Infectious Mononucleosis | Overview
This lymphatic system disease, which is often simply called “mono” or the “kissing disease,” can be caused by a number of bacteria or viruses, but the vast majority of cases are caused by the Epstein-Barr virus (EBV), a member of the herpes family.
▶ Signs and Symptoms
The symptoms of mononucleosis are sore throat, high fever, and cervical lymphadenopathy, which develop after a long incubation period (30 to 50 days). Many patients also have a grey-white exudate in the throat, a skin rash, and enlarged spleen and liver. A notable sign of mononucleosis is sudden leukocytosis, consisting initially of infected B cells and later T cells. Fatigue is a hallmark of the disease. Patients remain fatigued for a period of weeks. During that time, they are advised not to engage in strenuous activity due to the possibility of injuring their enlarged spleen (or liver). Eventually, the strong, cell-mediated immune response is decisive in controlling the infection and preventing complications. But after recovery, people usually remain chronically infected with EBV.
Although “mono” was first described more than a century ago, its most frequent cause was finally discovered through a series of accidental events starting in 1958, when Michael Burkitt discovered an unusual malignant tumour in African children (Burkitt’s lymphoma) that appeared to be infectious. Later, Michael Epstein and Yvonne Barr cultured a virus from tumours that showed typical herpes virus morphology. Evidence that the two diseases had a common cause was provided when a laboratory technician accidentally acquired mononucleosis while working with the Burkitt’s lymphoma virus.
The Epstein-Barr virus shares morphological and antigenic features with other herpesviruses; and in addition, it contains a circular form of DNA that is readily spliced into the host cell DNA. Scientists have long suspected a link between chronic EBV infection and other illnesses, especially Hodgkin’s lymphoma, but the connection is still controversial. Research in 2010 found the virus in 40% of Hodgkin’s lymphoma cases.
▶ Pathogenesis and Virulence Factors
The latency of the virus and its ability to splice its DNA into host cell DNA makes it an extremely versatile virus that can avoid the host’s immune response.
▶ Transmission and Epidemiology
More than 90% of the world’s population is infected with EBV. In general, the virus causes no noticeable symptoms, but the time of life when the virus is first encountered seems to matter. In the case of EBV, infection during the teen years seems to result in disease, whereas infection before or after this period is usually asymptomatic. Direct oral contact and contamination with saliva are the principal modes of transmission, although transfer through blood transfusions, sexual contact, and organ transplants is possible.
▶ Culture and Diagnosis
A differential blood count that shows excess lymphocytes, reduced neutrophils, and large, atypical lymphocytes with lobulated nuclei and vacuolated cytoplasm is suggestive of EBV infection. A test called the “Monospot test” detects heterophile antibodies—which are antibodies that are not directed against EBV but are seen when a person has an EBV infection. This test is not reliable in children younger than age 4, in which case a specific EBV antigen/antibody test is conducted.
▶ Prevention and Treatment
The usual treatments for infectious mononucleosis are directed at symptomatic relief of fever and sore throat. Hospitalization is rarely needed. Occasionally, rupture of the spleen necessitates immediate surgery to remove it.