Focus on sanitation and clean water may improve control of endemic cholera

UNIVERSITY PARK, Pa. — Public health strategies, from vaccination to improving sanitary conditions, may have different impacts depending on whether a disease is endemic — persisting in a particular location for an extended period of time — or a new outbreak, because the dominant transmission routes may differ, according to a new study. The study, which modeled cholera transmission after interventions in the Democratic Republic of the Congo (DRC), highlights how long-term strategies like improving access to clean water and sanitary infrastructure could be particularly effective in locations where cholera is endemic due to the contribution of an environmental reservoir.

A paper describing the research, led by researchers at Penn State, was published in the journal PLOS Neglected Tropical Diseases. 

“When conducting interventions for an infectious disease, public health officials must decide how to allocate resources and prioritize strategies that will have the most impact,” said Alexandre Blake, first author of the paper, who completed the work when he was a postdoctoral researcher in biology in the Penn State Eberly College of Science and the Center for Infectious Disease Dynamics (CIDD). “We are particularly interested in how to allocate resources in endemic situations. If an epidemic is like a forest fire, with transmission burning intensely before coming to a stop, then an endemic disease is more like a slow burn, with transmission constantly in the background with occasional flare ups. We used statistical modeling to better understand cholera transmission in the city of Kalemie in the DRC, where it is endemic. We specifically focused on the two years following a series of interventions to evaluate the impact of those interventions.”

Cholera is bacterial disease that can be transmitted when the bacteria are ingested. The disease can cause severe diarrhea, which can potentially contaminate water sources with the disease-causing bacteria and lead to increased transmission, especially in areas with poor sanitation or limited infrastructure to treat drinking water. Although many people who become infected have no symptoms or mild symptoms, symptomatic cases of cholera can progress quickly, leading to severe dehydration that can be fatal. One study estimated that 1.3 to 4 million cholera cases worldwide cause between 21,000 and 143,000 deaths each year. The Global Task Force on Cholera Control has created a road map to eliminate cholera in 20 countries by 2030, particularly targeting sub-Saharan Africa, where cholera has been endemic since the early 1970s. 

Interventions for cholera often include vaccination, which can be rapidly implemented, as well as water, sanitation and hygiene improvements, collectively called WASH. Large scale WASH strategies, such as improving infrastructure for sanitation and access to clean water, are more expensive and take months to years to implement.

“We know that WASH and vaccination are great tools to prevent cholera,” Blake said. “But the impact of vaccination has largely been studied in epidemic situations as a rapid response. WASH is slow and costly to implement which makes it hard to accomplish in low-income settings. To eliminate cholera, we need to better understand the many factors that drive transmission as well as the impact of different interventions in endemic settings.”

From late 2013 to early 2016, Doctors Without Borders led intervention efforts in Kalemie, including vaccination and WASH efforts. WASH efforts focused on improving access to clean water, for example by installing pipes, water reservoirs and public drinking fountains, as well as distributing water filters and, during outbreaks, chlorinating drinking water to kill bacteria. 

The researchers built several statistical models to determine which combination of factors best explained cholera transmission during this period. They used satellite images of light at night to track seasonal occupancy of the city, which impacts the number of people in the area capable of becoming infected with or transmitting the bacteria. They also included rainfall and other environmental variables that might impact the bacteria’s ability to survive in the nearby lake and infect more people.

Although the researchers did not find much impact of the seasonal movement of people in Kalemie, they did find that a large portion of transmission resulted from the environment, likely because the lake acts as a reservoir for the cholera bacteria.

“Because the bacteria persist in the lake, some residents of the city may have opportunities to be regularly exposed and may ultimately develop some level of immunity, which shapes transmission dynamics and eventually the impact of some interventions,” Blake said.

The researchers estimated that most of cholera transmission could be avoided if environmental exposure or contamination was prevented. The researchers also estimated that the majority of residents had some immunity during this period due to regular outbreaks and persistent exposure in the environment. Taken together, the researchers said that vaccination would have a smaller impact on preventing transmission compared to WASH in such settings.

Blake said that early in epidemic situations, most people in the area are assumed not to be immune to the disease, so fast-paced responses like vaccination can be a good choice for management — but endemic infections require a more comprehensive approach. 

“In an epidemic setting, it’s almost never a wrong decision to vaccinate because the majority of the population is susceptible to the disease, so vaccinating early in the outbreak can quickly increase immunity levels,” he said. “But in endemic settings, this assumption may not be true. Vaccines are ideal for people who do not already have some level of protection from previous exposure, but testing to see if an individual is already immune can be costly or impractical. So, in areas like Kalemie where there may be relatively high levels of immunity due to environmental transmission, we estimate a higher proportion of cases avoided from long-term strategies like WASH if both strategies are competing because of limited resources.”

An additional benefit of WASH, Blake said, is its positive impact on general health. 

“Whether it’s talking to people on the street about hygiene or improving sanitation and access to clean water, WASH provides basic needs and can reduce transmission of more than just cholera,” he said. “WASH efforts have greater up-front costs but have long-lasting impacts.”

Blake noted that studies to confirm the level of immunity in Kalemie and other endemic areas are needed. Ultimately, he said, this information could be used to refine intervention strategies targeting endemic diseases that have an environmental reservoir that contributes to transmission.

“Vaccination and WASH efforts are both critical tools in controlling the spread of diseases like cholera,” said Nita Bharti, associate professor of biology who is also affiliated with CIDD at Penn State and senior author of the paper. “But a ‘one size fits all’ approach is not going to work across epidemic and endemic situations. We need a location-specific understanding of the role of the environment and transmission routes to control cholera and other infectious diseases in endemic areas.”

In addition to Blake and Bharti, the research team at Penn State includes Adam Walder, who has since graduated with a doctoral degree in statistics, and Ephraim Hanks, associate professor of statistics, both affiliated with CIDD. The team also includes Placide Okitayemba Welo at Programme National d’Elimination du Choléra et de lutte contre les autres Maladies Diarrhéiques in the DRC; Francisco Luquero at Epicentre in France; and Didier Bompangue at the University of Kinshasa in the DRC. 

Funding from the U.S. National Institutes of Health, U.S. National Science Foundation and the U.S. National Institute of Food and Agriculture supported this research.