16 May 2026
Researchers identified a novel H5N1 clade 2.3.2.1e causing isolated brain infection in a child (1).
Historically, Highly Pathogenic Avian Influenza (HPAI) A(H5N1) has been characterized by severe respiratory failure in humans. While neurological complications have been documented, they almost always follow standard respiratory symptoms. However, in mid-April 2025, an eight-year-old boy from Vietnam’s Tay Ninh province was hospitalized with a 24-hour history of fever, severe headache, and vomiting. Despite a chest radiograph showing some lung consolidation, the patient exhibited no cough, no shortness of breath, and maintained normal oxygen saturation. The initial diagnosis was sepsis and meningoencephalitis. The investigation took a pivotal turn when clinicians analyzed the patient's cerebrospinal fluid (CSF). A multiplex PCR test detected Influenza A virus in the CSF with a high viral load, indicated by a cycle threshold (Ct) value of 19. Subsequent testing confirmed the subtype as H5N1. Remarkably, the virus was not detected in the patient’s throat swabs, blood, urine, or rectal swabs, even though serial CSF samples remained positive for 10 days. Further analysis showed that the boy had higher levels of H5-specific antibodies in his brain fluid than in his blood. This suggests intrathecal production. Scientists believe the virus likely reached the brain via cranial nerves, such as the olfactory nerve, essentially "skipping" the typical respiratory infection phase.
Genome sequencing identified the virus as a novel reassortant clade 2.3.2.1e. This strain is a genetic "hybrid," containing segments from both the older clade 2.3.2.1c and the globally dominant clade 2.3.4.4b. The virus carried two specific mutations in its hemagglutinin (HA) protein: S123P and R167K, which are associated with increased binding to receptors found in mammalian tissues. The patient, who had frequent close contact with his family’s fighting cocks prior to their sudden death, eventually made a full recovery after 19 days of intensive care and antiviral treatment with oseltamivir. This case mirrors and extends findings from other significant H5N1 events:
- The 2014 Canadian Case: A traveler returning from China succumbed to H5N1 meningoencephalitis. Like the Vietnamese case, this patient experienced severe CNS involvement, though it was accompanied by respiratory failure.
-Pathogenesis Studies: Research has increasingly shown that certain H5 strains possess a high "neuro-tropism," meaning they have a specific affinity for nervous tissue, as seen in experimental mammalian models (2).
1. Nguyen, P. N. T., et al. (2026). Central Nervous System Involvement by Novel Clade 2.3.2.1e H5N1 Avian Influenza Virus in a Paediatric Patient. Open Forum Infectious Diseases.
2. Bauer, et al. (2023) The neuropathogenesis of highly pathogenic avian influenza H5Nx viruses in mammalian species including humans. Trends in Neurosciences. 46.11: 953-970.
