Infectious Disease

Deer Tick, Lyme Disease VectorInfectious diseases like malaria, schistosomiasis, dengue fever, and zika virus are responsible for large burdens of disease globally and are highly sensitive to changes in environmental conditions, including temperature, soil moisture and precipitation patterns, deforestation, dams and irrigation projects, and others. It’s an urgent priority to better understand how land management practices alter the risk of these diseases in different settings and what types of interventions can reduce exposure to these diseases. Most emerging diseases globally are zoonotic diseases (with both human and animal hosts), and clearer understanding of anthropogenic influences on the emergence of zoonotic diseases (like HIV and Ebola) is another priority in planetary health research. Given the implications for food security and livelihoods, as well as for the state of global biodiversity, animal disease is also an important subtheme of disease ecology in the planetary health research context.

Learning Objectives

  • L1: Understand the environment-host-pathogen disease triangle and provide examples.
  • L2: Explain how environmental change can change the incidence, prevalence, geographical distribution, and/or severity of infectious diseases.
  • L3: Describe the criteria for an infectious disease hot spot and explain their characteristics with regard to environmental change.
  • L4: Recognize the interface between human and animal health in the contexts of environmental change and infectious diseases.

 

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Galvani AP, Bauch CT, Anand M, Singer BH, Levin SA. Human–environment interactions in population and ecosystem health. PNAS [Internet]. 2016;113 (51) :14502–14506. Publisher's VersionAbstract

As the global human population continues to grow, so too does our impact on the environment. The ingenuity with which our species has harnessed natural resources to fulfill our needs is dazzling. Even as we tighten our grip on the environment, however, the escalating extent of anthropogenic actions destabilizes long-standing ecological balances (12). The dangers of mining, refining, and fossil fuel consumption now extend beyond occupational or proximate risks to global climate change (3). Among a plethora of environmental problems, extreme climate events are intensifying (45). Storms, droughts, and floods cause direct destruction, but also have pervasive repercussions on food security, infectious disease transmission, and economic stability that take their toll for many years. For example, within weeks of the catastrophic wind and flood damage from the 2016 Hurricane Matthew in Haiti, there was a dramatic surge in cholera, among other devastating repercussions (67). In a world where 1% of the population possesses 50% of the wealth (8), those worst affected by extreme climatic events and the aftermath are also the least able to rebound.

Fiorella KJ, Milner EM, Salmen CR, Hickey MD, Omollo DO, Odhiambo A, Mattah B, Bukusi EA, Fernald LCH, Brashares JS. Human health alters the sustainability of fishing practices in East Africa. PNAS [Internet]. 2017;114 (16) :4171-4176. Publisher's VersionAbstract

Understanding feedbacks between human and environmental health is critical for the millions who cope with recurrent illness and rely directly on natural resources for sustenance. Although studies have examined how environmental degradation exacerbates infectious disease, the effects of human health on our use of the environment remains unexplored. Human illness is often tacitly assumed to reduce human impacts on the environment. By this logic, ill people reduce the time and effort that they put into extractive livelihoods and, thereby, their impact on natural resources. We followed 303 households living on Lake Victoria, Kenya over four time points to examine how illness influenced fishing. Using fixed effect conditional logit models to control for individual-level and time-invariant factors, we analyzed the effect of illness on fishing effort and methods. Illness among individuals who listed fishing as their primary occupation affected their participation in fishing. However, among active fishers, we found limited evidence that illness reduced fishing effort. Instead, ill fishers shifted their fishing methods. When ill, fishers were more likely to use methods that were illegal, destructive, and concentrated in inshore areas but required less travel and energy. Ill fishers were also less likely to fish using legal methods that are physically demanding, require travel to deep waters, and are considered more sustainable. By altering the physical capacity and outlook of fishers, human illness shifted their effort, their engagement with natural resources, and the sustainability of their actions. These findings show a previously unexplored pathway through which poor human health may negatively impact the environment.

Lamb JB, van de Water JAJM, Bourne DG, Altier C, Hein MY, Fiorenza EA, Abu N, Jompa J, Harvell CD. Seagrass ecosystems reduce exposure to bacterial pathogens of humans, fishes, and invertebrates. Science [Internet]. 2017;355 (6326) :731-733. Publisher's VersionAbstract

Plants are important in urban environments for removing pathogens and improving water quality. Seagrass meadows are the most widespread coastal ecosystem on the planet. Although these plants are known to be associated with natural biocide production, they have not been evaluated for their ability to remove microbiological contamination. Using amplicon sequencing of the 16S ribosomal RNA gene, we found that when seagrass meadows are present, there was a 50% reduction in the relative abundance of potential bacterial pathogens capable of causing disease in humans and marine organisms. Moreover, field surveys of more than 8000 reef-building corals located adjacent to seagrass meadows showed twofold reductions in disease levels compared to corals at paired sites without adjacent seagrass meadows. These results highlight the importance of seagrass ecosystems to the health of humans and other organisms.

Hunter DJ, Frumkin H, Jha A. Preventive Medicine for the Planet and Its Peoples. The New England Journal of Medicine [Internet]. 2017. Publisher's VersionAbstract

“Health is the human face of climate change” was the motivating idea behind the Climate and Health conference held at the Carter Center in Atlanta on Thursday, February 16, 2017. Originally scheduled by the Centers for Disease Control and Prevention (CDC), which then postponed it indefinitely, the meeting was resurrected by a coalition of nongovernmental organizations and universities and convened by former Vice President Al Gore. More than 300 attendees and a worldwide audience watching the live stream listened to more than 25 speakers addressing the health effects of climate change, the role of health professionals in adapting to these effects and communicating with the public and policymakers, and the health benefits of climate-change mitigation.

Faust CL, Dobson AP, Gottdenker N, Bloomfield LSP, McCallum HI, Gillespie TR, Diuk-Wasser M, Plowright RK. Null expectations for disease dynamics in shrinking habitat: dilution or amplification?. Philosophical Transactions of the Royal Society B [Internet]. 2017;372 (1722). Publisher's VersionAbstract
As biodiversity declines with anthropogenic land-use change, it is increasingly important to understand how changing biodiversity affects infectious disease risk. The dilution effect hypothesis, which points to decreases in biodiversity as critical to an increase in infection risk, has received considerable attention due to the allure of a win–win scenario for conservation and human well-being. Yet some empirical data suggest that the dilution effect is not a generalizable phenomenon. We explore the response of pathogen transmission dynamics to changes in biodiversity that are driven by habitat loss using an allometrically scaled multi-host model. With this model, we show that declining habitat, and thus declining biodiversity, can lead to either increasing or decreasing infectious-disease risk, measured as endemic prevalence. Whether larger habitats, and thus greater biodiversity, lead to a decrease (dilution effect) or increase (amplification effect) in infection prevalence depends upon the pathogen transmission mode and how host competence scales with body size. Dilution effects were detected for most frequency-transmitted pathogens and amplification effects were detected for density-dependent pathogens. Amplification effects were also observed over a particular range of habitat loss in frequency-dependent pathogens when we assumed that host competence was greatest in large-bodied species. By contrast, only amplification effects were observed for density-dependent pathogens; host competency only affected the magnitude of the effect. These models can be used to guide future empirical studies of biodiversity–disease relationships across gradients of habitat loss. The type of transmission, the relationship between host competence and community assembly, the identity of hosts contributing to transmission, and how transmission scales with area are essential factors to consider when elucidating the mechanisms driving disease risk in shrinking habitat.
Sokolow SH, Huttinger E, Jouanard N, Hsieh MH, Lafferty KD, Kuris AM, Riveau G, Senghor S, Thiam C, N'Diaye A, et al. Reduced transmission of human schistosomiasis after restoration of a native river prawn that preys on the snail intermediate host . PNAS [Internet]. 2015;112 (31) :9650–9655. Publisher's VersionAbstract

Eliminating human parasitic disease often requires interrupting complex transmission pathways. Even when drugs to treat people are available, disease control can be difficult if the parasite can persist in nonhuman hosts. Here, we show that restoration of a natural predator of a parasite’s intermediate hosts may enhance drug-based schistosomiasis control. Our study site was the Senegal River Basin, where villagers suffered a massive outbreak and persistent epidemic after the 1986 completion of the Diama Dam. The dam blocked the annual migration of native river prawns (Macrobrachium vollenhoveni) that are voracious predators of the snail intermediate hosts for schistosomiasis. We tested schistosomiasis control by reintroduced river prawns in a before-after-control-impact field experiment that tracked parasitism in snails and people at two matched villages after prawns were stocked at one village’s river access point. The abundance of infected snails was 80% lower at that village, presumably because prawn predation reduced the abundance and average life span of latently infected snails. As expected from a reduction in infected snails, human schistosomiasis prevalence was 18 ± 5% lower and egg burden was 50 ± 8% lower at the prawn-stocking village compared with the control village. In a mathematical model of the system, stocking prawns, coupled with infrequent mass drug treatment, eliminates schistosomiasis from high-transmission sites. We conclude that restoring river prawns could be a novel contribution to controlling, or eliminating, schistosomiasis.

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