The Hendra virus (HeV) is a zoonotic pathogen that can cause severe disease in both animals and humans. First identified in Australia in 1994, this virus is considered an emerging infectious disease because it can jump from its natural host species to domestic animals and subsequently to people. The virus is characterized by a high case fatality rate in horses and humans, requiring a clear understanding of its transmission cycle.
Understanding Hendra Virus
Hendra virus is classified as a member of the Henipavirus genus, which is contained within the Paramyxoviridae family of viruses. This genus also includes the closely related Nipah virus, both of which cause severe respiratory and neurological illness. HeV is an enveloped, single-stranded RNA virus, and its structure includes surface proteins that facilitate entry into host cells.
The virus was named after the Brisbane suburb of Hendra, Queensland, where it was first isolated following an outbreak at a racing stable in 1994. Since its discovery, all subsequent outbreaks have been confined to the coastal and forested regions of eastern Australia, primarily within Queensland and New South Wales. This geographic restriction is directly tied to the habitat of the virus’s natural reservoir host.
Clinical Manifestations in Hosts
The clinical presentation of Hendra virus infection can be variable, often making early diagnosis difficult in both horses and humans. The virus targets and damages blood vessel walls, leading to internal hemorrhaging, edema, and inflammation in multiple organs, particularly the lungs and brain. The incubation period in humans typically ranges from five to 21 days after exposure to an infected horse.
Symptoms in Horses
In horses, HeV infection usually presents with rapid deterioration, often leading to death or euthanasia within a few days. Initial signs involve a high fever, increased heart rate, and signs of respiratory distress. These respiratory symptoms can include difficulty breathing, rapid shallow breathing, and a characteristic frothy nasal discharge that may be clear or blood-tinged.
Neurological signs are also common and may manifest as muscle twitching, muscle weakness, head tilting, and an uncoordinated gait. Some infected horses display apparent vision loss, aimless walking, or unusual sleepiness, indicating inflammation of the brain. Any sudden, rapidly progressive illness in a horse should be treated with suspicion, especially where flying foxes are present.
Symptoms in Humans
In humans, the infection can present in one of two forms: a severe respiratory illness or a neurological disease. The initial symptoms are often non-specific and resemble a flu-like illness, including fever, headache, sore throat, and a dry cough. These initial symptoms can quickly progress to life-threatening complications.
The respiratory form involves the rapid development of pneumonia and pulmonary edema, leading to breathing difficulties. The neurological form, which is encephalitis, can cause symptoms like severe headache, confusion, dizziness, and unusual drowsiness. This may progress to convulsions, coma, and ultimately death.
Viral Ecology and Spillover
The natural reservoir for Hendra virus is the Australian flying fox, a type of fruit bat belonging to the genus Pteropus, which includes four mainland species. These bats carry the virus without showing signs of illness, maintaining the virus within their populations throughout their range in Australia. The virus is shed in the bat’s bodily fluids, particularly their urine, feces, and birthing products.
Spillover to the intermediate host is believed to occur when horses ingest feed or water contaminated by these bat excretions. This exposure often happens when horses graze beneath trees where flying foxes are feeding or roosting, allowing bat fluids to drop onto the pasture or into water troughs. Spillover events are more likely to occur during periods of bat stress or reproductive cycles when viral shedding may increase.
Horses then act as an amplifying host, producing high levels of the virus in their own bodily fluids, such as nasal discharge, blood, and urine. Humans become infected through close, direct contact with these infected horses, typically through exposure to their secretions or excretions, often during veterinary procedures or handling of a sick or deceased animal. There is currently no evidence of direct bat-to-human or human-to-human transmission.
Strategies for Control and Mitigation
Controlling the spread of Hendra virus relies on breaking the chain of transmission from bats to horses and from horses to humans. This requires a multi-pronged approach involving biosecurity, vaccination, and personal protection protocols. The primary measure to manage the risk is the vaccination of horses, which provides a public health benefit by preventing the virus from replicating in the intermediate host.
Biosecurity Protocols
Horse owners can reduce the risk of equine infection by implementing biosecurity protocols aimed at limiting flying fox contact with feed and water sources. This involves placing feed bins and water troughs under cover, such as in stables or sheds, and away from trees where bats are known to roost or feed. Fencing off horse-grazing areas from known bat roosting or feeding trees also helps minimize the chance of contamination.
Vaccination
The development of a registered vaccine for horses, Equivac HeV, has been important for Hendra virus control since its introduction in 2012. The vaccine is designed to prevent clinical signs of the disease in horses, thereby eliminating the primary source of human infection. Regular vaccination of horses is the most effective way to protect both the animals and the people who care for them.
Personal Protective Measures
For individuals who must handle sick horses, particularly veterinarians and handlers, the use of personal protective equipment (PPE) is a necessary precaution. This protective gear includes items like gloves, gowns, protective eyewear, and masks or respirators to prevent contact with potentially infectious body fluids. Any close contact with a horse suspected of infection must be avoided until a veterinarian can assess the animal and follow appropriate safety guidelines.