Mapping Reported Modes of Transmission of Highly Pathogenic Avian Influenza A (H5N1) to Humans: A Scoping Review.

Why it matters. A comprehensive scoping review mapping reported H5N1 transmission pathways to humans across 120 sources, providing structured evidence synthesis to inform public health understanding and prevention strategy development, though causal mechanisms remain unestablished.

Expert analysis

From the abstract

This scoping review identified and classified multiple reported H5N1 transmission pathways across 120 sources, with animal-to-human transmission (predominantly poultry-to-human) most frequently reported (90.8%), followed by environment-to-human (26.7%) and human-to-human (11.7%) transmission. The review maps plausible viral entry routes (mucosal, inhalation, ingestion, percutaneous) but does not establish causal mechanisms or relative likelihoods, indicating a need for mechanistic research to strengthen evidence-based prevention strategies.

Reported figures

Total sources included

120

Studies included

Grey literature included

Animal-to-human transmission sources

109 of 120 (90.8%)

Poultry-to-human transmission sources

100 of 120 (83.3%)

Cattle-to-human transmission sources

9 of 120 (7.5%)

Environment-to-human transmission sources

32 of 120 (26.7%)

Human-to-human transmission sources

14 of 120 (11.7%)

Key findings

Animal-to-human transmission pathways reported in 109 of 120 sources (90.8%)
Poultry-to-human transmission reported in 100 sources (83.3%)
Cattle-to-human transmission reported in 9 sources (7.5%)
Environment-to-human transmission pathways reported in 32 of 120 sources (26.7%)
Human-to-human transmission pathways reported in 14 of 120 sources (11.7%)

Population. Evidence from observational studies and grey literature reporting H5N1 transmission to humans; 120 sources screened from CINAHL, Embase, MEDLINE, Scopus, PubMed, and grey literature up to October 29, 2025, with no language restrictions

For practice. Clinicians should recognize that H5N1 exposure occurs through multiple overlapping pathways (animal contact, environmental exposure, and limited human-to-human transmission) via multiple entry routes, informing exposure assessment and infection control counseling. The evidence synthesized here supports comprehensive prevention messaging but does not yet quantify individual pathway risk or establish causal attribution.

Caveats

Narrative synthesis only; no formal quality assessment or risk of bias evaluation of included studies reported
Entry routes are biologically plausible but do not imply relative likelihood of transmission or causal attribution
Specific risk factors, measures of association, and causal mechanisms for individual pathways not established
Relative prevalence or importance of different transmission routes not quantified; heterogeneous evidence from observational studies and grey literature

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Evidence assessment

A multidimensional read, graded only from the abstract, not a single opaque score.

Study design

strong

Systematic scoping review with duplicate screening and cross-verified data extraction

Directness

strong

Directly addresses H5N1 transmission pathways to humans with comprehensive pathway classification

Consistency

moderate

120 sources synthesized narratively; heterogeneous evidence from observational studies and grey literature

Risk of bias

moderate

Observational studies and grey literature included; no formal quality assessment reported

Sample adequacy

strong

120 sources (70 studies, 50 grey literature) from multiple databases with no language restrictions

Provenance

Source: EuropePMC
Source type: aggregator
Content analyzed: abstract

Evidence grading reflects the portion of the source actually analyzed. Always confirm against the original before acting.

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