Marek's is shed in feather dander mostly, and birds get exposed to it mostly via inhalation of the dander. So anywhere the wind blows, people walk, chickens fly ect will have the virus. It is fairly hardy in the environment, but sanitation can really help if the thing you are trying to sanitize is non porous (metal, ceramic ect). It is fairly ineffective to try to disinfect organic material (unsealed wood, dirt, unsealed concrete). Moving a coop to the other side of the property is not helpful. Your bet bet IMO is cull symptomatic/suspect birds. Consider culling all of the birds from the bad source. Only bring in older pullets that are properly vaccinated (meaning vaccinated prior to exposure to actual Mareks, and kept away from the actual virus for several weeks or (better) longer). Older meaning over 4-5months old, after they have some age related immunity. Unvaccinated day old chicks are the worst thing to bring in. If you want to bring in chicks, hatch and vaccinate them yourself, or purchase vaccinated chicks from a hatchery- raise them far away from your existing flock and coop. Like in your house, or at friends house. Practice good disease control measures- hand washing, changing shoes, consider changing clothes ect.
