Insect Borne Viruses
The Department of Agriculture, Fisheries and Forests advises that wet spring and summers may result in an increased risk of insect borne virus or 'arbovirus' infections in horses. Heavy rains and flooding throughout the summer may led to high numbers of mosquitoes around metropolitan and regional Victoria. The most common mosquito-borne (arbovirus) in horses is Ross River Virus (RRV), which sporadically causes musculoskeletal disease in horses. Murray Valley Encephalitis is a zoonotic (can be transferred from the horse to humans) arbovirus. Although the majority of infected horses show few clinical signs, severely affected horses may develop encephalitis (inflammation of the brain) and in some cases death.

The most common clinical signs in horses include, but are not limited to:
  • a reluctance to walk
  • a stiff gait
  • ataxia (uncoordinated)
  • depression
  • tremors
Different symptoms from different viruses
How Arboviruses are involved
Laboratory testing of samples from horses showing muscle and joint soreness indicates that most infections are probably due to Ross River virus, an Alphavirus (arbovirus sub-group).

Laboratory testing of samples from horses with unusual nervous signs suggests that a majority of cases are due to infection with one of several Australian strains of mosquito-borne Flaviviruses that include viruses such as Murray Valley encephalitis virus and Kunjin virus. All testing to date has ruled out Japanese encephalitis and Hendra viruses.

It is likely that many horses may be infected with arboviruses but only a small proportion of infected horses will become ill. This means that a positive blood test must be carefully interpreted. Repeat testing will be needed to show that antibody levels are rising. 

Soreness signs are associated with Alphavirus infections
Reports of horses showing muscle and joint soreness have mainly originated from northern and central Victoria and South Australia. Signs are predominantly reluctance to walk, stiff gait and ataxia (uncoordinated). Affected horses usually recover with good husbandry and veterinary care.

Nervous signs are associated with Flavivirus infections
Reports of horses with nervous signs have originated from:
  • along the Murray River in Victoria
  • locations across South Australia from the Riverland, down the length of the Murray and areas both north and south of Adelaide from Port Pirie to the South East
  • various locations in NSW, including west of the Great Divide, from Mungindi in the north to the Murray River, and a significant cluster in the Hawkesbury Valley west of Sydney and also the Upper Hunter Valley.
Early signs of infection may include depression or mild colic. These initial signs are followed by nervous signs including lack of coordination, high stepping in front limbs, weakness in the hind quarters, muscle twitching and increased responsiveness to touch and sound. In some cases there has been facial paralysis or twitching, especially of the lips. Severely affected horses may fall repeatedly and in rare cases develop convulsions.

Most horses with clinical signs recover over several weeks with good husbandry and veterinary care, however, as of 4 April 2011, of the 112 cases in NSW, around 15 have died or have had to be euthanased for animal welfare reasons.

Animal health officers in each state are assisting with these investigations. 

Prevention in Horses
Horse owners should try to prevent their animals from being bitten by insects through measures including rugging, fly masks and using registered insect repellents. Ensuring that horses are in kept in good condition will help build a strong immune system and minimise the risk of infection with arboviruses. 

Human Health
Human health departments in most states and territories are advising the public to avoid being bitten by mosquitoes to minimise the risk of infection with these viruses. Check your state or territory's health department website for more information.

Owners who notice horses displaying unusual signs should contact their private veterinarian immediately.
Information for Vets
Sample collection and laboratory submissions
Veterinarians who would like further advice on submissions of samples should contact officers in their Primary Industries or Biosecurity department:
  • Biosecurity Queensland 13 25 23
  • Biosecurity SA 8226 0222
  • NSW State Diagnostic Laboratory 02 4640 6327
  • Victorian Department of Primary Industries 136 186
Samples are being tested at state animal health laboratories and the Australian Animal Health Laboratory at Geelong.
It is important that veterinarians and any assistants take stringent precautions when performing necropsies on horses showing neurological clinical signs. Great care should be exercised when handling brain and spinal cord tissue and appropriate personal protective equipment should be utilised as part of a risk management approach to personal safety.

This fact sheet was compiled by the Department of Agriculture, Fisheries and Forestry in conjunction with state and territory departments of primary industries and the Australian Animal Health Laboratory.
Atrial Fibrillation
Atrial fibrillation is not an uncommon cause of poor performance in racehorses. Typically, affected horses stop suddenly in the middle or second half of a race. They often recover and appear quite normal within a short period of time.

There are several possible causes. The majority of cases are idiopathic (no obvious or detectable underlying condition). Electrolyte imbalances may cause and/or contribute to the onset of atrial fibrillation. Uncommonly in racing horses underlying heart disease may exist, such as significant valvular insufficiencies (leaking valve) or myocarditis (inflamed heart muscle).

In the majority of cases the heart will return to a normal rhythm without veterinary intervention within 48 hours or less. Once the heart is back in a normal rhythm there is no ongoing effect on performance.

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Tying Up (Exertional Rhabdomyoysis)
Broadly, Tying Up is seen in horses for two main reasons: the horse has an underlying myopathy (muscle disease) or the horse has been overexerted.

Horses with underlying myopathies generally experience repeated episodes after short bouts of exercise. Recurrent Equine Rhadomyolysis (RER) is an underlying myopathy that occur in thoroughbred horses. In fact RER is reported to affect approximately 5% of racing thoroughbreds.
Exercise Induced Pulmonary Haemorrhage (EIPH)
Strenuous exercise is associated with haemorrhage from the pulmonary vessels into the airways.

EIPH has been detected in most breeds of horses, including thoroughbreds, standardbreds, quarter horses, and polo ponies.

EIPH may be evident after exercise as epistaxis (blood from the nostrils), and may be a cause of poor performance.

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Joint Infection/Septic Arthritis 
Joint infections are a common cause of lameness in horses. Lameness is usually severe and if left untreated may be permanent. Extreme cases may require euthanasia on humane grounds.

The most common causes of joint infections are puncture wounds or lacerations of the limb over or near a joint or tendon sheath. Wounds may enter into the joint directly or cause significant tissue damage around the joint that ultimately results in joint infection. Joint injections may also cause joint infection, however, are less common.

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Gastric Ulcers
Ulceration of the stomach is an important disease of athletic horses. The history and clinical signs of adult horses with gastric ulceration are often vague and not specific. Endoscopy of the stomach is the most accurate method for establishing a diagnosis.  Many factors contribute to the development of ulceration, including diet, exercise, housing, stress.  

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Acute Diarrhoea
Acute (sudden onset) severe diarrhoea (acute colitis) in adult horses is a potentially life-threatening disorder. There are a variety of known causes. Typically there is substantial inflammation of the large intestine (specifically large colon and caecum) and loss of large volumes of fluid (up to 100 litres) and protein. In severe disease, death may occur even before the onset of diarrhoea.

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Hendra is an emerging disease that was first identified in 1994 and has only been found in Australia. It occurs naturally in flying fox populations, and it’s thought to be transferred to horses through contaminated urine, faeces or foetal fluids. Humans become infected through close contact with infected horses. There’s been no evidence of direct infection from flying foxes to humans or of human-to-human infection. Horses can infect other horses as well as humans.

It has become clear that Hendra virus is now an endemic disease in Australia. All known cases in horses (more than 30) have occurred in Queensland or northern NSW (one case to date) from the Great Dividing Range east. However, it’s thought that there is a potential for disease wherever there are flying foxes given that antibodies to the virus have been detected in some proportion of all Australian flying fox populations.
Muscle Tear Management
Muscle strains account for more than 50% of musculoskeletal injuries in human athletes. However, the frequency with which they are definitively diagnosed (with proof they are the cause of lameness) in horses, is far lower. This is likely due to a number of reasons including difficulty in accurately palpating the deep muscle of horses and low levels of lameness caused by mild injuries.

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