The lethal Hendra virus returns

The health authorities of Queensland (Australia) have notified that on July 8 a horse from the Mackay area tested positive for Hendra virus infection. It is the first case in the region since 2017. As a preventive measure, the animal, which was very sick, has been euthanized.

And who is this Hendra virus (HeV)? It is a highly pathogenic emerging zoonotic virus that affects horses and humans, although experimental infections have also occurred in other mammals such as cats and guinea pigs.

The Hendra virus was discovered in 1994, after causing an infectious outbreak in an Australian farm located in the suburb of Hendra, in the city of Brisbane (Queensland). The incident led to the deaths of 14 horses and their trainer, a man named Vic Rail.

Vic Rail and his horses

Victory Robert Rail, nicknamed Vic Rail, was a racehorse trainer. In September 1994, Rail moved two horses into his stable from a suburban Brisbane paddock. Within a short time, both animals showed symptoms of an unknown respiratory illness. The horses received early veterinary treatment, but their condition worsened. What’s more, other horses in the barn began to get sick. Within a week, fourteen horses had died or been euthanized by local veterinary services.

Concern grew when Vic Rail developed symptoms similar to those his horses had exhibited. Urged by his partner, Lisa Symons, he sought medical treatment. Vic was admitted to Brisbane South Hospital. He died there a week later, on September 27, 1994.

Since then, more than 60 natural Hendra virus infections in horses have been recognized in Australia, resulting in the deaths of more than 100 animals. Also in this period, 7 human cases and 4 deaths have been confirmed.

Another virus transmitted by bats

Hendra virus belongs to the genus Henipavirus and the family Paramyxoviridae. The bat species of the genus Pteropuscalled flying foxes, appear to be the natural reservoirs of the virus.

The initially infected horses that Rail brought to his stable had likely sniffed or eaten vegetation contaminated with droppings or saliva from a flying fox carrying the Hendra virus. All confirmed human cases to date were infected after high-level exposures to bodily fluids from an infected horse, such as performing autopsies on horses without proper personal protective equipment or being extensively sprayed with respiratory secretions.

There is currently no evidence of human-to-human, bat-to-human, bat-to-dog, or dog-to-human transmission.

The infection causes severe acute respiratory and encephalitic disease with inflammation of the blood vessels (endothelial vasculitis). Hendra virus has a high case fatality rate, hovering around 90% in horses and 60% in humans. In addition, it causes chronic encephalitis among survivors.

Hendra virus is transmitted to horses by ingestion of fruit, grass, or water contaminated by secretions from infected bats.
Raul Rivas/USAL, Author provided

Aimless jogging, one of the symptoms

In horses, Hendra virus causes a wide variety of symptoms including fever, increased heart rate, and rapid deterioration with respiratory and/or neurological signs. Animals frequently exhibit muscle spasms, urinary incontinence, head tilt, vision loss, or wandering.

Symptoms in humans usually develop between 5 and 21 days after contact with an infectious horse. They initially include fever, cough, sore throat, headache, and tiredness. Later, affected people can develop meningitis or encephalitis, which causes a severe headache, high fever, drowsiness, and sometimes seizures and coma that lead to death in affected people.

Focus One Health

Vaccination is the most effective way to reduce the risk of Hendra virus infection in horses. The efficacy of an immunogen called HeV-sG led to the development and launch of the non-live, non-inactivated equine anti-HeV vaccine (Equivac® HeV), marketed by Zoetis Inc. in Australia in 2012.

The Australian Pesticides and Veterinary Medicines Authority (APVMA) granted full registration for Equivac® HeV in August 2015. The vaccine contains soluble forms of glycoprotein G (sG) of the Hendra virus, adjuvanted with an immunostimulatory complex. It is a safe and efficient vaccine.

At the moment there are no vaccines intended for administration in humans, but using the same immunogen, efforts are being made to develop efficient emergency vaccines to interrupt the possible spread of the disease in people.

Of course, the Hendra virus vaccine for horses is a great example of what the approach entails. One Health in the control of human diseases. In Australia, establishing a strategy One Health for the Hendra virus has resulted in more efficient and effective informed management. In fact, this approach allowed new Hendra virus genotypes to be identified in Australian flying foxes in October 2021.

The finding contemplates the possibility that, at any moment, new viruses emerge, demonstrating the importance of biosafety and surveillance programs.

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