Syndromic surveillance for mpox during a large religious mass gathering amid an ongoing outbreak, Uganda, 2025: a cross-sectional study

Large religious mass gatherings can facilitate infectious disease transmission through crowding, prolonged close contact, shared facilities, and extensive population movement. The 2025 Uganda Martyrs Day commemoration occurred during an ongoing national mpox outbreak. We describe the implementation of syndromic surveillance for mpox during a large religious mass gathering and the yield of mpox-compatible illness detected among pilgrims. We conducted a cross-sectional symptom-screening survey and retrospectively reviewed emergency medical services (EMS) records from the Namugongo Catholic and Protestant shrines during 29 May-5 June 2025. Pilgrims aged [≥]18 years were selected through systematic sampling at entrance gates and random sampling within demarcated zones. Mpox-compatible illness was defined as acute fever ([≥]38.5{degrees}C) with rash and at least one of headache, lymphadenopathy, back pain, myalgia, or profound weakness. We summarized participant characteristics, reported symptoms, mpox-compatible illness, and provisional diagnoses among pilgrims seeking care. Of 1,523 pilgrims approached, 1,299 participated (85.3% response rate); median age was 35 years (interquartile range: 25-49), and 752 (57.9%) were male. Among actively screened pilgrims, 73 (5.6%) reported [≥]2 mpox-related symptoms, and seven (0.5%) met the mpox-compatible illness definition. Among 5,216 pilgrims who sought care, 3,415 (65.5%) had documented presenting symptoms; of these, 628 (18.4%) had mpox-related syndromic presentations. Among all 5,216 care-seekers, the most common provisional diagnoses were peptic ulcer disease (n=563; 10.8%), respiratory tract infection (n=482; 9.2%), malaria (n=190; 3.6%), urinary tract infection (n=96; 1.8%), and gastritis (n=79; 1.5%). Active symptom screening suggested a low prevalence of mpox-compatible illness in the general pilgrim population, whereas EMS records identified a larger syndromic signal among pilgrims who sought care. Combining both approaches with standardized documentation, clinical triage, laboratory confirmation, and follow-up could improve detection during mass gatherings