STRUCTURE CLINICAL FEATURE AND LAB DIAGNOSIS OF HERPES SIMPLEX VIRUS

T.U 2058, 2060

STRUCTURE CLINICAL FEATURE AND LAB DIAGNOSIS OF HERPES SIMPLEX VIRUS

These viruses are members of the Alphaherpesvirinae subfamily of human

herpesviruses together with varizella-zoster virus, also called human herpesvirus. HSV is a large virus with a core containing double-stranded DNA within a coat, an icosahedron with 162 capsomeres. The envelope which surrounds the ‘naked’ particle is partly nuclear membrane derived, partly virally coded, with glycoprotein spikes. The diameter of a complete particle is 120–200 nm. The ‘naked’ virion measures about 100 nm.


The virus enters the cells through cellular membrane fusion after being attached to specific receptors through an envelope glycoprotein. The capsid is transported to nuclear pores where DNA circularizes after uncoating, and enters the nucleus. The genome is a 120–230 kbp double-stranded, linear DNA with repeated sequences located at each flank and certain other regions. Genomic rearrangements giving genomic ‘isomers’ may occur, but the biological significance of this is unknown. HSV1 and HSV2 show 50% sequence homology.

‘Fingerprinting’ of separated restriction enzyme digests shows differences between HSV1 and HSV2, and between strains within each type. This may allow epidemiological tracing. Expression of the genome is strictly regulated, and occurs in a certain order. Firstly alpha proteins then beta proteins and lately gamma proteins, mostly structural proteins, are produced. There are 35 structural proteins, and the genome may code for at least 65 additional proteins, e.g. viral enzymes not incorporated into the virion. Viral DNA is synthesized by the help of a large number of viral enzymes which may be targets for antiviral drugs. The newly synthesized viral genomes are packed into empty nucleocapsids in the cell nucleus. The synthesis of viral RNA is possible through the replicative cycle of viral DNA using cellular RNA polymerase II with viral factors participating. The filled nucleocapsids bud through an altered inner nuclear membrane.

Viral particles can then be released through the cellular membrane or through vacuoles on the surface as an enveloped virus. The herpes virus has an 18 hour replication cycle. Cellular macromolecular synthesis is shut off early during the replicative cycle, leading to cell death.

CLINICAL FEATURES

SYMPTOMS AND SIGNS

HSV1 infections usually affect the oral cavity, lips or face and occasionally genital region, and HSV2 infections are confined to genital region. In Herpes 1, labial herpes (‘cold sore’) is the most common manifestation.

Primary infection

The primary infection is symptomless in most cases, but may present as fever, enlarged submandibular lymph nodes, sore throat, gingivostomatitis with ulcers or vesicles, oedema, with associated anorexia, pain and malaise. This condition usually lasts for 10–21 days, and may be accompanied by inability to eat or drink. Dehydration may be a problem, especially in small children.

Symptomatic primary infection is most common in children of 1–5 years of age, with an incubation time of 2–12 days, mean about 4 days. There may be a prodrome of burning, itching or tingling pain for some hours followed by groups of vesicles usually on the external borders of the lips. Lesions may also be found in skin surrounding the lips; chin,  cheeks or nose. Within a few days the vesicles progress to pustules or ulcers with brownish-yellow crusts. Pain is most severe in the beginning and resolves during the next 4–5 days.

The Herpes 2 infection usually affects the genital regions. The primary genital infection may be severe, with illness usually lasting up to about 3 weeks (sometimes longer) with a shedding period of virus usually terminating shortly before or at the time of healing. The lesions are vesicles or ulcers localized to the cervix, vagina, vulva or perineum of the female, or the penis in the male. The lesions are painful, and may be associated with inguinal lymphadenopathy and dysuria. Systemic complaints, including fever and malaise, usually occur. Complicating extragenital affections, including aseptic meningitis, have been observed in about 10–20% of cases. Paraesthesia or dysesthesia may occur after the genital affection. Especially in women the severity of the primary infection may be associated with a high number of complications and frequent recurrences. Previous HSV1 infection reduces the severity and duration of primary HSV2 infection. The recurrent genital affection is usually milder, with fewer vesicles or ulcers and with duration of 7–10 days. The recurrent lesions seldom last more than 10 days, and the shedding of virus may terminate sooner. Sometimes, virus excretion can occur between active periods.

Neonatal herpes

Infection in the newborn may be acquired in utero, at or just after birth. The newborn has low resistance to this infection, and usually develops severe disease. The mortality rate of untreated disease is about 50%. Babies with neonatal herpes infection may develop:

. A disseminated generalized form with many affected organs, including the

central nervous system (CNS).

. Encephalitis with or without herpetic lesions of the skin.

. Herpetic lesions localized to skin, mouth and eyes.

The generalized form is especially serious, and is often combined with intravascular coagulopathy, hepatic and adrenal necrosis, pneumonitis and/or encephalitis followed by permanent neurological sequelae if the patient survives. The congenital infection may induce malformations such as microcephaly or microphthalmia, or other symptoms such as jaundice, hepatosplenomegaly, bleeding diathesis, seizures, irritability, chorioretinitisand herpetic vesicles of the skin.

HSV encephalitis.

In the USA HSV1 is considered the most common viral strain of fatal encephalitis.The lesion is usually a local process in the brain, consisting of haemorrhagic necrosis and oedema, mimicking a brain tumour. The localization is usually one of the temporal lobes. At later stages, however, the expansion retracts leaving scar tissue and midline structures deviating to the affecting side.

In immunocompromised patients the HSV infection may be severe, especially if the cellular immunity is reduced. This is true both in patients with a disease affecting the immune system, e.g. AIDS, and in patients under immunosuppressive treatment. Especially bone marrow, renal and cardiac transplant recipients are at risk for severe herpes infections. The lesions may be progressive, and involve unusual sites such as the respiratory tract, oesophagus, liver and intestinal mucosa, or occur as a disseminated infection in severely immunocompromised patients. The severity of the disease is directly related to the degree of immunosuppression, and will also last longer than usual, about 6 weeks. Malnourishment, especially in children, seems to aggravate symptoms. Even immunocompromised patients may discharge virus asymptomatically.

The initial infection with herpesvirus may be located in the eye, with severe

keratoconjunctivitis as a result. Recurrent infections of the eye may appear as ulcers of the cornea, sometimes dendritic ulcers, or as vesicles on the eyelids. Later chorioretinitis may develop. The cornea may develop opacifications after recurrence, indicating a progressive involvement. Blindness may be the consequence. Even herpetic necrosis of the retina has been observed as a very rare consequence of the infection.

LABORATORY DIAGNOSIS OF HERPES SIMPLEX VIRUS

Isolation in tissue culture is still useful. Electron microscopy or immunofluorescence is also useful in early cultures, and immunofluorescence when studying infected cells directly from the patient. Serology is of no use in recurrences except in cases of encephalitis. The encephalitic lesion may be visible early on in images made by computerized tomography (CT) or magnetic resonance imaging (MRI), and develops faster than a (malignant) neoplastic tumour. The aetiological diagnosis can be established by biopsy of affected brain tissue, but this procedure has been replaced by indirect methods such as tests for local antibody production and HSV PCR in CSF. Production of antibody to HSV within the brain starts as early as 2–3 days after clinical symptoms in a small fraction of patients. However, this fraction increases to nearly 100% over the next 2 weeks. A possible serological cross-reaction between HSV and VZV is of minor clinical importance as the treatment would be the same. The HSV PCR, using prepared CSF as template, is sensitive and will be positive early in the disease. After 5–6 days the sensitivity falls, probably as a result of the removal of virus by local anti-HSV antibodies. Thus, HSV PCR and CSF antibody tests complement each other. The PCR method can, by using specific probes, also identify the viral type. Diseases mimicking oral herpes infections are other vesicular or ulcerating lesions: herpangina caused by coxsackie virus, mononucleosis by Epstein–Barr virus, Stevens–Johnson syndrome, aphthous stomatitis, bacterial infections or lesions caused by drug intolerance, irradiation or immunosuppressive therapy.

The diagnosis of genital HSV infection should include chancroid, syphilis, genital lesions, Behc¸et syndrome, erythema multiforme, local candidiasis and simple erosions. The symptoms of neonatal herpes can be caused by other infections such as rubella, cytomegalovirus and toxoplasma, and sometimes by erythroblastosis.