Environmental Changes and Infectious Disease William R. Barnett PUBH 605

Environmental Changes and Infectious Disease

William R. Barnett

PUBH 605

Introduction Changes in the ecosystem are contributing to

the prevalence of infectious disease in some regions of the world.

These changes are brought on by either natural occurrence or through human interference.

Developing nations in Sub-Sahara Africa, Latin America, South America, and Asia are primarily affected by these environmental changes.

Overview of environmental changes

Climate:• Temperature

• Rise in average temperatures

• Rainfall• Increase in precipitation

• Intense weather patterns:• Heat waves

• Droughts

• Floods

Overview of environmental changes con’t

Land• Deforestation

• Loss of trees in tropical rainforests

• Soil degradation:• Soil erosion

• Overgrazing and cultivation

• Construction• Human settlements

• Roads

Overview of environmental changes con’t

Water• Natural water source diversion:

• Irrigation

• Reservoirs and dams

• Urban water management• Waste water systems

• Storm water systems

• Water chemistry:• Desalination and pH

Factors influencing environmental changes

Global warming:• Emission of “greenhouse gases” by

automobiles, industrial factories, and other sources.

Agricultural development:• Clearing of forests for farming and ranching.

• Efforts to irrigate water-poor lands.

Factors influencing environmental changes con’t

Human intrusion into environment:• Construction of reservoirs, dams, and roads

to accommodate development.

• Logging of tropical forests.

• Expansion of human population.

Specific infectious diseases

Vector-borne:• Mosquito

• Malaria

• Yellow fever

• Dengue fever

• Filariasis

• Other vectors• Leishmaniasis (sand flies)

• Trypanosomiasis (Tsetse fly)

Infectious disease con’t

• Onchocerciasis (Blackfly)

• Loiasis (Chrysops fly)

Parasitic (non vector-borne):• Schistosomiasis

• Cryptosporidium

• Giardia

Filariasis

Filariasis con’t

Sand fly

Leishmaniasis con’t

Tsetse Fly

Schistosomiasis

Onchocerciasis

Chrysops fly

Loiasis

Cryptosporidiosis

Giardia

Connecting environmental changes to infectious disease Environmental changes can adversely

affect human health by providing:• Breeding areas and habitat for vectors and

animal intermediates.• Adaptation of vectors to new habitats.• Additional animal species to the region,

which provide reservoirs for microorganisms.

• Increased interaction between pathogen and host.

Impact of environmental changes on infectious disease

Temperature and rainfall:• Small increases in existing low temperatures have

been shown to exert increased transmission of malaria.

• Malaria incidence is on the rise in higher altitude areas, such as highlands, due to temperature changes.

• Heavy rainfall has lead to contamination of water supplies by Cryptosopridium, which is resistant to chlorine treatment during these periods.

Source: Patz, et. al. (2000)

Deforestation

Impact of environmental changes on infectious disease con’t

Deforestation:• One of the major human activities associated with

the resurgence of malaria, yellow fever and other mosquito-borne viruses.

• Provides the opportunity for the “sylvatic” cycle of yellow fever in monkeys and mosquitoes to be transferred to humans in the “urban” transmission cycle through deforestation activities.

Source: Norris (2004)


Removal of the forest canopy has created pools for the expansion of mosquito breeding habitats.

New mosquito species, such as An. Gambiae, have become associated with malaria transmission around deforested areas.

The removal of intact forest has resulted in the emergence of newly recognized pathogens.

Source: Norris


Replacement of deforested areas with cocoa, coffee, oil palms, or mango trees provide a habitat for tsetse flies.

Although deforestation has reduced the prevalence of the Chrysops fly, the vector has adapted itself to rubber tree plantations.

Farmland grown over deforested regions has increased fox populations, which are excellent reservoirs for leishmaniasis.

Source: Patz, et. al.

Agriculture


Agricultural development:• Sedimentation and runoff in water sources

can block stream flow, decrease water depth and provide shallow water with no flow, which is an ideal mosquito habitat.

• Rice paddies over immense areas are habitats for development of immature mosquitoes.

Source: Norris


The prevalence of schistosomiasis parallels the degree of irrigation intensity.

Rice cultivation is associated with increased prevalence of filariasis

Raw sewage used to irrigate farmland has been contaminated with giardia.

Domestic animals, such as pigs and cows create shallow habitats for mosquitoes.

Source: Patz, et. al.


Increase in maize production has resulted in a supply of pollen to mosquito larvae.

Pesticides aimed at removing parasites have contributed to the growth of less dominant species of tsetse flies that carry trypanosomiasis.

Source: Sutherst (2001)

Aswan Dam (Egypt)


Construction:• Dams, reservoirs, and canals have

contributed to an increase in cases of schistosomiasis.

• Road construction leads to soil erosion and creation of ponds when the water rises creating mosquito breeding areas.

• Human activity in previously inaccessible areas provides more interaction between vectors and hosts.


Urbanization:• Storm water handling systems provide breeding

sites for mosquitoes when surface waters are scarce.

• Dengue virus exposure is greater for people living close to wells or service manholes.

• Discard materials, such as tires, cans, and barrels can provide a breeding habitat for mosquitoes.

Source: Norris (2004).


Other impacts:• Reduction of water salinity and conversion from acidic

to alkaline conditions increase the growth of freshwater snails, which contribute to the cycle of schistsomiasis.

• Movement by humans, who do not have protective immunity and are unaware of preventive measure against vectors, in remote areas.

• Animal and vector adaptability to changing environments and competition among species.

Source: Part, et. al.

Conclusion

Without a doubt, the changes in the world’s ecosystems contribute to the cycle of infectious disease.

In particular, human activities, such as deforestation, increased development agricultural lands, and movement of humans into previously inaccessible areas.

Changes in climate, such as greater rainfall, are another driver for increased cases of infectious disease.

Conclusion con’t

These activities and natural occurrences provide habitats for vectors and reservoirs, but also allow humans and animals more contact with infectious disease.

Still, most of these issues are driven by population growth, economic conditions, and the availability of food throughout the world.

References Norris, D. E. (2004). Mosquito-borne Diseases as a

Consequence of Land Use Change. EcoHealth, 1, 19-24.

Patz, J.A., et. al. (2000). Effects on Environmental Change on Emerging Parasitic Disease.

International Journal for Parasitiology, 30, 1395-1405.

Sutherst, R. W. (2001). The Vulnerability of Animal and Human Health to Parasites Under Global Change. International Journal for

Parasitology, 31, 933-948.

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Environmental Changes and Infectious Disease William R. Barnett PUBH 605