The international movement of horses and its influence on the

Rev. sci. tech. Off. int. Epiz.,
1985, 5 (1), 155-161.
The international movement of horses
and its influence on the spread
of infectious diseases
D . G. POWELL*
Summary: During the last thirty years a significant increase in the inter­
national movement of horses has occurred. The majority of horses are trans­
ported by air and this has contributed to the spread of certain infectious disea­
ses. Measures to control their transmission is the responsibility of government
veterinary services who certify that horses, and the locality from which they
are to be exported, are free of specified equine diseases. The regulations
appear to have been successful over the last fifteen years in preventing the
spread of several of the important notifiable diseases including African horse
sickness and Venezuelan equine encephalomyelitis. However, many of the less
severe equine infections which rarely cause mortality, such as influenza and
contagious équine metritis, have become widely disseminated as a result of
international horse movements.
In order to achieve greater control, continuous epidemiological surveil­
lance of the world's equine population is necessary. This must be supported
by competent laboratory facilities to confirm the cause of local outbreaks of
disease. Once this is achieved each outbreak should be reported nationally,
and also internationally, if it is considered that the outbreak poses a threat to
the trade of horses. Better communication needs to be established within each
country between the official veterinary services and the equine industries and
also at an international level to ensure that accurate information is provided
rapidly to prevent misconceptions from arising.
K E Y W O R D S : African horse sickness virus - Disease control - Disease sur­
veillance - E q u i n e encephalomyelitis virus - E q u i n e infectious anaemia virus E q u i n e influenza virus - H a e m o p h i l u s equigenitalis - Horse diseases - Interna­
tional t r a d e - P i r o p l a s m a .
INTRODUCTION
Horses are moved from one country to another for a number of reasons includ­
ing the sale of the animal, to compete at equestrian events and for breeding p u r p o ­
ses. During the last decade there has been a significant increase in the movement of
horses as a response to the greater role the horse plays in the leisure-time activities
that are increasingly available to m a n y people. These activities not only include the
traditional equestrian sports of racing and showing but other m o r e diverse ones
such as eventing, show j u m p i n g , trail riding and the organized equine programmes
for young children and teenagers. As a consequence there has been a tremendous
growth in equestrian competitions at both a national and international level. M a n y
of these competitions, especially the international events, are under the jurisdiction
* Department of Veterinary Science, College of Agriculture, University of Kentucky, Lexington,
KY 40546-0076, U S A .
— 156 —
of the International Equestrian Federation (IEF) which is responsible for their
coordination and supervision.
Also within the last decade, the t h o r o u g h b r e d horse has assumed great econo­
mic value as a consequence of its racing and breeding potential and has acquired
the status of a commodity to be traded as such on the international m a r k e t . In
order to improve its value, this select group of animals is frequently transported
a r o u n d the world to complete racing and breeding engagements. The majority of
horses accept transportation with equanimity, unperturbed by the extensive j o u r ­
neys which they undertake and adapting favourably to the constantly changing
environment.
Within a country, horse transportation is usually by r o a d b u t , for greater dis­
tances between continents, air transport plays an important part. Formerly sea
transport over long distances imposed a useful quarantine period but the increasing
use of air transport has contributed to the spread of equine disease, the most
obvious example being equine influenza.
The advancement of laboratory methods to assist in the diagnosis of equine
infectious disease has resulted in the recognition of a number of " n e w " equine
pathogens. They include the causal agents of contagious equine metritis (CEM),
P o t o m a c fever (equine monocytic ehrlichiosis) and Getah virus infection. The dis­
tribution of these diseases is gradually unfolding as laboratories t h r o u g h o u t the
world develop the techniques necessary to confirm a diagnosis.
RECENT PROBLEMS ASSOCIATED
WITH T H E M O V E M E N T OF HORSES
During the last twenty-five years there is documented evidence of the influence
that the movement of horses has had o n the spread of a n u m b e r of infectious and
contagious diseases (Table I). Other diseases including strangles, equine rhinopneumonitis and ringworm, which are endemic in m a n y horse populations and not rou­
tinely reported, are often disseminated as a result of international movement.
African horse sickness (AHS)
Following the successful eradication of A H S in Spain at the beginning of 1967,
there have been n o further reports of the disease occurring outside the endemic area
of Africa. As a further precaution, several countries, including many of the E u r o ­
pean nations prohibit the importation of horses from countries considered to be
infected with the disease or where vaccination against A H S is undertaken.
Contagious equine metritis (CEM)
After the initial reports of C E M in England and Ireland during the early part of
1977, the disease has become recognized in several E u r o p e a n countries, Scandina­
via, the United States of America, J a p a n and Australia (5). Effective control has
been achieved by the implementation of the Codes of Practice which recommend
methods for identification and treatment of infected animals. The greatest threat
that currently exists for the continued spread of C E M is in young colts about to
commence their breeding career and which were born to dams infected with the
disease at the time they nursed the foal. Between 1978 and 1982 Timoney and Powell
(8) documented the isolation of the C E M organism from sixteen colts which had no
previous sexual experience and further cases have occurred since that time. These
157
observations emphasize the importance of a t h o r o u g h bacteriological examination
of the genital tract of all colts prior to export as recommended in the International
Zoo-Sanitary C o d e published by the Office International des Epizooties (OIE). The
causal organism of C E M previously named Haemophilus
equigenitalis has now
been officially classified as Taylorella equigenitalis (type strain NCTC11184) (1).
TABLE I
Recent disease outbreaks following
the movement
of horses
Disease
Recipient
country(ies)
Date and source
African horse
sickness (AHS)
Middle East, S.W. Asia,
India, Cyprus
1959 Extension of
epidemic in Africa
Spain
1966 Extension of
epidemic in
N o r t h Africa
U K , Ireland
1977 I m p o r t a t i o n of
carrier m a r e
1977 I m p o r t a t i o n of
carrier animal
from E u r o p e
1978 I m p o r t a t i o n of
carrier stallions
from E u r o p e
Contagious equine
metritis ( C E M )
Australia
USA
Japan
1980 I m p o r t a t i o n of
carrier animal
from E u r o p e
Equine infectious
anaemia (EIA)
UK
1974 I m p o r t a t i o n of
carrier m a r e
Influenza
USA
1963 New subtype 2
imported from
South America
Singapore and
Malaysia
1977 Subtype 1
imported from
Europe
UK
1979 Subtype 2
imported during
international
horse show
Piroplasmosis
USA
1959 I m p o r t a t i o n of
C u b a n horses
infected with
Venezuelan equine
encephalomyelitis
(VEE)
USA
1971 Extension of
epidemic from
South a n d
Central America
B. caballi
Equine infectious anaemia (EIA)
Several countries including the USA, France and J a p a n have significantly redu­
ced the incidence of E I A a m o n g their equine population utilizing the immunodiffu-
— 158 —
sion (ID) diagnostic test in association with a regulatory p r o g r a m m e prohibiting the
movement of ID positive animals. In the United States the percentage of positive
samples has decreased from 3.8 percent in 1972 to 0.6 percent in 1982 (4). Follow­
ing the F o u r t h International Conference o n Equine Infectious Diseases held in
France during 1976, agreement was reached on the preparation of an international
reference serum to check the sensitivity of the ID test. This serum is available from
specified laboratories within the United States, J a p a n and France. Recent progress
in the diagnosis of E I A includes the development of an enzyme-linked immunosor­
bent assay (ELISA) (7) which is considered to be a sensitive method for detection of
E I A specific antibody. The assay also has the advantage of being able to test a large
number of samples within a short space of time.
Influenza
N o major pandemics of influenza have occurred since the emergence of the
subtype 2 strain in North America during 1963. Since that time epidemics have
occurred on several continents, notably N o r t h America, E u r o p e and Malaysia, at
irregular intervals attributable to either subtype 1 or 2. A study of recent subtype
2 isolates (3) from a variety of sources has indicated that the majority are antigenically distinguishable from the prototype Miami strain. Differences, however, are of
a minor nature and are thought unlikely to have been a major factor in the develop­
ment of outbreaks of influenza which occurred among vaccinated and nonvaccinated horses. Although it is accepted that vaccination has reduced the severity
of disease as well as its potential to be transmitted, protection has been shown to be
short-lived and levels of humoral antibody are low following primary and booster
vaccination, especially to subtype 2 antigens.
Piroplasmosis
Equine piroplasmosis is recognized as a disease occurring in tropical and sub­
tropical areas of the world but has also been diagnosed in temperate zones. It is
widely distributed and has been reported in the Americas, Asia, India, E u r o p e , the
Soviet Union, Australia, the Middle East, and the Caribbean. The disease is trans­
mitted by the tropical horse tick Dermacentor nitens which is found predominantly
in the ears of horses and also during heavy infestations in the nasal diverticulum,
m a n e , perineal region and along the ventral mid-line.
Venezuelan equine encephalomyelitis (VEE)
VEE ranks with A H S as having the potential for causing the greatest mortality
within a susceptible population and, in addition, is a zoonosis causing h u m a n fata­
lity. It is endemic in most tropical forests and swamp lands and has caused repeated
epizootics within northern South America between 1920 and 1967. In Florida,
USA, Mexico and South and Central American countries, VEE viruses have been
associated with enzootic cycles involving continuous low level transmission between
rodents and mosquitoes (6). A serological system for distinguishing between VEE
virus isolates has shown that antigenic classification correlates with epidemiological
and ecological features of the disease. Strains classified as IA, IB and IC have been
associated with epizootics whereas the remaining varieties of subtype I and subtypes
II, III and IV are associated with sporadic equine and h u m a n infection (9). By
determining the strain, especially those within the subtype I viruses, it has been pos­
sible to assess the potential for spread of an epizootic.
— 159 —
C U R R E N T M E T H O D S OF CONTROL
Horses are presently moved from one country to another under the authority of
a licence issued by the government veterinary service of the exporting country.
Within E u r o p e and Scandinavia horses m a y be moved under a general licence
which requires identification of the horse and also certifies freedom from certain
diseases. The licence also gives a declaration of the disease status of the country of
export — i.e. the country was free of a particular disease during a previously stipu­
lated period. T o export horses between continents it is the usual practice for the
importing country to request from the exporting country a specific licence for the
individual animal which covers one importation and is time limited. Each importing
country stipulates its own requirements usually based on guidelines outlined in the
most recent edition of the International Zoo-Sanitary C o d e .
Some countries have formulated a c o m m o n importation policy, a notable exam­
ple being the Tripartite G r o u p comprising the United Kingdom, France and Ire­
land. This informal agreement which has been in existence since 1974 is between the
respective government veterinary services and covers a c o m m o n list of notifiable
diseases and a c o m m o n import policy. A recent example of this c o m m o n policy was
the temporary suspension during the summer of 1984 of equine imports from the
United States as a result of the outbreak of equine viral arteritis a m o n g thorough­
bred mares and stallions in the state of Kentucky. This suspension was lifted allow­
ing horses to be imported from the United States under strict veterinary conditions.
Information concerning the incidence of specific equine diseases in various
countries is available through the O I E and via the many informal contacts which
exist between individuals involved in equine disease investigation from m a n y parts
of the world. T o facilitate the exchange of information at a scientific level, four
international conferences on equine infectious disease took place in 1966, 1969,
1972, and 1976 under the auspices of several organizations including the Grayson
Foundation (USA), the Société d'Encouragement (France) and the O I E . The p r o ­
ceedings of the meetings have been published and have provided an indispensable
reference for those seeking information on equine infectious disease.
F U R T H E R D E V E L O P M E N T OF T H E P R E S E N T SYSTEM
It would appear that the current regulations to control the spread of certain
major notifiable equine diseases, such as A H S and VEE, have been effective in limi­
ting their activity over the last fifteen years to their enzootic areas. Their continued
control is very much dependent on intensive surveillance within the enzootic area.
Definitive identification of VEE and A H S viruses is important epidemiologically
and serological tests using specialized antibody preparations coupled with bioche­
mical techniques have proved of great value in the characterization of VEE virus
(2). A similar argument applies to the epidemiological surveillance of equine
influenza viruses on a world-wide basis. Twenty-three years have elapsed since a
major antigenic shift or recombinant occurred within the equine influenza A virus.
On the basis of the behaviour of the virus in the h u m a n and other animal species
the potential for evolution of a third influenza A equine subtype is ever present.
Early identification is therefore imperative if a serious pandemic is to be averted in
what would be a completely susceptible population. The need for surveillance was
— 160 —
endorsed at a meeting held in Newmarket, England during 1983 under the auspices
of the World Health Organization ( W H O ) a n d the Animal Health Trust where it
was emphasized that all equine influenza isolates should be submitted to one of the
two W H O Collaborating Centres in either L o n d o n , England or Memphis, U S A .
The O I E has also established a reference laboratory for equine influenza at the
Veterinary School, Munich, Germany which will assist in surveillance a n d with the
preparation of reference reagents for diagnostic purposes.
The situation with respect t o other equine disease is less precise. Strangles, rhinopneumonitis, viral arteritis, piroplasmosis, salmonellosis, a n d C E M are a serious
cause of economic loss a n d a major outbreak of any one could seriously disrupt the
movement of horses. A more effective system of reporting these a n d some of the
newly emergent equine diseases is required in order t o institute proper control mea­
sures a n d t o allay some of the misconceptions which can so readily arise. A t a meet­
ing held in Ireland during 1984, sponsored by the Grayson F o u n d a t i o n , it was
agreed t o establish a system of communication for non-notifiable equine diseases.
Twenty countries with an interest in t h e international movement of horses, as well
as the O I E a n d F E I , have been asked to participate a n d a collating centre situated
in Europe has been proposed. T h e success of the project depends on the quality of
information generated at a national level supported by competent laboratory dia­
gnosis. T o this end better liaison needs to be established between the equine indus­
tries in each country a n d the government veterinary services responsible for export
certification. By providing a rapid a n d accurate flow of information concern­
ing t h e incidence of equine infectious diseases, a greater degree of control will b e
achieved without allowing bureaucratic a n d industrial over-reaction t o unnecessa­
rily inhibit the safe movement of horses.
*
**
REFERENCES
1.
2.
A N O N . (1984). — Validation of the publication of new names a n d new combinations pre­
viously effectively published outside t h e I J S B . Int. J. Syst. Bacteriol., 34, 5 0 3 - 5 0 4 .
CALISHER
CH.,
KINNEY
R.M.,
D E SOUZA
LOPES
O.,
TRENT
D.W.,
MONATH
T.P.
&
D . B . (1982). — Identification of a new Venezuelan equine encephalitis virus from
Brazil. Am. J. Trop. Med. Hyg., 31, 1260-1272.
FRANCY
3.
HINSHAW
V.S.,
NAEVE
C.W.,
WEBSTER
R.G.,
DOUGLAS
A.,
S H E K E L J.J.
&
BRYANS
J.
(1983). — Analysis of antigenic variation in equine 2 influenza A viruses. Bull. WHO, 61,
153-158.
4.
P E A R S O N J . E . & K N O W L E S R . C . (1984). — Standardization of the equine infectious anae­
mia immuno-diffusion test and its application to the control of the disease in the United
States. JAVMA, 1 8 4 , 2 9 8 - 3 0 1 .
5.
P L A T T H . & T A Y L O R C . E . D . (1983). — Contagious equine metritis. In Medical Microbio­
logy 1. Academic Press, 149-196.
6.
S U D I A W . D . & N E W H O U S E V . F . (1975). — Epidemic Venezuelan equine encephalitis in
North America. A summary of virus-vector-host relationships. Am. J. Epidemiology,
101, 1-13.
7.
SUZUKI
T . , U E D A S. & S A M E J I M A T . (1982). — Enzyme-linked immunosorbent assay for
diagnosis of equine infectious anaemia. Vet. Microbiol., 7, 307-315.
— 161 —
8.
P . J . & POWELL D . G . ( 1 9 8 2 ) . — Isolation of t h e contagious equine metritis
organism from colts a n d fillies in the United Kingdom a n d Ireland. Vet. Rec., 111,
478-482.
Timoney
9. YOUNG N . A . & JOHNSON K . M . (1969). — Antigenic variants of Venezuelan equine ence­
phalomyelitis virus: Their geographic distribution a n d epidemiologic significance. Am. J.
Epidemiol., 89, 2 8 6 - 3 0 7 .