Epstein-Barr virus (Infectious mononucleosis)

The Epstein-Barr virus, or EBV, also known as human herpesvirus-4, or HHV-4, is an enveloped virus of the Herpesviridae family, which contains linear, double-stranded DNA.

It is the most common cause of infectious mononucleosis, more commonly known as “mono.”

EBV infection begins when saliva or respiratory secretions that contain the virus from one person make their way into someone else’s mouth.

Often, this happens by sharing food or drinks, or by kissing, which is why mono is sometimes called the “kissing disease” and why it is most common in adolescents and young adults, aged 15-24, because young people are often doing the smooching.

When EBV reaches the new person’s mouth, it infects two types of cells in the oropharynx: epithelial cells, which line the surfaces of the oropharynx, and B cells, which are lymphocytes, or lymphoid cells, that create antibodies to fight off infections.

In the epithelial cells, the virus undergoes the lytic cycle, whereby its DNA gets transcribed and translated by cellular enzymes, which help to form viral proteins, which are packaged into new viruses, which can leave the host cell destroyed, or lysed, and subsequently infect neighboring epithelial cells.

The viruses that reach the lymphoid tissue of the oropharynx, the tonsils, infect their main target, B cells, by attaching to their CD21 receptor, also known as the CR2 receptor.

The infected B cells then enter the latent phase, in which the virus just sort of hangs out in the host cell instead of killing it.

They then carry and spread the infection along their normal trajectory to the other lymphoid tissues of the body, including the liver, spleen, and lymph nodes.

The body’s immune system reacts to the infection by mounting a humoral response, whereby the B cells create antibodies to fight off the virus, and a cellular response, in which cytotoxic (or CD8+) T cells, which are lymphocytes that mature in the thymus, work to kill the infected B cells.

This limits their ability to continue to spread the virus throughout the lymphoid tissues, thus stopping the infection from spreading.

Now, in most EBV-infected individuals, the infection is controlled, and, as a result, most people do not develop symptoms.

When someone infected with EBV does develop symptoms, that’s called infectious mononucleosis or mono.

The most common symptoms are fever, pharyngitis, or inflammation of the throat, and lymphadenopathy, or swollen lymph nodes.

The inflammation of the throat makes sense since the epithelial cells are infected, and swollen lymph nodes makes sense since infected B cells spread throughout the lymph tissue of the body, causing it to swell up.

The lymph nodes that are most commonly swollen as a result of an EBV infection are the posterior cervical lymph nodes in the back of the neck, because those lymph nodes drain the tonsils where the B cells are initially infected.

Another very common symptom of mono is fatigue, or extreme tiredness, which can be particularly severe in mono and can last for several months.

Other possible symptoms include tonsillitis, or inflammation of the tonsils, palatal petechiae, or red spots on the palate, hepatomegaly, or an enlarged liver, and splenomegaly, or an enlarged spleen.

The tonsillitis may be exudative or non-exudative, meaning there may or may not be white-ish or gray-ish fluid oozing out of the tonsils.

The palatal petechiae is a result of damage done to the infected epithelial cells on the palate, and the hepatomegaly and splenomegaly are a result of the liver and spleen being lymph tissues through which infected B cells flow as cytotoxic T cells try to kill them, causing those organs to swell up.

The splenomegaly, though rare, is of particular importance because an enlarged spleen is more susceptible to rupture, and splenic rupture can result in excessive bleeding and even death.

Another rare symptom of mono is a rash with faint, non-itchy, pink macules or patches that show up on the trunk and arms.

A different type of rash that can be similar to this can occur if an individual with mono is misdiagnosed with a group A streptococcus infection, more commonly known as strep throat.

In this case, the person is given the antibiotics, ampicillin or amoxicillin, and, for some unknown reason, they develop an itchy maculopapular rash.