Climate Change and Infectious Disease¶

Chapter 474 | Harrison's 22e · Parts 15-16 – Genetics, Genomics & Precision Medicine · Chapter 474

Key Clinical Points¶

Climate change is defined as multidecadal alterations in temperature, precipitation, wind, humidity, and other components of weather outside of the natural climate variability seen in comparable time periods.
The current climate period (Holocene) was characterized by stability (2–3°C range), but current changes are occurring at an unprecedented pace (not seen in 50 million years).
Climate change alters the epidemiology of infectious diseases by creating conditions that promote emergence or render areas currently suitable for certain pathogens unsuitable.
Rising temperatures directly impact vector biology, such as shortening the extrinsic incubation period (EIP) and larval development time for Aedes aegypti (dropping toward zero at 35°C).
Extreme weather events (e.g., heavy rainfall, flooding) increase the risk of waterborne diseases by facilitating the spread of E. coli and other enteric pathogens.
El Niño events create predictable shifts in global weather patterns that correlate with outbreaks of HPS, PL, MAL, DENG, RI, CHOL, and RVF.
Greenhouse gases (GHG) like CO2 and N2O are primary drivers of radiative forcing, leading to increased global temperatures.
Public health interventions include systemic changes such as universal health coverage, improved data in LMICs, and infrastructure improvements.
Mitigation strategies include transitioning to plant-based diets, improving urban design, and reducing fossil fuel reliance.
Environmental modeling (e.g., sea surface temperature, sea surface height, chlorophyll-a) can serve as predictive markers for cholera outbreaks.

DEFINITION & CLASSIFICATION¶

• Climate Change: multidecadal alterations in temperature, precipitation, wind, humidity, and other components of weather outside of the natural climate variability seen in comparable time periods.

• Historical Context: The Holocene (current period) was characterized by stability (2–3°C range), but current change is occurring at an unprecedented pace (not seen in 50 million years). The 5°C of warming that occurred at the end of the last ice age approximately 12,000 years ago took roughly twenty-first century to occur.

ETIOLOGY & PATHOPHYSIOLOGY¶

• Mechanism of Warming: Driven by greenhouse gases (GHG) which create a positive 'forcing' by increasing the amount of infrared radiation retained in the atmosphere. _Carbon dioxide (CO2) radiative forcing: 1.68 (1.33–2.03). _Nitrous oxide (N2O) radiative forcing: 0.17 (0.14–0.23).

• Impact on Vector Biology (e.g., Dengue): Increased temperature leads to: 1. Shorter time for larval development (dropping toward zero at 35°C). 2. Reduced extrinsic incubation period (EIP) starting at ~28°C. 3. Potential changes in the percentage of mosquitoes completing a blood meal.

• Impact on Water Quality: Heavy rainfall and flooding increase the concentration of pathogens like E. coli in water systems, especially during combined sewer overflow (CSO) events.

• Climate Drivers for Water-Related Illnesses: Algae/Cyanobacteria: Driven by temperature, ocean surface currents, ocean acidification, and hurricanes. Enteric bacteria and protozoan parasites (Salmonella enterica, Campylobacter species, toxigenic Escherichia coli, Cryptosporidium, Giardia): Driven by temperature (air and water), heavy precipitation, and flooding. Leptospira and Leptonema: Driven by flooding, temperature (increased water temperature), and heavy precipitation.

CLINICAL FEATURES¶

• Epidemiological Shifts: Climate change can promote the emergence of infectious diseases or render currently suitable areas unsuitable.

• Disease Correlation with Climate Events: El Niño events correlate with specific outbreaks: - Hantavirus Pulmonary Syndrome (HPS) - Plague (PL) - Malaria (MAL) - Dengue (DENG) - Rift Valley Fever (RVF) - Cholera (CHOL) - Other infections like influenza and COVID-19 are also influenced by environmental factors.

DIAGNOSTIC APPROACH¶

Environmental Monitoring & Modeling Utilize satellite measurements and environmental data to monitor: • Sea surface temperature • Sea surface height • Chlorophyll-a levels (indicator for phytoplankton abundance)
Predictive Analytics Correlation of environmental parameters with disease incidence (e.g., cholera) to identify high-risk periods.

MANAGEMENT & TREATMENT¶

Public Health Advocacy • Voice evidence-based understanding of climate-sensitive pathologies → Lobby for political action.
Systemic Healthcare Improvements • Achieve universal health coverage (including financial risk protection). • Ensure equitable access to quality essential health services, medicines, and vaccines. • Improve data collection, standardization, and integration in LMICs. • Anticipate/correct disaster-related failures (e.g., supply chain disruptions or power loss from extreme weather).
Environmental & Policy Interventions • Advocate for tighter particulate-matter air-quality standards. • Support institutional divestment from fossil fuels. • Promote rapid drawdown of emissions and negative emissions strategies. • Implement sustainable dietary guidelines (e.g., reduced meat consumption, shift to plant-based diets). • Improve urban design and increase access to public transportation.
Socioeconomic Support • Advocate for wealth-redistribution schemes (e.g., reparations, progressive taxation, debt cancellation, improved safety nets, underemployment insurance) → Empower disadvantaged populations to cope with climate hazards.

KEY PEARLS & HIGH-YIELD POINTS¶

• Greenhouse Gas Forcing (Table 1): Carbon dioxide (CO2) has a radiative forcing of 1.68 (1.33–2.03). Nitrous oxide (N2O) has a radiative forcing of 0.17 (0.14–0.23).

• Water-Related Illness Drivers (Table 2): Enteric pathogens (Salmonella enterica, Campylobacter species, toxigenic Escherichia coli, Cryptosporidium, Giardia) are primarily driven by temperature and heavy precipitation/flooding.

• Infrastructure Impact: Combined Sewer Overflow (CSO) events significantly increase E. coli levels compared to base flow or rain-only events.

• Mitigation Strategies: Include dietary shifts (less meat, more plant-based), urban design improvements, and transitioning away from fossil fuels.

• Risk of Bird-borne Ticks: In some regions, risk is limited to bird-borne "adventitious" ticks (e.g., Ixodes scapularis).

• Dengue Epidemic Potential: The term rVc represents the trend of annually averaged global dengue epidemic potential based on temperature and daily temperature range.

Reference Tables¶

TABLE 474-1 Greenhouse Gases: Sources, Sinks, and Forcings GAS Carbon dioxide (CO ) Methane (CH ) Nitrous oxide (N O)…¶

Harrison's 22e, p.3746

GAS	HUMAN SOURCES	SINKa	RADIATIVE FORCINGb (95% CONFIDENCE INTERVAL)
Carbon dioxide (CO) 2	Fossil fuel combustion, deforestation	Uptake by oceans (~30%), plants	1.68 (1.33–2.03)
	Fossil fuel production, ruminant animals, decomposition in landfills	Hydroxyl radicals in the troposphere
Nitrous oxide (NO) 2	Fertilizer, fossil fuel combustion, biomass burning, livestock manure	Photolysis in the stratosphere	0.17 (0.14–0.23)
	Refrigerants, electrical insulation, aluminum production	Hydroxyl radicals in the troposphere, sunlight in the stratosphere

Harrison's 22e, p.3752

PATHOGEN OR TOXIN PRODUCER	EXPOSURE PATHWAY	SELECTED HEALTH OUTCOMES AND SYMPTOMS	MAJOR CLIMATE CORRELATION OR DRIVER (STRONGEST DRIVERS LISTED FIRST)
Algae: Toxigenic marine species of Alexandrium, Pseudo-nitzschia, Dinophysis, Gambierdiscus; Karenia brevis	Shellfish, fish Recreational waters (aerosolized toxins)	Gastrointestinal and neurologic illness caused by shellfish poisoning (paralytic, amnesic, diarrhetic, neurotoxic) or fish poisoning (ciguatera). Asthma exacerbations, eye irritations caused by contact with aerosolized toxins (K. brevis).	Temperature (increased water temperature), ocean surface currents, ocean acidification, hurricanes (Gambierdiscus spp. and K. brevis)
	Drinking water Recreational waters	Liver and kidney damage, gastroenteritis (diarrhea and vomiting), neurologic disorders, and respiratory arrest.
Enteric bacteria and protozoan parasites: Salmonella enterica; Campylobacter species; toxigenic Escherichia coli; Cryptosporidium; Giardia	Drinking water Recreational waters Shellfish	Enteric pathogens generally cause gastroenteritis. Some cases may be severe and may be associated with long-term and recurring effects.	Temperature (air and water; both increase and decrease), heavy precipitation, and flooding
	Drinking water Recreational waters Shellfish	Most cases result in gastrointestinal illness. Severe outcomes may include paralysis and infection of the heart or other organs.
Leptospira and Leptonema bacteria	Recreational waters	Mild to severe flu-like illness (with or without fever) to severe cases of meningitis, kidney, and liver failure.	Flooding, temperature (increased water temperature), heavy precipitation
	Recreational waters Shellfish	Varies by species but include gastroenteritis (V. parahaemolyticus, V. cholerae), septicemia (bloodstream infection) through ingestion or wounds (V. vulnificus), skin, eye, and ear infections (V. alginolyticus).