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
- Tissue tropism is the ability of a pathogen to interact with a specific tissue or cell in a host. It can also refer to the range of tissues and cells in a host that support the growth of a pathogen.
- Tissue tropism is a key characteristic of viruses and can affect their virulence, pathogenesis, and susceptibility to antiviral drugs.
- For example, the influenza virus mainly infects the respiratory tract, while the Hepatitis B virus infects the liver.
Neurotropism
- Neurotropism refers to the ability of viruses, bacteria, fungi, and parasites to invade and survive in the nervous system.
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- Tissue Tropism Comparison:
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- 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.
December 18, 2024
