Hendra virus (HeV), also known as equine morbillivirus, is a frequently fatal viral infection that can affect both horses and people. [1] Hendra virus is most common in Australia, particularly in Queensland and New South Wales. [1] The “flying fox” bat species are the main reservoirs for Hendra virus.

Horses likely acquire Hendra virus through contact with contaminated bat urine. Once infected, horses develop symptoms such as difficulty breathing, fever, uncoordinated movement, and copious nasal discharge. Affected horses often die within 48 hours of developing symptoms.

There is no known treatment for Hendra virus, and many government bodies require euthanasia of affected horses to minimize the spread of disease. Diagnosis involves blood or tissue sampling to identify the virus.

Preventing Hendra virus infection involves a combination of vaccination, reducing exposure to flying foxes, and appropriate quarantine protocols during an outbreak.

Horse owners and veterinarians must be particularly vigilant regarding Hendra virus, as the virus is potentially fatal in humans exposed to infected horses. Of the seven cases of human Hendra virus reported so far, four of the patients died from the virus.

Causes of Hendra Virus in Horses

flying-fox-bats

The reservoir host of Hendra virus are “flying fox” bat species. [1] Reservoir hosts are species that carry a virus but typically do not develop symptoms of disease. [1] Up to 47% of flying foxes in Australia test positive for Hendra virus. [1]

How Hendra virus spreads from flying foxes to horses is currently unknown. [1] Most researchers believe that flying fox urine in the environment is the main source of horse exposure. [2][3] Studies show that horses living within 7 km of a flying fox roost are significantly more likely to develop HeV infection than other horses. [2]

Once a horse is infected with HeV, the virus can spread to other horses through direct contact with infected equine body fluids. [1] Humans are also susceptible to infection after contacting these body fluids. [1]

Veterinarians and veterinary assistants have a higher risk of infection due to increased exposure to equine body fluids. [2] All reported cases of human Hendra virus infection occurred after exposure to an affected horse, emphasizing the significance of this disease as a zoonosis (disease that spreads from animals to humans).

Although it is possible for HeV to spread between different animals, researchers consider it to have a low transmission rate. [1] Studies suggest that only around 10% of humans exposed to infectious equine body fluids develop disease. [1]

Effect of Viral Infection

Once the virus enters a horse’s body, it binds to neurons (nerve cells), smooth muscle cells, and cells lining blood vessels. [4] The virus enters the cell and starts replication. After the virus’ genetic material is copied, the infected cell ruptures and releases new viral particles into the body. [4]

Since infected cells rupture after viral replication, HeV infection results in widespread tissue damage. [4] This tissue damage results in the symptoms associated with Hendra virus.

Commonly affected tissues include: [4]

  • Small blood vessels within the lungs and brain
  • The brain and spinal cord
  • Spleen
  • Heart muscle
  • Lymph nodes
  • Kidneys

Symptoms

Symptoms of Hendra virus in horses typically develop 4-16 days after infection. [1] Symptoms include: [1][4]

  • Fever
  • Facial swelling
  • Uncoordinated movement (ataxia)
  • Large amounts of frothy nasal discharge
  • Difficulty breathing
  • Muscle twitching
  • Shifting weight between legs
  • Increased heart rate

Most horses die within 48 hours of symptoms developing. [1]

Neurological symptoms

Some outbreaks of HeV primarily affect the central nervous system, including the brain and spinal cord. [3] Neurological symptoms include: [3]

  • Circling
  • Disorientation
  • Head tilt
  • Facial paralysis
  • Difficulty urinating
  • Head pressing against walls or other surfaces
  • Uncoordinated movement

Diagnosis and Treatment

Diagnosis of Hendra virus requires laboratory testing to confirm the presence of the virus. [1] As the virus can infect humans, these diagnostic tests must be performed in high-biosecurity laboratories to minimize the risk of human disease. [1]

Appropriate samples for diagnosing HeV include: [1]

  • Blood
  • Nasal swabs
  • Tissue samples taken during necropsy

There is no known treatment for Hendra virus. [1] Due to the risk of human infection, many government bodies require euthanasia of infected horses after confirming a diagnosis. [1] Therefore, the overall prognosis of HeV infection in horses is unknown but is likely extremely poor. [1]

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Prevention

The main method of prevention for HeV is vaccination. [1] Horse owners must also consider reducing exposure to flying foxes and quarantine protocols as part of their prevention strategy. [1]

Vaccination

There are two main goals of Hendra virus vaccination: reducing the severity of disease and minimizing virus production in infected horses. [1] The most common vaccine used for Hendra virus is Equivac®. [4]

The Equivac® vaccine is a subunit vaccine, which means it contains only a small portion of viral proteins, rather than the complete virus. [4] These types of vaccines are safer compared to “live” vaccines since they have no risk of mutating back to an infectious strain of the virus. [4]

After vaccination, the horse’s immune system recognizes the viral proteins and produces an immune response specific to Hendra virus. [4]

Data on the HeV vaccine shows that immunized horses do not develop the disease, even after prolonged exposure to the virus. [1] Tissues from vaccinated and infected horses also show no evidence of virus production, suggesting that there is no risk to other animals from contacting these horses. [1]

Studies also show that vaccination does not impact performance in racing Thoroughbreds. [5] Therefore, researchers consider the HeV vaccine highly effective. [1]

Veterinarians can administer the Equivac® vaccine to horses as young as four months old. [4] The vaccination protocol involves an initial series of two vaccines, three to six weeks apart. [4][6] Annual booster vaccines maintain the immune response going forward. [4]

Side Effects

Side effects are uncommon with the Equivac® vaccine, with estimates suggesting only 0.001% of horses experience an adverse event. [6]

Reported side effects include: [4]

  • Swelling and pain at the injection site
  • Fever
  • Lethargy
  • Appetite loss
  • Muscle stiffness
  • Skin irritation
  • Colic

Vaccine Hesitance

Despite strong recommendation by the Australian government, use of the Hendra virus vaccine is limited. [7] Reports suggest only 13-20% of horses in high-risk areas are vaccinated. [7]

Studies examining the HeV vaccine suggest that owner interpretation and perception of certain factors are the main cause of low vaccination rates against HeV. The primary owner concerns against vaccination are related to: [7][8][9]

  • Cost: the HeV vaccine is often more expensive than the core equine vaccine protocols
  • Equine risk: they do not think their horses are at risk of developing disease, even if exposed to the virus
  • Human risk: if their horse develops disease, owners perceive they are not at a high risk of infection personally
  • Exposure rates: regardless of the presence of flying foxes in the area, some owners insist their horse gets no exposure due to lifestyle and management controls
  • International travel: some countries have limitations on transporting horses internationally after vaccination
  • Side effects: fear of adverse events after vaccination

There is often a conflict of interest between veterinarians and their client regarding use of the Hendra virus vaccine. [10]

Veterinarians who treat a sick, unvaccinated horse that subsequently tests positive for HeV can face workplace health and safety fines of up to $10 000 and may lose their veterinary license. [10] Therefore, veterinarians have a vested interest in vaccinating their client’s horses for their own safety and legal concerns.

Many veterinary hospitals also have strict policies preventing treatment of sick, unvaccinated horses until a negative HeV test result is confirmed. [10]

However, many clients oppose vaccination due to costs and other factors described above. [10] Studies suggest this conflict is having a negative impact on veterinarian-client relationships and the general public’s trust in veterinarians within Australia. [8][9]

Minimizing Flying Fox Exposure

As flying foxes are the main reservoir of HeV, reducing contact with flying foxes is presumed to reduce the risk of infection. [1]

Flying foxes are most common in woodland and open forest areas with a high prevalence of nectar, pollen, and fruit for consumption. [6] During droughts, flying foxes may expand their natural habitat to seek alternative food sources, increasing the chance of contact with horses. [6]

Strategies to reduce flying fox exposure include: [1][4][6]

  • Placing feeding and watering stations under cover
  • Avoiding feeding fruit or other feed that may attract flying foxes
  • Remove flowering trees and eucalyptus within horse pastures
  • Removing roosts in the local area

Quarantine Protocols

Quarantine strategies in the event of an infected horse are also vital to preventing spread of disease. [1]

Current protocols include: [1][2][4]

  • Immediate quarantine of the affected premises
  • Movement restrictions in the immediate area, preventing horses from leaving or entering the area
  • Diagnostic testing of all horses within the “outbreak zone” to determine the number of affected animals

Within the affected premises, strict isolation protocols are necessary for all affected horses. [1] These horses should be housed as far away from other horses as possible, within a well-demarcated perimeter designating the restricted area. [11]

Disinfection Protocols

Any surfaces previously contacted by affected horses should be thoroughly disinfected before any other horses enter the area. [11] Surfaces potentially contacted by handlers of affected animals should also be disinfected. [11]

Common surfaces requiring disinfection include: [11]

  • Water and feed buckets
  • Lead ropes and halters
  • Thermometers
  • Grooming supplies
  • Fences and gates
  • Light switches
  • Door handles
  • Wash rack and/or grooming area equipment

Isolation Protocols

Important isolation protocols include: [11]

  • Designated handlers for sick animals, who do not handle any healthy animals
  • Disposable Personal Protective Equipment (PPE) for all handlers
  • Designated equipment, such as buckets, wheelbarrows, and manure forks, for sick animals
  • Disposal of PPE, manure and bedding as recommended by government officials
  • Logs of people in contact with affected horses, including time, day, and duration of contact

Due to the high fatality rate of Hendra virus, horses may die shortly after isolation. Handlers must take particular care when handling the carcass, as the viral load is highest shortly after death. [3] Government policies for carcass removal and decontamination of the environment must be followed to prevent further spread of infection. [3]

Hendra Virus as a Zoonotic Disease

Hendra virus is a zoonotic disease, meaning it can transfer from animals to humans. [12] To date, all humans who developed Hendra virus infection acquired the disease from close contact with an infected horse. [12] There is no evidence of human-to-human or bat-to-human spread of the virus. [12]

Humans exposed to Hendra virus typically develop influenza-like illness. [10] In some cases, infection may progress to respiratory disease and inflammation in the brain. [10] Of the seven cases of human Hendra virus reported so far, four of the patients died due to their infection. [12]

Preventing Human Infection

Horse owners and veterinarians must be vigilant about biosecurity protocols and horse health to reduce their risk of infection. [1]

Several other equine diseases can appear similar to the early symptoms of Hendra virus, making awareness of the disease critical to preventing human infection. [1] Diseases with similar symptoms include colic and pneumonia, which are both more common than HeV infection. [1]

Personal Protective Equipment is recommended when handling any horses suspected of having HeV infection. [1] However, compliance with these suggestions is typically poor, as many perceive PPE use as impractical or constricting. [1]

Recommend PPE includes: [6]

  • Face shield
  • Face mask
  • Disposable gown
  • Gloves
  • Boot covers

These PPE materials should be disposable for Hendra virus-suspect cases due to the risk of zoonotic exposure during laundering. [11]

Summary

Hendra virus is a frequently fatal viral infection affecting horses and humans.

  • The virus originates in flying fox bat species and likely transfers to horses through bat urine
  • Infections are most common in Queensland and New South Wales in Australia
  • Symptoms include difficulty breathing, copious nasal discharge, fever, and uncoordinated movement
  • Hendra virus is usually rapidly fatal in horses and has no known treatment
  • Prevention involves a combination of vaccination, minimizing exposure to flying foxes, and appropriate quarantine protocols during outbreaks
  • Hendra virus can be fatal in humans, and all currently reported human cases resulted from exposure to an affected horse

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References

  1. Sellon. D. C. and Long. M. T., Eds., Equine Infectious Diseases. Second edition. Saunders/Elsevier, St. Louis, Missouri. 2013.
  2. Field. H. E., Hendra Virus Ecology and Transmission. Current Opinion in Virology. 2016.View Summary
  3. Middleton. D., Hendra Virus. Veterinary Clinics of North America: Equine Practice. 2014.View Summary
  4. Khusro. A. et al., Hendra Virus Infection in Horses: A Review on Emerging Mystery Paramyxovirus. Journal of Equine Veterinary Science. 2020.View Summary
  5. Schemann. K. et al., Investigation of the Effect of Equivac® HeV Hendra Virus Vaccination on Thoroughbred Racing Performance. Australian Veterinary Journal. 2018.View Summary
  6. Yuen. K. Y. et al., Hendra Virus: Epidemiology Dynamics in Relation to Climate Change, Diagnostic Tests and Control Measures. One Health. 2021.View Summary
  7. Wilson. S.-J. and Ward. M. P., Intangible and Economic Impacts of Hendra Virus Prevention Strategies. Zoonoses and Public Health. 2016.View Summary
  8. Wiethoelter. A. K. et al., “We’ve Learned to Live with It”—A Qualitative Study of Australian Horse Owners’ Attitudes, Perceptions and Practices in Response to Hendra Virus. Preventive Veterinary Medicine. 2017.
  9. Manyweathers. J. et al., “Why Won’t They Just Vaccinate?” Horse Owner Risk Perception and Uptake of the Hendra Virus Vaccine. BMC Veterinary Research. 2017.
  10. Wang. X. et al., Hendra Virus. Veterinary Clinics of North America: Equine Practice. 2023.
  11. General Biosecurity Guidelines. American Association of Equine Practitioners. 2022.
  12. Halpin. K. and Rota. P. A., A Review of Hendra Virus and Nipah Virus Infections in Man and Other Animals. Zoonoses: Infections Affecting Humans and Animals. Springer International Publishing, Cham. 2023.