Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Escherichia coli

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EEpstein-Barr virusPSTEIN-BARR VIRUS

Epstein-Barr virus (EBV) is part of the family of human her-

pes viruses. Infectious mononucleosis(IM) is the most com-

mon disease manifestation of this virus, which once

established in the host, can never be completely eradicated.

Very little can be done to treat EBV; most methods can only

alleviate resultant symptoms.

In addition to infectious mononucleosis, EBV has also

been identified in association with—although not necessarily

believed to cause—as many as 50 different illnesses and dis-

eases, including chronic fatigue syndrome, rheumatoid arthri-

tis, arthralgia (joint pain without inflammation), and myalgia

(muscle pain). While studying aplastic anemia (failure of bone

marrow to produce sufficient red blood cells), researchers

identified EBV in bone marrow cells of some patients, sug-

gesting the virus may be one causative agent in the disease.

Also, several types of cancer can be linked to presence of

EBV, particularly in those with suppressed immune systems,

for example, suffering from AIDSor having recently under-

gone kidney or liver transplantation. The diseases include

hairy cell leukemia, Hodgkin’s and non-Hodgkin lymphoma,

Burkitt’s lymphoma (cancer of the lymphatic system endemic

to populations in Africa), and nasopharyngeal carcinoma (can-

cers of the nose, throat, and thymus gland, particularly preva-

lent in East Asia). Recently, EBV has been associated with

malignant smooth-muscle tissue tumors in immunocompro-

mised children. Such tumors were found in several children

with AIDS and some who had received liver transplants.

Conversely, it appears that immunosuppressed adults show no

elevated rates of these tumors.

Epstein-Barr virus was first discovered in 1964 by three

researchers—Epstein, Achong, and Barr—while studying a

form of cancer prevalent in Africa called Burkitt’s lymphoma.

Later, its role in IM was identified. A surge of interest in the

virus has now determined that up to 95% of all adults have been

infected with EBV at some stage of their lives. In seriously

immunocompromised individuals and those with inherited

immune systemdeficiencies, the virus can become chronic,

resulting in “chronic Epstein-Barr virus” which can be fatal.

EBV is restricted to a very few cells in the host.

Initially, the infection begins with its occupation and replica-

tion in the thin layer of tissue lining the mouth, throat, and

cervix, which allow viral replication. The virus then invades

the B cells, which do not facilitate the virus’s replication but

do permit its occupation. Infected B cells may lie dormant for

long periods or start rapidly producing new cells. Once acti-

vated in this way, the B cells often produce antibodies against

the virus residing in them. EBV is controlled and contained by

killer cells and suppressor cells known as CD4 T lymphocytes

in the immune system. Later, certain cytotoxic (destructive)

CD8 T lymphocytes with specific action against EBV also

come into play. These cells normally defend the host against

the spread of EBV for the life of the host.

A healthy body usually provides effective immunityto

EBV in the form of several different antibodies, but when this

natural defense mechanism is weakened by factors that sup-

press its normal functioning—factors such as AIDS, organ

transplantation, bone marrow failure, chemotherapyand other

drugs used to treat malignancies, or even extended periods of

lack of sleep and overexertion—EBV escape from their homes

in the B cells, disseminate to other bodily tissue, and manifest

in disease.

Infection is determined by testing for the antibodies pro-

duced by the immune system to fight the virus. The level of a

particular antibody—the heterophile antibody—in the blood

stream is a good indicator of the intensity and stage of EBV

infection. Even though EBV proliferates in the mouth and

throat, cultures taken from that area to determine infection are

time-consuming, cumbersome, and usually not accurate.

Spread of the virus from one person to another requires

close contact. Because of viral proliferation and replication in

the lining of the mouth, infectious mononucleosis is often

dubbed “the kissing disease.” Also, because it inhabits cervi-

cal cells, researchers now suspect EBV may be sexually trans-

mitted. Rarely is EBV transmitted via blood transfusion.

EBV is one of the latent viruses, which means it may be

present in the body, lying dormant often for many years and

manifesting no symptoms of disease. The percentage of shed-

ding (transmission) of the virus from the mouth is highest in

people with active IM or who have become immunocompro-

mised for other reasons. A person with active IM can prevent

transmission of the disease by avoiding direct contact—such

as kissing—with uninfected people. However, shedding has

been found to occur in 15% of adults who test positive for

antibodies but who show no other signs of infection, thus

allowing the virus to be transmitted. Research efforts are

directed at finding a suitable vaccine.

The prevalence of antibodies against EBV in the general

population is high in developing countries and lower socio-

economic groups where individuals become exposed to the

virus at a very young age. In developed countries, such as the

United States, only 50% of the population shows traces of

antibodyby the age of five years, with an additional 12% in

college-aged adolescents, half of whom will actually develop

IM. This situation indicates that children and young persons

between the ages of 10 and 21 years are highly susceptible to

IM in developed countries, making it a significant health prob-

lem among students.

See alsoLatent viruses and diseases; Mononucleosis, infec-

tious; Viruses and responses to viral infection

ERYTHROMYCINS•seeANTIBIOTICS

EEscherichia coliSCHERICHIA COLI

Escherichia coliis a bacterium, which inhabits the intestinal

tract of humans and other warm-blooded mammals. It consti-

tutes approximately 0.1% of the total bacteriain the adult

intestinal tract. Its name comes from the name of the person,

Escherich, who in 1885 first isolated and characterized the

bacterium.

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