Studies show that exposed birds, even though they survive and are resistant still carry the virus in their dander.
This quote is directly from the Merck Veterinary Manual.
Transmission and Epidemiology
Marek disease is
highly contagious and readily transmitted among chickens. The virus matures into a fully infective, enveloped form in the epithelium of the feather follicle, from which it is released into the environment. It may survive for months in poultry house litter or dust. Dust or dander from infected chickens is particularly effective in transmission. Once the virus is introduced into a chicken flock, regardless of vaccination status, infection spreads quickly from bird to bird. Infected chickens continue to be carriers for long periods and act as sources of infectious virus. Shedding of infectious virus can be reduced, but not prevented, by prior vaccination. Unlike virulent strains of Marek disease virus, which are highly contagious, turkey herpesvirus is not readily transmissible among chickens (although it is easily transmitted among turkeys, its natural host). Attenuated Marek disease virus strains vary greatly in their transmissibility among chickens; the most highly attenuated are not transmitted. Marek disease virus is not vertically transmitted.
End of quote.
Then there is Typoid Mary to consider who was exposed to Typhoid Fever, never developed the disease but spread it to everyone she met because she was a carrier.
Chicken Pox lives on in the host's body, sometimes colonizing at the base of nerves to rear it's ugly head years later as shingles. The only way a person can get shingles is from the herpes zoster virus. Now, shingles aren't catching but if a person who hasn't had chicken pox comes in contact with the fluid from the blisters caused by shingles, then they can come down with chicken pox. Ask me how I know that one....which I learned the hard way and I'm a retired nurse, LOL. Chicken Pox can also be 'silent' in nature. Meaning you can have them and not know you have them. The symptoms might be really mild and you may even not have a rash...once again ask me how I know that one.
To think that resistant chickens cannot spread MD is a misconception of the disease. I will put it this way. IF you have Marek's disease in your flock, confirmed by a medical professional, then your entire flock is carriers of the disease whether or not they show symptoms. Resistant birds are carriers. As I stated earlier, resistance does not mean immunity. Resistance means they are less likely to be killed by the disease then birds that are susceptible.
My Egyptian Fayoumis, while resistant to Marek's Disease, are still carriers of the disease that infected our property. Vaccinated birds while resistant to the disease are still carriers of the disease.
Some breeds of chickens are more susceptible than others. Silkies, Seabrights, Cochins, Buff Orpingtons to name a few are VERY highly susceptible to Marek's Disease. Other birds like Fayoumis and naked necked Turkens are resistant to the disease. The closer the bird is to it's wild state (one of the reasons they suspect a Fayoumi is more resistant than a hybrid specialized breed) is because their genetics are closer to chicken genes in the wild state.
I suspect that Marek's Disease is a byproduct of science and breeders trying to create chickens that produce large amounts of eggs and look pretty while they are doing it.
