More recently, a sometimes overwhelming encephalitis has been recognized as a rare manifestation of influenza infection, particularly the H1N1 strain that has infected mainly children in Southeast Asian countries, but also other serotypes of influenza including mundane influenza viruses that cause yearly outbreaks. The disorder has been called an “encephalopathy” in research publications but convulsions, delirium, and coma suggest that the neurologic aspects are from encephalitis.
The relative frequency of the various viral infections of the nervous system can be appreciated from several studies. An early series from the Walter Reed Army Institute comprising 1,282 patients is particularly noteworthy in that a positive laboratory diagnosis was achieved in more than 60 percent of cases (Buescher et al)—a higher rate than in almost any subsequent study of comparable size. Aside from the poliovirus (some of the data were gathered before 1959) the common infective agents in cases of both aseptic meningitis and encephalitis were group B Coxsackie virus, echovirus, mumps virus, lymphocytic choriomeningitis virus, arboviruses, HSV, and Leptospira, in that order. In a later prospective virologic study of all children examined at the Mayo Clinic during the years 1974 to 1976, a diagnosis of aseptic meningitis, meningoencephalitis, or encephalitis was entertained in 42 cases and an infectious agent was identified in 30 of them (Donat et al). The California virus was isolated in 19 cases and one of the enteroviruses (echovirus types 19, 16, 21, or Coxsackie virus) in 8 cases; mumps, rubeola, HSV, adenovirus 3, and M. pneumoniae were detected in individual cases (several patients had evidence of combined infections). As mentioned, recent outbreaks of West Nile virus, close to 3,000 cases yearly in the United States, make it of more current import than some of the viral infections listed here. The related Japanese encephalitis virus is even more ubiquitous on a worldwide basis, causing 10,000 deaths in Asia each year.
In a more contemporary and impressively large series of viral infections of the nervous system from the United Kingdom involving more than 2,000 patients, viral identification in the CSF was attempted by means of PCR with positive results in only 7 percent, half of which were various enteroviruses (Jeffery et al). The other organisms commonly identified were HSV-1, followed by VZV, EBV, and other herpesviruses. In patients with HIV however, the relative frequencies of the organisms that cause meningoencephalitis are quite different and include special clinical presentations; this applies particularly to CMV infection of the nervous system, as discussed in the following text, under “Opportunistic Infections and Neoplasms of the CNS with HIV.” Our personal experience, predicated on practicing in New England, has been heavily biased toward HSV encephalitis, seasonal outbreaks of eastern equine or West Nile encephalitis, and HIV-related cases.