W11:Perilous Pathogens: Environmental Change and Zoonotic Diseases
Global Health Overview and the Rise of Zoonotic Pathogens
The landscape of global health is increasingly shaped by the emergence and re-emergence of pathogens that cross the species barrier. According to the World Health Organization (WHO) data from 2016, human mortality is dominated by non-communicable diseases and communicable conditions.
The top ten global causes of death include Ischaemic heart disease, stroke, chronic obstructive pulmonary disease, lower respiratory infections, Alzheimer's disease and other dementias, trachea, bronchus, and lung cancers, diabetes mellitus, road injury, diarrhoeal diseases, and tuberculosis.
Within this context, infectious diseases, particularly those of zoonotic origin, represent a significant and growing percentage of total human pathogens.
Of the species of infectious agents reported in humans—including viruses, prions, bacteria, rickettsia, fungi, protozoa, and helminths—approximately (") are zoonotic.
Furthermore, among those classified as "emerging" pathogens, which total roughly species, up to (") are zoonotic, illustrating that animal-to-human transmission is the primary source of new health threats.
Since 1900, the timeline of emerging zoonotic diseases has been marked by several catastrophic events and a steady increase in outbreaks. Major highlights include the Spanish Flu of 1918 (H1N1 influenza) which caused million deaths, and the emergence of HIV/AIDS in 1981, which has resulted in over million deaths. Other significant outbreaks include the Asian Flu in 1957 (H2N2, deaths), the Hong Kong Flu in 1968 (H3N2, deaths), and the Ebola virus identified in 1976 ( deaths initially). More recently, the H1N1 influenza pandemic of 2009 resulted in deaths, the 2014 Ebola outbreak caused deaths, and COVID-19 (SARS-CoV-2), emerging in late 2019, has caused more than deaths (as of earlier reporting phases). Minor yet significant transmission events include the Hendra virus in 1994 ( deaths), the Hendra virus in 1999 (H9N2, death), and the Nipah virus in 1999 ( deaths). This rising frequency of outbreaks suggests a fundamental change in the relationship between humans, animals, and the environment.
Driving Forces and Impact of Environmental Change
The emergence of pathogens is driven by a complex interplay of anthropogenic and ecological factors.
Overpopulation and population aging create vulnerable demographic pools, while breaches in public health measures—often exacerbated by poverty, social inequality, migration, and conflicts—facilitate the spread of disease.
Globalization and urbanization are also critical drivers. Urbanization modifies and destroys natural habitats, leading to wildlife infiltration into cities and a reduction in biodiversity that allows disease vectors to dominate.
Industrial livestock production and the global wildlife trade further intensify these risks. In man-made ecosystems,
herds are often permanently housed in crowded conditions (zero-grazing), which eliminates natural avoidance distances and predators, allowing sick animals to survive longer while excreting pathogens.
This contrasts sharply with natural ecosystems where mixed species grazing and predators maintain a balance between hosts and parasites.
Human interference has shifted the biomass of vertebrate land animals dramatically; years ago, wild animals constituted " of biomass, whereas today, humans and livestock dominate, leaving wild animals at only ".
Deforestation and land-use changes represent major ecological disruptions. As forests are cleared—specifically for palm oil production or urban expansion—the multi-layered structure of the forest (floor, understory, canopy, and emergent layer) is lost.
This loss of canopy protection leads to soil erosion and the formation of stagnant ponds that serves as breeding grounds for vectors.
Deforestation forces animals like bats to migrate toward human settlements, increasing the potential for zoonotic spillover.
A prime example is the 1998 Nipah virus outbreak in Malaysia, where deforestation for palm tree plantations led Pteropus fruit bats to feed on date palm sap and contaminate fruit eaten by pigs.
Pigs then acted as intermediate and amplifying hosts, transmitting the virus to humans, where it caused encephalitis.
Global data shows a clear correlation between the increase in zoonotic disease outbreaks (reaching approximately per year by 2020) and the rise in human population , per capita meat production, and global tree cover loss.
The Role of Climate Change in Disease Dynamics
Climate change significantly alters the distribution and behavior of disease vectors and reservoirs.
The average surface air temperature has increased by over the last century due to greenhouse gases like carbon dioxide ("), with levels now higher than they have been in million years.
The impacts are categorized as "warmer, wetter, and weirder," encompassing heat waves, heavy precipitation, and extreme cyclones.
These changes affect health directly through temperature extremes—causing heat-related cardiovascular and respiratory deaths—and indirectly by facilitating vector movement.
Warmer temperatures allow mosquitoes to inhabit previously cool areas and shorten the time required for young mosquitoes to become disease-spreading adults.
Floods and droughts also play roles; floods increase stagnant water for breeding, while droughts force humans and animals to congregate at limited water sources where vectors congregate.
Mora et al. (2022) demonstrated that over half of known human pathogenic diseases can be aggravated by climate change factors such as warming, precipitation, floods, drought, and fires.
As mammals shift their ranges to adjust to rising temperatures, species that have never encountered each other before will meet—an estimated new species pairs if warming is kept under .
Bats are expected to account for " of these new encounters, potentially leading to thousands of new viral crossing events.
Temperature specifically influences vectors by affecting their survival, feeding rates, and population growth, as well as pathogens by decreasing their incubation periods ( ") and increasing replication rates.
Case Studies in Vector-Borne Diseases: Malaria and Dengue
Malaria remains a highly climate-sensitive disease, with " of the world's population at risk. The transmission of Plasmodium falciparum is constrained by temperatures between and , while its primary vector, the Anopheles mosquito, has an optimal adult development range of . Modeling in Zimbabwe illustrates the potential for range expansion; under a scenario where temperatures increase by and precipitation increases by " by 2100, the fuzzy climate suitability for malaria changes drastically, shifting the burden to the fringes of endemic regions and highland areas that were previously protected by cooler climates. While control is currently good in some fringe areas, vulnerability remains extremely high in endemic regions where healthcare infrastructure is poor.
Dengue, transmitted by Aedes aegypti, is currently present in countries, with nearly four billion people at risk. These mosquitoes thrive in warm, wet environments. Their vectorial capacity—a quantitative measure of their potential to transmit pathogens—is highly temperature-dependent. For instance, no eggs are laid at temperatures below , and viral replication within the mosquito increases as temperature rises. The El Niño Southern Oscillation (ENSO) significantly influences dengue incidence; El Niño years tend to increase surface temperatures and alter rainfall patterns, leading to spikes in dengue cases months later. Predictive models use ENSO indices, such as sea surface temperature anomalies, to forecast outbreaks. Furthermore, drought conditions during El Niño can paradoxically increase dengue risk as humans store water in containers, providing breeding sites for mosquitoes.
Environmental Alterations and Specific Case Studies
Other notable examples of environmental change and disease include the Phocine distemper virus (PDV) and Usutu virus.
PDV, which originally caused massive mortality in Atlantic seals, was confirmed in sea otters in the North Pacific in 2004, likely facilitated by the loss of Arctic sea ice which allowed previously separated marine mammal populations to interact.
In the United Kingdom, the Usutu virus—a mosquito-borne killer—has caused a " decline in blackbird populations in Greater London since 2020.
This highlights how climate-driven shifts in vector ranges can devastate local wildlife.
Deforestation specifically skews wildlife communities toward "urban exploiters" or adapters. As biodiversity decreases, rarer species lose viability while dominant, peridomestic species (like rodents and certain bats) increase in density. This is central to the "dilution effect" hypothesis, which suggests that higher biodiversity can reduce disease transmission risk by distributing vector bites among a wider variety of hosts, some of which are poor reservoirs for the pathogen.
Biological Reservoirs: The Role of Rodents and Bats
Rodents and bats are the most significant mammalian reservoirs for zoonotic diseases. Rodents comprise over species and are responsible for more than diseases, including plague, leptospirosis, and hemorrhagic fever with renal syndrome. They are effective reservoirs due to their high diversity, opportunistic nature, high reproduction potential, and peridomestic affinity (the tendency to live near human dwellings). Bats (Chiroptera) also carry a high proportion of zoonotic diseases. They possess uniquely strong immune systems that allow them to tolerate high viral loads without succumbing to infection. Their ability to fly large distances makes them highly effective vectors. Current research estimates that there are approximately mammalian viruses awaiting discovery across nine virus families, many of which reside in these small mammal reservoirs.
Specific Health Threats: Lyme Disease, Anthrax, and Foodborne Pathogens
Lyme disease, caused by Borrelia burgdorferi and transmitted by blacklegged ticks (Ixodes scapularis), is highly responsive to climate drivers. Increased temperatures and shifting seasonal patterns lead to earlier tick activity and northward range expansion. The Representative Concentration Pathways (RCPs) used in climate modeling project different levels of risk: RCP 2.6 (strong mitigation) suggests minimal expansion, while RCP 8.5 ("business as usual," warming) suggests maximum range expansion and longer active seasons. Tick abundance and the prevalence of infection in host-seeking nymphs are the primary factors determining human risk.
In the Arctic, the thawing of -year-old permafrost presents a unique threat. In 2016, a summer heatwave in Siberia caused anthrax spores to emerge from melting permafrost, killing thousands of reindeer and sickening dozens of people. This was exacerbated by the cancellation of reindeer vaccination programs in 2007. Anthrax outbreaks are intensified by climate factors that increase spore exposure (flooding/drought) and animal crowding at limited water sources. Additionally, foodborne pathogens like O157:H7, Salmonella, and Campylobacter are affected by the environment. Higher temperatures increase bacterial growth rates (optimal at "), and heavy rainfall events increase the risk of wastewater overflow and contamination of drinking-water sources. In the EU, a positive correlation exists between mean weekly temperatures and the risk of campylobacteriosis and salmonellosis.
Conclusion and Key Epidemiological Findings
Epidemiological research has established several key findings with high confidence. First, climate change is expected to alter the geographic and seasonal distributions of vectors and vector-borne diseases. Second, vectors will show earlier seasonal activity and northward expansion, increasing human exposure. Third, rising temperatures and extreme weather will influence the abundance and infection rates of mosquitoes carrying West Nile virus and other pathogens. Finally, vector-borne pathogens will emerge or re-emerge due to the interaction of climate factors with other drivers like land-use changes. Crossing the species barrier remains a matter of pathogen "competence" or significant genomic changes that increase transmission efficiency. Human factors, including proximity to water bodies, international air travel, urban living standards, and water management practices (such as rainwater harvesting), will continue to be central in determining the scale and impact of future zoonotic disease events.