Epstein-Barr Virus (EBV)
|Genome: DNA Virus, dsDNA Virus||Structure: Enveloped Virus, Icosahedral Virus|
- EBV is transmitted via infected salivary secretions.
- During primary infection EBV initially replicates in the epithelia of the oral mucosa and pharyngeal mucosa. During this phase, local B-cells become infected and disseminate the virus throughout the body. The virus then enters latency in infected B-cells; however, the latent virus expresses proteins which promote B-cell growth and proliferation. Notably, the clinical consequences of EBV are due to the proliferation of latently-infected B-cells and not the direct cytotoxic effects of viral replication.
- Infection in children is usually subclinical while in young adults manifests as an infectious mononucleosis. Infection with EBV also increases the incidence of a number of malignancies but is likely not the sole etiological factor.
- Infectious Mononucleosis
- Infectious Mononucleosis is the result of primary infection with EBV and is characterized by local proliferation of the virus in the oropharyngeal mucosa together with exuberant proliferation of latently-infected B-cells. Infection is heralded by constitutional symptoms, severe fatigue, and a pharyngitis due to oropharyngeal viral proliferation. Excessive proliferation of B-cells results in extensive lymphadenopathy, often of the cervical lymph nodes, and splenomegaly which can increase the risk of traumatic splenic rupture. The peripheral blood displays a prominent leukocytosis (specifically a lymphocytosis) representing proliferative, infected B-cells. This is where the term "mononucleosis" is derived as there is an elevation of "mononuclear cells" (in reality lymphocytes) in the peripheral blood. These abnormally proliferative B-cells may escape mechanisms of peripheral [iImmune tolerance]] and tend to produce auto-antibodies that can be detected as "Heterophile Antibodies" that agglutinate sheep's blood. Cell-mediated immunity to the virus generates large numbers of antigen-specific CD8+ T-cells which kill infected B-cells and brings the infection under control. These abnormally large cytotoxic T-cells are termed "Atypical Lymphocytes" and their generation can contribute to the observed lymphocytosis
- Oncogenic Association
- Infection with EBV is associated with the development of a number of cancers although additional factors are clearly important as only a small percentage of EBV-infected individuals develop these diseases. However, it is thought that proliferation-inducing viral proteins help potentate oncogenic transformation and progression. Associated neoplasms include Nasopharyngeal Carcinoma, Hairy Leukoplakia, Burkitt Lymphoma.