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A new political and economic paradigm is emerging with the turn of the century, which is affecting the prevention, control, and eradication of animal and zoonotic diseases. We live in an era when ever more complex and intertwined
global political systems evolve, as exemplified by e.g. the European Union,
trading systems develop, as exemplified by e.g. the North American Free Trade Agreement,
food production, processing and distribution systems unfold, brought about by international trade in meat and poultry, dairy products and seafood and shellfish
information storage, retrieval, processing and dispersion systems become accessible through the internet.
These trends have lead to increased public awareness of disease risks and to the expectation that the veterinary profession should become the global steward of animal health, environmental quality, food safety, animal welfare and zoonotic disease control. All these responsibilities will require the application of the principles of preventive medicine: surveillance - increasingly based on international collaboration - and disease prevention and control.
Prevention and control strategies chosen must be in keeping with the characteristics of the virus, its transmission patterns and environmental stability, its pathogenesis and threat to animal health, productivity and profitability, zoonotic risk, etc. When available and legally permitted, the most valuable preventive measure is vaccination, not merely for the protection of the individual animal, but to build up a level of population immunity sufficient to break chains of transmission. Hygiene and sanitation measures are important methods of controlling fecal-oral infections in kennels and catteries, on farms and in commercial aquaculture facilities. Test-and-removal programs continue to be used on regional and country-wide bases to eradicate several viral diseases of livestock and poultry. The importation of exotic diseases (the term foreign animal diseases is used officially in some countries) into countries or regions is prevented by surveillance and quarantine programs. Finally, following the lead taken in human medicine to globally eradicate smallpox (accomplished in 1977) and polio (expected to be accomplished in 2002) the global eradication of rinderpest is now considered attainable.
Irrespective of all these concerted efforts, there is no chance that all pathogenic viruses will be definitely eliminated, ever. New diseases will strike time and again, and their causative viruses will be identified ever more quickly, either as being well-known agents that have undergone subtle genetic changes, or as recombinants with other viral or cellular genes. They may also turn out to be really new, hitherto undiscovered agents. Especially viruses possessing RNA genomes (which are replicated by error-prone replicases without proof-reading) will contribute to epidemics. As our knowledge increases, virus persistence in a host organism will turn out to be rather the rule than the exception; during this prolonged presence, changes in the viral genome take place, but most of them will pass unnoticed, as they have no pathogenetic corollary, or they only lead to lethal mutations. Every now and then, however, a 'killer' virus will emerge and wreak havoc - either to be controlled by veterinary efforts or peter out in virulence to become a harmless commensal.
The mutation rate in RNA viruses has been estimated at about 10?4, i.e. 1 out of 10,000 nucleotides is changed in any round of replication; since e.g. a coronaviral RNA holds about 30,000 nucleotides, any genome in a population would differ from the next in at least one site. In other words: no two particles are genomically identical ? a notion that has led to the 'quasispecies' concept for viruses. They possess the machinery to evolve more than a million times faster than cellular microorganisms, and one wonders how they can maintain their identities as pathogens over any evolutionarily significant period of time.
The answer is surprisingly simple: not individual viruses count biologically but a cloud of genetic variants centered around a consensus sequence. Like the cloud's unknown center also the consensus sequence is inscrutable; any published nucleotide sequence determined in the laboratory therefore is a random choice and may be more or less representative of the virus 'species'. New mutants appear at the periphery of the quasispecies distribution where they are produced by the erroneous copying of mutants. If these mutants have acquired new transmission, host spectrum, or virulence properties, they may start an epidemic, an emerging infection.
The term emerging diseases was used as a book title by the Food and Agricultural Organization (FAO) in 1966 to describe several viral diseases of veterinary importance, such as African swine fever, that appeared to have the potential to spread beyond their known geographical boundaries. In 1992, the Institute of Medicine, a branch of the National Academy of Sciences of the United States, in response to the recognition of the emergence of AIDS and nosocomial virus diseases such as Ebola hemorrhagic fever, but also to the re-emergence of e.g. tuberculosis, issued a report in which emerging infectious diseases were defined as diseases whose incidence has increased within the past two decades or threatens to increase in the near future. It was this 1992 report that drew the public's attention to the infectious diseases and the concept of emergence. Viral diseases, because of their rapidity of spread, feature prominently in any listing of emerging disease. Their prominence has been further highlighted by recognition of epidemics throughout the 1990s of Ebola hemorrhagic fever in central Africa, by an outbreak in 1994 of a new paramyxovirus disease in horses and humans in Australia and by identification in 1997 of a zoonotic outbreak of H5N1 avian influenza in Hong Kong. The infamous Bovine Spongiform Encephalopathy, though not caused by a virus, is another man-made emerging epidemic disease with a zoonotic potential.
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