Infectious mononucleosis, or mono, is a very common syndrome characterized by the triad of
fever, swollen lymph nodes - most frequently in the neck, and sore throat with inflamed tonsils.
Patients may also present with headache, fatigue, and enlarged spleen upon physical examination.
The syndrome can be caused by several different agents, but the most common is Epstein-Barr virus,
EBV; and the term “mononucleosis” usually refers to the disease caused by EBV.
EBV is ubiquitous in human. About 95% of all adults have antibodies against EBV,
likely from an infection during childhood. Symptomatic infections are most prevalent in
older teens and young adults, especially among college students. Infected young
children are often asymptomatic or have mild symptoms. Older adults are either immune to the
disease thanks to an earlier infection, or have atypical presentations that are misdiagnosed.
EBV is transmitted mainly via infected saliva. The virus is not very contagious,
it takes several exposures to high viral loads to acquire EBV. Hence, kissing is
the major route of transmission and mono is colloquially known as “the kissing disease”.
The incubation period is typically 3 to 5 weeks. The disease is self-limited and patients usually
recover after 2 to 6 weeks, but the virus may remain in the saliva for months. Recovered
patients may also shed virus periodically for life without developing symptoms. This is why
most people get infected by an asymptomatic person and often cannot recall being exposed to EBV.
After infecting the oral epithelial cells, EBV attacks lymphocytes, in particular B-cells,
in the tonsils. Infection then spreads throughout the lymphatic system,
causing a massive immune response that is responsible for most of the symptoms.
The immune response produces antibodies against EBV, providing lifelong immunity to EBV.
At the same time, infection by EBV causes B-cells to proliferate and become antibody-producing
plasma cells. Because B-cells are the source of antibodies of all kinds, NON-specific antibodies
that do not react to EBV antigens are also produced. These so-called heterophile antibodies
may be responsible for the mild thrombocytopenia, generalized rash, and antibiotic-related rash that
are occasionally associated with mononucleosis. As part of the immune defense, cytotoxic T-cells
are increased in numbers and activated to kill EBV-infected B-cells. These T-cells have
atypical morphology; they are known as Downey cells and are part of the diagnostic workup.
There are 2 antibody tests for mono: monospot test for heterophile antibodies,
and EBV-specific antibody test. The monospot test is highly specific, but may give false-negative
results in the first week of illness, and has low sensitivity, especially in children.
EBV antibody test is performed when monospot test is negative but mono is still suspected.
Mononucleosis is often misdiagnosed as strep throat,
and antibiotics may be given inappropriately.
Antibiotic treatment can cause a rash to develop and this is often mistaken for antibiotic allergy.
Mononucleosis is self-limited, most patients fully recover after a few weeks, although fatigue
may persist for months. Complications are rare but can be severe, sometimes life-threatening.
Treatment is supportive and includes bed rest, hydration, and fever and inflammation reducer.
Heavy lifting and active sports must be avoided for a month
to prevent splenic rupture. Corticosteroids can be helpful in certain complications,
such as impending airway obstruction, but are not usually recommended for uncomplicated disease.