Chickens were kept free of Marek's disease by ventilating a house with filtered air under positive pressure and restricting what could enter it: articles, materials and personnel all controlled. Across 741 days of operation there was no evidence of Marek's disease virus infection. The control condition is what makes the result strong. Marek's was enzootic in flocks housed on the same farm, so the pathogen was present on site for the entire period.
This is the poultry counterpart to the controlled filtration demonstrations in swine, and it predates them by roughly thirty years. Dee and colleagues showed in 2005 that MERV 14 media prevented PRRSV aerosol transmission in a dual chamber model, and the herd studies that followed took decades more to accumulate. The poultry question of whether filtered incoming air can exclude an airborne pathogen from a house under real disease pressure was answered in 1975, and the answer was yes.
Two qualifications govern how far it reaches. The pathogen is Marek's disease virus, not avian influenza, so this establishes the principle rather than an HPAI-specific result and should not be stretched into one. And filtration was never deployed alone: it was filtered positive pressure combined with strict entry restriction, which is the same integrated-programme conclusion the swine literature arrived at much later. The arrangement it describes survives today as standard practice in specific-pathogen-free and vaccine-egg production, where multi-stage filtration ending in HEPA is routine. What has never happened is the migration of that approach into commercial poultry production at scale.