Emerging Zoonotic Agents of Concern in Agriculture

Ricky Lee Langley and Carl John Williams

Key words: zoonoses, hepatitis E, hendra, manangle, lyme disease, erhichia, transmissible spongiform encephalopathy (TSE), hantavirus

Throughout the world, we are seeing unprecedented changes in our economic, social, and ecological systems that are having adverse impacts on plants, animals, and humans. These changes are leading to the resurgence of old diseases and the emergence of new ones. The landscape and diversity of animals in many regions are changing due to overgrazing and deforestation. Increasing pollution of water bodies by nitrogen-rich waste-water, fertilizers, and soil runoff and loss of wetlands and mangroves due to development and aquaculture, diking, and drilling is promoting growth of marine and freshwater algal blooms. These algal blooms may be toxic to animals and humans. Monitoring the patterns of temperature, wind, precipitation, and biodiversity has enormous implications for surveillance of disease vectors and reservoirs (1).

Human communicable diseases can be classified according to the source of infection as:

1. Anthroponoses: source is an infected human

2. Zoonoses: source is an infected animal

3. Sapronoses: source is an abiotic substrate (nonliving environment) (2)

A characteristic of most zoonoses and sapronoses is that once transmitted to a human, the epidemic chain is usually broken. However, a limited number of zoonoses are sometimes communicable from one person to another. Zoonotic diseases can be classified as either synanthropic zoonoses with an urban or domestic cycle in which the sources of infection are domestic animals or exoan-thropic zoonoses with a sylvatic (feral and wild) cycle in nature. However, some zoonotic diseases can circulate in both urban and natural cycles (2).

There are greater than 200 known zoonotic diseases in the world and more are being found as people move into or change environments that were previously uninhabited by humans, thus exposing them to new vectors. Zoonotic agents have a major economic impact on agriculture, especially in third world countries. In many areas, vaccines are not available to prevent diseases in domestic animals, thus the infected animal suffers and humans may either starve if there are large die-offs or may contract a zoonotic illness. In many cases where a zoonotic disease is found, whole herds are slaughtered to prevent the spread of disease outside the affected area. Mass slaughter has had a major economic impact on farmers in both developed and undeveloped nations. For example, thousands of cows in Great Britain were destroyed due to "mad cow disease." In Southeast Asia, millions of chickens have been slaughtered to prevent the spread of avian influenza. In Singapore and Malaysia, thousands of pigs have been killed to prevent the spread of Nipah virus (2).

This chapter covers a few of the emerging zoonotic diseases in developed nations, especially the United States. We have chosen hepatitis E, Hendra virus, Nipah virus, Menangle virus, hantavirus, Lyme disease, ehrlichiosis, and transmissible spongioform encephalopathies as emerging zoonotic agents to present in detail.

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