WAVLD Symposium Handbook_V4.indd - csiro

World Association of Veterinary Laboratory Diagnosticians – 13 th International Symposium, Melbourne, Australia, 11-14 November 2007
1745 - 1915 Special Plenary Sessions - Equine Influenza Mon
CHANGING DYNAMICS IN THE GLOBAL DISTRIBUTION OF EQUINE DISEASES
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P.J. Timoney* Department of Veterinary Science 108 Gluck Equine Research Center Lexington, KY 40546-0099
In an era of ever increasing globalization both with respect to travel and trade, the risk of spread of infectious
diseases of humans and animals has never been greater. Countries historically free of certain diseases of
public health or veterinary importance can no longer consider themselves remote from the risk of incursion of
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those diseases. The frequency with which diseases are disseminated either within or between countries
continues to escalate. As a consequence, fewer and fewer diseases can be considered geographically
restricted as was formerly the case.
Apart from humans, equids best exemplify the ease with which infectious diseases can be spread through
international movement. Horses are unique not only because of the longevity of the species, considerable
financial value of individual animals, but perhaps most importantly because of the frequency with which they
are shipped between countries for commercial and other reasons. Various industry initiatives over the past
30 to 40 years have contributed very significantly to the evolving nature of international trade in equids and
equine germplasm (semen and embryos), with the inevitable consequence of increasing the risk of global
spread of various diseases. These include continued proliferation in the number of prestigious and lucrative
racing and competition events, dual-hemisphere breeding of stallions, acceptance of artificial insemination by
the vast majority of breed registries, and the growing popularity of embryo transfer in certain horse breeds.
The objectives of this presentation are twofold: firstly, to increase awareness of what has been responsible
for the unprecedented growth in international trade in equids and equine germplasm and the resultant
enhanced risk of dissemination of certain diseases and secondly, to identify the major factors that can
influence the geographic distribution of those diseases.
Movement of equids has been confirmed as the single most important factor responsible for the spread of
equine diseases. This has been borne out by the numerous times specific diseases have been introduced or
re-introduced into countries or regions of the world through the importation of equids either incubating,
acutely infected, or persistently infected with particular pathogens. The countries at greatest risk of such
incursions are those with a significant import trade in equids and equine germplasm. Of the various modes
of disease transmission, spread by the respiratory route is widely accepted as the most rapid and efficient
means of dissemination of a pathogen. Spread of respiratory-borne diseases such as equine influenza,
equine rhinopneumonitis and strangles between countries through international movement has been
extensively documented. Equine influenza is the most important of these and the one that has resulted in
widespread epidemics of disease in naïve or inadequately protected horses. Over the past 40 to 50 years,
equine influenza virus has been responsible for a greater number of epidemics in equine populations
worldwide than any other pathogen, some of which have had a highly significant economic impact on the
affected country or countries. In individual instances, this has totaled many millions of dollars and caused
major disruption, especially of the racing and performance sectors of the industry. Aside from short-term
financial losses, the introduction of certain diseases into a country’s resident equine population can have a
considerable and sometimes long-term effect on international trade. This is exemplified by the restrictions
many countries impose on the importation of equids and equine germplasm from countries known to be
affected with African horse sickness.
Apart from international movement of equids and trade in equine germplasm, multi-national trade
agreements, emergent diseases, mutation of recognized equine pathogens, climate-related phenomena,
migration of amplifying/reservoir hosts or vectors of specific pathogens, availability of new vectors, vaccine
contamination and agroterrorism, have all been shown to affect or have the potential to alter the global
distribution of a wide range of equine infectious diseases.
While historically, countries have responded to the threat of introduction of infectious diseases by formulating
import controls that maximize disease exclusion measures, such overly restrictive policies are no longer
tenable in today’s global economic climate. This is especially pertinent to the equine industry worldwide, the
economic viability and success of which is critically dependant on the ability to ship horses within and
between countries without excessive restrictions on movement. Import policies should be based on the
principal control standards for preventing the international spread of equine diseases specified in the
Terrestrial Animal Health Code of the OIE. In view of the risks inherent in international trade in horses and
semen, countries need to ensure the adequacy of their post-entry disease containment and riskmanagement
control measures.
Disease surveillance and reporting at a national level and the timely exchange of accurate up-to-date
information on specific disease outbreaks at an international level are critical to reducing the risks of global
spread of the more important equine infectious diseases in the future.
Timoney PJ: Factors influencing the international spread of equine diseases. Vet Clin N Am Equine Pract 16:536-551, 2000.
Mon 12 November
World Association of Veterinary Laboratory Diagnosticians – 13 th International Symposium, Melbourne, Australia, 11-14 November 2007
EQUINE INFLUENZA: LEARNING LESSONS FROM OUTBREAKS
J R Newton, Animal Health Trust, Lanwades Park, Newmarket, Suffolk, United Kingdom, CB8 7UU
Among naïve horses equine influenza is a highly contagious respiratory disease which is characterized by
pyrexia, associated depression and anorexia, harsh dry cough, nasal discharge and secondary bacterial
respiratory infection. A novel H3N8 equine influenza A virus subtype, which first emerged in Miami, Florida in
1963, initiated a worldwide pandemic of equine respiratory disease and was the stimulus for development of
multivalent, adjuvanted influenza vaccines for horses. This early work, based on experience from human
vaccines, led to development of the now broadly standardized schedules for equine influenza vaccination.
These schedules recommend that a primary course of two doses of vaccines be given approximately four to
six weeks apart, followed by a booster vaccination six months after the end of the primary course and annual
boosters thereafter. The same schedules are still adopted today for the product datasheet recommendations
for the latest vaccines and are the basis for the regulatory rules for most international equine competitions.
It was recognized during several influenza outbreaks in the United Kingdom during the 1970’s especially in
the 1979 outbreak, that vaccinated horses generally suffered less severe disease than those that were
unvaccinated. Influenza vaccination of Thoroughbred racehorses in Great Britain became mandatory under
the Jockey Club Rules of Racing at the start of the flat racing season in March 1981 and was implemented
soon after in Ireland and France. Since 1981, British racing has not been cancelled because of equine
influenza but there have been continued seasonal peaks of infection among unvaccinated non-
Thoroughbred horses associated with increased mixing at shows in the summer months. Due to the absence
of systematic, consistent and long term surveillance data it is not possible to provide absolutely conclusive
evidence of the true impact of mandatory influenza vaccination on reducing the incidence of influenza virus
infection and associated disease. However, it is the widely held belief of many that this is indeed the case
and the markedly differing financial and welfare consequences of significant influenza outbreaks in
Thoroughbreds in 2003 in Great Britain and South Africa in which mandatory vaccination was and was not
respectively adopted, serve to highlight the benefits of vaccination in horses at risk of exposure to this highly
contagious disease agent. The outbreaks in Japan and Australia in 2007 perhaps provide a similar contrast.
With changes in equine H3N8 influenza A viruses due to antigenic drift, influenza outbreaks have, however,
caused periodic disruption to the training schedules of vaccinated Thoroughbreds in individual yards or
training centres in the UK, thereby bringing mandatory vaccination under periodic scrutiny. However,
investigations of these largely clinically mild and geographically limited outbreaks have permitted closer
assessment of factors associated with disease occurrence in vaccinated populations. Outbreaks of influenza
virus infection among racehorses vaccinated according to Jockey Club Rules, were investigated in
Newmarket in 1995 and 1998 to establish reasons for vaccine failure. Investigations showed that in 1995
horses with antibody levels above a threshold equivalent to that observed in previous experimental challenge
infections using nebulised aerosol, were protected from infection. Investigations in 1998 demonstrated no
such protective threshold. Subsequent characterization showed that the 1998 H3N8 virus was antigenically
distinct from the 1995 virus and those in available vaccines. These outbreaks highlighted the need for potent
vaccines to maintain protective antibody and inclusion of epidemiologically relevant viral strains in vaccines.
A more recent outbreak in racehorses in the UK in 2003 did, however, serve to confound some of the widely
held beliefs regarding vaccine breakdown in young horses and again highlighted the benefits of detailed
epidemiological and microbiological investigation. Between March and May 2003, equine influenza virus
infection was confirmed as the cause of clinical respiratory disease among both vaccinated and nonvaccinated
horses in at least 12 locations in the UK. In the largest outbreak, at least 21 training yards in
Newmarket, comprising more than 1300 racehorses, were variously affected with horses showing signs of
coughing and nasal discharge during a 9-week period. An American sub-lineage H3N8 equine influenza
virus, previously not identified in the UK, was responsible for the outbreak. On the basis of accepted criteria
there did not appear to be significant antigenic differences between the infecting virus and Newmarket/1/93,
the American lineage virus representative in the most widely used vaccine, to explain the vaccine failure.
Two-year-old horses were apparently less susceptible to infection than three-year-olds and older animals,
despite broadly equivalent antibody levels. However, multivariable analyses comparing infected and noninfected
animals showed that the apparently counter intuitive inverse age effect was explained by elements
in the vaccine history of these animals. Among factors in the vaccine history associated with influenza
infection, there was significantly increased risk of infection associated with male gender, and a period >3
months since the last vaccination. There was a significantly reduced risk of infection associated with an
increasing pre-infection antibody level and being first vaccinated after 6 months of age. There was also
significant variation in risk according to the last type of vaccine administered. Findings highlight the benefits
of relating vaccine history with risk of infection among vaccinated horses suffering influenza infection.