CHAPTER 69 Infectious Diseases and the International Movement of Horses
We are living in an era in which the world could be said to have become a “global village,” with “shrinking” national borders. No longer can countries consider themselves remote from the risk of incursion of a wide variety of infectious diseases of public health or veterinary significance.1,2 Globalization of trade and the ever-increasing volume of international movement, primarily of humans, have given rise to a major paradigm shift in the geographic distribution of many diseases. The frequency with which diseases are transferred from country to country and within countries continues to escalate. Fewer and fewer diseases can now be regarded as “geographically restricted” or “compartmentalized” to certain areas of the world.
The list of human and animal pathogens that have been accidentally introduced or reintroduced into regions or countries where they were never previously known to occur or from which they were eradicated in the past continues to grow.3 Examples of the more important human diseases and disease agents spread through international travel include tuberculosis, severe acute respiratory syndrome (SARS virus), influenza, malaria, cholera, Lassa fever, and Ebola virus. The roster of animal diseases with convincing evidence of spread through international trade is also extensive. Foot-and-mouth disease, hog cholera, exotic Newcastle disease, equine influenza, canine parvovirus, West Nile virus (WNV), and more recently, highly pathogenic avian influenza (H5N1 virus), well illustrate the point. Some animal disease pathogens (e.g., WNV) are important zoonoses and can give rise to significant morbidity and mortality in humans as well as in various animal species.
Many transboundary animal disease incursions are relatively transient in duration; the diseases are effectively controlled and eradicated within a limited time after their introduction. However, this is not always the case. Some disease pathogens are highly adaptable and very successful in establishing themselves in a new geographic environment. This is well illustrated by the behavior of West Nile virus subsequent to its first discovery in the northeastern United States in 1999.4 Over the span of relatively few years after its initial detection, WNV adapted itself remarkably well to the diversity of physical and climatic environments found not only in the continental United States, but also in Canada and numerous other countries in the Western Hemisphere. In only 5 years, WNV is considered to have become endemic in 48 of the 50 states in the United States.
ROLE OF HORSES AND THE EQUINE INDUSTRY
Since the 1960s, there has been an unprecedented upsurge in the growth of the horse industry in many countries, both for commercial and recreational purposes. The most significant factor in this development has been the favorable economic climate enjoyed by these countries over this period. National economics flourished from changes in the global market structure under various multinational trade agreements.5 Among the industries to benefit from the economic upswing has been the horse industry, with a resultant increase in the volume of international trade in equids and semen. The trend toward globalization of the horse industry received further impetus after establishment of the World Trade Organization (WTO) in January 1995.6 The primary goal of the WTO is to promote freer economic exchange between member countries through reduction or elimination of protectionist barriers to trade.7
Any consideration of the increased prominence of the equine industry in many countries must include the resurgence of interest in the horse as a leisure animal. The 2005 Economic Impact Study carried out in the United States identified recreation as the single largest equine-related activity, involving 3.9 million horses of an estimated national equid population of 9.2 million animals.8
INTERNATIONAL TRADE IN EQUIDS, SEMEN, AND EMBRYOS
An appreciation of the nature of international movement of equids is fundamental to understanding how trade in horses, apart from semen or embryos, contributes to the spread of equine diseases.9 Horses are shipped internationally for various reasons. Some are intended for permanent entry and others for temporary entry into the importing country. Most frequently, horse travel between countries is for the purpose of competing in a particular performance event. Among the more prominent and better known performance activities are racing, show jumping, dressage, eventing, driving, polo, vaulting, reining, and endurance riding.
Shipment of stallions and mares for breeding purposes is another important facet of the international movement of equids. This is especially significant in the case of the Thoroughbred industry worldwide, which bans the use of artificial insemination or embryo transfer in the breed. In the last 10 to 20 years, significant growth has occurred in the practice of “shuttling” stallions, the majority Thoroughbreds, between countries in the Northern and Southern Hemispheres.10 Financially, there is much to be gained from breeding a stallion in both northern and southern breeding seasons in the same calendar year. Horses are also shipped internationally to be sold at a commercial sale or in the case of a change in ownership.
A final category of horse movement that applies primarily to countries other than the United States is the shipment of horses for slaughter and the provision of meat, primarily for human consumption. Several hundred-thousand horses are shipped annually from eastern Europe and northern Africa to various countries in western Europe, including (but not only) Italy, Germany, France, and Belgium, for this purpose.3
As with live animal movements, the international trade in equine semen and embryos has expanded significantly in recent years.9 This is largely the result of acceptance of artificial insemination in all the major horse breeds except Thoroughbreds.11,12 Another contributing factor has been the technical advances in successfully cryopreserving equine sperm and embryos, enabling them to be shipped to countries worldwide.12
Air Transportation
Frequently taken for granted, the advent of commercial jet aircraft transportation has been the single most important factor in helping to bring about the unparalleled growth in international trade in equids, semen, and embryos that has taken place over the past 40 years.9 It revolutionized the speed and ease with which horses could be shipped between or within countries and largely replaced other means of transporting horses over considerable distances by land or by sea.
Economic Significance
A second major contributory factor to the importance of international trade in equids, semen, and embryos has been the increased economic significance of the equine industry in a growing number of countries worldwide. The horse has joined the list of animal and plant commodities that are traded globally.3
To date, relatively few countries have attempted to assess the impact of their respective industries on their national economies. Limited studies have been done in the United Kingdom, Ireland, and Mexico, all of which underscore the important cultural and economic value of the horse in these countries.13 Australia and the United States have carried out comprehensive studies of their respective equine industries.8,14–16 The American Horse Council Foundation commissioned an economic impact study of the horse industry in the United States in the mid-1990s and again 10 years later.8,15 Comparison of the findings of the two studies revealed that the U.S. equid population had increased considerably over the intervening period, from an estimated 6.9 million in the mid-1990s to 9.2 million in 2005. Both impact studies dramatically demonstrated the major contribution of the horse industry to the national economy. In the 2005 study, the combined contributions of the direct, indirect, and induced effects of the industry (in terms of Gross Domestic Product) were assessed at more than $102 billion.8 The industry provides considerable employment, estimated at 1.4 million full-time equivalent jobs annually. Revenue from the sale of equids has risen steadily over the last several years, with the annual figure currently in excess of $2 billion.
Industry Trends
Not surprisingly, changing trends in the horse industry over the past 30 to 40 years have been a major influence on the evolving nature of international trade in equids, semen, and embryos.5 As already mentioned, horses are most frequently transported between countries to compete in various performance events, racing, show jumping, and other types of equestrian sports. The number of highly prestigious and lucrative equine competitive events has proliferated around the world in the past 10 to 15 years.3 This is well illustrated by the increase in number of major equestrian events recognized by the International Equestrian Federation (FEI); these have nearly doubled over the past 8 years. Attracted by the considerable prize monies involved, owners are shipping their horses many thousands of miles, sometimes from the Northern to the Southern Hemisphere, or vice versa, to compete in international events such as The Breeders Cup, Dubai World Cup, Prix de l’Arc de Triomphe, Japan Cup, Melbourne Cup, and Rolex Three Day Event. Transporting horses over significant distances for competition purposes has inherent risks (e.g., traumatic injuries, shipping fever). Nonetheless, the number of prestigious international events will likely continue to increase, with escalation in the volume of movement of horses, despite the health risks involved.
Another important economic trend that has grown in volume, especially over the past 10 years, is the practice of dual-hemisphere breeding of stallions, where a stallion fulfills a breeding season in both hemispheres in the same calendar year.10,11 This is more popular and economically more advantageous, for both stallion and mare owner alike, than transporting mares between hemispheres for breeding purposes (e.g., shipping a mare from Southern to Northern Hemisphere to be bred Southern Hemisphere time). The vast majority of “shuttle stallions” are shipped from the Northern to the Southern Hemisphere, with most of them Thoroughbreds. They originate principally in Ireland, the United Kingdom, France, the United States, Canada, and Japan and travel primarily to Australia and to a lesser extent, New Zealand, Argentina, Chile, Brazil, Columbia, Peru, Venezuela, and South Africa.
As noted, acceptance of artificial insemination by an increasing number of breed registries has been another major development in the horse industry in recent years.11,12 Technical advances in preserving the fertilizing capability of equine sperm and the viability of equine embryos have provided the basis for the expanding trade in fresh-cooled and frozen semen being shipped internationally. There is greater demand among breeders for access to particular preeminent bloodlines among those breeds in which artificial insemination is permitted.
FACTORS AFFECTING GEOGRAPHIC DISTRIBUTION OF EQUINE INFECTIOUS DISEASE
A diversity of related and unrelated factors have been identified with the potential to influence the global distribution of equine infectious diseases.3,9 These include the international trade in equids, semen, and embryos; multinational trade agreements; emergent diseases; variants of established equine pathogens; climatic disturbances; availability of new vectors; migration of amplifying/reservoir hosts or vectors of specific pathogens; and acts of agroterrorism.
International Trade in Equids, Semen, and Embryos
It has been repeatedly shown that movement of equids and trade in semen represent the most important factor responsible for the spread of equine diseases.3,11,17 With the progressive growth in international trade in equids, semen, and embryos, especially in recent years, the risk of dissemination of a variety of equine diseases, both within and between countries, has increased commensurately. This has been amply borne out by the significant number of times that specific diseases have been introduced or reintroduced into countries or geographic regions of the world through the importation of equids or semen. Most vulnerable to the risk of disease incursions are countries such as the United States, with a significant import trade in equids, semen, and embryos.
The risk of incursion of transboundary equine diseases can be influenced in part by whether the equids being imported are approved for temporary or permanent entry into a country.3 An exception would be those diseases transmitted primarily by the respiratory route, such as equine influenza, equine rhinopneumonitis, strangles, and glanders. Spread of respiratory infections can occur regardless of whether importation is temporary or permanent. On the other hand, in the case of equine diseases characterized by the carrier state, the risk of transfer of these diseases is much greater in horses taking up permanent residence in a country. This applies especially, but not exclusively, to infections that are transmitted primarily or solely by the venereal route (e.g., CEM, EVA, dourine).
Clearly, the mode(s) of transmission of individual equine diseases has an important influence on how successfully they can be transmitted to and spread within a naive or unprotected equine population in an importing country. Any shipment of equids may include one or more animals incubating or subclinically infected with a particular agent or that are asymptomatic carriers of a specific pathogen. Examples include equine herpesviruses (EHV-1, -3, and -4), equine arteritis virus (EAV), equine infectious anemia virus, Streptococcus equi subsp. equi, Taylorella equigenitalis, Burkholderia mallei, Trypanosoma equiperdum, Babesia equi, and Babesia caballi.3,5
Respiratory transmission by an acutely infected equid is widely considered one of the most efficient means of disease transfer. Respiratory-borne diseases such as equine influenza, equine rhinopneumonitis, and strangles have been spread repeatedly between countries through international movement of horses (Table 69-1
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