Dr Patricia Ellis
Principal Veterinary Officer, Horse Industry Programs
Unit of Chief Veterinary Officer
Department of Natural Resources and Environment
475 Mickleham Rd
Attwood, Victoria 3049

Australia can still be called a lucky country as it remains free of many important horse diseases which cause serious production losses, interfere with international trade and/or cause public health problems in other parts of the world.

Historically, our relative freedom from serious diseases can be attributed to our geographic isolation as an island continent. There were no horses in Australia before 1788. In the early colonial period, the long voyage by sea was, in itself, an effective quarantine barrier. Sick animals tended to die or completely recover by landfall. In more recent times, application of sound quarantine procedures has successfully prevented the introduction of major diseases by imported horses or semen.

Few foreign countries have identical health status and therefore health conditions for the import of horses from different countries vary. The basic template for an import protocol covers the following aspects:

• Approved country or region of origin of acceptable health status supported by adequate reporting mechanisms, reliable veterinary services and efficient diagnostic laboratories.
• Minimum residency of two months in an approved country or region.
• Country, regional and premises freedom from important diseases.
• Three weeks pre-export quarantine isolation in approved premises during the residency period (permanent imports).
• Fourteen days pre-export quarantine isolation in approved premises during the residency period (temporary imports for competition).
• Vaccination against diseases such as equine influenza and Japanese encephalitis.
• Testing for diseases such as Contagious Equine Metritis, Vesicular Stomatitis, Piroplasmosis, Equine Viral Arteritis and Equine Infectious Anaemia (according to country of origin).
• Travel by an approved route. Disinfection of vehicles and disinsection of aircraft.
• Two weeks quarantine on arrival (both permanent and temporary imports) under government supervision commencing from the time of admission of the last imported horse.
• Further quarantine surveillance and testing of pregnant mares until at least 20 days after foaling to avoid the introduction of Equine Herpes Virus 1 and CEM.

Horses imported temporarily for competition purposes are kept under quarantine surveillance during their visit to Australia and are not permitted to mate or be resident on stud premises. For this reason they are not tested for diseases which are transmitted by the venereal route and can complete pre-embarkation quarantine 7 days earlier than permanent imports. In all other respects current quarantine conditions for the import of competition horses are exactly the same as for permanent imports. Competition horses imported for major races and events like the Olympic Games are registered animals, travel on passports and have a known residency, vaccination and disease testing history. They are kept under constant veterinary supervision before, during and after the event.

Diseases recognised internationally as being of significant importance to trade are defined by the Office International des Epizooties (OIE) as List A diseases. These are diseases that can spread very rapidly and have serious socioeconomic or public health consequences. OIE also defines a second group of diseases (List B diseases) that are considered to be of socioeconomic or public health importance within infected countries and are significant in international trade.

We are extremely fortunate that the Australian horse population is currently free of all OIE List A diseases and most OIE List B diseases.

Australia is free of the following List A diseases of horses:

• vesicular stomatitis (never occurred)
• African horse sickness (never occurred)

and the following List B diseases:

• contagious equine metritis (detected in 1977; declared freedom in 1985)
• dourine (never occurred)
• epizootic lymphangitis (never occurred)
• equine encephalomyelitis (Western and Eastern encephalomyelitis)(never occurred)
• equine influenza (virus type A) (never occurred)
• equine piroplasmosis (detected in 1976; did not establish due to lack of suitable tick vector)
• glanders (only cases occurred in post entry quarantine in 1891)
• horse pox (never occurred)
• Salmonella abortus equi (never occurred)
• surra (never occurred)
• Venezuelan encephalomyelitis (never occurred)

• Aujeszky’s disease
• rabies
• screw-worm fly

Over the years however, some viral diseases of horses have penetrated our defences. See Table 1.

Equine infectious anaemia (EIA) is an infectious disease of horses caused by a virus. It is spread naturally by large blood sucking insects such as March flies. For this reason it is common in low-lying, humid and swampy areas. Hence its nick-name – "Swamp Fever". Contaminated surgical instruments, hypodermic needles and blood transfusions, associated with infected horses, can also readily transmit it. The virus is also shed in semen and milk, and can pass from mare to foal during pregnancy.

The clinical signs of EIA are highly variable. It can cause intermittent bouts of fever, anaemia and ill thrift. These signs may be accompanied by jaundice, pinpoint haemorrhages under the tongue and swelling of the lower legs and abdomen. However in many cases the signs are so mild they are overlooked. Infected horses are life long carriers of the virus and are potentially a continuing source of infection for other horses. There is no treatment or vaccine for the disease.

The disease has a worldwide distribution. It was first recognised in Australia in Queensland in 1950 but undoubtedly was present before then. The disease is well established in certain areas of Queensland associated with inland river systems but occurs only occasionally in NSW, Northern Territory and Victoria. The last positive case detected in Victoria was in 1977.

Detection of two cases in thoroughbred horses in NSW this year sparked an intensive surveillance effort in the thoroughbred breeding populations of NSW and Victoria, but only one further case has been detected, in a horse which was a contact of one of the index cases. EIA is a notifiable disease under legislation within Australia.

EHV-1 (also known as equine abortion virus) is the most serious viral cause of abortion in horses and is the main cause of abortion storms, ie. multiple cases of abortion on a single farm. EHV-1 can also cause stillbirths, neonatal deaths and infrequently in Australia, respiratory disease in weaned foals and yearlings. Some strains of EHV-1 are also known to cause outbreaks of paralysis of the hind limbs, and sometimes the front limbs.

EHV-1 is endemic in most countries. However abortion storms and deaths of new-born foals only happened in Australia for the first time in NSW in 1977. The virus is thought to have been introduced at that time by an imported thoroughbred mare. Since then outbreaks have occurred sporadically in major horse breeding areas of Australia. Serious abortion storms occurred on two Victorian thoroughbred studs in 1989 and severe outbreaks were reported in NSW in 1993 and in NSW and South Australia in 1994.

Equine viral abortion is a notifiable disease in most Australian States. The Australian Equine Veterinary Association has developed guidelines for control of the disease. Victoria operates a financial incentive scheme to encourage the submission of aborted foetuses for EHV1 screening. The incentive is matched by Thoroughbred Breeders Victoria which may explain why most of the foetuses submitted for screening are thoroughbred. However the disease does not discriminate between breeds and cases in trotting horses have certainly occurred in Victoria.

Nervous disease associated with EHV-1 infection has been described in USA, Canada, continental Europe and Great Britain but is extremely rare in Australia. The only confirmed incident was a single case reported in Victoria in 1982.

Another closely related equine herpes virus, EHV-4, also occurs worldwide. It occasionally causes single abortions but not abortion storms. EHV-4 is more important in Australia as a common cause of respiratory disease in young horses in autumn and winter. Older horses can be a source of infection when inapparent shedding occurs after reactivation of latent virus.

Equine herpes virus 3 (EHV3) causes a venereal disease of horses called coital exanthema. Within a few days of service, watery blisters appear on the penis and prepuce of stallions and the vulva and perineum of mares. The blisters progress to become circular ulcers and may be associated with severe inflammation and swelling in stallions. Lesions may last 1-2 weeks in mares and 3 weeks in stallions.

Affected stallions are reluctant to serve, but otherwise fertility is not affected. After the lesions have healed they leave depigmented areas (white spots) which persist for a long time. There is some evidence that in some recovered horses the virus can persist in a latent state for long periods and light up intermittently with consequent shedding and transmission. This happens with nearly all herpes viruses.

Coital exanthema is widespread internationally. In Australia it has been reported in New South Wales, Victoria and Queensland but clinically recognised episodes are not common. A survey of 500 horses in Queensland in the early 1970’s found antibody to EHV3 virtually absent in horses up to 3 years of age; after that the frequency increased steadily with age, reaching 34% in horses 8 years of age and older. Two outbreaks in ponies have been reported in Victoria in the past two years.

Equine viral arteritis virus (EAV) is a virus that is widespread in trotting horse populations internationally - but disease in trotting horses is rare. Nevertheless the virus achieved global notoriety when an outbreak of equine viral arteritis (EVA) occurred in Thoroughbred horses in Kentucky in 1984.

The response of horses to EAV infection is extremely variable and many horses do not get sick at all. When the virus is associated with clinical disease, respiratory signs, inflamed conjunctivae (pink-eye) and swollen legs are typical clinical signs. The major reservoirs responsible for transmission of the virus are carrier stallions which spread it to mares in their semen. Spread by the respiratory route can also occur. Respiratory signs, pink-eye and carrier stallions possibly provide important clues to the history of the introduction of EAV to Australia.

Australia’s experience with EAV provides a fascinating example of the difficulties in retrospectively unravelling the history of the introduction of an "emerging" disease to a virgin continent.

The virus did not achieve prominence here until 1988 when it became evident that carrier stallions might have entered Australia via New Zealand. Subsequent blood testing demonstrated that many Australian trotters appeared to have been exposed to EAV but clinical disease was not evident. In 1989 the presence of the virus in Australia was confirmed when EAV was isolated from the semen of 12 imported trotting stallions from North America based in several Australian States. Retrospective blood testing indicated that the Australian trotting horses had been exposed to EAV by at least 1975 or earlier.

As the endemic strain appeared to be of low virulence (infection was not causing clinical disease) and the infection was already widespread in the trotter population, eradication was not attempted in Australia. Quarantine restrictions are maintained to prevent the introduction of more pathogenic strains.

The early history of introduction of viral respiratory diseases to Australia is clouded by a lack of specific diagnoses. The viral respiratory diseases that are today internationally known as EHV, EVA and equine influenza had yet to be identified as different diseases caused by different viruses. In our early years any respiratory disease outbreak was called "equine influenza". Historical records indicate that "equine influenza" also known as "equine fever", "equine catarrh" and "pink-eye" occurred intermittently throughout Australia from about 1842 to 1912 and there were several severe outbreaks during this period. Frequently 10 to 15% of affected horses died. Heavy losses occurred in horse teams engaged in railway construction. After about 1912 the disease apparently disappeared. Whether outbreaks were due to a single disease or several diseases is impossible to fathom retrospectively.

However equine influenza (EI) is a disease, which with the benefit of hindsight, I believe has never occurred in Australia. I base this opinion on knowledge of the extremely short incubation period of EI, the brief time virus is shed from the respiratory tract and the quarantine practices that have been in place since the advent of colonial settlement. This virus does not establish a carrier status and any infection in horses imported by sea would have burnt out long before landfall. A further critical safeguard, important now that most horses are imported by air, is that pre-import and post-arrival quarantine periods, longer than the period for which an infected horse sheds the virus, were early and integral parts of Australian quarantine procedures for horses and remain so to this day.

One historical account by William Tyson Kendall (one of the founding fathers of veterinary science in Australia) of an "equine influenza"outbreak in 1886-1887 relates:

Sound familiar? I bet that EVA was part of this mystery disease complex and that it was introduced to Australia far earlier than 1975. Virulent at first, like the original EAV isolate from Bucyrus, Ohio in 1953, the virus may have gradually lost its "sting" and subsequently caused only subclinical disease. While it might easy to suggest it was introduced to Australia by imported trotters – our earliest import was Vermont Boy from Boston in 1863 - horses imported for draught work might have been responsible, as transmission of a similar disease by travelling Clydesdale stallions was described in Scotland in 1888, and 1892. Unfortunately we will never know – the answer is lost in the mists of time.

In Australia, the virus apparently only circulates in the trotting horse population, possibly because the breeding population is closed. Very few trotting stallions or mares (if any) are mated to other breeds of horse in Australia today, although the practice was common last century. In other countries, disease outbreaks have occurred when the virus spills into other breeds. It is worth keeping this mind in particularly if horses of different breeds are commingled at artificial insemination centres.

This newly emergent virus has been already been discussed in detail by a previous speaker. Only a brief summary is given in this paper to emphasise that the horse is not a reservoir (carrier) host of Hendra virus (HeV) and, that disease in horses is a very unusual event in Australia.

HeV causes acute and rapidly fatal viral pneumonia in horses characterised by copious frothy nasal discharge when the horse nears death. The virus also affects humans. The maintenance hosts of HeV in nature are fruit bats. On only three occasions (twice in 1994 and once in early 1999) have other species (horses and humans) been involved. In the two cases where humans were affected, infection only occurred after close contact with infected horses. Fourteen horses were infected in the Hendra outbreak whereas the incidents at Mackay and Cairns involved two and one horse/s respectively.

Over 4000 horses from regions in which the three HeV incidents have occurred have been tested for antibody to the virus with negative results. Similarly 130 people, all with close contact with fruit bats, have tested also negative. Retrospective studies of specimens submitted to laboratories in Queensland for years before HeV was identified have found no further cases. Clearly the very few clinical cases that have occurred in Australia are not manifestations of more widespread subclinical infection.

However an on-going watch is maintained and suspicious deaths in horses are investigated to exclude HeV as the cause of death.

Japanese encephalitis (JE) is a viral disease widespread in eastern Asia which is transmitted by mosquitos. It is mainly associated with stillbirth in pigs and encephalitis (inflammation of the brain) in humans and, to a lesser extent, horses.

Inapparent infections in horses are far more common than recognisable cases of disease. If disease does occur there may be fever, loss of appetite, depression, sluggish movement, jaundice, impaired swallowing, stiff neck, muscular tremors, incoordination, staggering and falling and in severe cases (which are uncommon), aimless wandering, blindness, hyper-excitability and death. Only about 5% of clinically affected horses die but in severe outbreaks the case fatality rate can be 30-40%. There is a safe and effective vaccine for the disease, which is widely used in Asia.

JE virus multiplies and is maintained in nature in a mosquito/bird cycle. Water birds (herons and egrets) are the main reservoir and multipliers of the virus. Pigs are important amplifying hosts as, unlike other animals, they develop a strong and lengthy viraemia (virus circulation in the blood) following infection. Close association with dense pig populations increases the risk of mosquito-borne infection of humans and horses. While infection occasionally causes death of humans and horses but they do not spread the disease as insufficient virus circulates in their blood to infect mosquitos.

The first case of JE on the Australian mainland was reported in a fisherman from the Mitchell River area of western Cape York in March 1998. The virus was subsequently found to have infected pigs in the region. Establishment of the virus in mainland Australia has not yet been clearly demonstrated and it is probable that the 1998 JE incursion has died out. The consensus of expert opinion is that the 1998 incursion into the mainland was probably due to windblown mosquitos from Papua New Guinea. Seasonal contributing factors included a drought in PNG which allowed very large numbers of mosquitos to breed in the resulting stagnant water, and strong northwesterly winds capable of transporting these mosquitos to the Torres Strait and Cape York Peninsula.

Sentinel animal herds maintained by the Northern Australian Quarantine Strategy played a pivotal role in investigating and responding to the 1998 incident and are extremely important for future surveillance. No JE activity was detected in sentinels during the wet season of 1998/1999 in northern Australia.

JE activity in northern Australia is a significant threat to the Australian horse industry. Southern spread from the remote north into populated areas could have a major impact on human health, the horse industry, and governments by disrupting tourism and racing activities. Mosquitos and water birds capable of maintaining a JE cycle are widely spread throughout most of the Australian mainland, and feral pigs are fairly common.

Contingency plans to deal with an outbreak of JE are already in place with an Australian Veterinary Emergency Plan (AUSVETPLAN) disease strategy already prepared. The AUSVETPLAN recognises that if JE becomes established in northern Australia, eradication is unlikely because of the role of wild birds and wild pigs in the spread of the disease. The AUSVETPLAN recommends vaccination at industry expense as the major strategy to protect the horse industry.

I would also like to say a few words about the much-talked about Ross River virus (RRV). RRV is a virus spread by mosquitos that causes disease primarily in humans. The virus can infect other mammals, including horses. Human infection is characterised by clinical signs of rash, lethargy, muscle pain and swollen joints.

Most infections in horses do not cause disease, although it is suspected, based on analogy with human disease, that the virus can cause encephalitis, muscle stiffness and joint swelling in horses. However such a broad range of non-specific signs and conditions are ascribed to RRV by owners and veterinarians, there is a serious danger of the virus becoming "a refuge for the diagnostically destitute", particularly if the diagnosis is "confirmed" by finding RRV antibody in a single blood sample. If paired blood samples taken two weeks apart demonstrate a four-fold rise in the antibody level, the horse can be deemed to have a current infection. Unfortunately paired sera are rarely received by diagnostic laboratories.

The problem with single blood samples is that there is a high rate of background exposure to the virus, seemingly without any associated clinical disease. For instance in one survey, 30% of 112 apparently normal healthy horses from the Gippsland area of Victoria had positive RRV blood test results.

Antibody to the virus persists for a very long time after exposure so interpreting a single positive result is difficult. A single positive test only indicates that the horse has come into contact with the virus at some time previously. But did the infection occur last week or last year! Impossible to say! The take home message is that if you suspect Ross River virus, test paired sera particularly if the first test is negative.

The viral diseases brought to Australia by imported horses since 1788 are the ones where symptomless carriers are common (eg equine infectious anaemia, equine viral arteritis and the equine herpes viruses). Most introduced diseases were diagnosed retrospectively as better knowledge about the behaviour of the disease or reliable tests became available – but the stable door was wide open and the diseased horse had bolted long before diagnostic police were in pursuit so the disease became established!

In more recent times it seems that our luck may be wearing a bit thin. Even the tightest quarantine barrier can’t control the migration of free-living bats and birds, or mosquitoes blown on the wind. New and emerging diseases, such as Hendra virus and Japanese encephalitis, diseases that also can affect humans, have occurred in and to the north of the Australia mainland respectively. Fortunately the horse is not a reservoir host for either of these diseases. Intensive investigations have revealed that HeV infection of horses appears to be a very unusual event and JE has not infected horses on the Australian mainland …….. yet!

As we move toward the new millennium, stringent and on-going surveillance is in place to give early warning of future JE outbreaks. Contingency plans have been developed. We have a national emergency response plan called AUSVETPLAN - the Australian Veterinary Emergency Plan – which served us well during the HeV incident and which can be applied to other disease emergencies. It is regularly reviewed to take into account changing circumstances.

Over the past decade, sound and scientifically based quarantine protocols have allowed significant progress to be made in international movement of horses and at the same time protected the Australian horse industry from exotic diseases. Hopefully this will continue. Quarantine could well be our safest barrier. But an exotic disease could also drift in from the north with a bird or mosquito so it is best to be prepared. We cannot afford to be complacent.

Failing to plan is planning to fail!

Animal Health in Australia Home Page http://www.brs.gov.au/aphb/aha/

Australian Equine Veterinary Association (1997) AEVA guide to management of EHV1 abortion Australian Equine Veterinarian 15:37-40

Azuolas J (1998) Ross River virus disease of horses Australian Equine Veterinarian 16:56-58

Beveridge WIB (1986) Animal Health in Australia, Volume 1:Viral Diseases of Farm Livestock (Second Edition), Australian Agricultural Health and Quarantine Service, Department of Primary Industry, Canberra p113-144

Clark J (1892) Transmission of pink-eye from apparently healthy stallions to mares Journal of Comparative Pathology 5:261-264

Department of Primary Industries and Energy (1997) Japanese encephalitis. In: AUSVETPLAN. DPIE, Canberra

Dyer K - Personal communication on imports and breeding practices during the last century

Ellis PM (1998) Import of horses to Australia In: Proceedings 298. Equine Infectious Diseases, Post Graduate Foundation, Sydney p193-195

Huntington PJ, Forman AJ and Ellis PM (1990) The occurrence of equine viral arteritis virus in Australia Australian Veterinary Journal 67:432-435

Kendall WT (1906) History of stock diseases in Australia. Paper given at Fourth Convention of the Victorian Chamber of Agriculture, July 1906. Journal of Agriculture 4:525-532

Pottie A (1888) The propagation of influenza from stallions to mares Journal of Comparative Pathology 1:37-38

Seddon HR (1952) Diseases of Domestic Animals in Australia Part 4. Protozoan and Viral Diseases, Department of Health, Canberra p 159-160