or orally (20 mg/kg once or twice followed by 2 g/calf/day for 10 days),37-39 florfenicol injected intramuscularly (two 20 mg/kg injections, 48 hours apart), or subcutaneously (40 mg/kg one time),40,41 ceftiofur crystalline-free acid administered via subcutaneous injection (6.6 mg of ceftiofur equivalents/kg one time) in the posterior aspect of the pinna,42 and tulathromycin (2.5 mg/kg) injected subcutaneously.43

For vaccination against IBK, most producers utilize commercially available bacterins. Many different commercial vaccines are available for producers; due to strain differences, these may or may not be effective in particular herds. Where commercial vaccines are not effective, some vaccine companies offer autogenous bacterins, which anecdotally are effective. Other anecdotal evidence suggests that the use of autogenous vaccines against non-M. bovis isolates of gram-negative cocci from IBK-affected eyes may provide an alternative strategy to prevent IBK versus conventional commercial M. bovis vaccines where such vaccines are ineffective. One vaccine study that examined the use of an M. bovis pilin-cytotoxin recombinant vaccine against IBK reported that pilin-MbxA vaccinated calves had the lowest overall cumulative proportion of ulcerations.44 Ocular culture result data suggested that vaccination with an M. bovis antigen had an effect upon the particular Moraxella species isolated from an ulcerated cornea. Moraxella bovis was cultured more often from the eyes of control calves than from the eyes of calves vaccinated with cytotoxin (recombinant carboxy terminus of cytotoxin [MbxA]) and pilin-MbxA.44 Moreover, the vaccination of calves with MbxA and pilin-MbxA resulted in a higher prevalence of non M. bovis species of Moraxella (Moraxella bovoculi) in ocular cultures.44 Studies are currently underway to evaluate the efficacy of both subunit and whole M. bovoculi and M. bovis vaccines to prevent IBK.

Based on current published as well as anecdotal evidence, it is likely that herds of cattle from which both M. bovis and M. bovoculi are present would benefit from vaccines that incorporate antigens from M. bovis as well as M. bovoculi to prevent IBK.

References

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Speaker Information
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John Angelos, DVM, PhD, DACVIM
University of California - Davis
Davis, CA


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