IMPORTANCE OF EMERGING DISEASES TO THE HEALTH OF THE WORLD
- Corrie Brown, DVM, PhD
- Professor and Coordinator of International Activities
- University of Georgia
- Athens, GA 30602-7388, USA
- corbrown@vet.uga.edu
- 706-542-5842
- FAX: 706-542-5828
We are entering a new era in public and veterinary health as disease emergence continues to increase in both human and animal populations. The factors responsible for new diseases are varied but also synergistic. The interface between animal and human health is becoming increasingly blurred as new diseases cross from one species to another and all the myriad factors involved in the creation of new diseases continue to intertwine and increase. It is imperative that medical and veterinary fields interact and communicate in a fluid manner, so that we can respond effectively to each new disease as it arises, and continue to protect our global populations of all species.
An emerging disease is defined as a new disease, a new presentation of an old problem, or an existing disease that shows up in a new geographic area. The term was first used to describe several new entities in humans that surfaced in the early 1980's, the most notable example being acquired immunodeficiency syndrome. Since then, the number of emerging diseases in humans has continued to increase. However, the number of diseases that have surfaced in the human population pales in comparison with the number of emerging diseases of animals. These new animal disease problems are having pleiotropic effects - on animals populations, on the environment, and on the health of humans, both directly, through transfer of zoonotic agents, and also indirectly, through trade compromises and decreases in the availability of animal protein.
Several underlying factors inherent in modern society are responsible for the increase in emerging diseases, notably increased movement of people and animals, changing environments, crossing species boundaries, and changes in lifestyles and animal husbandry. Each of these reasons is discussed briefly below.
The first and foremost factor contributing to the occurrence of new diseases is the expansion of the human population and the attendant increase in traffic of people and animals, bringing bodily ecosystems with all their microflora and potential pathogens to new areas and animals. All three global pandemics of bubonic plague can be traced to travel and trade, transporting infected rodents and their fleas into contact with susceptible human populations. Today, human travel is at an all-time high. Foreign travel is the fastest growing sector of the global economy. One in four people in the developing world takes an international trip each year. So, it is not surprising that diseases like dengue hemorrhagic fever are expanding in range to impact novel populations. Traffic of pathogen-carrying animals is a well-recognized historical problem, with clear records of the invading armies of Genghis Khan, Attila the Hun and Napoleon spreading contagious bovine pleuropneumonia and rinderpest into conquered territories. The establishment of the OIE, in 1924, was specifically in response to a post-conflict dissemination of rinderpest throughout Europe. At the time, it was thought that with good information reporting regarding the presence of disease, neighboring countries could remain free of animal diseases provided that border security was well maintained. And, for many decades, twentieth century border controls were thought to be an excellent line of defense against occurrence of "transboundary" diseases of animals. However, current levels of global trade make the concept of intact border security less reliable. The number of animals and animal products crossing international borders is at an astronomical level and the establishment of free trade throughout much of the world ensures that the possibilities for halting the entrance of a transboundary disease will become increasingly problematic. Foot-and-mouth disease in the United Kingdom and classical swine fever in the Netherlands are both recent examples of disease emergence as a result of escalating international trade.
A second reason for new disease emergence is related to environmental changes. Lyme disease may be the most notable example in human medicine. Although Borrelia burgdorferi had probably existed in mouse populations for centuries, it was only when suburban development and burgeoning white-tailed deer populations brought the spirochete-infected tick populations into close enough contact with humans to result in the numbers of cases that merited serious investigation and subsequent recognition. Habitat destruction, causing animal populations to cluster in evolutionarily incompatible areas or where novel disease possibilities abound has created many problems in recent years. The emergence of Hendra virus affecting horses and humans, and Menangle and Nipah viruses affecting pigs and humans, are presumably related to habitat changes, causing fruit bats to exist ever closer to humans and their domestic animals. Changes to an existing environment can result in the emergence of a toxic microbe, as Pfiesteria piscicida, a dinoflagellate responsible for extensive fish mortality and human morbidity, is thought to have emerged as a result of anthropogenic loading of watersheds along the eastern seaboard of the U.S. Climatic events presaging changes in vector populations can also lead to the emergence of disease. The 1998 Rift Valley fever epizootic in east Africa was in part determined by El Nino-Southern Oscillation phenomenon creating increased precipitation and amplification of mosquito vector populations.
A third underlying factor in disease emergence is crossing species boundaries. As new species are being brought into contact with one another for a variety of reasons - tourism and migration in the case of humans and ecological disruption, show, trade, or efficiency of production in the case of animals - agents from one species may move into another, with subsequent disease and dissemination in the new host population. Perhaps the most notorious example in human medicine is the shift of human immunodeficiency virus from African primates into humans and then subsequent spread among humans. Because of the nature of animal populations and the sheer number of species, the opportunities for this transfer to take place are exponentially greater in the animal world and these opportunities continue to increase as we move species around and crowd them into ever-dwindling natural available spaces. Canine distemper in lions in the Serengeti is a prominent example as a normally canine-only virus virus migrated from domesticated dog populations into large cats to cause disease. Influenza viruses moving from wild bird populations or perhaps from mammalian reservoirs into poultry are a constant concern with respect to disease emergence. Mycobacterium bovis and its transfer from cattle to deer and then back again from captive or winterfed deer back to cattle is an all-too familiar regulatory problem in those areas where deer and cattle are likely to overlap.
A fourth underlying factor in disease emergence involves lifestyle changes in humans, with the correlate in animal populations being husbandry and technological changes. Intravenous drug usage has certainly resulted in emergence of various diseases among that population of individuals. In animals, bovine spongiform encephalopathy is a poignant and painful example of how seemingly simple changes in agricultural technology can have far-reaching impacts on animal agriculture, human health, and overall economies. Emergence of antibiotic-resistance strains of bacteria is being attributed, whether factually based or not, to the feeding of growth-promoting levels of antibiotics to animals. Aquaculture and the stocking of streams for anglers are not exempt from disease emergence. Streptococcus iniae, a newly described bacterial organism, has been associated with epizootics of meningitis in farmed fish over the last decade. Whirling disease, caused by Myxobolus cerebralis, has become a major threat to the survival of wild rainbow trout in many streams in the western United States, as transmission from one area to another has occurred through movement of infected hatchery fish.
Given the interconnectedness of today's world, trying to separate diseases into the categories of "human" and "non-human" has become increasingly artificial. In fact, as we continue to change the environment, modify habitats, move species around, and alter the way we live and raise animals, pathogens from the animal and human worlds will not only continue to proliferate and mutate but also collide in a multitude of unpredictable ways. It is important that we craft adequate mechanisms for communication and control and prepare to meet the current and future challenges of both animal and human medicine.