Continued...

Discussion :-

Encephalitis is an infection of the parenchyma of the brain (in contrast to meningitis, an infection of the meninges). The most common cause of viral meningitis is enterovirus infection, whereas one of the more common causes of viral encephalitis is HSV. The major manifestations of viral meningitis include fever, headache, irritability, mild lethargy or drowsiness, and some degree of nuchal rigidity. On the other hand, encephalitis often presents with significant neurologic signs and symptoms in addition to an acute febrile illness. In the usual case of viral meningitis, the disease is relatively benign and self-limiting. Encephalitis, however, is a disease which can be associated with significant morbidity and mortality (1). It is not unusual for encephalitis and meningitis to co-exist in the same patient as meningoencephalitis. Encephalitis must also be distinguished from cerebritis. Cerebritis is a highly destructive inflammatory condition of the brain, which may be bacterial in origin, often leading to abscess formation. Encephalitis is most commonly viral in nature and the extent of parenchymal damage is variable. Arboviruses are the most common cause of episodic viral encephalitis. Two of the most common forms of arboviral encephalitis are:

1. St. Louis encephalitis, which is mainly found in urban areas around the Mississippi River, and
   
2. California encephalitis, which is principally found in rural areas of the northern Midwest and East. Eastern equine encephalitis is one of the most severe causes of arboviral encephalitis, but it is uncommon (mostly in the New England area). Western equine encephalitis is most prevalent in the rural areas west of the Mississippi (2). The arboviruses tend to have a seasonal preference for the summer and early fall months because they are largely transmitted via arthropod vectors.
   HSV is the most common cause of sporadic viral encephalitis in the United States, accounting for approximately 10 to 20 percent of all cases. It is estimated that about 2 persons per million per year will suffer from HSV encephalitis. HSV-1 is acquired more frequently and earlier in life than HSV-2. By the fifth decade of life, more than 90 percent of adults will be HSV-1 seropositive. It has been also estimated that about 22 percent of the adults in the United States are HSV-2 seropositive. HSV-1 accounts for more than 95 percent of all cases of HSV encephalitis (3). Unlike other sources of viral encephalitis, HSV encephalitis has no seasonal pattern. HSV encephalitis is most prevalent in the neonatal, 5 to 30, and over 50 age groups.
   
   Neonates (infants less than 6 weeks old) have the highest incidence of HSV visceral and/or CNS infection among all age groups. Untreated, neonatal herpes infection disseminates or invades the CNS in greater than 70 percent of cases. In contrast to childhood/adult HSV encephalitis, 70 percent of neonatal HSV infection is caused by HSV-2, acquired from contact with infected genital secretions at time of birth
   
   
3. The incidence is about 1:3500-5000 deliveries, with signs and symptoms manifesting 4-11 days after delivery