I don't think you understand how Marek's spreads or presents. If a bird has it, even if there are NO symptoms then in all likelihood the entire FLOCK will be exposed....
Killing a sick bird does little or NOTHING to protect the other flocks members from Marek's.... Plenty of chicken breeders around here have told me they believe they have seen Marek's in their flocks, that means they ARE breeding for resistance as the birds that did not get sick but were exposed DO have some natural resistance and the birds that are very sucsceptible will like get sick and die of it....
I would expect that every chicken not hatched and living in an isolation chamber all its life has been exposed to Mareks Disease at sometime in its life. Mareks Disease is rampant in the wild bird population and in every nook and cranny of every chicken coop. Furthermore there are several, maybe up to 5 or more strains of the Mareks virus and each strain requires its own separate vaccine. Normally only the vaccine for the strain most prevalent in your local is administered at the hatchery. I mentioned that I though that a Mareks test would never be available. However a blood test for the active form of Mareks will prove the only way to breed Mareks resistant chickens. Without this test you could just as well try and breed a red chicken without ever knowing what color the feathers are on every generation of fowl that you breed. In this case how dose anyone know if they are making progress toward the goal of a Mareks free flock? You can't because the only goal that matters is if your chickens are currently Mareks free, X # of days after being exposed to the Mareks virus or viruses. Besides the total load of the Mareks virus that a hen is exposed to seems to play a large role in whether a chicken will develop active Mareks Disease. In this case culling a Mareks Mary who is shedding the Mareks virus in large amounts seems like the wise thing to do.
As for testing for Mareks Disease, the only diagnostic test currently in use for Mareks is a Necropsy of any chicken that is suspected of dying from an active case of Mareks. The Necropsy looks for enlarged nerves and a special type of tumor or cancerous growth that is associated with Mareks Disease. As far as I am aware, no blood test yet exists like the specific antigen test used to identify Pullorum - Typhoid disease. In fact the Mareks virus is so widespread that it should be found in virtually every chicken tested for Mareks.
There is however a wonderful tool available to geneticist that should provide chickens with complete immunity against Mareks. The problem with this tool is that it will goad every nut case to flop around like a fish out of water screaming "Franken Food, Franken Food, Franken Food." Of course I am talking about DNA Sequencing, otherwise known as "Genetic Engineering." In 5 to 10 years time Mareks could be totally eradicated from domestic chicken populations but only at the cost of possibly reduced sales of domestic chickens and eggs.
Currently there is a Genetically Modified rice verity that has been awaiting approval for 15 years. This rice verity is called Golden Rice and Golden Rice has beta carotene, a precursor of Vitamin A. No one like the evil Monsanto is even proposing to sell this GM rice verity to poor small farmers in Third World countries. They want to give it away to the Third World, like for free. Third World farmers would even be allowed to save their Golden Rice seed from year to year or crop to crop and replant Golden Rice to their hearts content. However during the 15 years that Golden Rice has been in existence certain groups has prevented its release. During this time 45,000,000 innocent children in impoverished countries have died, or became permanently blind from Vitamin A Deficiency.
The reason I mentioned this is because Genetically Modification seems like the perfect way to confer total Mareks resistance to chickens. The only reason that I can see that this isn't being currently attempted is because if a group of people (like those resisting the release of free Golden Rice seeds to the world's poor) would condemn 30,000,000 human children to a life of perpetual blindness and the even more everlasting death, how much more happy will those people be to condemn the lowly chicken to a slow death from Mareks Disease by opposing raising and eating a chicken variety or the eggs from a chicken variety that has undergone Genetic Modification?
