Herpes Simplex Virus Proteins

The HSV genome is large enough to encode approximately 80 proteins. Only half the proteins are required for viral replication; the others facilitate HSV’s interaction with different host cells and the immune response. The HSV genome encodes enzymes, including a DNA-dependent DNA polymerase and scavenging enzymes, such as deoxyribonuclease, thymidine kinase, ribonucleotide reductase, and protease. Ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides, and thymidine kinase phosphorylates the deoxyribonucleosides to provide substrates for replication of the viral genome. The substrate specificities of these enzymes and the DNA polymerase differ significantly from those of their cellular analogues and thus represent potentially good targets for antiviral chemotherapy.

HSV encodes at least 10 glycoproteins that serve as viral attachment proteins (gB, gC, gD, gE/gI), fusion proteins (gB, gH/gL), structural proteins, immune escape proteins (gC, gE, gI), and other functions. For example, the C3 component of the complement system binds to gC and is depleted from serum. The Fc portion of immunoglobulin G (IgG) binds to a gE/gI complex, thereby camouflaging the virus and virus-infected cells. These actions reduce the antiviral effectiveness of antibody.

Replication

HSV can infect most types of human cells and even cells of other species. The virus generally causes lytic infections of fibroblasts and epithelial cells and latent infections of neurons (see Chapter 44, Figure 44-12, for a diagram).

HSV-1 binds quickly and efficiently to cells through an initial interaction with heparan sulfate, a proteoglycan found on the outside of many cell types, and then through a tighter interaction with receptor proteins at the cell surface. Penetration into the cell requires interaction with nectin-1 (herpesvirus entry mediator C), an intercellular adhesion molecule that is a member of the immunoglobulin protein family and similar to the poliovirus receptor. Nectin-1 is found on most cells and neurons. Another receptor is HveA, a member of the tumor necrosis factor receptor family that is expressed on activated T cells, neurons, and other cells. HSV can penetrate the host cell by fusion of its envelope with the cell surface membrane. On fusion, the virion releases its capsid into the cytoplasm, along with a protein that promotes the initiation of viral gene transcription, a viral-encoded protein kinase, and cytotoxic proteins. The capsid docks with a nuclear pore and delivers the genome into the nucleus.

The immediate early gene products include DNA-binding proteins, which stimulate DNA synthesis and promote the transcription of the early viral genes. During a latent infection of neurons, the only region of the genome to be transcribed generates the latency-associated transcripts (LATs). These RNAs are not translated into protein but encode micro-RNAs that inhibit expression of important immediate early and other genes.

The early proteins include the DNA-dependent DNA polymerase and a thymidine kinase. As catalytic proteins, relatively few copies of these enzymes are required to promote replication. Other early proteins inhibit the production and initiate the degradation of cellular messenger RNA (mRNA) and DNA. Expression of the early and late genes generally leads to cell death.

The genome is replicated as soon as the polymerase is synthesized. Circular, end-to-end concatameric forms of the genome are made initially. Later in the infection, the DNA is replicated by a rolling-circle mechanism to produce a linear string of genomes that, in concept, resembles a roll of toilet paper. The concatamers are cleaved into individual genomes as the DNA is sucked into a procapsid.

Genome replication triggers transcription of the late genes from which structural and other proteins are encoded. Many copies of the structural proteins are required. The capsid proteins are then transported to the nucleus, where they are assembled into empty procapsids and filled with DNA. DNA-containing capsids associate with viral protein-disrupted nuclear membranes and bud into and then out of the endoplasmic reticulum into the cytoplasm. The viral glycoproteins are synthesized and processed like cellular glycoproteins. Tegument proteins associate with the viral capsid in the cytoplasm, and then the capsid buds into a portion of the trans-Golgi network to acquire their glycoprotein-containing envelope. The virus is released by exocytosis or cell lysis. Virus can also spread between cells through intracellular bridges, which allows the virus to escape antibody detection. Virus-induced syncytia formation also spreads the infection.

HSV infection of neurons may result in virus replication or establishment of latency, depending on which viral genes the neuron is capable of transcribing. Transcription of the LAT and no other viral gene will result in latency. As for other alphaherpesviruses, HSV encodes a thymidine kinase (scavenging enzyme) to facilitate replication in nondividing cells, such as neurons. HSV also encodes ICP34.5, a unique protein that has multiple functions to facilitate virus growth in neurons. ICP34.5 removes a cellular block to protein synthesis activated in response to virus infection or as part of the response to interferon-α.

Pathogenesis and Immunity

The mechanisms involved in the pathogenesis of HSV-1 and HSV-2 are very similar (Box 51-2). Both viruses initially infect, replicate in mucoepithelial cells, cause disease at the site of infection, and then establish latent infection of the innervating neurons. HSV-1 is usually associated with infections above the waist, and HSV-2 with infections below the waist (Figure 51-3), consistent with the means of spread for these viruses. HSV-1 and HSV-2 also differ in growth characteristics and antigenicity, and HSV-2 has a greater potential to cause viremia with associated systemic flulike symptoms.

Figure 51-3 Disease syndromes of herpes simplex virus (HSV). HSV-1 and HSV-2 can infect the same tissues and cause similar diseases but have a predilection for the sites and diseases indicated.

HSV can cause lytic infections of most cells and latent infection of neurons. Cytolysis generally results from the virus-induced inhibition of cellular macromolecular synthesis, the degradation of host cell DNA, membrane permeation, cytoskeletal disruption, and senescence of the cell. Visible changes in the nuclear structure and margination of the chromatin occur, and Cowdry type A acidophilic intranuclear inclusion bodies are produced. Many strains of HSV also initiate syncytia formation. In tissue culture, HSV rapidly kills cells, causing them to appear rounded.

HSV initiates infection through mucosal membranes or breaks in the skin. The virus replicates in the cells at the base of the lesion and infects the innervating neuron, traveling by retrograde transport to the ganglion (the trigeminal ganglia for oral HSV and the sacral ganglia for genital HSV) (see Figure 51-5 later). CD8 T cells and interferon-γ are important to maintaining HSV in latency. Upon reactivation, the virus then returns to the initial site of infection, and the infection may be inapparent or may produce vesicular lesions. The vesicle fluid contains infectious virions. Tissue damage is caused by a combination of viral pathology and immunopathology. The lesion generally heals without producing a scar.

Innate protections, including interferon and natural killer cells, may be sufficient to limit the initial progression of the infection. T-helper 1 (TH1)–associated and CD8 cytotoxic killer T-cell responses are required to kill infected cells and resolve the current disease. The immunopathologic effects of the cell-mediated and inflammatory responses are also a major cause of the disease signs. Antibody directed against the glycoproteins of the virus neutralizes extracellular virus, limiting its spread, but is not sufficient to resolve the infection. In the absence of functional cell-mediated immunity, HSV infection is likely to recur, be more severe, and may disseminate to the vital organs and the brain.

HSV has several ways to escape host protective responses. The virus blocks the interferon-induced inhibition of viral protein synthesis and encodes a protein to plug the transporter-associated-with-processing (TAP) channel, preventing delivery of peptides into the endoplasmic reticulum (ER), which blocks their association with class I major histocompatibility complex (MHC I) molecules and prevents CD8 T-cell recognition of infected cells. The virus can escape antibody neutralization and clearance by direct cell-to-cell spread and by going into hiding during latent infection of the neuron. In addition, the virion and virus-infected cells express antibody (Fc) and complement receptors that weaken these humoral defenses.

Latent infection occurs in neurons and results in no detectable damage. A recurrence can be activated by various stimuli (e.g., stress, trauma, fever, sunlight [ultraviolet B]) (Box 51-3). These events trigger virus replication in an individual nerve cell within the bundle and allow the virus to travel back down the nerve to cause lesions to develop at the same dermatome and location each time. The stress triggers reactivation by promoting replication of the virus in the nerve, by transiently depressing cell-mediated immunity, or by inducing both processes. The virus can be reactivated despite the presence of antibody. However, recurrent infections are generally less severe, more localized, and of shorter duration than the primary episodes because of the nature of the spread and the existence of memory immune responses.

Epidemiology

Because HSV can establish latency with the potential for asymptomatic recurrence, the infected person is a lifelong source of contagion (Box 51-4). As an enveloped virus, HSV is transmitted in secretions and by close contact. The virus is very labile and is readily inactivated by drying, detergents, and the conditions of the gastrointestinal tract. Although HSV can infect animal cells, HSV infection is exclusively a human disease.

HSV is transmitted in vesicle fluid, saliva, and vaginal secretions (the “mixing and matching of mucous membranes”). The site of infection, and hence the disease, is determined primarily by which mucous membranes are mixed. Both types of HSV can cause oral and genital lesions.

HSV-1 is usually spread by oral contact (kissing) or through the sharing of drinking glasses, toothbrushes, or other saliva-contaminated items. HSV-1 can infect the fingers or body through a cut or abraded skin. Autoinoculation may also cause infection of the eyes or fingers.

HSV-1 infection is common. More than 90% of people living in underdeveloped areas have the antibody to HSV-1 by 2 years of age.

HSV-2 is spread mainly by sexual contact or autoinoculation or from an infected mother to her infant at birth. Depending on a person’s sexual practices and hygiene, HSV-2 may infect the genitalia, anorectal tissues, or oropharynx. The incidence of HSV-1 genital infection is approaching that of HSV-2. HSV may cause symptomatic or asymptomatic primary genital infection or recurrences. Neonatal infection usually results from the excretion of HSV-2 from the cervix during vaginal delivery (Clinical Case 51-1) but can occur from an ascending in utero infection during a primary infection of the mother. Neonatal infection results in disseminated and neurologic disease with severe consequences.

Initial infection with HSV-2 occurs later in life than infection with HSV-1 and correlates with increased sexual activity. The current statistics indicate that 25% of adults in the United States are infected with HSV-2, which amounts to approximately 45 million people, with up to 1 million newly infected people per year.

Clinical Syndromes

HSV-1 and HSV-2 are common human pathogens that can cause painful but benign manifestations and recurrent disease. In the classic manifestation, the lesion is a clear vesicle on an erythematous base (“dewdrop on a rose petal”) and then progresses to pustular lesions, ulcers, and crusted lesions (Figure 51-4). Both viruses can cause significant morbidity and mortality on infection of the eye or brain and on disseminated infection of an immunosuppressed person or a neonate.

Figure 51-4 Clinical course of genital herpes infection. The time course and symptoms of primary and recurrent genital infection with herpes simplex virus type 2 are compared. Top, Primary infection; bottom, recurrent disease.

(Data from Corey L, et al: Genital herpes simplex virus infection: clinical manifestations, course and complications, Ann Intern Med 98:958–973, 1983.)

Oral herpes can be caused by HSV-1 or HSV-2. The lesions of herpes labialis or gingivostomatitis begin as clear vesicles that rapidly ulcerate. The vesicles may be widely distributed around or throughout the mouth, involving the palate, pharynx, gingivae, buccal mucosa, and tongue (Figure 51-5). Many other conditions (e.g., Coxsackie virus lesions, canker sores, acne) may resemble HSV lesions.

Figure 51-5 A, Primary herpes gingivostomatitis. B, Herpes simplex virus establishes latent infection and can recur from the trigeminal ganglia.

(A, From Hart CA, Broadhead RL: A color atlas of pediatric infectious diseases, London, 1992, Wolfe. B, Modified from Straus SE: Herpes simplex virus and its relatives. In Schaechter M, Eisenstein BI, Medoff G, editors: Mechanisms of microbial disease, ed 2, Baltimore, 1993, Williams & Wilkins.)

Infected people may experience recurrent mucocutaneous HSV infection (cold sores, fever blisters) (Figure 51-6), even though they never had a clinically apparent primary infection. The lesions usually occur at the corners of the mouth or next to the lips. Recurrent facial herpes infections are generally activated from the trigeminal ganglia. As noted earlier, the symptoms of a recurrent episode are less severe, more localized, and of shorter duration than those of a primary episode. Herpes pharyngitis is becoming a prevalent diagnosis in young adults with sore throats.

Figure 51-6 Cold sore of recurrent herpes labialis. It is less severe than that of primary disease.

(From Hart CA, Broadhead RL: A color atlas of pediatric infectious diseases, London, 1992, Wolfe.)

Herpetic keratitis is almost always limited to one eye. It can cause recurrent disease, leading to permanent scarring, corneal damage, and blindness.

Herpetic whitlow is an infection of the finger, and herpes gladiatorum is an infection of the body. The virus establishes infection through cuts or abrasions in the skin. Herpetic whitlow often occurs in nurses or physicians who attend patients with HSV infections, in thumb-sucking children (Figure 51-7), and in people who have genital HSV infections. Herpes gladiatorum is often acquired during wrestling or rugby.

Figure 51-7 Herpetic whitlow.

(From Emond RTD, Rowland HAK: A color atlas of infectious diseases, ed 3, London, 1995, Mosby.)

Eczema herpeticum is acquired by children with active eczema. The underlying disease promotes the spread of the infection along the skin and potentially to the adrenal glands, liver, and other organs.

Genital herpes can be caused by HSV-1 or HSV-2. In male patients, the lesions typically develop on the glans or shaft of the penis and occasionally in the urethra. In female patients, the lesions may be seen on the vulva, vagina, cervix, perianal area, or inner thigh and are frequently accompanied by itching and a mucoid vaginal discharge. Anal sex can lead to HSV proctitis, a condition in which the lesions are found in the lower rectum and anus. The lesions are usually painful. In patients of both sexes, a primary infection may be accompanied by fever, malaise, and myalgia, which are symptoms related to a transient viremia. The symptoms and time course of primary and recurrent genital herpes are compared in Figure 51-4.

Recurrent genital HSV disease is shorter in duration and less severe than the primary episode. In approximately 50% of patients, recurrences are preceded by a characteristic prodrome of burning or tingling in the area in which the lesions eventually erupt. Episodes of recurrence may be as frequent as every 2 to 3 weeks or may be infrequent. Unfortunately, any infected person may shed virus asymptomatically. Such individuals may be important vectors for spread of this virus.

Herpes encephalitis is usually caused by HSV-1. The lesions are generally limited to one of the temporal lobes. The viral pathology and immunopathology cause the destruction of the temporal lobe and give rise to erythrocytes in the cerebrospinal fluid, seizures, focal neurologic abnormalities, and other characteristics of viral encephalitis. HSV is the most common viral cause of sporadic encephalitis and results in significant morbidity and mortality, even in patients who receive appropriate treatment. The disease occurs at all ages and at any time of the year. HSV meningitis may be a complication of genital HSV-2 infection; symptoms resolve by themselves.

HSV infection in the neonate is a devastating and often fatal disease caused most often by HSV-2. It may be acquired in utero but more commonly is contracted either during passage of the infant through the vaginal canal (possibly at the baby’s scalp-monitor site) because the mother is shedding herpesvirus at the time of delivery, or it is acquired postnatally from family members or hospital personnel. The baby initially appears septic, and vesicular lesions may or may not be present. Because the cell-mediated immune response is not yet developed in the neonate, HSV disseminates to the liver, lung, and other organs, as well as to the central nervous system (CNS). Progression of the infection to the CNS results in death, mental retardation, or neurologic disability, even with treatment.

Laboratory Diagnosis

Treatment, Prevention, and Control

HSV encodes several target enzymes for antiviral drugs (Box 51-5) (see Chapter 48). Most antiherpes drugs are nucleoside analogues that inhibit the viral DNA polymerase, an enzyme essential for viral replication and the best antiviral drug target. Treatment prevents or shortens the course of primary or recurrent disease. None of the drug treatments can eliminate latent infection.

The prototype anti-HSV drug is acyclovir (ACV). Valacyclovir (the valyl ester of ACV), penciclovir, and famciclovir (a derivative of penciclovir) are related to ACV in their mechanisms of action but have different pharmacologic properties. Vidarabine (adenosine arabinoside [Ara A]), idoxuridine (iododeoxyuridine), and trifluridine, also U.S. Food and Drug Administration (FDA)-approved for treatment of HSV, are less effective. Although cidofovir and adefovir are active against HSV, cidofovir is only approved for treatment of CMV.

ACV is the most prescribed anti-HSV drug. Phosphorylation of ACV and penciclovir by the viral thymidine kinase and cellular enzymes activates the drug as a substrate for the viral DNA polymerase. These drugs are then incorporated into and prevent the elongation of the viral DNA (see Chapter 48, Figure 48-2). ACV, valacyclovir, penciclovir, and famciclovir are (1) relatively nontoxic, (2) effective in treating serious presentations of HSV disease and first episodes of genital herpes, and (3) also used for prophylactic treatment.

The most prevalent form of resistance to these drugs results from mutations that inactivate the thymidine kinase, thereby preventing conversion of the drug to its active form. Mutation of the viral DNA polymerase also produces resistance. Fortunately, resistant strains appear to be less virulent.

Ara-A is less soluble, less potent, and more toxic than ACV. Trifluridine, penciclovir, and ACV have replaced iododeoxyuridine as topical agents for the treatment of herpetic keratitis. Tromantadine, an amantadine derivative, is approved for topical use in countries other than the United States. It works by inhibiting penetration and syncytia formation. Various nonprescription treatments may be effective for specific individuals.

Avoidance of direct contact with lesions reduces the risk of infection. Unfortunately, the symptoms may be inapparent, and thus the virus can be transmitted unknowingly. Physicians, nurses, dentists, and technicians must be especially careful when handling potentially infected tissue or fluids. The wearing of gloves can prevent the acquisition of infections of the fingers (herpetic whitlow). People with recurrent herpetic whitlow disease are very contagious and can spread the infection to patients.

HSV is readily inactivated by soap, disinfectants, bleach, and 70% ethanol. Washing with soap readily disinfects the virus.

Patients who have a history of genital HSV infection must be instructed to refrain from sexual intercourse while they have prodromal symptoms or lesions and to resume sexual intercourse only after lesions are completely reepithelialized because virus may be transmitted from lesions that have crusted over. Condoms may be useful and are undoubtedly better than nothing but may not be fully protective.

A pregnant woman who has active genital HSV infection or who is asymptomatically shedding the virus in the vagina at term may transmit HSV to the neonate if the infant is delivered vaginally. Such transmission can be prevented by cesarean section.

No vaccine is currently available for HSV. However, killed, subunit, vaccinia hybrid, and DNA vaccines are being developed to prevent acquisition of the virus or to treat infected people. The glycoprotein D is being used in several of these experimental vaccines.

Structure and Replication

VZV has the smallest genome of the human herpesviruses. VZV replicates in a similar manner but slower and in fewer types of cells than HSV. Human diploid fibroblasts in vitro and activated T cells, epithelial cells, and epidermal cells in vivo support productive VZV replication. Like HSV, VZV establishes a latent infection of neurons, but unlike HSV, several viral RNAs and specific viral proteins can be detected in the cells.

Pathogenesis and Immunity

VZV is generally acquired by inhalation, and primary infection begins in the tonsils and mucosa of the respiratory tract. The virus then progresses via the bloodstream and lymphatic system to the cells of the reticuloendothelial system (Box 51-6; Figures 51-8 and 51-9). A secondary viremia occurs after 11 to 13 days and spreads the virus throughout the body and to the skin. The virus infects T cells, and these cells can home to the skin and transfer virus to skin epithelial cells. The virus overcomes inhibition by interferon-α, and vesicles are produced in the skin. The virus remains cell associated and is transmitted on cell-to-cell interaction, except for terminally differentiated epithelial cells in the lungs and keratinocytes of skin lesions, which can release infectious virus. Virus replication in the lung is a major source of contagion. The virus causes a dermal vesiculopustular rash that develops over time in successive crops. Fever and systemic symptoms occur with the rash.

Figure 51-8 Mechanism of spread of varicella-zoster virus (VZV) within the body. VZV initially infects the respiratory tract and is spread to the reticuloendothelial system and T cells and then by cell-associated viremia to the skin.

Figure 51-9 Time course of varicella (chickenpox). The course in young children, as presented in this figure, is generally shorter and less severe than that in adults.

The virus becomes latent in the dorsal root or cranial nerve ganglia after the primary infection. The virus can be reactivated in older adults when immunity wanes or in patients with impaired cellular immunity. On reactivation, the virus replicates and is released along the entire neural pathway to infect the skin, causing a vesicular rash along the entire dermatome, known as herpes zoster, or shingles. This damages the neuron and may result in postherpetic neuralgia.

Interferon-α, interferon-stimulated protections, and natural killer and T cells limit the spread of the virus in tissue, but antibody is important for limiting the viremic spread of VZV. Passive immunization with varicella-zoster immune globulin (VZIG) within 4 days of exposure is protective. Cell-mediated immunity is essential for resolving the disease. The virus causes more disseminated and more serious disease in the absence of cell-mediated immunity (e.g., in children with leukemia) and may recur on immunosuppression. Although important for protection, cell-mediated immune responses contribute to the symptomatology. An overzealous response in adults is responsible for causing more extensive cell damage and a more severe manifestation (especially in the lung) in primary infection than that seen in children. T-cell and antibody levels decrease later in life, allowing VZV recurrence and herpes zoster disease.

Epidemiology

VZV is extremely communicable, with rates of infection exceeding 90% among susceptible household contacts (Box 51-7). The disease is spread principally by the respiratory route but may also be spread through contact with skin vesicles. Patients are contagious before and during symptoms. More than 90% of adults in developed countries have the VZV antibody. Herpes zoster results from the reactivation of a patient’s latent virus. The disease develops in approximately 10% to 20% of the population infected with VZV, and the incidence rises with age. Herpes zoster lesions contain viable virus and therefore may be a source of varicella infection in a nonimmune person (child).

Clinical Syndromes

Varicella (chickenpox) is one of the five classic childhood exanthems (along with rubella, roseola, fifth disease, and measles). The disease results from a primary infection with VZV; it is usually a mild disease of childhood and is normally symptomatic, although asymptomatic infection can occur (see Figure 51-9). Varicella characteristics include fever and a maculopapular rash that appear after an incubation period of approximately 14 days (Figure 51-10). Within hours, each maculopapular lesion forms a thin-walled vesicle on an erythematous base (“dewdrop on a rose petal”) that measures approximately 2 to 4 mm in diameter. This vesicle is the hallmark of varicella. Within 12 hours, the vesicle becomes pustular and begins to crust, after which scabbed lesions appear. Successive crops of lesions appear for 3 to 5 days, and at any given time, all stages of skin lesions can be observed.

Figure 51-10 Characteristic rash of varicella in all stages of its evolution.

(From Hart CA, Broadhead RL: A color atlas of pediatric infectious diseases, London, 1992, Wolfe.)

The rash spreads across the entire body but is more prevalent on the trunk and head than on the extremities. Its presence on the scalp distinguishes it from many other rashes. The lesions itch and cause scratching, which may lead to bacterial superinfection and scarring. Lesions on the mucous membrane typically occur in the mouth, conjunctivae, and vagina.

Primary infection is usually more severe in adults than in children. Interstitial pneumonia may occur in 20% to 30% of adult patients and may be fatal. The pneumonia results from inflammatory reactions at this primary site of infection.

As noted earlier, herpes zoster (zoster means “belt” or “girdle”) is a recurrence of a latent varicella infection acquired earlier in the patient’s life. Severe pain in the area innervated by the nerve usually precedes the appearance of the chickenpox-like lesions. The rash is limited to a dermatome and resembles varicella (Figure 51-11). A chronic pain syndrome called postherpetic neuralgia, which can persist for months to years, occurs in as many as 30% of patients in whom herpes zoster develops.

Figure 51-11 Herpes zoster (“shingles”) in a thoracic dermatome.

VZV infection in immunocompromised patients or neonates can result in serious, progressive, and potentially fatal disease. Defects of cell-mediated immunity in such patients increase the risk for dissemination of the virus to the lungs, brain, and liver, which may be fatal. The disease may occur in response to a primary exposure to varicella or because of recurrent disease.

Laboratory Diagnosis

The CPEs in VZV-infected cells are similar to those seen in HSV-infected cells and include Cowdry type A intranuclear inclusions and syncytia. These cells may be seen in skin lesions, respiratory specimens, or organ biopsy specimens. Syncytia may also be seen in Tzanck smears of scrapings from a vesicle’s base. A direct fluorescent antibody to membrane antigen (FAMA) test can also be used to examine skin lesion scrapings or biopsy specimens. Antigen and genome detection are sensitive means of diagnosing VZV infection. PCR techniques are especially useful for systemic and neuronal disease.

Isolation of VZV is not routinely done because the virus is labile during transport to the laboratory and replicates poorly in vitro.

Serologic tests that detect antibodies to VZV are used to screen populations for immunity to VZV. However, antibody levels are normally low, so sensitive tests such as immunofluorescence and enzyme-linked immunosorbent assay (ELISA) must be performed to detect the antibody. A significant increase in antibody level can be detected in people experiencing herpes zoster.

Treatment, Prevention, and Control

Treatment may be appropriate for adults and immunocompromised patients with VZV infections and for people with shingles, but no treatment is usually necessary for children with varicella. ACV, famciclovir, and valacyclovir have been approved for the treatment of VZV infections. The VZV DNA polymerase is much less sensitive to ACV treatment than the HSV enzyme, requiring large doses of ACV or the improved pharmacodynamics of famciclovir and valacyclovir (see Box 51-5). There is no good treatment, but analgesics and other painkillers, topical anesthetics, or capsaicin cream may provide some relief from the postherpetic neuralgia that follows zoster.

As with other respiratory viruses, it is difficult to limit the transmission of VZV. Because VZV infection in children is generally mild and induces lifelong immunity, exposure of children to VZV early in life is often encouraged. However, high-risk people (e.g., immunosuppressed children) should be protected from exposure to VZV.

Immunosuppressed patients susceptible to severe disease may be protected from serious disease through the administration of varicella-zoster immunoglobulin (VZIG). VZIG is prepared through the pooling of plasma from seropositive people. VZIG prophylaxis can prevent viremic spread leading to disease but is ineffective as a therapy for patients already suffering from active varicella or herpes zoster disease.

A live attenuated vaccine for VZV (Oka strain) has been licensed for use in the United States and elsewhere and is administered after 2 years of age on the same schedule as the measles, mumps, and rubella vaccine. The vaccine induces the production of protective antibody and cell-mediated immunity. A stronger version of this vaccine is available for adults older than 60 years; it boosts antiviral responses to limit the onset of zoster.

Structure and Replication

EBV is a member of the subfamily Gammaherpesvirinae, with a very limited host range and a tissue tropism defined by the limited cellular expression of its receptor. The primary receptor for EBV is also the receptor for the C3d component of the complement system (also called CR2 or CD21). It is expressed on B cells of humans and New World monkeys and on some epithelial cells of the oropharynx and nasopharynx.

EBV infection has the following three potential outcomes:

EBV encodes more than 70 proteins, different groups of which are expressed for the different types of infections.

EBV in saliva infects epithelial cells and then naïve resting B cells in the tonsils. The growth of the B cells is stimulated first by virus binding to the C3d receptor, a B-cell growth-stimulating receptor, and then by expression of the transformation and latency proteins. These include Epstein-Barr nuclear antigens (EBNAs) 1, 2, 3A, 3B, and 3C; latent proteins (LPs); latent membrane proteins (LMPs) 1 and 2; and two small Epstein-Barr–encoded RNA (EBER) molecules, EBER-1 and EBER-2. The EBNAs and LPs are DNA-binding proteins that are essential for establishing and maintaining the infection (EBNA-1), immortalization (EBNA-2), and other purposes. The LMPs are membrane proteins with oncogene-like activity. The genome becomes circularized; the cells proceed to follicles that become germinal centers in the lymph node, where the infected cells differentiate into memory cells. EBV protein synthesis ceases, and the virus establishes latency in these memory B cells. EBNA-1 will be expressed only at cell division to hold onto and retain the genome in the cells.

Antigen stimulation of the B cells and infection of certain epithelial cells allow the transcription and translation of the ZEBRA (peptide encoded by the Z-gene region) transcriptional activator protein, which activates the immediate early genes of the virus and the lytic cycle. After synthesis of the DNA polymerase and replication of DNA, the structural and other late proteins are synthesized. They include gp350/220 (related glycoproteins of 350,000 and 220,000 Da), which is the viral attachment protein, and other glycoproteins. These glycoproteins bind to CD21 and MHC II molecules, receptors on B cells and epithelial cells, and also promote fusion of the envelope with cell membranes.

The viral proteins produced during a productive infection are serologically defined and grouped as early antigen (EA), viral capsid antigen (VCA), and the glycoproteins of the membrane antigen (MA) (Table 51-3). An early protein mimics a cellular inhibitor of apoptosis, and a late protein mimics the activity of human interleukin-10, which enhances B-cell growth and inhibits TH1 immune responses.

Table 51-3 Markers of Epstein-Barr Virus (EBV) Infection

Pathogenesis and Immunity

EBV has adapted to the human B cell and manipulates and uses the different phases of B-cell development to establish a lifelong infection in the individual, and it still promotes its transmission. The diseases of EBV result from either an overactive immune response (infectious mononucleosis) or the lack of effective immune control (lymphoproliferative disease and hairy cell leukoplakia).

The productive infection of B cells and epithelial cells of the oropharynx, such as in the tonsils (Box 51-8 and Figure 51-12), promotes virus shedding into saliva to transmit the virus to other hosts and establishes a viremia to spread the virus to other B cells in lymphatic tissue and blood.

Figure 51-12 Progression of Epstein-Barr virus (EBV) infection. Infection may result in lytic, latent, or immortalizing infection, which can be distinguished on the basis of production of virus and expression of different viral proteins and antigens. T cells limit the outgrowth of the EBV-infected cells and maintain the latent infection. CD, Cluster of differentiation; EA, early antigen; EBER, Epstein-Barr–encoded RNA; EBNA, Epstein-Barr nuclear antigen; LMPs, latent membrane proteins; LP, latent protein; MA, membrane antigen; VCA, viral capsid antigen; WBCs, white blood cells; ZEBRA, peptide encoded by the Z gene region.

EBV proteins replace host factors that normally activate B-cell growth and development. In the absence of T cells (e.g., in tissue culture), EBV can immortalize B cells and promote the development of B-lymphoblastoid cell lines. In vivo, B-cell activation and proliferation occurs and is indicated by the spurious production of an IgM antibody to the Paul-Bunnell antigen, termed the heterophile antibody (see later discussion of serology).

The outgrowth of the B cell is normally controlled by T cells responding to proliferation indicators on the B cell and to EBV antigenic peptides. B cells are excellent antigen-presenting cells and present EBV antigens on both MHC I and MHC II molecules. The activated T cells appear as atypical lymphocytes (also called Downey cells) (Figure 51-13). They increase in number in the peripheral blood during the second week of infection, accounting for 10% to 80% of the total white blood cell count at this time (hence the “mononucleosis”).

Figure 51-13 Atypical T-cell (Downey cell) characteristic of infectious mononucleosis. The cells have a more basophilic and vacuolated cytoplasm than normal lymphocytes, and the nucleus may be oval, kidney shaped, or lobulated. The cell margin may seem to be indented by neighboring red blood cells.

Infectious mononucleosis is essentially a “civil war” between the EBV-infected B cells and the protective T cells. The classic lymphocytosis (increase in mononuclear cells), swelling of lymphoid organs (lymph nodes, spleen, and liver), and malaise associated with infectious mononucleosis results mainly from the activation and proliferation of T cells. A great amount of energy is required to power the T-cell response, leading to great fatigue. The sore throat of infectious mononucleosis is a response to EBV-infected epithelium and B cells in the tonsils and throat. Children have a less active immune response to EBV infection and therefore have very mild disease.

During productive infection, antibody is first developed against the components of the virion, VCA, and MA, and later against the EA. After resolution of the infection (lysis of the productively infected cells), antibody against the nuclear antigens (EBNAs) is produced. T cells are essential for limiting the proliferation of EBV-infected B cells and controlling the disease (Figure 51-14). EBV counteracts some of the protective action of TH1 CD4 T-cell responses during productive infection by producing an interleukin-10 analogue (BCRF-1) that inhibits the protective TH1 CD4 T-cell responses and also stimulates B-cell growth.

Figure 51-14 Pathogenesis of Epstein-Barr virus (EBV). EBV is acquired by close contact between persons through saliva and infects the B cells. The resolution of the EBV infection and many of the symptoms of infectious mononucleosis result from the activation of T cells in response to the infection.

The virus persists in at least one memory B cell per milliliter of blood for the infected person’s lifetime. EBV may be reactivated when the memory B cell is activated (especially in the tonsils or oropharynx) and may be shed in saliva.

Epidemiology

EBV is transmitted in saliva (Box 51-9). More than 90% of EBV-infected people intermittently shed the virus for life, even when totally asymptomatic. Children can acquire the virus at an early age by sharing contaminated drinking glasses. Children generally have subclinical disease. Saliva sharing between adolescents and young adults often occurs during kissing; thus EBV mononucleosis has earned the nickname “the kissing disease.” Disease in these people may go unnoticed or may manifest in varying degrees of severity. At least 70% of the population of the United States is infected by age 30.

The geographic distribution of some EBV-associated neoplasms indicates a possible association with cofactors. Malaria appears to be a cofactor in the progression of chronic or latent EBV infection to AfBL. The restriction of nasopharyngeal carcinoma to people living in certain regions of China indicates a possible genetic predisposition to the cancer or the presence of cofactors in the food or environment. More subtle mechanisms may facilitate the role of EBV in 30% to 50% of cases of Hodgkin disease.

Transplant recipients, patients with the acquired immunodeficiency syndrome (AIDS), and genetically immunodeficient people are at high risk for lymphoproliferative disorders initiated by EBV. These disorders may appear as polyclonal and monoclonal B-cell lymphomas. Such people are also at high risk for a productive EBV infection in the form of hairy oral leukoplakia.

Clinical Syndromes (Clinical Case 51-2)

Heterophile Antibody–Positive Infectious Mononucleosis

The triad of classic symptoms for infectious mononucleosis is lymphadenopathy (swollen glands), splenomegaly (large spleen), and exudative pharyngitis accompanied by high fever, malaise, and often hepatosplenomegaly (large liver and spleen). A rash may occur, especially after ampicillin treatment (for the sore throat). The major complaint of people with infectious mononucleosis is fatigue (Figure 51-15). The disease is rarely fatal in healthy people but can cause serious complications resulting from neurologic disorders, laryngeal obstruction, or rupture of the spleen. Neurologic complications include meningoencephalitis and the Guillain-Barré syndrome. Mononucleosis-like syndromes can also be caused by CMV, HHV-6, Toxoplasma gondii, and human immunodeficiency virus (HIV). Similar to infections caused by other herpesviruses, EBV infection in a child is much milder than infection in an adolescent or adult. In fact, infection in children is usually subclinical.

Clinical Case 51-2

Epstein-Barr Virus (EBV) in the Immunocompromised Individual

Purtilo and associates (Ann Intern Med 101:180–186, 1984) reported on a boy with Duncan disease who presented with reduced levels of IgA, a history of thrush, and recurrent episodes of otitis media. This member of the Duncan family had an X-linked recessive, progressive, combined, variable immunodeficiency disease caused by a mutation in the SH2D1A protein, which prevents proper communication between B and T cells. After exposure to EBV at age 11 years, the boy did not develop antibodies to EBV, but generic serum IgM levels increased, and EBNA-positive immortalized B-cell lines readily grew from his peripheral blood. Establishment of the B-cell lines is indicative of aberrant T-cell control of the virus-induced B-cell proliferation. At age 18 years, he was treated with packed red cells for red cell aplasia, and then 9 weeks later, he developed infectious mononucleosis with fever, generalized lymphadenomegaly, tender liver and swollen spleen, lymphocytosis with a predominance of atypical lymphocytes, and a positive monospot test. Within another 6 months, he was agammaglobulinemic with no detectable B cells and suffered from Haemophilus influenzae and Mycobacterium tuberculosis pneumonias. After an additional 5 months, B cells were again detected. The onset of infectious mononucleosis at age 18 years may have resulted from new infection or a reactivation of the earlier infection. This case illustrates the unusual nature of EBV and other virus infections when the immune response is compromised.

Figure 51-15 Clinical course of infectious mononucleosis and laboratory findings of those with the infection. Epstein-Barr virus infection may be asymptomatic or may produce the symptoms of mononucleosis. The incubation period can last as long as 2 months. EA, Early antigen; EBNA, Epstein-Barr nuclear antigen; VCA, viral capsid antigen.

Laboratory Diagnosis

EBV-induced infectious mononucleosis is diagnosed on the basis of the symptoms (Box 51-10), the finding of atypical lymphocytes, the presence of lymphocytosis (mononuclear cells constituting 60% to 70% of the white blood cell count, with 30% atypical lymphocytes), heterophile antibody, and antibody to viral antigens. Virus isolation is not practical. PCR and DNA probe analysis for the viral genome and immunofluorescent identification of viral antigens are used to detect evidence of infection.

Atypical lymphocytes are probably the earliest detectable indication of an EBV infection. These cells appear with the onset of symptoms and disappear with resolution of the disease.

Heterophile antibody results from the nonspecific, mitogen-like activation of B cells by EBV and the production of a wide repertoire of antibodies. These antibodies include an IgM heterophile antibody that recognizes the Paul-Bunnell antigen on sheep, horse, and bovine erythrocytes but not that on guinea pig kidney cells. The heterophile antibody response can usually be detected by the end of the first week of illness and lasts for as long as several months. It is an excellent indication of EBV infection in adults but is not as reliable in children or infants. The horse cell (Monospot) test and ELISA are rapid and widely used for the detection of the heterophile antibody.

Serologic tests for antibody to viral antigens are a more dependable method than heterophile antibody to confirm the diagnosis of EBV mononucleosis (Table 51-4; see Figure 51-15). EBV infection is indicated by the finding of any of the following: (1) IgM antibody to the VCA, (2) the presence of VCA antibody and the absence of EBNA antibody, or (3) elevation of antibodies to VCA and early antigen. The finding of both VCA and EBNA antibodies in serum indicates that the person had a previous infection. Generation of antibody to EBNA requires lysis of the infected cell and usually indicates T-cell control of active disease.

Table 51-4 Serologic Profile for Epstein-Barr Virus (EBV) Infections

Treatment, Prevention, and Control

No effective treatment or vaccine is available for EBV disease (see Box 51-5). The ubiquitous nature of the virus and the potential for asymptomatic shedding make control of infection difficult. However, infection elicits lifelong immunity. Therefore, the best means of preventing infectious mononucleosis is exposure to the virus early in life because the disease is more benign in children.

Structure and Replication

CMV is a member of the subfamily Betaherpesvirinae. It has the largest genome of the human herpesviruses. In contrast to the traditional definition of a virus, which states that a virion particle contains DNA or RNA, CMV carries specific mRNAs into the cell in the virion particle to facilitate infection. Human CMV replicates only in human cells. Fibroblasts, epithelial cells, granulocytes, macrophages, and other cells are permissive for CMV replication. Virus replication is much slower than for HSV, and CPE may not be seen for 7 to 14 days. This may facilitate the establishment of latent infection in myeloid stem cells, monocytes, lymphocytes, the stromal cells of the bone marrow, or other cells.

Pathogenesis and Immunity

The pathogenesis of CMV is similar to that of other herpesviruses in many respects (Box 51-11). CMV is an excellent parasite and readily establishes persistent and latent infections rather than an extensive lytic infection. CMV is highly cell associated and is spread throughout the body within infected cells, especially lymphocytes and leukocytes. The virus is reactivated by immunosuppression (e.g., corticosteroids, infection with HIV) and possibly by allogeneic stimulation (i.e., the host response to transfused or transplanted cells).

Cell-mediated immunity is essential for resolving and controlling the outgrowth of CMV infection. However, CMV is an expert at immune evasion and has several means for evading innate and immune responses. CMV infection alters the function of lymphocytes and leukocytes. The virus prevents antigen presentation to both CD8 cytotoxic T cells and CD4 T cells by preventing the expression of MHC I molecules on the cell surface and by interfering with cytokine-induced expression of MHC II molecules on antigen-presenting cells (including the infected cells). A viral protein also blocks natural-killer-cell attack of CMV-infected cells. Similar to EBV, CMV also encodes an interleukin-10 analogue that would inhibit TH1 protective immune responses.

Epidemiology and Clinical Syndromes

In most cases, CMV replicates and is shed without causing symptoms (Table 51-5). Activation and replication of CMV in the kidney and secretory glands promote its secretion in urine and bodily secretions. CMV can be isolated from urine, blood, throat washings, saliva, tears, breast milk, semen, stool, amniotic fluid, vaginal and cervical secretions, and tissues obtained for transplantation (Table 51-6 and Box 51-12). Virus can be transmitted to other individuals by means of blood transfusions and organ transplants. The congenital, oral, and sexual routes; blood transfusion; and tissue transplantation are the major means by which CMV is transmitted. CMV disease is an opportunistic disorder, rarely causing symptoms in the immunocompetent host but causing serious disease in an immunosuppressed or immunodeficient person, such as a patient with AIDS or a neonate (Figure 51-16).

Table 51-5 Sources of Cytomegalovirus Infection

Table 51-6 Cytomegalovirus Syndromes

^* Complication of mononucleosis or posttransfusion syndrome.

Figure 51-16 Outcomes of cytomegalovirus (CMV) infections. The outcome of CMV infection depends very heavily on the immune status of the patient. Ab, Antibody; AIDS, acquired immunodeficiency syndrome.

Laboratory Diagnosis

Histology

The histologic hallmark of CMV infection is the cytomegalic cell, which is an enlarged cell (25 to 35 mm in diameter) that contains a dense, central, “owl’s eye,” basophilic intranuclear inclusion body (Table 51-7; Figure 51-17). Such infected cells may be found in any tissue of the body and in urine and are thought to be epithelial in origin. The inclusions are readily seen with Papanicolaou or hematoxylin-eosin staining.

Table 51-7 Laboratory Tests for Diagnosing Cytomegalovirus Infection

Test	Finding
Cytology and histology*	“Owl’s-eye” inclusion body Antigen detection
	In situ DNA probe hybridization
	PCR
Cell culture	Cytologic effect in human diploid fibroblasts (slow)
	Immunofluorescence detection of early antigens (faster)
	PCR (faster)
Serology	Primary infection

PCR, Polymerase chain reaction.

^* Samples taken for analysis include urine, saliva, blood, bronchoalveolar lavage specimens, and tissue biopsy specimens.

Figure 51-17 Cytomegalovirus-infected cell with basophilic nuclear inclusion body.

Treatment, Prevention, and Control

Ganciclovir (dihydroxypropoxymethyl guanine), valganciclovir (valyl ester of ganciclovir), cidofovir, and foscarnet (phosphonoformic acid) have been approved by the FDA for the treatment of specific diseases resulting from CMV infections of immunosuppressed patients (see Box 51-5). Ganciclovir is structurally similar to ACV; it is phosphorylated and activated by a CMV-encoded protein kinase, inhibits the viral DNA polymerase, and causes DNA termination (see Chapter 48). Ganciclovir is more toxic than ACV. Ganciclovir can be used to treat severe CMV infections in immunocompromised patients. Valganciclovir is a prodrug of ganciclovir that can be taken orally, is converted to ganciclovir in the liver, and has better bioavailability than ganciclovir. Cidofovir is a phosphorylated cytidine nucleoside analogue that does not require a viral enzyme for activation. Foscarnet is a simple molecule that inhibits the viral DNA polymerase by mimicking the pyrophosphate portion of nucleotide triphosphates.

CMV spreads mainly by the sexual, tissue transplantation, and transfusion routes, and spread by these means is preventable. Semen is a major vector for the sexual spread of CMV to both heterosexual and homosexual contacts. The use of condoms or abstinence would limit viral spread. Transmission of the virus can also be reduced through the screening of potential blood and organ donors for CMV seronegativity. Screening is especially important for donors of blood transfusions to be given to infants. Although congenital and perinatal transmission of CMV cannot effectively be prevented, a seropositive mother is least likely to produce a baby with symptomatic CMV disease. No vaccine for CMV is available.

Pathogenesis and Immunity

HHV-6 infection occurs very early in life. The virus replicates in the salivary gland, is shed, and transmitted in saliva.

HHV-6 primarily infects lymphocytes, especially CD4 T cells. HHV-6 establishes a latent infection in T cells and monocytes but may replicate on activation of the cells. Cells in which the virus is replicating appear large and refractile and have occasional intranuclear and intracytoplasmic inclusion bodies.

Similar to the replication of CMV, the replication of HHV-6 is controlled by cell-mediated immunity. Similar to CMV, the virus is likely to become activated in patients with AIDS or other lymphoproliferative and immunosuppressive disorders and cause opportunistic disease.

Clinical Syndromes (Box 51-13)

Exanthem subitum, or roseola, is caused by either HHV-6B or HHV-7 and is one of the five classic childhood exanthems previously mentioned (Figure 51-18). It is characterized by the rapid onset of high fever of a few days’ duration, which is followed by a rash on the trunk and face, and then it spreads and lasts only 24 to 48 hours. The presence of infected T cells or the activation of delayed-type hypersensitivity T cells in the skin may be the cause of the rash. The disease is effectively controlled and resolved by cell-mediated immunity, but the virus establishes a lifelong latent infection of T cells. Although usually benign, HHV-6 is the most common cause of febrile seizures in childhood (age 6 to 24 months).

Figure 51-18 Time course of symptoms of exanthem subitum (roseola) caused by human herpesvirus 6 (HHV-6). Compare these symptoms and this time course with those of fifth disease, which is caused by parvovirus B19 (see Chapter 53).

HHV-6 may also cause a mononucleosis syndrome and lymphadenopathy in adults and may be a cofactor in the pathogenesis of AIDS. Similar to CMV, HHV-6 may reactivate in transplant patients and contribute to the failure of the graft. HHV-6 has also been associated with multiple sclerosis and chronic fatigue syndrome.

Human Herpesvirus 8 (Kaposi Sarcoma–Associated Herpesvirus)

HHV-8 DNA sequences were discovered in biopsy specimens of Kaposi sarcoma, primary effusion lymphoma (a rare type of B-cell lymphoma), and multicentric Castleman disease through the use of PCR analysis. Kaposi sarcoma is one of the characteristic opportunistic diseases associated with AIDS. Genome sequence analysis showed that the virus was unique and a member of the subfamily Gammaherpesvirinae. Similar to EBV, the B cell is the primary target cell for HHV-8, but the virus also infects a limited number of endothelial cells, monocytes, and epithelial and sensory nerve cells. Within the Kaposi sarcoma tumors, endothelial spindle cells contain the virus.

HHV-8 encodes several proteins that resemble human proteins and promote the growth and prevent apoptosis of the infected and surrounding cells. These proteins include an interleukin-6 homologue (growth and antiapoptosis), a Bcl-2 analogue (antiapoptosis), chemokines, and a chemokine receptor. These proteins can promote the growth and development of polyclonal Kaposi sarcoma cells in AIDS patients and others. HHV-8 DNA is present and is associated with peripheral blood lymphocytes, most likely B cells, in approximately 10% of immunocompetent people. HHV-8 is more prevalent in certain geographic areas (Italy, Greece, Africa) and in patients with AIDS. Kaposis sarcoma is the most common cancer in sub-Saharan Africa. The virus is most likely a sexually transmitted disease but may be spread by other means.

Herpesvirus simiae (B virus) (subfamily Alphaherpesvirinae, the simian counterpart of HSV), is indigenous to Asian monkeys. The virus is transmitted to humans by monkey bites or saliva, or even by tissues and cells widely used in virology laboratories. Once infected, a human may have pain, localized redness, and vesicles at the site where the virus entered. An encephalopathy develops and is often fatal; most people who survive have serious brain damage. PCR or serologic tests can be used to establish the diagnosis of B-virus infections. Virus isolation requires special facilities.