To say "there is no cure," one goes beyond the realm of scientifically supportable statements, because one of the cardinal rules of science is that it is not possible to prove the non-existence of something. I choose to believe there is a cure, even if we may not have found it yet. "Proof" is also rather difficult to come by in matters of science, which is why words like "evidence" and "theory" are so prevalent.There is no cure for Marek's, the hypericum treatment only has anecdotal evidence of effectiveness, no proof.
From what I have understood, I believe you are correct here.Any bird carrying Marek's will spread it
I also understand this to be correct.It's hard to kill outside of a host, it can withstand extreme temperatures and dry air and live in the soil for potentially up to a year (lots of debate on the actual timeframe, but the consensus is that it is highly persistent).
And this is thanks to the Marek's vaccine which has greatly furthered the spread by keeping the carriers of the disease alive to shed the contagion to all their neighbors.It has become ubiquitous, any State lab or knowledgeable vet will agree.
Now, cure or no cure, if a chicken develops actual immunity to the disease, in theory, at least, that chicken should no longer be shedding the virus. This would be because immunity entails having an immune system that correctly recognizes, responds to, and defeats or neutralizes the enemy. For example, I had chicken pox as a child (a herpes virus as is Marek's). I am not spreading chicken pox to all of my contacts. I am immune, and do not become symptomatic again when in contact with others who are suffering an acute case of it. Naturally, the mode of transmission differs between chicken pox and Marek's, but immunity has always meant that, in addition to being protected oneself, one also does not become a vector of transmission for others, and if one were immune to Marek's, continued transmission would cease.
(This is the point of the supposed "herd immunity" that true vaccines are supposed to confer.)
But Marek's vaccine is a "leaky vaccine," one that does not confer immunity. And just here is where an interesting phenomenon of the immune system is worth consideration--one of which the vaccine appears to take advantage.
The immune system will typically exhibit symptoms for just one disease at a time--even if the body hosts multiple simultaneous infections. For example, suppose a person were infected with both dengue fever and the common cold (influenza) at the same time. It is likely that the cold symptoms would be trumped by, and masked by, those of the dengue fever--which was the greater danger. Or suppose one has a cold prior to the onset of gastroenteritis. The gastroenteritis might make it appear that the cold had vanished, as its symptoms came to the fore. Many people would be hard-pressed to think of a time when they were sick with multiple agents at the same time because the immune system typically is symptomatic for only the most urgent of them, and they would not notice the others.
The Marek's vaccine prevents the chickens from becoming symptomatic--even should they acquire the disease. And these are more than mere "carriers" of it--they are "super-spreaders."