African biostatisticians have found cholera does not spread only through contaminated water or direct contact with infected people but can also be rapidly transmitted by flies, making outbreaks faster and more unpredictable.
The findings form part of a study by researchers linked to the University of the Witwatersrand (Wits) which examined how flies act as environmental vectors by carrying the cholera-causing bacterium Vibrio cholerae between contaminated environments and human food.
According to the university, this mode of transmission significantly complicates efforts to predict and control outbreaks.
Scientists analysed how frequently flies pick up bacteria from contaminated water, how easily they transfer it to food and how long the insects survive in different environments.
Prof Romain Glèlè Kakaï, co-project investigator for the Sub-Saharan Africa Consortium for Advanced Biostatistics (SSACAB) at the Wits School of Public Health and a professor at the University of Abomey-Calavi in Benin, said the modelling revealed a clear link between fly activity and outbreak intensity.
“When these fly-related transmission factors were high, cholera outbreaks were more likely to take off. When flies die quicker or don’t pick up or transmit bacteria as efficiently, outbreaks are far more likely to fade out,” he said.
By the end of the first quarter of 2025, 25 countries had reported cholera or acute watery diarrhoea, with 116,574 cases and 1,514 deaths, most of them in Africa.
Additional modelling data showed flies are easily contaminated with cholera bacteria and can transmit it physically to many people in a short time.
“The model suggests cholera transmission can be highly explosive. Even a small initial contamination may lead to a large outbreak when environmental vectors such as flies are active. Once contaminated, flies can mechanically transmit the bacteria to many individuals in a short time, producing outbreak dynamics that resemble sparks igniting dry grass,” said Kakaï.
The findings come amid a sharp resurgence of cholera in Africa and other regions.
The Africa Centres for Disease Control and Prevention recorded about 300,000 confirmed and suspected cholera cases in 2025 in 20 countries, marking the worst outbreak on the continent in 25 years.
According to the National Institute for Communicable Diseases (NICD), the World Health Organisation (WHO) reported 804,721 cholera cases and 5,805 deaths globally in 2024 in 33 countries spanning five WHO regions.
“The Eastern Mediterranean and African regions bore the brunt of the burden. With growing spread, rising case numbers and logistical constraints in global response systems, WHO has maintained a grade 3 emergency designation since January 2023, its highest level of alert,” the NICD said.
By the end of the first quarter of 2025, 25 countries had reported cholera or acute watery diarrhoea, with 116,574 cases and 1,514 deaths, most of them in Africa.
Wits said fragile water systems, poor sanitation, rapid urbanisation and climate shocks that contaminate water sources and overwhelm health infrastructure all increase the complexity and unpredictability of cholera transmission.
Alongside modelling fly-driven transmission, researchers also assessed the impact of vaccination in stopping outbreaks.
Wits said another study confirmed that to interrupt “runaway” transmission, where one infected person can infect more than three others, at least 70% of people in high-risk groups must be vaccinated.
In less-exposed populations, coverage of about 62% to 65% is required to halt transmission.
The university said while long-term investments in clean water, sanitation, climate resilience and community health systems are essential, vaccination remains the fastest and most direct way to break chains of transmission once outbreaks are under way.
“Vaccination can have immediate and measurable effects on cholera transmission, reducing outbreak intensity in the short-term, while improvements in water and sanitation act more gradually and contribute to longer-term control,” said Kakaï.
Wits confirmed a phase 1 safety clinical trial is under way at the university for an oral cholera vaccine manufactured in South Africa.
“If successful, it will be the first time Africa has produced its own cholera vaccine, rather than relying on imported supplies,” the university said.
SSACAB principal investigator and head of the Wits School of Public Health Prof Tobias Chirwa said local scientific capacity was critical.
“Our modelling reveals outbreaks move fast and unpredictably, so response tools must be local, reliable and ready,” he said.
In their study, researchers Leul Mekonnen Anteneh, Mahouton Norbert Hounkonnou and Kakaï stressed the importance of understanding how cholera spreads in complex environments.
“Infectious diseases have a tremendous impact on human health. Thus, it is essential to research the mechanisms of disease transmission and the control of diseases such as cholera,” they said.
“Food-borne diarrhoeal illness, or cholera, is one of the most serious infectious diseases. Though cholera is mostly a waterborne disease, fly vectors can indirectly spread it.”
They explained flies contaminate food by crawling, feeding and laying eggs on meals, exposing vulnerable populations to infection.
Despite extensive research and established prevention measures, cholera continues to pose a serious public health threat in developing countries due to climate change, conflict, displacement, urbanisation and population growth.
The researchers noted cholera transmission via flies is largely random and influenced by factors such as environmental contamination, human immunity, treatment effectiveness and seasonal changes.
“A vector-borne disease’s activity might be influenced by a specific time of year or seasonal change. As a result, seasonal fluctuation and the initial population infection rate influence the likelihood of a disease outbreak,” they said.
Chirwa said strengthening African scientific capacity was essential to effective outbreak response.
“Africa needs more than access to vaccines; it needs the scientific capacity to decide when, where and how to use them most effectively.
“By training African biostatisticians and data scientists, SSACAB is helping to ensure outbreak response is guided by local expertise, local data and models that reflect African realities.”
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