Category Archives: Floods

Viruses and bacteria are spread by floodwater – evidence from the 2011–2012 La Niña floods in Peru

A flooded street in Santa Clara de Nanay, April 2, 2012 (courtesy of Asociación Benéfica Prisma)

Josh Colston of the Division of Infectious Diseases and International Health, University of Virginia School of Medicine, Charlottesville, presented his findings on the connection between floods and enteric pathogens in Peru at the 2019 meeting of the American Society of Tropical Medicine and Hygiene. Below he has shared us with a summary of his work and findings. A link to the recently published work is also provided.

Climate change represents an impending global public health threat since extreme weather events like floods can cause injury and drowning, toxic exposure, and the spread of infectious diseases. Poor people living in unplanned settlements with inadequate infrastructure are most vulnerable to these impacts. Outbreaks of gastroenteritis often occur following floods and can be particularly serious for young children. But there are many different bugs that can cause this illness and it’s not yet known which of them are most prone to contaminating floodwater. However, a newly published paper in the International Journal of Environmental Research and Public Health may shed some light thanks to a small piece of serendipity in an otherwise devastating natural disaster.

Location of the study site

The 2010-2012 La Niña event (the colder counterpart of El Niño) caused huge disruption to weather patterns over several continents. The Amazonian region of Peru around the city of Iquitos was particularly badly hit by heavy rains. It’s a low-lying area particularly prone to flooding since it’s situated at the confluence of several Amazon tributaries. Waterways are the main transport routes in the region, so most of the population lives close to the banks of the rivers. Following months of heavy rainfall in late 2011 and early 2012, three of the rivers – the Ucayali, Marañón, and Nanay – burst their banks, causing widespread flooding and forcing many locals to abandon their homes and evacuate to drier areas. By the end of the disaster, an estimated 50,000 people had been made homeless.

It just so happened that, in a quiet fishing town on the outskirts of Iquitos called Santa Clara de Nanay, an epidemiologic surveillance study was being carried out. Around 300 babies had been recruited and field workers were taking regular measurements and biological samples to see how they were growing and what bugs they were catching. Using a special epidemiologic method known as ‘causal inference’ researchers were able to compare the samples of the infants’ poop before, during and after the flood to see how the rates of infection changed.

Estimated prevalence rates of four viruses and three bacteria before, during and after the flood

Interestingly, two viruses showed substantial upticks during the flood. Rates of rotavirus were 5 times, and sapovirus 2.5 times the normal level for that time of year. What’s more, the rotavirus cases seemed to be caused by unusual virus strains that were not common in the area and which are less preventable by vaccine. Meanwhile, three bacteria – Campylobacter, Shigella and a type of E. coli called ST-ETEC – showed smaller increases. It’s common to catch Campylobacter from poultry and, since a lot of households in Santa Clara keep chickens in their yards, it’s possible that the mini-outbreak was cause by floodwater washing chicken droppings out of the coops and into the wider environment.

What’s clear from this and other recent studies, is that we need to start thinking bigger when it comes to drainage and sanitation solutions. Traditional low-cost, household-level improvements to water sources and sanitation facilities may not be up to the task in the face of climate events that may suddenly and unexpectedly expose entire communities to large amounts of untreated sewage. Investments in more ambitious, municipal-level water, wastewater, and drainage infrastructure – the kind that historically brought about massive, society-wide child health improvements in high income countries – may be the only sure route to climate resilience.