Emerging infectious diseases that afflict people, animals or both, are being talked about with increasing frequency nowadays. The list has grown during the last 15 to 20 years and contains AIDS, mad cow disease and bird flu.
“Emerging” is a complex concept because the word includes very different situations. In the first place, it should be noted that the term “emergent” has specific geographical connotations. What is emergent in one area may be chronic in another. From a local point of view, the appearance of a new infection, unknown until that moment, may be used to include it among emerging diseases. But when the disease is already known, what emerges may be the onset of a more serious variety, the spread of the disease to new territories or the transmission to a species that did not formerly suffer that particular illness. As examples we can mention outbreaks of African horse sickness, which reached Spain through wild hosts; classic swine fever, caused by bringing infected animals into Spain from other countries; or foot-and-mouth disease outbreaks in Britain.
In such varied situations the intervening factors are also varied. The most important one is the enormous capacity of infectious agents to change, thanks to the mutation and recombination of their genetic material. This facilitates their disease-causing potential and allows them to infect other species, such as in canine parvovirus, which appeared in dogs around 1978-79 in a great worldwide epidemic. This variety of the disease seems to derivate from a feline disease that only needed a few changes to adapt and cause the disease in dogs. Other examples of cross-species transmission are distemper, which now also affects larger felines and seals; and AIDS in humans.
Other factors also assist this problem: changes in productive systems, international commerce, animal and human travels and environmental changes, such as climate change, because they influence the vectors that transmit these diseases.
In conclusion, reality has shown us that diseases are dynamic and this must force us to be prepared for unexpected situations, to not disregard hazards from beyond our borders and to work on prevention.
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