Applied Ecology/Disease Transmission
In 1999, a study group on Veterinary Public Health (VPH), convened jointly by the World Health Organization (WHO), the Food and Agriculture Organization of the United nations (FAO), and the Office International des epizooties (OIE), and including twenty-eight experts from eighteen countries, defined veterinary public health as "the contribution to the complete physical, mental, and social well-being of humans through an understanding and application of veterinary science." The emphasis was on the human ecosystem where the contribution of veterinary science to human health has been fundamental and sustained over millennia. It is not generally appreciated that this contribution pertains not only to livestock and food production, animal traction, and transportation, which have laid the basis for most urban societies around the world. The study and management of animal diseases have also laid the basis for much of what we know about the dynamics and management of infectious human diseases, and the promotion of environmental quality.
Defined as diseases transmitted between vertebrate animals and humans, zoonotic diseases (zoonoses) include bubonic plague, Lyme disease, salmonella, and rabies. Disease-carrying animals, called reservoirs, infect humans through several pathways: when they are eaten by humans, when they bite humans, or when arthropods that have fed on them, such as mosquitoes or ticks, then feed on a human host. It has been estimated that over 60% of infectious diseases impacting humans are zoonotic in origin and zoonoses are on the rise globally, accounting for over 75% of emerging diseases.
Many important human diseases have originated in animals, and so changes in the habitats of animals that are disease vectors or reservoirs may affect human health, sometimes positively and sometimes negatively. For example, the Nipah virus is believed to have emerged after forest clearance fires in Indonesia drove carrier bats to neighbouring Malaysia, where the virus infected intensively farmed pigs, and then crossed to humans. Intensive livestock production, while providing benefits to health in terms of improved nutrition, has also created environments favourable to the emergence of diseases. Greater human contact with wild species and 'bush meat' from encroachment in forests and changes in diet also create opportunities for disease transmission. Trends ranging from forest clearance to climate-induced habitat changes also appear to have impacted certain populations of mosquitoes, ticks and midges, altering transmission patterns for diseases like malaria and lyme disease.
Until recently, zoonotic diseases have not been treated as part of ecological systems. In response to the prevalence of zoonoses, the multidisciplinary field of disease ecology has emerged. It involves the study of any ecological system that includes pathogens and incorporates the complexity of multiple interactions. The research area covers basic processes underlying the linkages between climate, ecosystems, and infectious disease, particularly the different ways that climate can influence the emergence and transmission of infectious disease agents. For example Mexican researchers adopted an ecosystem approach to better understand the complex set of factors that influenced the incidence and spread of malaria in Oaxaca. This project includes the molecular biology of the vector and the parasite, community perceptions of malaria, statistical analyses, and a geographic information-based surveillance system.
Mammals are the most common reservoirs for zoonotic diseases, with rodents leading the pack. The plague (Yersinia pestis), Lyme disease (Borrelia burgdorferi), Hantavirus pulmonary syndrome and Rocky Mountain spotted fever (Rickettsia rickettsii) all owe their spread to the presence of rodents. From an ecological perspective, rodents occupy the middle rung of the food chain. Primarily herbivores, with diets rich in plant matter, they are a food source for vertebrate predators such as foxes, and owls.
Aquatic animals that carry human parasites are also a source of disease and death. The human cost is high: The World Health Organization (WHO 2004, 2) estimates that globally "1.8 million people die each year from diarrhoeal diseases, 200 million people are infected with schistosomiasis and more than 1 billion people suffer from soil-transmitted helminthes infections."
Unfortunately, agricultural systems, especially irrigated ones, have long been associated with manifestations of extreme human ill-health arising from water-related diseases. The major reason is that public health and disease control programs have not been concerns of the water resources sector, which typically has focused on potential economic benefits of water bodies in terms of food production and power generation. This theme of applied ecology aims to increase knowledge of the relationship between water, human health and ecosystems; and to develop practical measures to reduce negative environmental health impacts by:
- mitigating adverse impacts due to malaria and other water-related parasitic diseases through water and land management strategies;
- managing the agricultural use of polluted water sources (including urban and industrial waste) so as to optimize food production and livelihoods benefits and minimize adverse health and environmental impacts;
- exploring the trade-offs necessary to provide for environmental water requirements in river basins, the wise use of wetland ecosystems, and the conservation of biodiversity through the application of eco-agricultural principles.
Another important issue is the dangerous integration of circumstances when animals and consumers from different ecosystems come into contact. The lack of resistance to new pathogens makes humans and animals replicating reservoirs, for viruses and bacteria to adapt and rapidly mutate. Further, the staggering numbers of animals and people in contact change one-in-a-million odds of a disease transfer into almost a daily possibility. Even under the most hygienic conditions, this pool of viruses, bacteria, and other pathogens creates optimal conditions for diseases to multiply rapidly and jump between species to exploit new potential hosts; something the most "successful" diseases do all too well. Under this scenario, two problems are created. First is the high risk of new diseases spreading into human populations. Second is that this can create a "fear factor" amongst people - their concern that wildlife is unhealthy might cause them to try to remove the threat by killing the wildlife. Shooting flying foxes was proposed in Southeast Asia when they were thought to be carrying nipa virus, even though the link has not been definitively proven and the disease is rarely found in flying foxes.
The global trade in wildlife provides disease transmission mechanisms that not only cause human disease outbreaks but also threaten livestock, international trade, rural livelihoods, native wildlife populations, and the health of ecosystems. Outbreaks resulting from wildlife trade have caused hundreds of billions of dollars of economic damage globally.
In almost all cases, eradication schemes are not cost efficient or effective means to reduce disease spread when compared to health education, sanitation, and controlling animal movement. Moreover, eradication schemes do not address the fundamental problem of our creating conditions, which maximize opportunities for disease build-up and cross-species transmission. Much research is still needed on the links between viruses in different species and human disease, and means of transmission between the two. Rather than attempting to eradicate pathogens or the wild species that may harbour them, a practical approach would include decreasing the contact rate among species, including humans, at the interface created by the wildlife trade. Since wildlife marketing functions as a system of networks with major hubs, these points provide control opportunities to maximize the effects of regulatory efforts.
Intensive production has also given rise to new disease problems, such as Bovine Spongiform Encephalopathy (Mad Cow Disease) and Avian flu. These indicate the potential for disease transmission through human food chains that are now often extended halfway across the globe. Industrialized animal production systems are now major features of human ecology. They have considerable impacts on the quality of the atmosphere, water and soil due to nutrient overloads; they impact terrestrial ecosystems directly and indirectly; in addition, disruption of marine fisheries occurs locally with pollution and runoff from production facilities and globally in terms of depletion of fish stocks where fishmeal has become a large commodity in the production of livestock feeds.
The response to the fact that several vector-borne, parasitic or zoonotic diseases have (re)-emerged and spread in Europe with major health, ecological, socio-economical and political consequences, has been the establishment of EDEN (Emerging Diseases in a changing European Environment). Most of these outbreaks are linked to global and local changes resulting from climate change, human-induced landscape changes or the activities of human populations. Europe must anticipate, prevent and control new emergences to avoid major societal and economical crises (cf. SARS in Asia, West Nile in the USA). EDEN offers a unique opportunity to prepare for uncertainties about the future of the European environment by exploring the impact of environmental. Other aims are to identify, evaluate and catalogue European ecosystems and environmental conditions linked to global change, which can influence the spatial and temporal distribution and dynamics of human pathogenic agents. The project will develop and co-coordinate at the European level a set of generic methods, tools and skills such as predictive emergence and spread models, early warning, surveillance and monitoring tools and scenarios, which can be used by decision makers for risk assessment, decision support for intervention and public health policies both at the EU and at the national or regional level. Part of EDEN's innovation will be to combine spatial data (earth observation data, GIS etc.) with epidemiological data.
EDEN has selected for study a range of indicator human diseases that are especially sensitive to environmental changes and will be studied within a common scientific framework (involving Landscapes, Vector and Parasite bionomics, Public Health, and Animal Reservoirs). Some of these diseases are already present in Europe (tick- and rodent-borne diseases, leishmaniasis, West Nile fever); others were present historically (malaria) and so may re-emerge, whilst others are on the fringes of Europe (Rift Valley fever) in endemic regions of West and Northern Africa.