Avian influenza culling can itself create human exposure: how wide should ring culling go?

Ring culling is designed to contain highly pathogenic avian influenza by removing infected flocks and nearby poultry before the virus spreads. But a new modelling study highlights a neglected One Health trade-off: every additional flock culled also creates human contact with poultry. In the model, a small culling ring sharply reduced both epidemic size and overall poultry losses, whereas wider rings delivered progressively smaller epidemiological gains while exposing culling teams to increasing numbers of uninfected birds.

The study simulated avian influenza spread among 260 poultry holdings within a 40-km square and tested ring-culling radii from 0 to 8 km, with 300 simulated epidemics for each radius. Human exposure was expressed as hazard-weighted person-days combining three sources: depopulation of infected flocks, depopulation of uninfected flocks within the ring, and routine husbandry on farms remaining in production.

Two kilometres captured most of the epidemiological benefit

Without ring culling, simulated epidemics affected an average of 17 flocks, lasted 36 days and resulted in 439,460 birds being destroyed.

A 1-km ring reduced infections to 12.1 flocks. At 2 km, infections fell to 5.8 flocks, while total birds destroyed actually decreased to 353,816, 85,644 fewer birds than with no ring culling. Beyond 2 km, however, returns diminished rapidly.

At 4 km, infections fell further to 4.0 flocks, but 558,040 birds were destroyed. At 8 km, only 2.5 flocks became infected, but total culling exceeded 1.02 million birds.

Expressed differently, at 4 km each additional flock infection prevented cost about 9,091 birds culled. At 8 km, that figure rose to 40,073 birds per infection averted.

The cull itself becomes a source of exposure

The study’s more unusual contribution is to include occupational exposure in the equation. Depopulating an infected flock is considered a particularly intense form of human exposure because workers spend prolonged periods handling large numbers of birds, often in enclosed environments where viral loads and aerosol generation can be high. But as culling rings expand, an increasing proportion of that work involves farms that were never infected.

At a 2-km radius, uninfected farms accounted for 16% of depopulation-related exposure. At 8 km, that proportion reached 70%.

Under the study’s base assumptions, total human exposure still declined as the radius widened, because rapid epidemic control prevented later infections and reduced prolonged routine exposure. But that result depended strongly on one uncertain parameter: how risky is depopulating an uninfected flock compared with depopulating an infected one?

One missing number changes the optimal radius

The model assumed that depopulating an uninfected flock carried 12% of the relative hazard associated with an infected flock. When researchers varied that assumption from 2% to 50%, the radius producing the lowest human exposure shifted from 8 km to 4 km. The estimated reduction in exposure compared with no ring culling also fell from 73% to 45.7%. The optimal ring radius changes substantially depending on a hazard value that has not been directly measured.

The authors therefore call for serological follow-up of culling personnel, distinguishing workers who handled confirmed infected flocks from those involved only in preventive depopulation.

A model, not a prescription for national policy

The findings should not be interpreted as evidence that every country should adopt a 2-km ring. The model uses an illustrative poultry landscape rather than data from a specific outbreak, and results depend on assumptions about farm density, transmission distance, detection delays and occupational hazard.

Culling policies should be assessed not only by the number of infections prevented, but also by the poultry destroyed and the occupational exposure generated by the intervention itself.

For HPAI preparedness, that means treating culling teams as a specific occupational-risk group and incorporating worker exposure into decisions about how aggressively preventive depopulation should be used.