When we lost one of our chicks to questionable Marek's a few weeks ago I was in touch with a vet from Cornell Veterinary College and talked about disinfecting my coop. (She was listed as a good contact person in some literature I had gotten from my local cooperative extension.)
I asked about Virkon S (which had been suggested by someone I knew who has had chickens, ducks, etc) and she said that it's a very good product. It has a long list of pathogens that it is effective against (including Marek's, Coccidiosis,Bordatella---even Parvo) works quickly (needs only 5-10 minutes of contact time), and you can move the chickens back in after it dries. In lower concentration it can even be used in their drinking water so I am figuring that it is pretty safe stuff.
Since Marek's can last up to 65 weeks in the environment I wanted to do all I could to reduce any exposure that my remaining chicks might have.
I bought it on
Amazon (it's cheaper elsewhere but I wanted faster delivery so I could get going on cleanup). It comes in powder form and tabs---I bought the tabs. Fifty in a bottle and each makes 16 oz of solution. I think I ended up using six or eight of the tabs---plenty left for any future issues. I think the bottle was about $27 but I have seen it for about $10 less on other sites.
I used a spray bottle and saturated all surfaces after scrubbing the surfaces with soap and water. The inside of my coop is painted so cleanup was relatively easy but Virkon S can be used on porous surfaces as well. I cleaned all my chicken stuff with it too---feeders, brooders, etc. Yeah, I am a nut, lol.
I am by no means an expert in chicken keeping---this is my first bunch of chicks so I may be overdoing it here but I am trying to do my best. Others here have far more experience with these things but I thought I would share.
Hope this helps.
Do you know what happened to the chickens that were in the coop before?