Is Influenza A the same as Avian Influenza?

Is Influenza A the Same as Avian Influenza?

No, influenza A and avian influenza are not the same—avian influenza refers to specific subtypes of influenza A viruses that primarily infect birds and can occasionally infect humans, whereas influenza A is a broader category that includes multiple subtypes circulating in humans, birds, pigs, and other species. 1

Understanding the Relationship

Influenza A as the Broader Category

• Influenza A viruses are classified by their surface proteins: hemagglutinin (HA) with 16 subtypes (H1-H16) and neuraminidase (NA) with 9 subtypes, creating 144 possible combinations 2
• All influenza A subtypes have been found in aquatic birds, which serve as the natural reservoir for these viruses 2, 3
• Common human influenza A subtypes include H1N1 and H3N2, which cause seasonal flu 1

Avian Influenza as a Specific Subset

• Avian influenza specifically refers to influenza A viruses that are adapted to and primarily circulate in bird populations 4, 2
• Multiple avian influenza subtypes can infect humans, including H5N1, H7N7, H7N2, H7N3, and H9N2 1
• These viruses are divided into highly pathogenic (HPAI) and low pathogenic strains based on their ability to cause severe disease in poultry 4

Critical Clinical Distinctions

Highly Pathogenic H5N1 (The Most Concerning Avian Strain)

• Sporadic human infections with highly pathogenic avian influenza A (H5N1) have caused severe illness with an overall case-fatality proportion of approximately 60% 1
• Human cases occur primarily among persons with direct or close unprotected contact with sick or dead birds 1
• Severe lower respiratory illness with multiorgan failure characterizes fatal H5N1 cases 1
• Limited, nonsustained human-to-human transmission has likely occurred in some case clusters 1

Other Avian Influenza Subtypes in Humans

• Low pathogenic avian influenza viruses cause a range of clinical presentations from conjunctivitis (H7N7, H7N2) to uncomplicated influenza-like illness (H9N2) 1
• Highly pathogenic H7N7 infections typically present with influenza-like illness or conjunctivitis, though severe infections including one fatal case have been reported 1

Key Clinical Pitfalls to Avoid

Vaccination Misconceptions

• Current seasonal influenza vaccines are NOT expected to provide protection against human infection with avian influenza A viruses, including H5N1 1, 5
• Seasonal flu vaccination does not protect against swine influenza viruses either 5
• However, reducing seasonal influenza risk through vaccination in persons potentially exposed to nonhuman influenza viruses may theoretically reduce the risk of viral recombination 1, 5

Diagnostic Considerations

• Clinicians must maintain high suspicion for avian influenza in patients with influenza-like illness who report direct contact with sick or dead birds 1
• Human infection with novel or nonhuman influenza A virus strains is a nationally notifiable disease requiring rapid identification and investigation 1
• The clinical presentation of avian influenza can range from asymptomatic infection to severe respiratory failure, making exposure history critical 1

Transmission and Pandemic Risk

Current Transmission Patterns

• Avian influenza viruses derived from strains circulating in poultry have the potential to recombine with human influenza A viruses 1
• To date, no evidence exists of genetic reassortment between human influenza A and H5N1 viruses, though this remains a theoretical concern 1
• Aquatic birds transmit influenza A viruses through fecal-oral transmission in untreated water 3

Species Barrier Considerations

• Avian influenza viruses are transmitted to humans less frequently than seasonal human influenza A strains due to species-specific adaptations 3, 6
• Pigs can serve as intermediate hosts where avian and human influenza viruses may reassort, potentially creating pandemic strains 3