Infectious Mononucleosis in Athletes

The Epstein-Barr virus is a herpesvirus spread mainly in saliva, hence the popular label of the kissing disease, and also through shared drinks and utensils, which is relevant in team environments. After a long incubation of roughly four to six weeks, the virus infects B lymphocytes and provokes a vigorous immune response, which produces the characteristic reactive, or atypical, lymphocytes and a generalised enlargement of lymphoid tissue, including the lymph nodes, the tonsils and the spleen.

The spleen enlarges in a large proportion of cases through this lymphoid proliferation and congestion, and as it swells its capsule is stretched and the organ becomes softer and more fragile, so it is vulnerable to rupture from relatively minor trauma or a sudden rise in intra-abdominal pressure. Splenic rupture is rare but is the most feared complication, and the risk is highest in the first weeks of the illness. Importantly, the degree of enlargement does not track reliably with how unwell the athlete feels or with examination findings, and rupture can occur without an obvious blow.

Most people have been infected with the virus by adulthood, often without symptoms in childhood, and symptomatic mononucleosis is more common when first infection is delayed to adolescence or early adulthood, after which the virus persists in a latent state for life.

The classic picture is a prodrome of malaise followed by the triad of fever, an often severe sore throat with tonsillar enlargement and exudate, and tender lymphadenopathy, characteristically in the posterior cervical chain. Marked fatigue and malaise are prominent and can be prolonged, and there may be splenomegaly, sometimes hepatomegaly, palatal petechiae and puffy eyelids. In an athlete it often presents as a bad throat with fatigue and underperformance out of keeping with a simple cold, and in a contact-sport player that combination should prompt thought about the spleen.

The acute symptoms usually settle over two to four weeks, but the fatigue can linger for weeks to months, which is frequently the rate-limiting factor for return. The important clinical skill is to recognise the illness, to keep the spleen in mind, and to distinguish it from a straightforward bacterial throat, since that distinction changes both prescribing and advice about sport.

Mononucleosis is often a clinical diagnosis supported by tests where needed. A full blood count (FBC) typically shows a lymphocytosis with reactive, atypical lymphocytes, and liver function tests (LFTs) are frequently mildly deranged. The heterophile antibody test, widely known as the Monospot, supports the diagnosis but has notable false negatives, particularly in the first week, so a negative test early does not exclude the illness and may need repeating.

Where the heterophile test is negative or the timing is unclear, EBV-specific serology is more sensitive and specific: viral capsid antigen (VCA) immunoglobulin M (IgM) indicates acute infection, while Epstein-Barr nuclear antigen (EBNA) antibody appears later, so its presence points to past rather than current infection. A crucial practical point is to avoid amoxicillin and ampicillin, which commonly provoke a florid maculopapular rash in mononucleosis, so it is worth being confident of the diagnosis before prescribing an antibiotic for a sore throat.

Ultrasound can measure the spleen, but a normal or unknown result does not confirm it is safe to return, because normal spleen size varies, an individual's baseline is usually unknown, and there is no validated size threshold for clearance. The corollary matters too: where a scan has been done and shows persistent significant splenomegaly, contact and collision sport remain contraindicated until it resolves, even once the usual three to four week point has passed.

Treatment is supportive: rest, plenty of fluids, and simple analgesia such as paracetamol for fever and sore throat, with most people recovering without specific treatment. Antibiotics are not indicated for the viral illness and are reserved for a genuine bacterial complication, and amoxicillin and ampicillin are specifically avoided because of the rash. Corticosteroids are not used routinely and are reserved for specific complications such as impending airway obstruction from grossly enlarged tonsils, where a short course with specialist input may help, rather than for fatigue.

While the spleen is enlarged, contact and activities that raise intra-abdominal pressure, such as heavy lifting and straining, are avoided, and it is reasonable to avoid aspirin and non-steroidal anti-inflammatory drugs (NSAIDs) during that period given bleeding risk, and to avoid alcohol while liver tests are deranged. Urgent assessment or admission is needed for suspected splenic rupture, airway compromise, an inability to swallow with dehydration, significant hepatitis or diagnostic uncertainty. Fatigue is managed with reassurance and a gradual, graded increase in activity, recognising that a small number develop prolonged post-viral fatigue.

The central issue is splenic rupture, which is rare but potentially fatal and is highest in the first weeks of illness, with the great majority of ruptures occurring within roughly the first three weeks of symptom onset. Rupture can follow contact, collision or a sudden rise in intra-abdominal pressure from heavy lifting or straining, and occasionally happens without obvious trauma.

A pragmatic and widely used approach is to avoid all strenuous exercise, contact or collision sport and heavy lifting in the early illness. Light, non-contact aerobic activity can usually be reintroduced gradually from around three weeks after symptom onset, provided the athlete is afebrile, systemically well and clearly recovering. Contact and collision sport, heavy lifting and marked straining are generally deferred until at least three to four weeks from symptom onset and only once the athlete is clinically well, individualised and longer where symptoms persist or splenomegaly is suspected.

Return is guided chiefly by time from onset and clinical recovery rather than by imaging, since a normal or unknown scan cannot clear an athlete, but where a scan has been performed and shows persistent significant splenomegaly, contact sport stays contraindicated until it resolves, even beyond the usual three to four week point. Fatigue often outlasts the splenic risk and can itself limit progress, so load is built up gradually while watching for a relapse of tiredness. Return is a shared, documented decision, with the athlete understanding the small residual risk, and since there is no single universally agreed protocol, it is sensible to align with current guidance and any governing-body rules.

• •United Kingdom primary care and sports medicine guidance on glandular fever covers diagnosis, the amoxicillin caution and avoiding contact sport for a period because of splenic rupture risk.
• •Sport and exercise medicine consensus statements and reviews address the return-to-sport timeline, the window of splenic rupture risk, and the limited role of imaging in timing return.
• •Guidance on heterophile and Epstein-Barr virus-specific testing supports appropriate investigation and interpretation.

• •Infectious mononucleosis (glandular fever) is usually caused by the Epstein-Barr virus and combines sore throat, fever, lymphadenopathy and marked fatigue.
• •The spleen commonly enlarges and can rupture, a rare but life-threatening complication, with the risk highest in the first weeks.
• •Avoid amoxicillin and ampicillin, which cause a widespread rash in mononucleosis.
• •The heterophile (Monospot) test can be falsely negative early, so a negative result does not exclude it.
• •Contact and collision sport and heavy lifting are avoided for at least the first three to four weeks from onset and until clinically recovered.
• •A scan cannot confirm safe return, but persistent significant splenomegaly on a scan delays contact sport until it resolves.