Herpes Simplex Type 1 Encephalitis

Abstract

Encephalitides, an acute infection of the brain parenchyma, are characterized by fever, headache and altered consciousness. Neurological deficits and focal or generalized epileptic seizures may also be seen. There are important differences in clinical presentations between encephalitides caused by viruses. While some viral encephalitides, such as Herpes simplex virus type-1 (HSV type-1) encephalitis, cause sporadic infection; others, such as Japanese B encephalitis virus and Eastern equine encephalitis virus, cause epidemic infections with specific geographic distribution. Some viruses like HSV cause fulminant encephalitis leading to death within a couple of days whereas viruses such as Measles virus can cause progressive subacute sclerosing panencephalitis lasting several months and years. HSV type-1, HSV type-2, LaCrosse encephalitis virus, St. Louis encephalitis virus usually causes encephalitis in healthy individuals, whereas HSV type-1, Cytomegalovirus, Varicella-zoster virus, Epstein-Barr virus, Human herpes virus type-6 and Enteroviruses are associated with encephalitides in immunodeficient or immunocompromised patients (Mathewson Commission, 1929; Meyer et al.,1960; Roos,1999). Herpes simplex virus (HSV) is the most common cause of sporadic fatal encephalitis (Mathewson Commission, 1929; Meyer et al.,1960; Smith et al., 1941). Smith et al. detected inclusion bodies consistent with HSV infection from a newborn’s brain with encephalitis and virus was isolated from brain tissue then (Smith et al., 1941). The first adult case of HSE was reported by Zarafonetis et al. (Zarafonetis et al., 1944). The pathological findings in this patient's brain were prominent perivascular cuffing of lymphocytes and a large number small hemorrhages in left temporal lobe. Later in several studies, this temporal lobe localization was reported to be characteristic for HSE in patients older than 3 months. In the mid 1960s, Nahmias and Dowdle found two distinct antigenic type of HSV, as HSV type-1 and HSV type-2 (Nahmias & Dowdle, 1968). The HSE, observed in adults, is caused by HSV type-1 predominantly (Dennett et al., 1997; Whitley & Lakeman, 1995). HSV type-2 is rarely seen in healthy adults and usually causes benign CNS infection, whereas severe meningoencephalitis is seen in immunosuppressed individuals (Mommeja-Marin et al., 2003). Herpes neonatorum, transmitted from perinatal area, causes severe encephalitis in neonates (Corey et al., 1983). HSV type-1 and HSV type-2 are from Herpesviridae family. The common feature of Herpesviridae family is that they stay in life-long latent (persistent) form in the organism and they can reactivate later leading recurrent infections. Also other viruses from Herpesviridae group may lead CNS diseases (Garcia-Blanco et al., 1991).