Re-emergent Equine Diseases
The terms “emergent” and “re-emergent” frequently are used interchangeably without apparent appreciation of the meaning of each term as a descriptor of a specific infectious disease. It is important to make a distinction between the terms insofar as they are not synonymous. “Emergent” refers to the first recorded appearance and recognition of a disease in a population for which there are no previous published reports. “Re-emergent” diseases, on the other hand, are those that have been experienced in the past but either have reappeared in a more virulent form or have occurred in a different epidemiological setting.
A wide range of factors involving infectious agent, host, or the environment can contribute to the occurrence of an emergent or re-emergent disease. These can include microbial change and adaption, host susceptibility to infection, modulation of climate, altered ecosystems, population demographics, trends in international trade, and land use. The number of emergent and re-emergent diseases of humans, animals, and plants continues to increase as more sophisticated technologies become available that enable detection of previously undiscovered infectious agents in the host and the environment.
There are various examples of re-emergent diseases, some of which are host restricted to members of the family Equidae, whereas others can infect different domestic and/or wildlife species besides the horse. The majority of the better-known examples are viral diseases. The emergence of a strain of equine influenza virus, influenza-A/equine/ Jilin/89 (H3N8), in China in 1989 gave rise to very high morbidity and associated case-fatality rates in the equine population exposed to this particular strain of virus. Very fortunately, the Jilin/89 strain of H3N8 virus did not spread outside of China.
Other examples that qualify as re-emergent were the outbreaks of Venezuelan equine encephalomyelitis caused by subtype 1E strains of the virus that occurred in Chiapas and Oaxaca states in Mexico in 1993 and 1996. Both disease events were associated with unprecedented clinical-attack rates and moderate case-fatality rates. Prior to these occurrences, subtype 1E strains of the virus were not known to cause significant disease and losses in
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