Viral meningitis is the most frequent infectious cause of aseptic meningitis, occurring at a rate of about 17.8 per 100,000 persons per year.1 Viral meningitis is more common in infants less than 1 year old and is more frequent in the summer months.1,9 The most common cause of viral meningitis in children are enteroviruses.11 Two common types of enteroviruses are coxsackie and echoviruses. Additionally, nonviral infectious causes of aseptic meningitis include mycobacterial and spirochetal (eg, Borrelia burgdorferi).2
Viral encephalitis is the least frequent of the CNS infections discussed here, with traditional estimates approximating about 6.3 per 100,000 person-years.1 These numbers are likely slightly higher due to the introduction of newer viruses, especially West Nile virus over the past decade.12 Unfortunately, the majority of encephalitis cases have an unknown etiology.13 The most frequent virus identified is herpes simplex virus (HSV), which has been estimated to occur in about 1 in every 250,000 to 500,000 persons per year.14,15 Other viral causes include: enteroviruses, other herpes viruses (eg, varicella, cytomegalovirus, Epstein-Barr virus, and human herpes virus 6), adenoviruses, arboviruses (eg, West Nile virus, St. Louis encephalitis virus, Eastern equine encephalitis virus, and Western equine encephalitis virus), measles, mumps, and rubella viruses, influenza viruses, bunyaviruses, reoviruses, arenaviruses, and rabies viruses.1,2,11,16
Pathogens that cause aseptic meningitis and viral encephalitis enter the body as they would usually to cause infection (eg, orally for enterovirus, tick bite for B. burgdorferi).17 The pathogen however moves from the usual site of infection via the blood to the CNS where it leads to encephalitis or meningitis.17 Herpes viruses are unusual as they can travel to the CNS by either the blood or travel along the neuron. Additionally, herpes viruses are also unique as they can cause meningitis/encephalitis during initial disease or by reactivation of disease.17 A summary of the typical infectious causes of meningitis and encephalitis by age are shown in Table 24-1.2-9,11-16,18