Yemen cholera epidemic update, Forecasting the outbreak

August 28, 2017

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Current status

The number of cholera cases are dropping in Yemen, with the weekly number of reported new cases declining by one third since June. Despite this good news, there are still many cases reported on a daily basis. As of Aug. 26. the number of cases has reached 569,032, including 2020 deaths since Apr. 27.

Massive collective efforts to treat the sick and improve water and sanitation systems have helped slow the spread of the disease, according to UNICEF.

“We had difficulties managing the number of patients that came to us – many of them with severe conditions,” said Dr. Nahla Arishi, Deputy Manager and Head of the cholera treatment center at Alsadaqah Hospital in Aden City. “The hospital is crowded and beds and essential medicines are in short supply. But I can’t close the hospital’s doors and not accept children because there aren’t enough beds – I am a doctor and a mother too.”

A nationwide cholera awareness campaign is currently underway, mobilizing over 40,000 volunteers going house-to-house and reaching over 2.7 million families so far – approximately 80 per cent of households in Yemen. Through the campaign:

Nearly 12.5 million people across the country have received information on water disinfection, handwashing, sanitation and food safety, and
250,000 children and adults suffering from diarrhoea have been provided with oral rehydration salts and referred for treatment.

“Many of the children I have visited in their homes are thin and weak,” said Muthab Alburaik Salem, a community health volunteer working on the campaign. “It’s crucial to spread awareness among vulnerable communities so they are spared additional suffering. I fear that my own children will be exposed to diseases – so I treat all children I’m working with in Yemen as if they were my own,” adds Muthab, a mother of two.

Despite these recent gains, the fight against cholera is far from over. Amid continued violence, water and sanitation systems are collapsing, and more than half of Yemen’s health facilities are out of service, cutting off almost 15 million people from safe water and access to basic healthcare. The country remains on the brink of famine, with an estimated 385,000 children suffering from severe acute malnutrition, putting them at heightened risk of acute watery diarrhea and cholera.

Forecasting the outbreak

Researchers from Hokkaido University developed a new mathematical model which accurately forecasted that a devastating cholera epidemic in Yemen would peak by early July, the 26th week of 2017 and the cumulative incidence would be the order of 700-800 thousand cases.

After the cholera outbreak in Yemen, the team compiled a real-time forecast based on weekly data collected by the World Health Organization (WHO) about suspected cases and fatalities between April 16 (16th week of the outbreak) and July 1 (26th week). The team incorporated reporting delays–time lags between the onset of the disease and the reporting of cases–in the mathematical model by analyzing the epidemic curve that was updated every week. It also discovered a method, through the study of weekly death rates, to adjust the ascertainment bias–the tendency that more cholera cases likely will be reported after many cases have already been reported rather than in the initial phase of the outbreak. Incorporated in the epidemic curve is a logistic curve or generalized logistic (Richards) curve.

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The team estimated the cumulative cholera cases at the end of the epidemic would be 790,778 on the logistic model and 767,029 on the Richards model. The researchers estimated the epidemic curve would peak by the 26th week of 2017 and then drop monotonically in the subsequent weeks. The forecasted monotonic decline has been actually seen in WHO data by mid-August 2017.

“Our model succeeded in excluding two biases for the first time and the resulting forecast has been proven reliable so far. Real-time forecasting could assist enhancing situation awareness about the ongoing epidemic communication between experts and citizens while avoiding excessive pessimism, in addition to crafting future measures against cholera,” says Hiroshi Nishiura of the research team.