Viral Encephalomyelitis

Viral encephalomyelitis is an important cause of morbidity and mortality worldwide, and many encephalitic viruses are emerging and re-emerging due to changes in virulence, spread to new geographic regions, and adaptation to new hosts and vectors [1]. The term encephalomyelitis refers to inflammation in the brain and spinal cord that results from the immune response to virus infection. In humans, the viruses most commonly identified as causes of viral encephalomyelitis are herpesviruses and RNA viruses in the enterovirus (e.g., polio, enterovirus 71), rhabdovirus (e.g., rabies), alphavirus (e.g., eastern equine, Venezuelan equine, and western equine encephalitis), flavivirus (e.g., West Nile, Japanese encephalitis, Murray Valley, and tick-borne encephalitis), and bunyavirus (e.g., La Crosse) families. Other virus families with members that can cause acute encephalitis are the paramyxoviruses (e.g., Nipah, Hendra) and arenaviruses (e.g., lymphocytic choriomeningitis, Junin). However, this is certainly not a complete list, because for most cases of human viral encephalitis the etiologic agent is not identified, even when heroic attempts are made [2]. The primary target cells for most encephalitic viruses are neurons, although a few viruses attack cerebrovascular endothelial cells to cause ischemia and stroke or glial cells to cause demyelination, encephalopathy, or dementia [3–5]. Widespread infection of neurons may occur or viruses may display preferences for particular types of neurons in specific locations in the central nervous system (CNS). For instance, herpes simplex virus (HSV) type 1 often infects neurons in the hippocampus to cause behavioral changes, while poliovirus preferentially infects motor neurons in the brainstem and spinal cord to cause paralysis and Japanese encephalitis virus infects basal ganglia neurons to cause symptoms similar to those of Parkinson’s disease. Because infections with encephalitic viruses are initiated outside the CNS (e.g., with an insect bite, skin, respiratory, or gastrointestinal infection), innate and adaptive immune responses are usually mounted rapidly enough to prevent virus entry into the CNS. Therefore, most viruses that can cause encephalitis more often cause asymptomatic infection or a febrile illness without neurologic disease, and encephalomyelitis is an uncommon complication of infection.