Listeria monocytogenes is ubiquitous in the farm environment and can reach counts of 108 cfu /g wet weight of silage in some poorly fermented silages. Listerial keratoconjunctivitis (‘silage eye’) is a wide spread problem in ruminants causing economic losses to farmers and impacting on animal welfare. It has been long established that L. monocytogenes can infect rabbit and guinea pig conjunctiva leading to internalisation into epithelial cells and neutrophils and can also progress to the infection of corneal epithelial cells.
The aims of this study were to determine whether L. monocytogenes can be isolated from healthy cattle eyes and if they can infect conjunctiva explants. In addition a range of different isolates were tested to determine any strain specific invasiveness of these cells.
The detection limit for conjunctiva swabs (inoculated with known amounts of Listeria into conjunctiva post slaughter) was found to be 3 x 103 cfu per conjunctiva. Interestingly, three L. monocytogenes isolates were isolated from healthy bovine eyes (n=33) suggesting that this organism can be present without causing disease. Conjunctiva explants were obtained and cultured from cattle eyes post slaughter and infected for 20 h with different L. monocytogenes isolates including two of the healthy bovine eye isolates. Most isolates were recovered at level of log10 2-5 cfu per explant, except for one of the healthy bovine eye isolates that was only recovered at a very low level from 1 out of 7 explants infected.
In summary, L. monocytogenes was present at detectable levels in 10% of healthy bovine eyes. In addition, we have established a bovine conjunctiva explant model for bacterial infection. Using this model different L. monocytogenes were found to be recovered at different levels, however, more work is required to determine whether this represents tissue tropism or is due to variation within infection model.