Description

Hendra virus was discovered following an outbreak of illness in a large racing stable in the suburb of Hendra, Brisbane, Australia in 1994. It is an emerging disease that so far has been found only in Australia. The virus occurs naturally in flying fox (bat) populations, and it is thought to be transferred to horses through contaminated urine, feces, or fetal fluids. 

Hendra virus is a zoonotic disease that can be transmitted from horses to humans. Seven confirmed Hendra virus infections in humans, all in Queensland, have occurred and four of the infected people died, the most recent in 2009. The seven confirmed human cases all became infected following close contact with respiratory secretions and/or blood from a horse infected with Hendra virus.

Many people have reported similar contact with infected horses but have remained well and their blood tests have shown no evidence of infection.

Spill-over of Hendra virus from fruit bats to horses is actually quite rare, but so far, more than 50 horses have been infected with the virus. These animals have either died as a direct result of their infection or been euthanized.

Horses are the only species of domestic animal that can be naturally infected with Hendra virus. A single dog showed evidence of exposure to the virus following contact with an infected horse in July 2011. However, testing of many other animals and insects has shown no evidence of the Hendra virus.

Infections in horses range from asymptomatic infection to fatal respiratory and neurological syndrome.  Symptoms are generally similar to those seen with other respiratory and neurological illnesses. Hendra should be suspected if there is also the proximity of bats, or the presence of human cases of acute respiratory distress syndrome or encephalitis. The incubation period or interval from infection to onset of symptoms in horses varies between five and 16 days. The course of fatal cases is short, with death occurring within 1-3 days.

While the exact route of infection is unknown, it is thought that horses may contract Hendra virus infection from eating food recently contaminated by flying fox urine, saliva, or birth products. Spread of infection to other horses can then follow.

Spreading of the virus happens more often when the sick horse is kept with other horses in a stable, but is possible wherever horses have close contact with body fluids from an infected horse. Small amounts of the virus may be present in a horse's body fluids, particularly nasal secretions, for a few days before they become sick.

Symptoms

• Fever
• Increased heart rate
• Respiratory distress
• Neurological signs, such as uncoordinated gait and muscle twitching
• Frothy, clear or blood-tinged nasal discharge
• Jaundiced mucous membranes
• Depression
• Anorexia
• Pneumonia

Causes

Epidemiologic, serologic, and virologic evidence implicates fruit bats of the Pteropodidae family as the source of Hendra virus. The bats are also known as flying foxes. Field and experimental proof exists of transmission from bats to horses, with isolates recovered from the uterine fluid and fetal tissues of a grey-headed flying fox, Pteropus poliocephalus and a black flying fox, P alecto.

The infrequent occurrence and sporadic nature of outbreaks suggest that exposure of horses to Hendra virus is a chance event. While horses may have been infected through contact with food or water contaminated with material from infected pteropid bats, including secretions, excreta, or tissues from mothers or fetuses, the actual mechanism of spread remains to be determined.

Prevention

Steps can be taken to decrease the risk of Hendra infection in horses. It is important to protect horse food from contamination by flying fox fluids. Horse feed and water troughs should be relocated to areas away from where bats feed or roost.

Sick horses should be isolated while awaiting test results. If an outbreak is suspected, the horse premises should be quarantined immediately. Culling of infected animals – with close supervision of the burial or incineration of carcasses – may be necessary to reduce the risk of transmission to people. Restricting or banning the movement of horses from infected stables to other areas can reduce the spread of the disease.

Protective equipment, such as gloves, gowns, masks, and protective eyewear should be worn while handling sick animals or their tissues, and during post-mortems.

Hendra virus is killed by heat, drying, and cleaning with detergents. All surfaces that come into contact with a sick horse or bodily products of the horse should be thoroughly cleaned and dried before other horses are allowed into the area. Some disinfectant products are also effective against the virus.

Treatment

Although there is no specific antiviral treatment and no vaccine for the disease caused by Hendra virus, research into the development of a horse vaccine against Hendra virus is well underway.

At the first signs of symptoms of the disease, a veterinarian should be called in to accurately diagnose the illness. To date, antiviral medications have not been effective in treating Hendra virus infection in horses or people, but three people have recovered from infections with general medical support.