Infectious Mononucleosis

Infectious mononucleosis (IM) is a condition caused by infection with the Epstein Barr virus (EBV). It is commonly known as the “kissing disease”, “glandular fever” or “mono”. This virus is found in the saliva of infected individuals. Infection may be spread by kissing or by sharing cups, toothbrushes and other equipment that transmits saliva.

EBV is secreted in the saliva of infected individuals and can be infectious several weeks before the illness begins and intermittently for the remainder of the patient’s life. Most people are infected with EBV as children, when it causes very few symptoms. When infection occurs in teenagers or young adults, it causes more severe symptoms. It is the symptomatic infection with EBV that is called infectious mononucleosis. Typical symptoms are fever, sore throat and fatigue.

TOM TIP: Look out for the exam question that describes an adolescent with a sore throat, who develops an itchy rash after taking amoxicillin. Mononucleosis causes an intensely itchy maculopapular rash in response to amoxicillin or cefalosporins. 



  • Fever
  • Sore throat
  • Fatigue
  • Lymphadenopathy (swollen lymph nodes)
  • Tonsillar enlargement
  • Splenomegaly and in rare cases splenic rupture


Heterophile Antibodies

In certain diseases (such as HIV) we can test for specific antibodies to the disease. That way we know the body has come in contact with the disease and launched an immune response to it. In infectious mononucleosis, the body produces something called heterophile antibodies, which are antibodies that are more multipurpose and not specific to the EBV antigens. It takes up to 6 weeks for these antibodies to be produced.

We can test for these heterophile antibodies using two tests:

  • Monospot test: this introduces the patient’s blood to red blood cells from horses. Heterophile antibodies (if present) will react to the horse red blood cells and give a positive result.
  • Paul-Bunnell test: this is similar to the monospot test but uses red blood cells from sheep.

These tests are almost 100% specific for infectious mononucleosis, however not everyone who has IM produces heterophile antibodies, and it can take up to six weeks for the antibodies to be produced. Therefore they are only 70 – 80% sensitive.


Specific Antibody Tests

It is possible to test for specific EBV antibodies. These antibodies target something called viral capsid antigen (VCA):

  • The IgM antibody rises early and suggests acute infection
  • The IgG antibody persists after the condition and suggests immunity


Management and Prognosis

Infectious mononucleosis is usually self limiting. The acute illness lasts around 2 – 3 weeks, however it can leave the patient with fatigue for several months once the infection is cleared.

Patients are advised to avoid alcohol, as EBV impacts the ability of the liver to process the alcohol. Patients are advised to avoid contact sports due to the risk of splenic rupture. Emergency surgery is usually required if splenic rupture occurs.



  • Splenic rupture
  • Glomerulonephritis
  • Haemolytic anaemia
  • Thrombocytopenia
  • Chronic fatigue

EBV infection is associated with certain cancers, notable Burkitt’s lymphoma.


Last updated January 2020