Infectious mononucleosis (IM) is a condition caused by infection with the Epstein-Barr virus (EBV). The virus is found in the saliva of infected individuals, and infection may be spread by kissing or sharing cups, cutlery, toothbrushes, or anything contaminated with saliva. It is commonly known as the “kissing disease”, “glandular fever” or “mono”.
EBV can be shed in saliva before symptoms, for months after infection, and intermittently later in life. Most people are infected with EBV as children, when it causes minimal symptoms. When infection occurs in teenagers or young adults, it causes more typical symptoms of infectious mononucleosis.
Presentation
The typical presentation is an adolescent or young adult with an acute illness featuring:
- Low-grade fever
- Sore throat
- Fatigue
- Bilateral posterior cervical lymphadenopathy (swollen lymph nodes in the posterior triangle of the neck)
- Tonsillar enlargement with “whitewash” exudate (a white coating)
- Splenomegaly (and a risk of splenic rupture)
TOM TIP: Infectious mononucleosis can characteristically cause an itchy maculopapular rash in response to amoxicillin. Look out for the exam question that describes an adolescent with a sore throat who develops an itchy rash after taking amoxicillin.
Testing
The first-line tests in the NICE Clinical Knowledge Summaries (updated November 2024) are:
- Full blood count with differential white cell count
- Heterophile antibody test (monospot test) in the second week of illness
The heterophile antibody test involves mixing the patient’s blood with red blood cells from horses (monospot test) or sheep (Paul-Bunnell test). Antibodies produced in response to EBV create a reaction in response to the animal red blood cells, giving a positive result. A negative result may occur in the first week of illness because there are insufficient antibodies to elicit a reaction. In this scenario, repeating the test in 5-7 days is recommended.
Findings in infectious mononucleosis are:
- Raised lymphocytes
- Atypical or “reactive” lymphocytes
- Positive heterophile antibody test
Epstein-Barr virus serology is the alternative test used in children under 4 years old, in immunocompromised patients, and when the heterophile antibody test is negative or unsuitable.
- IgM antibodies rise early and suggest acute or recent infection (M for Miserable)
- IgG antibodies persist for life after the acute illness (G for Got it before)
Management and Prognosis
Infectious mononucleosis is usually self-limiting. The acute illness lasts about 2-3 weeks. However, it can leave the patient with fatigue for several months once the infection is cleared.
During the acute illness, patients are advised to avoid:
- Excess alcohol (EBV can cause hepatitis and impaired liver function)
- Heavy lifting and contact sports for at least 3 weeks (due to the risk of splenic rupture)
Complications
Complications include:
- Prolonged fatigue
- Hepatitis
- Pericarditis and myocarditis
- Interstitial nephritis
- Thrombocytopenia and haemolytic anaemia
- Encephalitis, Guillain-Barré syndrome and Bell’s palsy
- Splenomegaly and splenic rupture
EBV infection is associated with certain cancers, notably Burkitt lymphoma.
Last updated May 2026
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