HPAI biosecurity in poultry farms: small operational details may determine whether farms are repeatedly infected

A German case-control study suggests that overall biosecurity scores may be less informative than specific weaknesses in day-to-day farm management. In Lower Saxony poultry farms repeatedly affected by highly pathogenic avian influenza, two factors stood out in multivariable analysis: poorly maintained vegetation around poultry houses and the absence of dedicated systems for removing carcasses without reopening or crossing sensitive hygiene areas.

Lower Saxony is Germany’s largest poultry-producing state, with around 77.6 million birds, and has also been one of the country’s most frequently affected regions for HPAI. Between October 2025 and February 2026, it recorded more than twice as many poultry outbreaks as North Rhine-Westphalia, the second most affected German state. The study deliberately focused on farms with a history of repeated infection. Farms were classified as cases if they had experienced at least two confirmed HPAI outbreaks between 2016 and 2023, while controls had no outbreaks during that period. Ultimately, 15 case farms and 26 control farms were included.

Overall biosecurity scores did not separate affected and unaffected farms

Researchers assessed each farm using a specially developed checklist covering eight areas: farm premises, hygiene sluices, protective clothing, poultry houses, feed and litter storage, pest control, carcass storage and documentation. The maximum score was 76.5 points.

Case farms averaged 42.2 points, compared with 45.3 points in control farms, but this difference was not statistically significant. It suggests that a farm can have a broadly acceptable biosecurity profile while still retaining one or two critical vulnerabilities capable of allowing viral entry.

The authors note several possible explanations: farmers with better biosecurity may have been more willing to participate, farms may have improved after previous outbreaks, and strong infection pressure may occasionally overwhelm even relatively good biosecurity.

Vegetation around poultry houses emerged as a key factor

One of the strongest findings concerned grass management. Control farms were significantly more likely to maintain short-cut grass around poultry houses, whereas repeatedly affected farms more often had taller vegetation. In multivariable analysis, short-cut grass remained strongly associated with lower outbreak status. The mechanism is biologically plausible. Tall grass and dense vegetation can provide habitat for rodents and small wild birds, both of which may mechanically or biologically contribute to pathogen introduction around poultry buildings.

The authors note that previous research has also associated infrequent mowing around poultry farms with increased H5N1 risk. They therefore recommend maintaining short vegetation particularly ahead of and during the colder months, when HPAI activity typically increases.

Carcass removal may represent an overlooked biosecurity breach

The second major factor was how dead birds are removed from poultry houses. Only 33.3% of case farms had dedicated carcass-removal facilities, compared with 69.2% of control farms. The presence of such facilities remained associated with lower outbreak status in the multivariable model. This finding is particularly interesting because the authors found little previous research addressing this specific issue.

Dedicated flaps or openings allow dead birds to be removed without opening the main poultry-house entrance or carrying carcasses through a hygiene sluice.

Without these systems, carcass removal may repeatedly create opportunities for cross-contamination through footwear, equipment, hands or surfaces. Opening the main poultry-house gate may also create a direct route for environmental contamination to enter.

Feed, litter and manure deliveries also matter

Several additional factors were associated with outbreak status in univariable analysis. One concerned storage design. Farms where suppliers had to enter the clean area to deliver feed, litter or access manure storage were more likely to belong to the repeatedly affected group. Storage configured so that suppliers could operate without entering clean production areas was associated with lower outbreak risk.

This reinforces a fundamental biosecurity principle: each additional vehicle or person crossing into the clean farm zone creates another possible route for virus introduction.

The authors therefore recommend designing storage and delivery areas so that external suppliers remain outside sensitive production zones whenever possible.

Shoe-disinfection trays produced a surprising result

The study also found a more unexpected association. Farms using disinfection trays rather than mats had significantly higher odds of belonging to the case group in univariable analysis. One explanation may be behavioural: trays may be more cumbersome, less consistently used, or poorly maintained. Farms with both mats and trays did not show the same association. Previous studies have also shown that neither footbaths nor disinfectant mats should be relied on as stand-alone biosecurity measures. Changing footwear may be more effective than simply stepping through disinfectant.

Proximity to roads produced contradictory signals

Geography produced less straightforward results. Case farms were located closer to motorways but further from federal highways than control farms. Longer distance from federal highways remained significant in univariable analysis, but neither road variable remained in the final multivariable model.

Motorways could correlate with poultry transport or other movement-related risks, while federal highways may simply act as proxies for different land-use patterns, poultry density or wild-bird habitat.

Importantly, the presence of migratory birds near the farm itself was not significantly associated with outbreak status in this small study.

The study also found no significant difference in the total number of conventional biosecurity measures implemented between farms with and without repeated outbreaks. Small changes could still have immediate value.