What is influenza zoonotic or novel strain?

Influenza Zoonotic or Novel Strain

Influenza zoonotic or novel strains are influenza A viruses that cross species barriers from animals to humans, or represent new viral subtypes with pandemic potential due to limited or no pre-existing immunity in the human population. 1

Definition and Characteristics

• Zoonotic influenza refers to influenza viruses that have jumped from animals (typically birds or swine) to humans, while novel strains are new influenza variants that emerge through genetic changes and have not previously circulated widely in humans 1, 2
• These viruses are nationally notifiable diseases in the United States, requiring rapid identification and investigation to determine exposure sources, identify additional cases, and evaluate human-to-human transmission potential 1
• Influenza A viruses are categorized into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N), with frequent genetic changes occurring through:
  • Antigenic drift: Point mutations during viral replication causing gradual changes 1
  • Antigenic shift: More dramatic changes resulting in new influenza A subtypes with pandemic potential 1, 3

Sources of Zoonotic and Novel Influenza

Avian Influenza

• Highly pathogenic avian influenza A(H5N1) has caused sporadic severe and fatal human cases in Asia, Africa, Europe, and the Middle East 1
• Human infections typically occur through direct or close unprotected contact with sick or dead birds 1
• Limited, non-sustained human-to-human transmission of H5N1 has likely occurred in some case clusters 1
• Other avian influenza viruses causing human illness include:
  • Low pathogenic viruses: H7N7 (conjunctivitis), H7N2 (respiratory disease), H9N2 (influenza-like illness) 1
  • Highly pathogenic viruses: H7N7 (including one fatal case in the Netherlands), H7N3 (conjunctivitis) 1

Swine Influenza

• Novel influenza A(H1N1) that emerged in 2009 was derived partly from influenza A viruses circulating in swine 1
• This virus was antigenically distinct from human influenza A(H1N1) viruses in circulation since 1977 1
• Swine are considered important "mixing vessel" hosts due to their ability to be infected by both avian and human influenza viruses 2, 4

Pandemic Potential

• New influenza A subtypes have pandemic potential when they can:
  • Cause human illness
  • Demonstrate efficient human-to-human transmission
  • Encounter little or no pre-existing immunity in humans 1
• Even when not a completely new subtype (like 2009 H1N1), novel strains can have pandemic potential if humans lack pre-existing antibodies to key viral hemagglutinin epitopes 1
• The emergence of novel influenza viruses with pandemic potential is unpredictable and can outpace current vaccine development technologies 5, 6

Clinical Impact and Surveillance

• Human infections with zoonotic influenza can range from mild illness to severe disease with high mortality 1, 2
• Age distribution of severe cases often differs from seasonal influenza - the 2009 H1N1 pandemic affected younger populations more severely than typical seasonal influenza 1
• Human infection with novel or nonhuman influenza A virus strains requires rapid identification and investigation to:
  • Determine possible exposure sources
  • Identify additional cases
  • Evaluate human-to-human transmission potential 1

Treatment Considerations

• Neuraminidase inhibitors (zanamivir, peramivir) can be used to treat influenza infections, but resistance can develop 7, 8
• Cross-resistance between different neuraminidase inhibitors has been observed in some viral strains 7, 8
• Amino acid substitutions in viral neuraminidase or hemagglutinin proteins can confer reduced susceptibility to antiviral medications 7, 8

Prevention and Control

• Coordinated surveillance at the animal-human interface is essential for pandemic preparedness 5, 3
• A "One Health" approach involving practitioners in human health, animal health, and environmental sciences is needed 5, 6
• Changes in travel, livestock trade, animal husbandry practices, and wet markets contribute to increased risk of novel influenza virus emergence 5, 4
• Strategic virus surveillance of both animal and human populations is critical for early detection and response 2, 4