H5N1 transmission to humans: poultry exposure dominates, but environmental and cattle pathways are increasingly documented

A new review of 120 scientific and grey-literature sources provides one of the most detailed maps yet of how highly pathogenic avian influenza A(H5N1) has been reported to reach humans. Animal-to-human exposure dominated the evidence, particularly contact with poultry, but the review also identified environmental exposure, cattle-associated transmission and a smaller number of suspected human-to-human events.

Poultry remains the main documented pathway

Animal-to-human transmission was reported in 109 of 120 sources, or 90.8%. Within this group, 100 sources (83.3% of the total) described poultry-to-human transmission, confirming that direct or indirect exposure to infected birds remains the dominant pathway documented in the literature. Activities associated with exposure included handling sick or dead birds, slaughtering or defeathering poultry, preparing carcasses, working in live-bird markets and contact with contaminated materials or surfaces.

The review distinguishes the exposure event from the actual route by which virus enters the body. Possible entry routes include the eyes, nose and mouth, inhalation of droplets or aerosols, ingestion and percutaneous exposure through damaged skin.

Cattle add a newer transmission interface

Nine of the 120 sources, or 7.5%, reported cattle-to-human transmission pathways. This category reflects the emergence of H5N1 in dairy cattle in North America and the subsequent infections detected among exposed farm workers. Although numerically much smaller than poultry-associated transmission, it represents an important change in H5N1 epidemiology because cattle create repeated and prolonged contact between infected mammals and people.

The review therefore captures the transition of H5N1 from a predominantly avian occupational risk to a broader livestock and farm-environment problem.

Environmental exposure appears in more than one quarter of sources

Environment-to-human transmission pathways were reported in 32 sources, or 26.7%. These included contact with contaminated surfaces, water, faeces, secretions, equipment and other environments where infected animals had been present. This matters for prevention because exposure does not necessarily require directly touching an infected animal. A single event may combine several mechanisms, for example, handling infected poultry while simultaneously inhaling contaminated droplets or touching the eyes with contaminated hands.

Human-to-human transmission remains exceptional

Fourteen sources (11.7%) reported possible human-to-human transmission pathways. However, the authors stress that the presence of such reports does not demonstrate efficient or sustained person-to-person spread. That distinction remains fundamental. H5N1 continues to cause predominantly zoonotic infections, and sustained human transmission has not been established.

Exposure pathway does not equal proven transmission mechanism

The authors repeatedly caution that the mapped routes are reported or biologically plausible pathways, not estimates of relative risk and not necessarily proof of causation. The review therefore cannot say that one route of entry is more likely than another. What it does show is that H5N1 exposure is more complex than a simple “contact with infected birds” model.

For occupational protection, that supports measures addressing several routes simultaneously: gloves, eye and respiratory protection, hand hygiene, environmental decontamination and reduction of direct contact with infected animals.