Hendra virus (HeV) is a BSL-4 zoonotic paramyxovirus (genus Henipavirus) endemic to eastern Australia, with flying foxes (Pteropus spp.) as the natural reservoir and horses as the amplifying intermediate host. Only 7 confirmed human cases have been documented since its emergence in 1994, with a 57% case fatality rate, presenting as severe respiratory disease or progressive encephalitis. [1-2] No approved antiviral therapy or human vaccine currently exists; management is intensive supportive care. [3]
1. History
- Exposure history is paramount: Ask specifically about contact with horses (especially sick, dying, or dead horses) in eastern Australia (Queensland, New South Wales) [2]
- Inquire about participation in equine autopsy, veterinary procedures, or handling of equine respiratory secretions, blood, or urine [2]
- Timing: Incubation period ranges from 5 to 21 days [2]
- Initial symptoms are nonspecific: fever, myalgia, headache, lethargy (influenza-like illness) [2]
- Progression: Ask about worsening headache, confusion, drowsiness, seizures, cough, dyspnea — suggesting encephalitic or respiratory progression [2][4]
- Important negatives: No bat-to-human transmission has been documented; no human-to-human transmission reported [2]
2. Alarm Features
- Rapid neurological decline: altered consciousness, seizures, focal neurological deficits — suggests encephalitis [2][4]
- Severe respiratory distress with bilateral infiltrates (ARDS-like picture) [2]
- Any febrile illness within 21 days of close contact with a sick horse in an endemic area [2]
- Relapsing encephalitis can occur months after initial infection (documented at 13 months post-initial illness) — a unique and dangerous feature [2]
3. Medications
- No approved antiviral therapy exists [3]
- Ribavirin: Used on compassionate basis; in-vitro activity but inadequate serum levels achieved clinically and no proven efficacy in animal models or human cases [2]
- Remdesivir: Protective in NHP models against related Nipah virus when initiated within 24 hours of exposure; not tested in human HeV infection [3]
- Favipiravir: Protective in hamster models against Nipah virus; no human data [5]
- Monoclonal antibody m102.4: Most promising post-exposure prophylaxis; cross-reactive against HeV and Nipah; used compassionately in 14 individuals; phase 1 trial showed safety at doses up to 20 mg/kg [3][6]
- Chloroquine/hydroxychloroquine: No efficacy in animal models despite in-vitro activity; not recommended [2]
- Corticosteroids: Used empirically in early cases; no evidence of benefit
4. Diet
- No specific dietary triggers or recommendations
- Standard ICU nutritional support for critically ill patients with encephalitis or respiratory failure
- Maintain adequate hydration, particularly in febrile patients
5. Review of Systems
- Neurological: Headache, confusion, drowsiness, seizures, focal deficits, neck stiffness
- Respiratory: Cough, dyspnea, chest tightness, hemoptysis
- Constitutional: Fever, rigors, myalgia, fatigue, malaise
- Cardiovascular: Signs of vasculitis (the virus targets vascular endothelial cells via ephrin-B2 receptors) [1]
- Renal: Kidney involvement documented in fatal cases (virus isolated from kidney tissue) [2]
6. Collateral History and Family History
- Occupational history is critical: veterinarians, equine handlers, stable workers, horse owners, and those performing equine necropsies are at highest risk [2]
- Determine adequacy of PPE used during horse contact [2]
- Contact tracing of all individuals exposed to the same infected horse(s) [2]
- Family history is not relevant (no genetic predisposition); however, household contacts of infected individuals should be monitored given theoretical transmission risk
- No evidence of asymptomatic human infection from serosurveillance studies [2]
7. Risk Factors
- Direct close contact with infected horses — the sole documented route of human infection [2]
- Exposure to equine respiratory secretions, blood, urine, or body fluids without adequate PPE [2]
- Performing invasive procedures (autopsy, nasogastric intubation, dental procedures) on infected horses [1]
- Geographic: Eastern Australia, particularly coastal Queensland and New South Wales [2][7]
- Seasonal: Peak equine spillover events coincide with winter bat shedding peaks [1][8]
- Ecological: Proximity of horse paddocks to flying fox roosts; horses grazing under roosting trees [2]
- Urbanization and habitat fragmentation increasing bat-human-horse interface [9]
8. Differential Diagnosis
- Influenza and other respiratory viruses (early nonspecific phase)
- Nipah virus infection (closely related henipavirus; distinguished by geography and exposure history) [10]
- Japanese encephalitis, Murray Valley encephalitis, West Nile virus (other encephalitides in the Australian/Indo-Pacific region)
- Bacterial meningitis/encephalitis (bacterial vs. viral CSF profile)
- Herpes simplex encephalitis (temporal lobe predominance on MRI)
- Australian bat lyssavirus (bat exposure history; rabies-like presentation)
- Leptospirosis (animal exposure, renal involvement)
- Q fever (Coxiella burnetii; livestock exposure in Australia)
- Severe community-acquired pneumonia (if respiratory-predominant presentation)
- Key distinguishing feature: horse contact in endemic area + influenza-like prodrome progressing to encephalitis or severe pneumonia [2][11]
9. Past Medical History
- No specific comorbidities identified as increasing susceptibility (extremely limited case series)
- Document any immunosuppression (theoretical concern for viral clearance)
- Prior episodes of unexplained encephalitis or meningitis in individuals with equine exposure should raise suspicion for prior undiagnosed HeV (relapsing encephalitis documented) [2]
- Vaccination status of horses the patient had contact with (Equivac HeV vaccine) [1]
10. Physical Exam
- Vital signs: Fever (often high), tachycardia, tachypnea, hypoxia in respiratory cases
- Neurological: Altered mental status, meningismus, cranial nerve palsies, focal motor deficits, seizures, reduced GCS [2][4]
- Respiratory: Crackles, reduced breath sounds, signs of consolidation or ARDS
- Skin: Examine for signs of vasculitis (petechiae, purpura) — pathology driven by endothelial tropism [1]
- General: Myalgia, lymphadenopathy (nonspecific)
- PPE: Clinicians examining suspected cases must use full BSL-4 equivalent precautions — gown, gloves, N95/P2 respirator, eye protection [2]
11. Lab Studies
- RT-qPCR (quantitative real-time PCR): Diagnostic method of choice for acute infection — detects viral RNA in blood, respiratory secretions, CSF, urine [2]
- Serology: IgM-capture ELISA (early diagnosis), indirect IgG ELISA (convalescent); useful for epidemiological surveillance but not reliable for acute diagnosis due to rapid disease course [2]
- Virus neutralization test: Reference standard but requires BSL-4 laboratory [2]
- Routine labs: CBC, CMP, LFTs, coagulation studies, lactate — to assess organ dysfunction
- CSF analysis: Lymphocytic pleocytosis expected in encephalitic cases; send for HeV PCR
- Blood cultures: To exclude bacterial sepsis
- All specimens from suspected cases must be handled under BSL-4 conditions; notify the laboratory before sending samples [2]
12. Imaging
- Chest X-ray/CT chest: May show bilateral airspace infiltrates, diffuse alveolar shadowing, or be normal [2]
- MRI brain (gold standard for encephalitis):
- Multifocal hyperintense cortical, subcortical, and brainstem foci on T2/FLAIR [2]
- Matching diffusion restriction
- Leptomeningeal enhancement
- In relapsing encephalitis: extensive but predominantly cortical hyperintense lesions [2]
- CT head: Less sensitive than MRI but may be used for initial assessment in the ED
- Imaging is essential in any patient with neurological symptoms and relevant exposure history
13. Special Tests
- Immunohistochemistry: On tissue from fatal cases (brain, lung, kidney, spleen) — can confirm diagnosis post-mortem [2]
- Virus isolation: Possible on Vero cells but requires BSL-4 containment [2]
- Luminex multiplex microsphere assay: Can differentiate HeV from Nipah virus antibody responses; does not require BSL-4 [2][12]
- Surrogate virus neutralization tests: Using pseudotyped viruses in BSL-2 labs [2]
- Point-of-care lateral flow assays: Under development using ephrin-B2 capture ligands; not yet widely deployed [13]
- Electron microscopy: Can identify paramyxovirus morphology in tissue samples [2]
14. ECG
- No specific ECG findings described for Hendra virus infection
- ECG monitoring is indicated in critically ill patients for arrhythmia surveillance
- Myocarditis is a theoretical concern given the virus's endothelial tropism and vasculitis pathology [1]
- Rule out other causes of encephalitis with cardiac involvement (e.g., enteroviral)
15. Assessment
- Severity stratification:
- Mild: Self-limited influenza-like illness (2 of 7 cases recovered completely without progression) [2]
- Severe: Progression to encephalitis and/or severe pneumonia/ARDS (4 of 5 who progressed died) [2]
- Typical presentation: Nonspecific febrile illness → progressive encephalitis or respiratory failure over days [2][4]
- Atypical: Acute encephalitis without antecedent influenza-like prodrome (1 case) [2]
- Complications:
- Relapsing encephalitis months to over a year after initial infection (suggests viral latency/persistence) [2]
- ARDS, multiorgan failure
- Vasculitis and thrombosis (driven by ephrin-B2 receptor binding on endothelium) [1]
- Death (57% CFR overall) [1]
16. Treatment Plan
- Initial stabilization: Airway management, hemodynamic support, seizure control
- Intensive supportive care is the current standard — there is no approved specific therapy [3]
- Mechanical ventilation for respiratory failure
- ICP management for cerebral edema
- Anticonvulsants for seizures
- Experimental therapies (compassionate use/investigational):
- Monoclonal antibody m102.4: Most promising agent; administer as early as possible post-exposure; dosing from phase 1 trial: up to 20 mg/kg IV, with option for repeat dose at 72 hours [3][6]
- Ribavirin IV: Has been used but efficacy is uncertain; basal serum levels achieved were below the in-vitro IC90 of 64 mg/L [2][4]
- Remdesivir: Consider based on NHP data against Nipah; must be initiated within 24 hours of exposure for potential benefit [3]
- Post-exposure prophylaxis for high-risk contacts: m102.4 (~19 mg/kg single dose) has been given compassionately [2-3]
- Infection control: Strict contact and droplet precautions; BSL-4 handling of all specimens [2]
- Notify public health authorities immediately upon suspicion [2]
17. Disposition
- Admission criteria: Any symptomatic patient with confirmed or suspected HeV exposure should be admitted to an isolation facility with negative-pressure capability [2]
- ICU admission: Encephalitis, respiratory failure, hemodynamic instability, altered consciousness
- Observation: Asymptomatic high-risk contacts should be monitored for 21 days post-exposure with daily symptom checks [2]
- Specialist consultation triggers:
- Infectious disease (mandatory)
- Public health authorities (mandatory — this is a notifiable disease)
- Neurology (encephalitis)
- Pulmonology/critical care (respiratory failure)
- Infection control team
- Discharge: Only after clinical resolution, negative viral PCR, and in coordination with public health; interim restrictions on blood/tissue donation may apply [2]
18. Follow Up / Return Precautions
- Survivors must be monitored long-term for relapsing encephalitis — documented up to 13 months after initial infection [2]
- Follow-up MRI brain at intervals to monitor for new or evolving lesions
- Serial serological testing for contacts and survivors
- Return immediately for: new-onset headache, confusion, personality change, seizures, fever, respiratory symptoms — at any time within months of initial exposure/infection
- Counseling on the risk of late relapse and the importance of reporting any neurological symptoms
- Psychological support — given the high fatality rate and occupational nature of exposure
- Contacts should avoid blood/tissue donation during the surveillance period [2]
- Equine contacts: Ensure horse vaccination with Equivac HeV and reinforce biosecurity/PPE practices [1-2]
References
1. Three Decades of Discovery: An Overview of Hendra Virus, the Original Henipavirus. — Halpin K, Gómez Román R. PLoS Neglected Tropical Diseases. 2026.
2. Hendra Virus: An Emerging Paramyxovirus in Australia. — Mahalingam S, Herrero LJ, Playford EG, et al. The Lancet. Infectious Diseases. 2012.
3. Medical Countermeasures Against Henipaviruses: A Review and Public Health Perspective. — Gómez Román R, Tornieporth N, Cherian NG, et al. The Lancet. Infectious Diseases. 2022.
4. Human Hendra Virus Encephalitis Associated With Equine Outbreak, Australia, 2008. — Playford EG, McCall B, Smith G, et al. Emerging Infectious Diseases. 2010.
5. Favipiravir (T-705) Protects Against Nipah Virus Infection in the Hamster Model. — Dawes BE, Kalveram B, Ikegami T, et al. Scientific Reports. 2018.
6. Safety, Tolerability, Pharmacokinetics, and Immunogenicity of a Human Monoclonal Antibody Targeting the G Glycoprotein of Henipaviruses in Healthy Adults: A First-in-Human, Randomised, Controlled, Phase 1 Study. — Playford EG, Munro T, Mahler SM, et al. The Lancet. Infectious Diseases. 2020.
7. Flying-Fox Species Density--a Spatial Risk Factor for Hendra Virus Infection in Horses in Eastern Australia. — Smith C, Skelly C, Kung N, Roberts B, Field H. PloS One. 2014.
8. Hendra Virus Ecology and Transmission. — Field HE. Current Opinion in Virology. 2016.
9. The Impact of Human Population Pressure on Flying Fox Niches and the Potential Consequences for Hendra Virus Spillover. — Walsh MG, Wiethoelter A, Haseeb MA. Scientific Reports. 2017.
10. Henipavirus Zoonosis: Outbreaks, Animal Hosts and Potential New Emergence. — Li H, Kim JV, Pickering BS. Frontiers in Microbiology. 2023.
11. Hendra Virus: A One Health Tale of Flying Foxes, Horses and Humans. — Hazelton B, Ba Alawi F, Kok J, Dwyer DE. Future Microbiology. 2013.
12. Henipavirus Microsphere Immuno-Assays for Detection of Antibodies Against Hendra Virus. — McNabb L, Barr J, Crameri G, et al. Journal of Virological Methods. 2014.
13. Development of a Point-of-Care Immunochromatographic Lateral Flow Strip Assay for the Detection of Nipah and Hendra Viruses. — Jia J, Zhu W, Liu G, et al. Viruses. 2025.