Investigation of a Cholera Outbreak in Monze District, Southern Province, Zambia, November 2025: A Case-Control Study

Abstract

Background
Cholera remains a recurrent public health problem in Zambia, which recorded 29 outbreaks between 1977 and 2018. On 4 November 2025, Monze District in Southern Province reported a cluster of acute watery diarrhoea among residents and among travellers who had arrived by train for a funeral. We investigated to confirm the causative organism, describe the outbreak, measure the timeliness of the response against the 7-1-7 framework (7 days for detection, one day for notification and 7 days for early response actions) and identify factors associated with infection.

Methods
We conducted an unmatched case-control study with a ratio of four controls to each case in Monze District between 6 and 17 November 2025. We collected participant demographic, clinical and environmental exposure data using a structured questionnaire. We enrolled all cases; controls were residents of the same facility catchment areas without acute watery diarrhoea, drawn from neighbouring households and close contacts of cases. We collected stool samples for laboratory confirmation on 33 patients. Statistical analyses were performed using R version 4.3.3. Logistic regression was used to identify factors associated with cholera infection. The 7-1-7 framework was used to assess timeliness in detection, notification and response.

Results
We enrolled 166 participants (33 cases and 133 controls). Median age (Interquartile range) was 26 years (15, 47 years) among cases and 31 years (22, 42 years) among controls. Three community cases died (9.1% case fatality rate) out of 33 cases. All 33 cases had acute watery diarrhoea and 32 (97.0%) had dehydration. Onsets began on 2 November, peaked the same day with six cases, and formed a second cluster of four cases on 11 November. Vibrio cholerae O1 serotype Ogawa was confirmed by culture. Four exposures were independently associated with infection: travel in the 7 days before onset (adjusted odds ratio (aOR) 6.28, 95% CI 2.43–16.91), a household member with diarrhoea (aOR 5.54, 95% CI 1.65–20.18), untreated household drinking water (aOR 6.50, 95% CI 1.82–35.50) and no soap in the household (aOR 2.72, 95% CI 1.11–6.83). Detection took 2 days, notification 0 days and early response 0 days, within the targets of 7, 1 and 7 days, respectively.

Conclusion
The findings are consistent with travel-associated introduction followed by household and community transmission in areas with unsafe water and limited access to sanitation, although the precise source and transmission pathways could not be conclusively established. Strengthening household water treatment, reliable access to soap, prompt protection of households in which a case occurs, and travel advice at transport hubs are the measures most likely to prevent similar events. All three deaths occurred in the community without the patient reaching care, highlighting the need to investigate barriers to timely care-seeking and access to emergency treatment.