Hendra Virus Infection: Symptoms, Treatments, Medications and Prevention

Hendra virus infection is a zoonotic disease caused by the Hendra virus (HeV), a highly lethal virus in the Henipavirus genus. Discovered in 1994 in Hendra, a suburb of Brisbane, Australia, this virus primarily affects horses and can spread to humans. Both horses and humans can suffer from severe respiratory and neurological diseases, often leading to high mortality rates. Flying foxes (fruit bats of the genus Pteropus) are the natural hosts of the Hendra virus, and horses contract the virus through contact with contaminated bat bodily fluids. While human cases are rare, the fatality rate is high, making Hendra virus a significant public health concern in Australia and other regions where flying foxes are present.

What is Hendra Virus Infection?

The Causative Agent: Hendra Virus (HeV)

Hendra virus infection is caused by the Hendra virus (HeV), which belongs to the Henipavirus genus in the Paramyxoviridae family. Hendra virus is closely related to the Nipah virus (NiV), another deadly zoonotic virus. Hendra virus primarily affects horses and can be transmitted from horses to humans, causing severe illness in both.

• Natural reservoir: The natural reservoir of the Hendra virus is flying foxes (fruit bats of the Pteropus genus). These bats carry the virus asymptomatically and excrete it in their urine, saliva, and feces.
• Transmission to horses: Horses are typically infected by consuming food, water, or pasture contaminated with bat urine, saliva, or birth fluids. The virus can also be transmitted through close contact with infected horses.
• Transmission to humans: Human infection occurs through close contact with an infected horse’s bodily fluids, such as blood, saliva, nasal discharge, or urine. There is no evidence of human-to-human transmission of Hendra virus.

Mode of Transmission

The transmission of Hendra virus involves complex interactions between bats, horses, and humans:

• Bat-to-horse transmission: Horses are infected through exposure to food, water, or pasture contaminated by the bodily fluids of infected bats, particularly during the birthing season when bat populations are higher.
• Horse-to-human transmission: Humans become infected through close contact with an infected horse’s bodily fluids during procedures such as handling the animal, administering care, or performing necropsies (post-mortem examinations).
• No human-to-human transmission: There has been no documented evidence of human-to-human transmission of Hendra virus.

The Lifecycle of the Hendra Virus

Hendra virus primarily resides in flying foxes, which excrete the virus in their urine, saliva, and other fluids. Horses, which graze in areas contaminated by these fluids, can contract the virus. Infected horses show symptoms of respiratory distress and neurological illness, and humans may become infected through close contact with the sick animals. The virus primarily targets the endothelial cells (cells that line blood vessels) in both horses and humans, leading to vasculitis, edema, and organ failure.

Hendra Virus in Animals

Horses are the primary victims of Hendra virus infection. Once infected, they develop severe respiratory and neurological symptoms. Infected horses typically die within days if untreated, and they serve as the main source of transmission to humans. Flying foxes, the natural hosts, carry the virus asymptomatically and transmit it to horses through contaminated food, water, or pasture.

Who is at Risk of Hendra Virus Infection?

Hendra virus infection is primarily a risk for individuals who are in close contact with infected horses. Certain populations, geographic regions, and activities increase the risk of exposure to Hendra virus.

Geographic Risk Factors

The Hendra virus is endemic to Australia, particularly in areas where flying foxes are common. The virus has primarily been reported in Queensland and New South Wales, though flying foxes are distributed across northern, eastern, and southeastern Australia, and thus, the risk of infection is present in these regions.

1. Australia

Hendra virus has only been detected in Australia, specifically in areas where flying foxes are abundant. The most commonly affected areas are:

• Queensland: Most cases of Hendra virus infection have occurred in southeast Queensland, where both horses and flying foxes are common.
• New South Wales: Hendra virus outbreaks have also been reported in northern New South Wales.

2. Flying Fox Populations

Regions with large populations of flying foxes are at higher risk of Hendra virus outbreaks. Flying fox species that carry Hendra virus include:

• Pteropus alecto: The black flying fox.
• Pteropus poliocephalus: The grey-headed flying fox.
• Pteropus conspicillatus: The spectacled flying fox.
• Pteropus scapulatus: The little red flying fox.

Occupational and Behavioral Risk Factors

Individuals who work with or care for horses are at the highest risk of Hendra virus infection due to the potential for close contact with infected animals.

1. Veterinarians and Veterinary Staff

Veterinarians and veterinary assistants are at high risk of Hendra virus infection due to their close contact with horses during diagnosis, treatment, and post-mortem examinations. Procedures such as intubation, suction, or necropsies increase the risk of exposure to infectious fluids.

2. Horse Owners and Trainers

Horse owners, trainers, and stable hands are also at increased risk of Hendra virus infection if they come into contact with sick horses. Feeding, grooming, or riding an infected horse can result in exposure to the virus.

3. Animal Health Workers

Individuals involved in the care, transport, or handling of horses, including animal health officers and equine farm workers, are at risk of contracting Hendra virus if they work in regions where the virus is present.

Risk from Contact with Bats

While human cases of Hendra virus infection have only been associated with contact with infected horses, individuals who come into close contact with flying foxes (e.g., bat carers, wildlife rescue workers) may indirectly increase their risk of exposure, especially if they handle bats in regions where Hendra virus is present.

Immunocompromised Individuals

Individuals with weakened immune systems may be more susceptible to severe outcomes if they contract Hendra virus. While there is no direct evidence of increased risk of infection among immunocompromised individuals, their weakened immune response may increase the likelihood of complications if they become infected.

Symptoms of Hendra Virus Infection

Hendra virus infection can present with a wide range of symptoms, from mild flu-like illness to severe respiratory and neurological complications. The incubation period (the time between exposure to the virus and the onset of symptoms) is typically between 5 to 16 days.

1. Initial Symptoms

The early symptoms of Hendra virus infection are non-specific and often resemble those of other viral illnesses. These symptoms may appear within 5 to 16 days after exposure and include:

• Fever: A sudden onset of high fever is one of the earliest and most common symptoms.
• Headache: A severe headache typically accompanies the fever.
• Muscle pain (myalgia): Generalized muscle aches, particularly in the back and legs, are common.
• Fatigue: Profound fatigue or weakness often occurs early in the illness.
• Dizziness: Some patients report dizziness or lightheadedness.

2. Respiratory Symptoms

As the infection progresses, respiratory symptoms often develop, which can range from mild to severe. These symptoms may include:

• Cough: A dry cough is commonly reported, and it may worsen over time.
• Shortness of breath (dyspnea): Difficulty breathing or rapid breathing is a sign of respiratory involvement.
• Chest pain: Pain or discomfort in the chest may occur, particularly during coughing or deep breathing.
• Pulmonary edema: Fluid accumulation in the lungs, known as pulmonary edema, can lead to severe respiratory distress.

3. Neurological Symptoms

In some cases, Hendra virus infection can affect the central nervous system, leading to neurological complications. Neurological symptoms may include:

• Confusion: Altered mental status, including confusion or disorientation, is common in severe cases.
• Seizures: Patients may experience seizures due to inflammation of the brain.
• Encephalitis: Inflammation of the brain (encephalitis) can lead to coma or death in severe cases.
• Paralysis: In rare instances, paralysis or motor dysfunction may develop as the virus attacks the nervous system.

4. Severe Complications

Without prompt treatment, Hendra virus infection can lead to life-threatening complications, including:

• Acute respiratory distress syndrome (ARDS): Severe respiratory failure due to widespread inflammation of the lungs.
• Multiple organ failure: The virus can spread to other organs, causing liver, kidney, or heart failure.
• Septic shock: Severe infection can result in septic shock, characterized by dangerously low blood pressure and organ dysfunction.
• Death: Hendra virus has a high mortality rate, with many severe cases resulting in death.

Diagnosis of Hendra Virus Infection

Diagnosing Hendra virus infection requires a combination of clinical evaluation, patient history, and laboratory testing. Because the early symptoms of Hendra virus infection are non-specific, it can be challenging to diagnose based on clinical symptoms alone.

1. Clinical Evaluation

The first step in diagnosing Hendra virus infection is a thorough clinical evaluation, including a detailed medical history and physical examination. Physicians will inquire about potential exposure to infected horses or flying foxes, recent travel to endemic areas, and the presence of respiratory and neurological symptoms.

2. Laboratory Tests

Several laboratory tests are used to confirm a diagnosis of Hendra virus infection. These tests aim to detect the presence of the virus or the body’s immune response to the infection.

1. Polymerase Chain Reaction (PCR)

PCR testing is the most reliable and sensitive method for detecting Hendra virus. This molecular technique is used to detect the viral RNA in clinical samples, such as blood, throat swabs, urine, or cerebrospinal fluid (CSF).

• Advantages: PCR can detect the virus early in the infection, even before antibodies are detectable, making it the preferred diagnostic tool.

2. Serological Tests

Serological tests detect antibodies produced by the immune system in response to Hendra virus infection. These tests are useful for confirming a diagnosis in the later stages of the disease.

• Enzyme-linked immunosorbent assay (ELISA): ELISA detects IgM and IgG antibodies specific to Hendra virus. A significant rise in antibody titers between acute and convalescent samples confirms the diagnosis.
• Neutralization assays: These tests measure the ability of patient antibodies to neutralize the virus in vitro.

3. Virus Isolation

In specialized laboratories, the virus can be isolated from clinical samples and cultured. However, this method is rarely used due to the need for biosafety level 4 (BSL-4) containment, as Hendra virus is highly contagious and dangerous.

3. Imaging Studies

In severe cases with respiratory symptoms, chest X-rays or CT scans may be performed to assess the extent of lung damage. In cases of suspected encephalitis, brain imaging studies such as MRI or CT scans can reveal signs of inflammation or swelling in the brain.

4. Differential Diagnosis

Hendra virus infection must be differentiated from other respiratory or neurological conditions, such as influenza, COVID-19, pneumonia, encephalitis, or other rickettsial infections. A history of exposure to infected horses or flying foxes can provide critical diagnostic clues.

Treatments for Hendra Virus Infection

There is no specific antiviral treatment for Hendra virus infection. Supportive care and early intervention with experimental therapies may improve outcomes. Treatment is focused on managing symptoms, providing supportive care, and preventing complications.

1. Supportive Care

Supportive care is the mainstay of treatment for Hendra virus infection. This includes:

• Oxygen therapy: Patients with severe respiratory symptoms or acute respiratory distress syndrome (ARDS) may require oxygen therapy or mechanical ventilation to maintain adequate oxygen levels in the blood.
• Intravenous fluids: IV fluids are administered to maintain hydration and electrolyte balance, particularly in patients experiencing severe vomiting or diarrhea.
• Medications for fever and pain: Antipyretics (such as acetaminophen) may be used to control fever, and pain relievers may help alleviate muscle pain or headaches.
• Management of neurological symptoms: Patients with seizures or encephalitis may require anticonvulsant medications and intensive care.

2. Experimental Therapies

In the absence of a specific antiviral treatment for Hendra virus, experimental therapies are being investigated to improve survival rates.

1. Monoclonal Antibodies

One of the most promising experimental treatments for Hendra virus is the use of monoclonal antibodies that target the virus.

• m102.4 monoclonal antibody: This antibody has shown efficacy in neutralizing the Hendra virus in animal models and has been used in emergency settings for post-exposure prophylaxis in humans. It works by binding to the virus and preventing it from entering host cells.

2. Ribavirin

Ribavirin, an antiviral drug that has been used to treat other viral infections, has shown some effectiveness against Hendra virus in vitro. However, its effectiveness in humans remains unclear, and it is not widely used due to its potential side effects.

3. Treatment of Complications

Severe cases of Hendra virus infection may require additional treatments to manage complications, such as multi-organ failure, septic shock, or respiratory failure.

• Mechanical ventilation: Patients with severe acute respiratory distress syndrome (ARDS) may require mechanical ventilation to support breathing.
• Hemodialysis: Patients with kidney failure may require hemodialysis to filter waste products from the blood.

Most Common Medications for Hendra Virus Infection

There are no specific medications approved for the treatment of Hendra virus infection. The most common treatments include:

1. Supportive Care

Supportive care, including oxygen therapy, intravenous fluids, and antipyretics, is the main treatment for Hendra virus infection. These measures help manage symptoms and prevent complications.

2. Monoclonal Antibodies

Monoclonal antibodies, such as m102.4, have been used experimentally in emergency situations to neutralize the virus and prevent it from spreading in the body. However, this treatment is not yet widely available.

3. Antiviral Drugs

Ribavirin, an antiviral drug, has been used in some cases, but its effectiveness in treating Hendra virus infection is still under investigation.

Where is Hendra Virus Infection Most Prevalent?

Hendra virus infection is primarily found in Australia, where the natural reservoir, flying foxes, are common. The virus is endemic in regions with large populations of flying foxes and where horses are frequently in contact with these bats.

1. Australia

Hendra virus is endemic in Queensland and New South Wales, though cases have also been reported in Northern Australia and other parts of the country where flying fox populations exist.

• Queensland: The majority of Hendra virus outbreaks in horses and humans have occurred in southeast Queensland.
• New South Wales: Northern New South Wales has also experienced several outbreaks of Hendra virus.

2. Geographic Risk Areas

Hendra virus is primarily associated with areas where flying fox populations are high, particularly in subtropical and tropical regions of Australia. These areas include:

• Southeast Queensland
• Northern New South Wales
• Northern Territory

Flying foxes are also found in parts of Western Australia and Victoria, but no human cases of Hendra virus have been reported in these regions to date.

Prevention of Hendra Virus Infection

Preventing Hendra virus infection requires reducing exposure to the virus, particularly for individuals who work with or care for horses in endemic regions. The following strategies can help reduce the risk of Hendra virus transmission.

1. Vaccination for Horses

One of the most effective prevention strategies for Hendra virus is the Hendra virus vaccine for horses, which was introduced in 2012.

• Equivac HeV: This vaccine is available for horses and significantly reduces the risk of Hendra virus transmission to both horses and humans. Regular vaccination of horses in endemic areas is strongly recommended.

2. Avoiding Contact with Infected Horses

Horse owners, veterinarians, and trainers should take precautions to avoid contact with horses showing signs of illness. If a horse is suspected of being infected with Hendra virus, the following measures should be taken:

• Isolate the horse: Sick horses should be isolated from other animals and humans to prevent the spread of the virus.
• Wear protective equipment: Individuals caring for sick horses should wear personal protective equipment (PPE), including gloves, masks, goggles, and coveralls, to prevent contact with bodily fluids.

3. Minimizing Exposure to Flying Foxes

Since flying foxes are the natural reservoir for Hendra virus, it is important to reduce the likelihood of horse-bat interactions.

• Remove food sources: Horse owners should remove fruit trees or cover water sources and feed bins to prevent contamination by bat urine or feces.
• Stable horses at night: Horses should be stabled at night, especially during the bat birthing season when the risk of virus transmission is higher.

4. Public Health Education

Raising awareness about the risks of Hendra virus and the importance of prevention measures is crucial, especially in endemic areas.

• Educating veterinarians and horse owners: Public health campaigns should focus on educating veterinarians, horse owners, and trainers about the symptoms of Hendra virus and the importance of vaccination and protective measures.
• Wildlife management: Efforts to manage and monitor flying fox populations can help reduce the risk of Hendra virus transmission to horses.

5. Handling Bats Safely

While human infection has not been directly linked to flying foxes, individuals who handle bats (such as wildlife carers) should take precautions to avoid potential exposure to the virus.

Use PPE: Wildlife carers should always wear gloves and protective clothing when handling bats in regions where Hendra virus is present.