Epstein-Barr Virus

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University of Oklahoma Study Abroad Microbiology in Arezzo, Italy[1]

Epstein-Barr budding in a B cell From: msdiscovery.org [2]

Etiology

Taxonomy

| Order = Herpesvirales | Family = Herpesviridae | Subfamily = Gammaherpesvirinae | Genus = Lymphocryptovirus | Species = Human Herpesvirus 4

NCBI: [3] Genome: [4]

Description

The Epstein-Barr virus (EBV) is one of the two human host-specific viruses in the subfamily Gammaherpesvirinae, along with Kaposi's Sarcoma, a virus normally associated with lesions in AIDS patients. It is the most common virus among the human population. More than 95% of the human population contain EBV antibodies, meaning that they have come into contact with the virus at some point in life and it is lying latent. Many people are asymptomatic when infected, but under certain stresses, diseases, such as mononucleosis, can arise. EBV was found to be the main virus responsible for Burkitt's Lymphoma in 1964, and later on was found to be correlative with Hodgkin's Lymphoma and nasopharyngeal carcinoma. EBV is not the sole cause of these cancers, but it does play an important role in their development. A defining marker of a virus is that it requires a host to replicate and survive, and when EBV was discovered, it was very difficult to grow on any medium.

Herpesviruses are characteristically icosahedral, 20-sided, and 70-100 microns in diameter, which led Epstein to characterize EBV with this family, however because he could not run standard tests of the other herpesviruses at the time, he concluded it must be a new strain [1].

Genome

EBV is a double-stranded DNA virus, making it more stable and less likely for mutations than RNA viruses. EBV is 184 kb pairs in length. The genome shows around 70 predictable open reading frames and there are two different strains. The strains differ in their latent proteins, but they are not associated with any specific disease. Analyzing repeat genes can be used in studying outbreaks [2]. The genome codes for latent and lytic proteins, and there are RNA transcription proteins whose functions are still unknown. It is an enveloped herpes virus, which means it can cause a life-long latent infection. EBV is very large for a herpesvirus and is surrounded by an outer layer comprised of cellular membranes from infected cells, adding an extra 50-100 nm to its size. The envelope is necessary for infectivity and sensitivity.

Clinical Features

Clinical features can be hard to discern as most immunocompetent patients are asymptomatic [3]. In immunocompetent individuals, symptoms will mainly correspond to infectious mononucleosis (IM) in young adults, though these symptoms could also presuppose leukemia or lymphoma. In immunocompromised patients, EBV may exacerbate their autoimmunity; this results in high morbidity and mortality rates. Most symptoms in IM and other diseases caused by EBV are a direct result of cytotoxic T cells attacking the EBV in B cells.

Symptoms

Infectious Mononucleosis

The symptoms of IM can begin quickly and develop rapidly. The main, specific, symptoms to IM are sore throat and neck swelling, though there are non-specific symptoms such as vague discomfort, headaches, chills, and fever [2]. Spleen tenderness and rashes are some of the more commonly serious symptoms to suggest IM. EBV replicates in B cells and epithelial cells and the sore throat is usually caused by lysis of the oropharyngeal epithelial cells. The swollen neck is due to enlarged lymph nodes as the infected B cells replicate to normal cells. IM can be caused by a variety of pathogens, however EBV is the cause of over 90% of reported cases. Teenagers are at the highest risk of IM when exposed to EBV for the first time. Primary exposure of EBV in infants is usually accompanied by flu-like or no symptoms whatsoever. Chronic Fatigue Syndrome was once thought to be associated with EBV, but as of recent research, no correlation has ever been found [4]. IM caused by EBV has been shown to be a risk factor for those with Chronic Fatigue Syndrome, but EBV itself does not cause CFS.

Other Diseases/Cancers

EBV is an aggressor and stimulator of other, more serious and life-threatening, diseases and cancers. There is not always a direct correlation between EBV and these symptoms, but they can be worsened if EBV becomes lytic. Most of these ailments are found in the lymphatic system, as that is where the infected B cells reside.

• Burkitt's Lymphoma: EBV is found in nearly all endemic patients in Africa. Chronic malaria is thought to reduce resistance to EBV, allowing it to cause tumors, mostly around the jaw and facial bones. This is one of the first cases of viral infections being correlated to cancers [5].

• Hodgkin's Lymphoma: Being infected with IM increases your chance of contracting this cancer, but a precise correlation cannot be found. Symptoms include enlarged lymph nodes, spleen, and liver, as well as back pain and weight loss. This is a serious cancer that usually possesses a survival rate of 5 years, though treatments are always improving [6].

• Nasopharyngeal Carcinoma: There are a variety of factors that can be attributed to this cancer, and the main viral component is associated with EBV. As stated previously, this is the most common virus among the human population, so there are many reasons this virus could be found in many cancer patients. EBV may aggravate the cancer, but it has not been proven to be a direct cause. Studies have only shown that patients produce a higher level of antibodies towards EBV, and this can be used as a marker for NPC [7]. Once again, main symptoms include swollen lymph nodes, a soft palate, pain, and hearing loss.

• Autoimmune Complications:

• • Hairy Leukoplakia-This is a white lesion that cannot be scraped off in the oral cavity. It is directly caused by EBV and is usually seen in HIV or otherwise immunocompromised patients [8].

• • Multiple Sclerosis-Neurological functions are severely compromised, making everyday tasks challenging. It has been found that those infected with EBV during adolescence are more likely to develop MS than those exposed during childhood or not at all. Some hypothesize that MS could be a serious complication of IM, though there are some outlying patients that test negative for EBV. EBV has a mechanism that prevents infected B cells from being killed, and some of these errant cells could make it to the central nervous system, where antibodies could mistake the myelin sheath for EBV. Though much research still needs to be done to determine specific pathways that could link EBV to MS [9].

• • Pretty much any disease that can cause swelling in the lymph nodes could be associated with EBV in some way. Symptoms are relatively similar across these various diseases, and because so much of the population is infected with EBV, correlations are prevalent, but suspicious.

References

1. Cancer Virus: The Story of Epstein-Barr Virus

2. Principles and Practice of Clinical Virology

3. Routine Epstein-Barr Virus Diagnostics

4. CDC Chronic Fatigue Syndrome

5. Burkitt's Lymphoma, Malaria, and Epstein-Barr

6. Hodgkin's Disease and Epstein-Barr Virus

7. Relationship Between Epstein-Barr and Nasopharyngeal Carcinoma

8. Managing HIV/AIDS

Created by Jordan Abney, student of Tyrell Conway at the University of Oklahoma.