I'm in the north east of England so experiencing the same weather conditions.... mild and wet.... yuk!
Marek's is spread via dander dust from an infected bird which is then inhaled by other birds. It can be carried on the wind, on birds feathers, on your clothing, shoes and hair. Possibly even as simply as picking up a bag of feed at the feed store that someone else, whose flock has Marek's, has brushed against..... it can be pretty much everywhere. A bit like cold sores with people, birds can be infected and appear perfectly healthy but are carriers for life and shed the virus when they have an outbreak. If however, you have added new birds in the past few months, they are the most likely source.
6 months is a prime time for it to exhibit especially at this time of year and adolescent cockerels have almost certainly been a significant factor triggering outbreaks in my flock.
Mine started with a lame pullet and I too assumed she had injured herself as I have very high roosts, but a few days later I had two more lame. Some got better pretty quickly (a matter of days) and a percentage of those had secondary attacks up to a year later. Others deteriorated and had to be culled. I also had some young birds which died unexpectedly without lameness which I now suspect were also Marek's.... perhaps visceral tumours that caused heart attack. When I have time. I now do post mortem exams on the ones that die as it helps me understand the disease and how best to deal with it's vicitms.... I feel better when I have culled a bird and then find huge tumours inside it....it kind of vindicates my decision.
This last season I have only had only one chick exhibit symptoms out of 28 broody reared chicks, 5 of them have just started laying and I've processed 8 of the cockerels that were all healthy. The sick one presented with a dropped wing this time. It has improved and is holding it's own but not growing and thriving although it eats and poops normally,.... it still free ranges with the flock though the day though, so not an invalid as such but I put it in the infirmary at night where it has easy access to food and water without risk of bullying..... I've found that Marek's sick birds benefit hugely from the company of other chickens.... having 2 sick with it at once is a blessing as they motivate each other to eat and fight it. Isolation causes them to get depressed and they eventually give up fighting it. It affects the immune system, so being happy and eating well is very important. I had one last year that was on her side and couldn't get up. I had her in the infirmary which is inside the hen house but through the day she was in there on her own and after a couple of weeks I was on the point of culling her.... she was propped up in a nest and constantly soiling herself etc. O put another lame pullet in with her one day and to my shock and amazement she attacked it, despite being badly disabled. Thankfully, once they sorted out the pecking order they became firm friends and both showed dramatic improvement over the next few months, so that when spring came I was able to put them out on the grass in the sunshine in a cage.... after that improved even more quickly and a month or so later they were gimpy still but free ranging with the flock and even laying the odd egg....just so you know there is hope.
Marek's birds will often be bright eyed and eat well but waste away as the tumours develop. I had to euthanize a legbar cock a few weeks ago that had his first attack a year ago and fully recovered a few days later. He has been absolutely fine for a year but went down with it again a month ago and this time he succumbed. You get to know when to give tlc and when it's time to end their suffering. Usually, if they are keen to eat and show some fight, I will give supportive care.
Anyway let's hope it isn't Marek's but at least you are now reasonably genned up if it is. Hopefully I have answered all your queries on it but shout up if I missed anything. There are several extensive threads about Marek's on this forum and from reading those and talking to other poultry people here in the UK, it seems that the Marek's here is not as virulent, whereas people in the US have lost over 50% (some 100%) of their young birds to it.
Edited by rebrascora - 1/4/16 at 7:37am