Changing climate can change disease outbreak trends

Extreme weather can reshape the ecology of diseases, according to a latest Mongabay-India (MI) analysis. Heatwaves, droughts, erratic rainfall and flash floods, in combination or alone, can affect how long pathogens survive, change the distribution of disease-carrying insects and bring people, animals and pathogens into closer contact, increasing the risk of outbreaks in new places at unexpected times, says the report.

The MI report points out that experts say that India is working on climate-health surveillance and early-warning systems, but gaps in data and infrastructure could limit its ability to predict and prevent outbreaks. Some bacterial diarrhoeal diseases often peak during warmer, wetter conditions. Stating a 21-year study from Kolkata, covering data from 1999 to 2019, the MI report highlights that the study found a biannual pattern in cholera cases, with peaks coinciding with rising summer temperatures and the onset of the monsoon.

Hepatitis A and E often see a rise in cases during the monsoon and post-monsoon months. In Delhi and Mumbai, major hepatitis E outbreaks have been linked to sewage entering water supplies after heavy monsoon rains. Recent studies have also found that climate change is making these seasonal patterns unpredictable; it can change when and where infections appear, the MI report states. Warmer conditions can allow mosquitoes to survive and breed for longer, potentially expanding the geographic and seasonal window for diseases such as dengue, malaria and Zika.

The MI report draws attention to a 2023 study found that as water sources shrink, people, livestock and wildlife may increasingly congregate around the same limited sources, increasing the chances of pathogen transmission. Other studies have linked overcrowding with the increase of pathogen transmission. Communities may also be forced to rely on unsafe or alternative sources of water, including those shared with domestic animals, increasing opportunities for contact and zoonotic disease transmission. Less water for washing, food preparation and sanitation creates further opportunities for infection. Water collected from tankers or communal taps stored in large containers for long periods creates new sites for disease vectors such as mosquitoes to breed,

The MI report says the challenge is to move from treating climate-related diseases as a series of unexpected outbreaks to recognising it as a predictable and increasingly preventable consequence of a changing climate. India has already started building a framework to address this. In 2018, the Indian government launched the National Action Plan for Climate Change and Human Health (NAPCCHH), which aims to strengthen preparedness through disease surveillance, early-warning systems, capacity building and climate-resilient healthcare infrastructure. The National Centre for Disease Control (NCDC) has also developed district-level climate and health risk assessments, which require states to use daily surveillance data, local civil-registration data on temperature-related mortality and weather information to establish local heat-health thresholds.

A World Health Organization report highlights that climate change is directly contributing to humanitarian emergencies. Research shows that 3.6 billion people already live in areas highly susceptible to climate change. Between 2030 and 2050, climate change is expected to cause approximately 250 000 additional deaths per year, from undernutrition, malaria, diarrhoea and heat stress alone. The direct damage costs to health (excluding costs in health-determining sectors such as agriculture and water and sanitation) is estimated to be between $2–4 billion per year by 2030. Areas with weak health infrastructure — mostly in developing countries — will be the least able to cope without assistance to prepare and respond. Reducing emissions of greenhouse gases through better transport, food and energy use choices can result in very large gains for health, particularly through reduced air pollution.

The WHO report warns that climate change presents a fundamental threat to human health. It affects the physical environment as well as all aspects of both natural and human systems — including social and economic conditions and the functioning of health systems. It is therefore a threat multiplier, undermining and potentially reversing decades of health progress. As climatic conditions change, more frequent and intensifying weather and climate events are observed, including storms, extreme heat, floods, droughts and wildfires. These weather and climate hazards affect health both directly and indirectly, increasing the risk of deaths, noncommunicable diseases, the emergence and spread of infectious diseases, and health emergencies.