Infectious diseases and climate change: The perfect storm

Storms and vectors

Extreme weather events resulting from climate change can include strong winds and heavy precipitation in dramatic and destructive storms. Following storms, the accumulation of stagnant water in open containers and low-lying areas provides convenient breeding sites for mosquitoes and other insect populations, leading to an increase in incidence of vector-borne diseases among human and animal populations – including Zika, malaria, dengue, West Nile virus, Rift Valley fever and more. It doesn’t take long for vector populations to boom – under the right conditions mosquito eggs can hatch into larvae within just days of getting wet again, and can become biting adults less than two weeks later.

Waterborne diseases

Many dysentery diseases also show an uptick in incidence with the flooding following extreme storms. Heavy rains and floods regularly bring about an increase in outbreaks of cholera, typhoid, hepatitis-A and leptospirosis, particularly in areas without sufficient sewage systems in place to prevent rapid multiplication of bacteria in contaminated water. Water- and food-borne dysentery diseases can persist within the environment for long periods of time, within which the bacteria multiplies and through which human infections occur via direct contact (wading, swimming) or consumption (both water and food – exacerbated by the inevitable shortage of potable water following disasters). In these settings, the sudden influx of large volumes of water can increase the potential for bacterial growth, and increase the rate of contact between humans and the contaminated source, depending on disease- and locale-specific conditions.

High-density contacts

A contact is considered effective for disease transmission if the interaction is sufficient to transfer   infectious material from one individual to another. This is, of course, highly dependent upon the disease under consideration: a sneeze from three feet away won’t transmit cholera, but it could transmit chickenpox. A shift in contact rates, as can happen when large populations are suddenly condensed into small physical spaces, can easily upset what was previously a stable system, suddenly increasing transmission rates of an already-circulating disease, altering normal seasonal transmission patterns, or introducing brand-new diseases and mutations. 
Relief facilities and shelters often cluster sizeable groups of people into close quarters for potentially long periods of time following extreme storms and other climate change-related events, providing the perfect setting for person-to-person disease transmission. There’s a wide variety of diseases that can be transmitted person-to-person, specifically by airborne transmission, and the potential for rapid spread varies widely. For example, one person infected with seasonal flu is likely to transmit the virus to 1-2 other people before recovering; in contrast, one person suffering from chickenpox can transmit the virus to 12 others. For measles, this jumps to 18. Within groups temporarily housed in relief shelters following storms, this can easily lead to rapid spread of illnesses among already stressed populations.

Overtaxed medical facilities and staff

When extreme storms, flooding, heat waves and other climate change-related weather events occur, the public health workforce is called in to address the needs of affected populations injured and displaced by the disaster. Often there simply aren’t enough responders available to help, even with assistance pouring in from outside the affected area. The high demand for (and shortage of) adequate medical care – particularly when disasters occur in developing countries – means that more people who catch an illness either from environmental exposure or contact with other infected individuals might not be treated, will in turn infect other people, and could have a greater chance of dying. This isn’t only dependent upon the availability of doctors and nurses, but also ambulances, hospital beds and even the simplest of medical supplies like latex gloves.

So now what?

Given the unpredictable nature of the extreme weather events associated with climate change, the ability to both explore potential outcomes and evaluate recovery strategies is critical. Exploring these in advance of the event can have an enormous impact on the overall number of illnesses and deaths due to disease following such events, since time is of the essence when disasters are occurring in real time.

At Ramboll we employ high-resolution climate models to evaluate how extreme storms can change in the future. This gives us the ability to assess the risk and magnitude of potential future storm events, and with this information we can project disease impacts based on local conditions. Once we understand what the public health impacts might be, we can perform scenario analyses to explore prevention and response measures, and develop the materials to communicate the risks and potential solutions to a wide variety of audiences.

For one of our clients, we performed scenario analyses to evaluate utilization of vaccination and mosquito control strategies to determine which combination of interventions could have the greatest impact on preventing Zika cases. In another project, we evaluated vaccination and environmental clean-up scenarios to prevent norovirus outbreaks in high-density populations.

Ramboll’s team of health experts have performed similar analyses for a wide variety of other conditions which are affected by climate change including: measles, rubella, varicella (chickenpox), cholera, dengue, hepatitis A and B, and many more. By combined expertise in climate modeling, health, environment, and risk communication we can get a unique perspective on the health impacts of climate change, and how we can respond.

Online publications

Sustainability Strategy & Management

Taking Climate Action