The morphology of the bacteria is consistent with that of Listeria monocytogenes. Listeria monocytogenes is a gram-positive, facultative, anaerobic bacillus. It is ubiquitous in the environment, particularly in temperate zones, and has been isolated from fish, birds and mammals. The bacterium is psychrophilic and some of its virulence factors such as hemolysin are induced or enhanced at low temperature. Coupled with increased stress to ruminants in winter months, the psychrophilia is thought to be responsible for the increased number of cases seen during this time.
Listerial encephalitis is almost solely a disease of adult animals.
Listeria has an affinity for the brain stem, the lesions being most severe in the medulla and pons and less severe anteriorly in the thalamus and posteriorly in the cervical parts of the spinal cord. The proposed route of infection is by entry through a defect in the oral mucous membrane, penetration into a nerve and centripetal spread to the CNS via the cranial nerves. The trigeminal nerve is particularly implicated. There is correlation between listerial encephalitis and trigeminal neuritis and between the severity of the trigeminal neuritis and ipsilateral lesions in the brain stem. L. monocytogenes has been demonstrated in myelinated axons of the trigeminal nerve and fiber tracts in the brain stem and in the cytoplasm of medullary neurons.