Infectious Diseases in Horses
Viral infections of the equine respiratory tract are a veterinary challenge. They occur frequently and result in major economic loss to the horse industry. Currently available vaccines are not completely effective in controlling respiratory diseases, and the growth in international travel contributes to their spread.
The most common and important causes of viral respiratory disease in the horse are equine influenza and equine herpesviruses. Both were discussed extensively during the eighth International Conference on Equine Infectious Diseases, held in Dubai in March. In addition, the issues of disease control raised by a more internationally mobile horse population were addressed.
Vaccine effectiveness was among the hot topics in the equine influenza presentations. Problems include the short duration of protection that vaccination provides and variations in the potency of the different products. Some contain viral strains that are no longer epidemiologically relevant.
It was suggested that more studies to identify the best vaccine and a greater effort to make the information public ("naming names") would be beneficial. Minimum standards must be met to get a vaccine licensed in the United States. However, because the government does not release the data that is turned in by the pharmaceutical companies seeking vaccine approval, the main sources of information about vaccine effectiveness are independent research and the experiences of vaccine users.
A step forward in the control of equine influenza is the development of procedures that can detect how well a horse is protected against equine influenza. The antibody levels can be measured in serum, and scientists also can differentiate between antibody responses stimulated by infection and those resulting from vaccination.
Such technology could be used on a widespread basis to monitor populations of horses, to assure a general level of protective antibody among individuals, and to monitor the response to vaccination. Serum testing also could be used to predict outbreaks of influenza, and, perhaps, to minimize the risk of such outbreaks.
There also has been an important technological advance in the monitoring of equine herpesviruses. In the past, it has been difficult to determine whether a horse is suffering from equine herpesvirus type 1 (EHV-1) or equine herpesvirus type 4 (EHV-4). However, a new serological test developed in Australia can distinguish between the antibodies of the two strains.
"You can tell whether a horse has both viruses, one or the other of the viruses, or neither of the viruses," said Dr. Michael J. Studdert, professor of veterinary virology and director of the Centre for Equine Virology at the University of Melbourne.
In addition to respiratory disease, EHV-1 causes abortion in pregnant mares, early neonatal death, and neurologic disease. EHV-4 rarely causes disease outside the respiratory tract.
One reason these two herpesviruses are difficult to prevent is that a horse's immune response frequently fails to clear them from the body. Consequently, the majority of clinically recovered horses remain infected for life. The animals can become chronic carriers of the disease, excreting the virus under stressful circumstances such as overcrowding, shipping, and weaning.
SVANOVA Biotech of Sweden will begin marketing the new test, which uses ELISA technology, later this year. According to a brochure published by the company, the test will give horsemen information that will allow mares to be segregated into EHV-1 positive and negative groups. The test also will provide insight into the extent EHV-1 abortion could be a problem in a particular area. In Studdert's opinion, the test will be useful to veterinarians in determining when to begin the vaccination of foals against EHV-1 and EHV-4.
Investigations into the prevalence and incidence of EHV-1 and EHV-4 have been conducted with the aid of the new test. They have yielded some interesting results. In general, it has been assumed that both types of herpesviruses have a similar prevalence in the horse population. However, according to SVANOVA, studies conducted in Australia, Iceland, Sudan, and Sweden showed an almost 100% prevalence of EHV-4 antibodies, but only an 8-62% prevalence for EHV-1.
One weakness of the test is that it cannot distinguish between antibodies caused by infection and those resulting from vaccination. Studdert said that researchers are working to develop a procedure that will be more discriminating.
On The Move
Jet travel has made the horse population more mobile. The number of international events is on the rise. The horse industry is growing in many countries, and stallions are covering mares in the both the Northern and Southern Hemispheres in a single year. In addition, there are more shipments of semen and other equine biological products. All have made the control of infectious diseases more difficult and have contributed to the emergence of new diseases, according to Peter Timoney, FRCVS, PhD, the chairman and director of the Maxwell H. Gluck Equine Research Center at the University of Kentucky.
Efforts to facilitate international trade overall, he said, also have added to the challenge, "creating a certain element of conflict." Such ventures have increased, and they are exerting a growing influence over disease prevention and control policies. One key development, according to Timoney, was the establishment of the World Trade Organization, with more than 100 member countries, in 1995. It is no longer acceptable, he said, for countries to maintain a "zero risk" importation policy on the basis of safeguarding their resident horse populations without providing scientific and other justification in support of such a restrictive approach.
Traditional thinking also has been altered by less restrictive forms of disease control, such as risk assessment analysis and regionalization. With those strategies as alternatives, countries are not automatically or necessarily subjected to a complete ban on horse exportation when a disease emerges.
"All this means that there is a potential for increased risk of intercontinental movement of diseases unless we adopt the necessary safeguards," said Timoney, who served on the conference's International Committee. "We're going to have to be more vigilant; we're going to have to be more effective at being able to identify animals who could carry or who could be subacutely infected with particular organisms. We're going to have to look more carefully at pre-export testing and screening and post-import testing and screening. And we're going to have to come up with a better means of emergency disease preparedness.
"We can't afford to be complacent," he concluded. "We could have a disease introduced very quickly, and, unless we are willing and prepared to deal with it, we could find ourselves in a very difficult situation."
An important point of Timoney's talk, as well as a recurring theme during the conference, was the need for worldwide standardization of disease control, an effort that would include vaccination efficacy, laboratory procedures, quarantine requirements, and interpretation of scientific information.
The four-day International Conference on Equine Infectious Diseases was conducted in conjunction with the March 28 Dubai World Cup (UAE-I), an international Thoroughbred race that was captured by Silver Charm, last year's champion 3-year-old colt in the United States. The conference attracted more than 300 delegates, who represented more than 40 countries.
Numerous scientific papers were presented on a wide variety of equine maladies, among them African horse sickness, equine infectious anemia, equine leptospirosis, rabies, and strangles. New treatment regimens and new approaches to vaccination also were discussed and evaluated. Quite a bit of work is being done on DNA (deoxyribonucleic acid) vaccinations. But the presenters who discussed this topic indicated that while it is promising, there are several complications that must be ironed out before this type of protection becomes practical in the horse.
The program included a workshop on Borna disease, which causes neurologic problems and decreases performance in the horse. It is one of the oldest known equine viral diseases, but it affects many other animals, including cats, cattle, and sheep. There is a growing interest in the malady because the virus also has made humans ill.
Most of the equine cases discussed were found in Germany and other European countries, but one paper reported that antibodies specific to the disease had been detected in Iran, Israel, Japan, and the United States. However, the paper also noted that since such antibodies are found clinically in healthy horses, it can be assumed that natural infections in the animals remain subclinical in a majority of cases.
One conference speaker, Dr. Charles Frank, made some comments that might be disturbing to American breeders of Thoroughbreds. An adviser to Britain's Thoroughbred Breeders Association, he spoke out strongly about the possible negative effect of equine protozoal myeloencephalitis (EPM) on equine trade between the United States and Europe.
According to Frank, some European Thoroughbred breeders are concerned about EPM, a potentially fatal neurologic disease, to the extent that they are "seriously considering abandoning the future importation of horses from Kentucky and elsewhere in the United States."
Frank said after his talk that he based his warning primarily on the worries expressed by two breeders in particular "who buy yearlings and who are very influential." He declined to provide their names.
In some parts of the United States, more than 50% of horses test positive for EPM in their blood, which is considered a sign of exposure, but does not mean they have the disease. According to Frank, European horsemen would like to know how many of those animals eventually will develop clinical signs of EPM, information that is not yet available.
"That's what we want, the risk factor," he said. "My main objective was to encourage the American researchers to come up with an answer. This is a problem that warrants a lot of intensive research in order to find more solutions."
Research indicates that horses contract EPM only when they are based in the Western Hemisphere. The disease has been found in other parts of the world, but only in imported horses.
EPM is not passed from horse to horse. Instead, the animals become ill after eating food or drinking water contaminated by the droppings of opossums, which are the definitive hosts of the EPM-causing parasite, a protozoan.
The conference ended with delegates establishing research priorities for individual infectious diseases. They also took a wider look at the entire infectious disease problem, drawing conclusions and making recommendations. They called for the following:
Increased efforts in the standardization of diagnostics and the development of reference preparations.
Standardization of vaccine efficacy with respect to minimum requirements that equine vaccines need to meet.
Increased epidemiological effort with respect to both induction of disease surveillance programs as well as transmission studies.
Greater development of skills in risk assessment at the government agency level and in other institutes as they apply to infectious diseases of the horse.
The delegates also expressed the need for the formation of a "specialist subgroup" from among their colleagues to deal with issues related to specific important diseases.
The conference delegates also passed a resolution to make a request to the director general of the Office Internationale des Epizooties (OIE) to constitute an ad hoc group to assist with the prioritization of equine health problems and to propose measures for their resolution.
The OIE is the adviser to the World Trade Organization on animal disease issues that affect international trade. Its primary responsibility is to collect, process, and disseminate data relating to important animal diseases. It also provides guidelines and norms for the prevention and control of diseases, according to information published in the conference's handbook. Since 1995, the traditional role of the OIE has been expanded to enable member countries to be recognized internationally as free of a specific disease, but so far, this has been done for only a limited number of maladies.
The decision-making body of the OIE is the International Committee, which is made up of the chief veterinary officers of member countries. To assist the committee, according to the handbook, there are specialist commissions, collaborating centers, and reference laboratories as well as working groups.
The University of Melbourne's Studdert and Dr. Jenny Mumford of the Animal Health Trust in Newmarket, England, were the co-recipients of the Dubai Equine Award, which recognized outstanding contributions to the understanding and control of equine infectious diseases. The award was funded by a $40,000 donation from Sheikh Mohammed bin Rashid al Maktoum, the Crown Prince of Dubai and the Minister of Defense for the United Arab Emirates. Mumford, who served as the chairman for the conference's Scientific and Editorial Committee, is widely known in the research community for her work in equine influenza.
About the Author
Deirdre Biles is the Bloodstock Sales Editor for The Blood-Horse magazine.