EBV causes infectious mononucleosis. It is associated with Burkitt's lymphoma, other B-cell lymphomas, and nasopharyngeal carcinoma. EBV is also associated with hairy leukoplakia, a whitish, nonmalignant lesion on the tongue seen especially in AIDS patients.
EBV is structurally and morphologically identical to other herpesviruses but is antigenically different. The most important antigen is the viral capsid antigen (VCA), because it is used most often in diagnostic tests. The early antigens (EA), which are produced prior to viral DNA synthesis, and nuclear antigen (EBNA), which is located in the nucleus bound to chromosomes, are sometimes diagnostically helpful as well. Two other antigens, lymphocyte-determined membrane antigen and viral membrane antigen, have been detected also. Neutralizing activity is directed against the viral membrane antigen.
Humans are the natural hosts. EBV infects mainly lymphoid cells, primarily B lymphocytes. In latently infected cells, multiple copies of EBV DNA are found in the cytoplasm of infected B lymphocytes. Some, but not all, genes are transcribed, and only a subset of those are translated into protein.
Summary of Replicative Cycle
The cycle is similar to that of HSV (see Herpes Simplex Viruses). EBV enters B lymphocytes at the site of the receptor for the C3 component of complement.
EBV is transmitted primarily by the exchange of saliva, e.g., during kissing. The saliva of people with a reactivation of a latent infection as well as people with an active infection can serve as a source of the virus. In contrast to CMV, blood transmission of EBV is very rare.
EBV infection is one of the most common infections worldwide; more than 90% of adults in the United States have antibody. Infection in the first few years of life is usually asymptomatic. Early infection tends to occur in individuals in lower socioeconomic groups. The frequency of clinically apparent infectious mononucleosis, however, is highest in those who are exposed to the virus later in life, e.g., college students.
Pathogenesis & Immunity
The infection first occurs in the oropharynx and then spreads to the blood, where it infects B lymphocytes. Cytotoxic T lymphocytes react against the infected B cells. The T cells are the "atypical lymphs" seen in the blood smear. EBV remains latent within B lymphocytes. A few copies of EBV DNA are integrated into the cell genome; many copies of circular EBV DNA are found in the cytoplasm.
The immune response to EBV infection consists first of IgM antibody to the VCA. IgG antibody to the VCA follows and persists for life. The IgM response is therefore useful for diagnosing acute infection, whereas the IgG response is best for revealing prior infection. Lifetime immunity against second episodes of infectious mononucleosis is based on antibody to the viral membrane antigen.
In addition to the EBV-specific antibodies, nonspecific heterophil antibodies are found. The term "heterophil" refers to antibodies that are detected by tests using antigens different from the antigens that induced them. The heterophil antibodies formed in infectious mononucleosis agglutinate sheep or horse red blood cells in the laboratory. (Cross-reacting Forssman antibodies in human serum are removed by adsorption with guinea pig kidney extract prior to agglutination.) Note that these antibodies do not react with any component of EBV. It seems likely that EBV infection modifies a cell membrane constituent such that it becomes antigenic and induces the heterophil antibody. Heterophil antibodies usually disappear within 6 months after recovery. These antibodies are not specific for EBV infection and are also seen in individuals with hepatitis B and serum sickness.
Infectious mononucleosis is characterized primarily by fever, sore throat, lymphadenopathy, and splenomegaly. Anorexia and lethargy are prominent. Hepatitis is frequent; encephalitis occurs in some patients. Spontaneous recovery usually occurs in 2–3 weeks. Splenic rupture, associated with contact sports such as football, is a feared but rare complication of the splenomegaly.
In addition to infectious mononucleosis, EBV causes two other diseases. One is a severe, often fatal, progressive form of infectious mononucleosis that occurs in children with an inherited immunodeficiency called X-linked lymphoproliferative syndrome. The mutated gene encodes a signal transduction protein required for both T-cell and NK-cell function. The mortality rate is 75% by age 10. Bone marrow or cord blood transplants may cure the underlying immunodeficiency. The other disease is hairy leukoplakia, a whitish lesion on the tongue of AIDS patients.
The diagnosis of infectious mononucleosis in the clinical laboratory is based primarily on two approaches:
In the hematologic approach, absolute lymphocytosis occurs and as many as 30% abnormal lymphocytes are seen on a smear. Theseatypical lymphs are large and have a lobulated nucleus and a vacuolated, basophilic cytoplasm. They are cytotoxic T cells that are reacting against the EBV-infected B cells.
In the immunologic approach, there are two types of serologic tests. (a) The heterophil antibody test is useful for the early diagnosis of infectious mononucleosis because it is usually positive by week 2 of illness. However, because the antibody titer declines after recovery, it is not useful for detection of prior infection. The Monospot test is often used to detect the heterophil antibody; it is more sensitive, more specific, and less expensive than the tube agglutination test. (b) The EBV-specific antibody tests are used primarily in diagnostically difficult cases. The IgM VCA antibody response can be used to detect early illness; the IgG VCA antibody response can be used to detect prior infection. In certain instances, antibodies to EA and EBNA can be useful diagnostically.
Although EBV can be isolated from clinical samples such as saliva by morphologic transformation of cord blood lymphocytes, it is a technically difficult procedure and is not readily available. No virus is synthesized in the cord lymphocytes; its presence is detected by fluorescent-antibody staining of the nuclear antigen.
No antiviral therapy is necessary for uncomplicated infectious mononucleosis. Acyclovir has little activity against EBV, but administration of high doses may be useful in life-threatening EBV infections.
There is no EBV vaccine.
Association with Cancer
EBV infection is associated with cancers of lymphoid origin: Burkitt's lymphoma in African children, other B-cell lymphomas, nasopharyngeal carcinoma in the Chinese population, and thymic carcinoma in the United States. The initial evidence of an association of EBV infection with Burkitt's lymphoma was the production of EBV by the lymphoma cells in culture. In fact, this was how EBV was discovered by Epstein and Barr in 1964. Additional evidence includes the finding of EBV DNA and EBNA in the tumor cells. EBV DNA and antigens are found in nasopharyngeal and thymic carcinoma cells also. EBV can induce malignant transformation in B lymphocytes in vitro. The role of EBV in carcinogenesis is unclear.
HUMAN HERPESVIRUS 8 (KAPOSI'S SARCOMA–ASSOCIATED HERPESVIRUS)
In 1994, it was reported that a new herpesvirus, now known as HHV-8, or Kaposi's sarcoma–associated herpesvirus (KSHV), may be the cause of Kaposi's sarcoma (KS), the most common cancer in patients with AIDS. The idea that a virus other than HIV is the cause of KS arose from epidemiologic data that showed that KS was common in patients who acquired HIV sexually but rare in patients who acquired HIV via blood transfusion. A second virus transmitted sexually appeared likely to be the cause.
The initial evidence that HHV-8 was involved was the finding that most KS cells taken from AIDS patients contain the DNA of this virus, but tissues taken from AIDS patients without KS had very little viral DNA. The DNA of this virus was also found in KS cells that arose in non–HIV-infected patients. On DNA analysis, HHV-8 resembles the lymphotropic herpesviruses, e.g., Epstein-Barr virus and herpesvirus saimiri, more than it does the neurotropic herpesviruses, such as herpes simplex virus and varicella-zoster virus.
Additional support was provided by serologic studies showing that most HIV-infected patients with KS had antibodies to HHV-8, whereas considerably fewer HIV-infected patients without KS had antibodies to the virus and very few patients with other sexually transmitted diseases, but who were not HIV-infected, had these antibodies. The current estimate of HHV-8 infection in the general population ranges from about 3% in the United States and England to about 50% in East Africa.
HHV-8 causes malignant transformation by a mechanism similar to that of other DNA viruses (such as human papillomavirus), namely, inactivation of a tumor suppressor gene. A protein encoded by HHV-8 called nuclear antigen inactivates the RB (retinoblastoma) tumor suppressor protein, which causes the cells to grow in an uncontrolled manner.
Transmission of HHV-8 is primarily sexually, but it is also transmitted in transplanted organs such as kidneys and appears to be the cause of transplantation-associated KS. The DNA of HHV-8 is found in the cells of transplantation-associated KS but not in the cells of other transplantation-associated cancers.
KS in AIDS patients is a malignancy of vascular endothelial cells that contains many spindle-shaped cells and erythrocytes. The lesions are dark purple, flat to nodular, and often appear at multiple sites such as the skin, oral cavity, and soles (but not the palms). Internally, lesions occur commonly in the gastrointestinal tract and the lungs. The extravasated red cells give the lesions their purplish color. HHV-8 also infects B cells, inducing them to proliferate and produce a type of lymphoma called primary effusion lymphoma.
Laboratory diagnosis of KS is often made by biopsy of the skin lesions. HHV-8 DNA and RNA are present in most spindle cells, but that analysis is not usually done. Virus is not grown in culture.
The type of treatment depends upon the site and number of the lesions. Surgical excision, radiation, and systemic drugs, such as alpha interferon or vinblastine, can be used. There is no specific antiviral therapy and no vaccine against HHV-8.
The poxvirus family includes three viruses of medical importance: smallpox virus, vaccinia virus, and molluscum contagiosum virus (MCV). Poxviruses are the largest and most complex viruses.
Smallpox virus, also called variola virus, is the agent of smallpox, the only disease that has been eradicated from the face of the Earth. Eradicationis due to the vaccine. There is concern regarding the use of smallpox virus as an agent of bioterrorism. Pox viruses of animal origin, such as cowpox and monkey pox, are described in Chapter 46.
Poxviruses are brick-shaped particles containing linear double-stranded DNA, a disk-shaped core within a double membrane, and a lipoprotein envelope. The virion contains a DNA-dependent RNA polymerase. This enzyme is required because the virus replicates in the cytoplasm and does not have access to the cellular RNA polymerase, which is located in the nucleus.
Smallpox virus has a single, stable serotype, which is the key to the success of the vaccine. If the antigenicity varied as it does in influenza virus, eradication would not have succeeded. Smallpox virus infects only humans; there is no animal reservoir.
Summary of Replicative Cycle
Vaccinia virus, a poxvirus virtually nonpathogenic for humans, is used for studies on poxvirus replication and as a vector in certain gene therapy experiments. After penetration of the cell and uncoating, the virion DNA-dependent RNA polymerase synthesizes early mRNA, which is translated into early, nonstructural proteins, mainly enzymes required for subsequent steps in viral replication. The viral DNA then is replicated, after which late, structural proteins are synthesized that will form the progeny virions. The virions are assembled and acquire their envelopes by budding from the cell membrane as they are released from the cell. Note that all steps in replication occur in the cytoplasm, which is unusual for a DNA virus.
Transmission & Epidemiology
Smallpox virus is transmitted via respiratory aerosol or by direct contact with virus either in the skin lesions or on fomites such as bedding.
Prior to the 1960s, smallpox was widespread throughout large areas of Africa, Asia, and South America, and millions of people were affected. In 1967, the World Health Organization embarked on a vaccination campaign that led to the eradication of smallpox. The last naturally occurring case was in Somalia in 1977.
Pathogenesis & Immunity
Smallpox begins when the virus infects the upper respiratory tract and local lymph nodes and then enters the blood (primary viremia). Internal organs are infected; then the virus reenters the blood (secondary viremia) and spreads to the skin. These events occur during the incubation period, when the patient is still well. The rash is the result of virus replication in the skin, followed by damage caused by cytotoxic T cells attacking virus-infected cells.
Immunity following smallpox disease is lifelong; immunity following vaccination lasts about 10 years.
After an incubation period of 7–14 days, there is a sudden onset of prodromal symptoms such as fever and malaise. This is followed by the rash, which is worse on the face and extremities than on the trunk (i.e., it has a centrifugal distribution). The rash evolves through stages from macules to papules, vesicles, pustules, and, finally, crusts in 2–3 weeks.
In the past when the disease occurred, the diagnosis was made either by growing the virus in cell culture or chick embryos or by detecting viral antigens in vesicular fluid by immunofluorescence.
The disease was eradicated by global use of the vaccine, which contains live, attenuated vaccinia virus. The success of the vaccine is dependent upon five critical factors: (1) smallpox virus has a single, stable serotype; (2) there is no animal reservoir, and humans are the only hosts; (3) the antibody response is prompt; therefore, exposed persons can be protected; (4) the disease is easily recognized clinically; therefore, exposed persons can be immunized promptly; and (5) there is no carrier state or subclinical infection.
The vaccine is inoculated intradermally, where virus replication occurs. The formation of a vesicle is indicative of a "take" (success). Although the vaccine was relatively safe, it became apparent in the 1970s that the incidence of side effects such as encephalitis, generalized vaccinia, and vaccinia gangrenosa exceeded the incidence of smallpox. Routine vaccination of civilians was discontinued, and it is no longer a prerequisite for international travel. Military personnel are still vaccinated.
In response to the possibility of a bioterrorism attack using smallpox virus, the U.S. federal government has instituted a program to vaccinate "first responders" so that they can give emergency medical care without fear of contracting the disease. To protect the unimmunized general population, the concept of "ring vaccination" will be used. This is based on the knowledge that an exposed individual can be immunized as long as 4 days after exposure and be protected. Therefore, if an attack occurs, people known to be exposed will be immunized as well as the direct contacts of those people and then the contacts of the contacts, in an expanding ring. Several military personnel and civilians have experienced myocarditis following vaccination and, as of this writing, caution has been urged regarding expanding this program to the general population.
Vaccinia immune globulins (VIG), containing high-titer antibodies against vaccinia virus, can be used to treat most of the complications of vaccination. In the past, methisazone was used to treat the complications of vaccination and could be useful again. Rifampin inhibits viral DNA-dependent RNA polymerase but was not used clinically against smallpox.
MOLLUSCUM CONTAGIOSUM VIRUS
Molluscum contagiosum virus (MCV) is a member of the poxvirus family but is quite distinct from smallpox and vaccinia viruses. The lesion of molluscum contagiosum is a small (2–5 mm.), flesh-colored papule on the skin or mucous membrane that is painless, nonpruritic, and not inflamed. The lesions have a characteristic cup-shaped crater with a white core. Note that these lesions are different from warts, which are caused by papillomavirus, a member of the papovavirus family.
MCV is transmitted by close personal contact, including sexually. The disease is quite common in children, in whom lesions often occur around the eyes and on the trunk. Adults often have lesions in the genital area. The lesions can be widespread in patients with reduced cellular immunity. In immunocompetent patients, the lesions are self-limited but may last for months.
The diagnosis is typically made clinically; the virus is not isolated in the clinical laboratory, and antibody titers are not helpful. Removal of the lesions by curettage or with liquid nitrogen is often effective. There is no established antiviral therapy, but cidofovir may be useful in the treatment of the extensive lesions that occur in immunocompromised patients. In AIDS patients, anti-retroviral therapy may restore sufficient immunity to cause the lesions to resolve. There is no vaccine.
HEPATITIS B VIRUS
Hepatitis B virus, a DNA-enveloped virus, is described in Chapter 41 with the other hepatitis viruses.