Bovine H5N1

A study investigated and compared the tissue tropism of a 2004 H5N1 strain isolated from a fatal human case and the 2024 bovine strain of H5N1 that is currently circulating in mice and found that the two strains displayed significantly different abilities to infect different tissues.

• H5N1 Virus Evolution: The study investigated two strains of the H5N1 virus:
  • The 2004 H5N1 strain isolated from a fatal human case.
  • The 2024 H5N1 bovine strain (clade 2.3.4.4b), currently circulating in cattle.
    The strains showed significant differences in tissue tropism and disease outcomes in animal models.

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• Tissue Tropism Comparison:
• • The 2004 H5N1 strain replicated largely in the respiratory tract, with limited involvement of the central nervous system (CNS). It predominantly caused respiratory disease.
  • The 2024 bovine H5N1 strain exhibited enhanced neurotropism. It replicated in both the respiratory tract and brain, causing inflammation in the brain and nasal cavity.Observations in Dairy Cattle
• Transmission Hypotheses: The H5N1 transmission within and between cattle herds is hypothesized to be linked to milking practices.
• Virus Replication in Cows: The virus in dairy cattle has been limited to the mammary glands of lactating cows. 
  • Unlike in mice, central nervous system involvement has not been observed in dairy cows through either natural or experimental infection.

Concerns and Implications

• Cross-Species Barrier: The growing list of susceptible mammalian hosts demonstrates the enhanced adaptability of H5N1. This raises serious concerns for wildlife, domestic animals, and public health.
• Shift in Virus Behavior: The clade 2.3.4.4b exhibits pronounced neurotropism compared to previous H5N1 strains. 
  • This shift is characterized by high viral loads in the brain and minimal presence in the lungs, accompanied by mutations linked to enhanced CNS involvement.

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About Avian influenza (bird flu)

• H5N1 is a type of influenza virus that causes severe respiratory disease in birds.
• Transmission: Humans are also at risk but they rarely contract bird flu. 
  • Most of the cases of human infection involve people who have. prolonged or close exposure to H5N1-infected animals such as farm/poultry workers, are at a higher risk. 
• Cases of Human Infection: Human infections with H5N1 were first reported in 1997 during a poultry outbreak in Hong Kong.
• Signs and Symptoms: Mild illness, manifested through eye redness (conjunctivitis) or mild flu-like upper respiratory symptoms, to severe (such as pneumonia requiring hospitalisation).
• Treatment: The antiviral Oseltamivir
  • There are also vaccine candidates that have been developed, but not readily available, for H5N1.
• Mortality: This could be as high as 60 per cent.
• Potential of Pandemic: Avian influenza is not on the WHO priority list of pathogens of pandemic potential.