Walsh & Hoyt: Cause

The cause of EL is unknown but is thought to be an infectious organism. Von Economo was able to produce the disease in monkeys by subdural injection of filtered brain tissue from affected patients; however, he was unable to identify the agent responsible. Most investigators favor a viral etiology, and the most likely candidate for both epidemic and sporadic EL is the influenza virus. Other viruses implicated in the production of EL include HSV, Japanese B encephalitis virus, Western equine encephalitis virus, central European tick-borne encephalitis virus, coxsackievirus group B, lymphocytic choriomeningitis virus, and HIV or other retrovirus. To date, although occasional cases with some, but not all, of the characteristics of EL have been shown to be caused by a particular agent, such as coxsackievirus B4, no viruses, viral nucleic acids, viral antigens, or antiviral antibodies have been detected with regularity in patients with classic acute EL or in patients with PEP, even using sensitive techniques such as reverse-transcriptase PCR.