Okay, here are my thoughts....
First don't panic. Do NOT assume this is Marek's and your flock is doomed, doomed, doomed.
Without a necropsy (which obviously you can't do on a live bird) there is NO way of knowing for certain if this is Marek's. The best you can do is make an educated guess without lab tests to rule out other possibilities.
In that line of thought...
I personally think Marek's is less likely in your RSL Ginny for these reasons:
1. RSL are bred to be Marek's resistant
2. She was vaccinated for Marek's (which does not offer full immunity but does help)
3. She is still able to lift the leg rather than have it drag behind or hang limply
4. No one else is exhibiting symptoms (but that is not a definitive test by any means but if you have 2 or 3 develop limping symptoms at the same time, that IS indicative)
There are other things that present with limping: Injury, vitamin deficiency, toxins, or some kind of muscular/skeletal defect. I will link the Great Marek's thread from BYC below that really discusses all the alternatives:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
As to my own experiences, I have had 2 birds, my Barnevelder rooster and his daughter exhibit limping. For the rooster, he exhibited at about 16 weeks of age, limped for several weeks, then was completely whole. I attributed it to fishing line I found unfurled. No other exhibited symptoms.
His daughter, (RSL hen/Barnevelder) hatched about a year later, did the same thing though, at about the same age, on the same leg. Hmmmm. Attributed it to being just introduced to the flock and madly scurrying away as she was lower on the pecking order (I had witnessed that behavior). No others exhibited symptoms.
About a year later, I lost another of his daughters, a Cream Legbar (hen)/Barnevelder to suspicious behavior. She was fine, then one day hobbling on the ground unable to walk, flapping to move from one spot to the next in a classic Marek's behavior. She was about 10 to 12 weeks of age. We were having a heat wave so I hoped it was possibly lurking coccidia, as I had had symptoms of that in a prior chick batch in that coop, with dehydration, but no one else exhibited symptoms.
A winter passed, that coop was cleaned then left fallow with hard freezing cold for at least 4 months. I replaced my remaining broody bantam Cochin hen (the other was killed by a hawk during this time) with a purchased Silkie pullet (who had been first isolated in that coop before introducing my remaining bantam to her).
After 4 months, this Silkie pullet exhibited classic Marek's symptoms. She limped with drawn up limp leg that did not react to stimulation (weak gripping). Though very suspicious and classic, I hoped for lurking coccidia and/or parasite build up, I treated her for both (amprolium and Ivermectin). She rallied quickly and within a week was walking normally. I was breathing a sigh of relief several weeks later (even setting eggs under her coop mate) when I found her early one morning, about 3 weeks after initial symptoms had fully resolved, flapping her last torturous flaps in the run with twisted neck. She died before I could put her out of her misery.
I am now marking off the days to see if anyone else comes down with it considering my flock "closed" as no one is moving in or out. It has been about 4 1/2 weeks since her death.
This Silkie had help nanny 2 Cream Legbar chicks (which are now about 14 weeks of age) and I also have 2 mixed Barnevelder chicks that hatched just after her demise, all in that run together with the bantam Cochin who was her flock mate.
This brooding hutch has fence exposure to the flock for integration. I also have five 16 week old Barney mixes hatched in another coop but with dander and fence exposure.
To date, no one is showing any symptoms. I am hopeful that the Cochin is resistant. I purchased her as a young pullet from a turkey farm with resistance in mind (the vaccine is developed from the milder turkey virus, which almost all turkeys have...an old timer's trick was to raise turkeys with chickens to protect from Marek's). I am hopeful my 2 Barnevelder babes are resistant as I think the father may have exhibited a mild case and fully recovered. We will see about this strain of Cream Legbar from a different line, purchased breeder eggs (2 hatched of the 6 set).
All told? Marek's is an illusive disease. Its virus is morphing so that it can be mild or virulent. You can have different strains in the same flock.
As most BYC folk have mixed flocks of different breeds, often vaccinated, you likely won't see the flock wide devastation that can happen in large commercial flocks of a single heritage. (Sort of like the blight killing all the potatoes in Ireland as they were all the same strain of potatoes).
So even if your Ginny has Marek's, it is not a death sentence to the rest of the flock. Most literature I read says that Marek's is so prevalent in the environment that you must consider your flock exposed, which begs the thought that healthy resistant birds are not a risk to other healthy resistant birds.
Since the vaccination is "leaky," preventing the symptoms of Marek's but not the disease itself, many in the industry feel it is best to simply breed for resistance and are not recommending vaccinations any more to prevent a Super Marek's strain.
So Marek's is not the end of the world for your flock.
As to Ginny, my gut instincts tell me it is not likely Marek's. She does not want to use that leg rather than the leg is not usable. I suspicion injury or muscular/skeletal issues such as weak tendon groove or hip joint.
But time will tell. She may rally and be just fine. Keep TLC care up. What you are doing is very helpful.
Also add garlic in her food mash. If you are really attached to her, and you want a "Hail Mary" pass, put 2 tabs of 700 mg St. John's Wort 3% hypericum into her mash once a day for 5 days.
There is actual research which shows that St. John's Wort can help resolve viral diseases in chickens. The link below shows it being used for Infectious Bursal Disease and was injected rather than ingested which is different than tabs in food (but injection of pure hypericum is not something I am able to do), so I will go herbal tablets in mash. There is also a LOT of colloquial anecdotes about St. John's Wort and Marek's that shouldn't be taken seriously, one repeating BYC article is quoted again and again that shows no evidence of cure or even that Marek's was originally present.
But, if I have it, and it does strike (again) in my flock, I will resort to St.John's Wort to see if I can help those affected before culling.
I wish you the best for your little flock.
LofMc
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3384280/
