The bio-security that I do is far from perfect!
I try not to bring in started bird from non NPIP places, but have done so and probably WON'T in the future. And I have bought CHICKS because Marek's can't be passed to the egg... only to find out that some BLEEP BLEEP's actually let their broody hatch them and stole the chicks!
So if I do get chicks from non NPIP places I will be sure to ask if they were broody hatched. However... Marek's is NOT part of the NPIP standard.
I don't hang bird feeders to avoid attracting "extra" birds or wild life to my yard. They are still a major problem though!
And my dogs chase of anything that doesn't belong here. Keeping rodent populations under control I think is also important.
I hadn't really thought about passing something on from my place.. until we were at the feed store picking up chicks a while back. And the signs say "wait for an employee to help you." Well, I'm there all the time and lot's of experience so I thought, meh I will help myself and save the employee the time plus I get to pick my own chicks better that way. Before long my brain clicked and I realized that it was also to protect the birds from disease contamination from my place! Just imagine if everybody showed up and handled the chicks. Nope, I don't wash my hands before going to the feed store. So even if rules don't seem to make sense, should probably abide by them as there are likely things we don't yet realize.
Prior to that and you mentioning not tracking it off your property, I hadn't really ever considered my place as being a source for disease.
No I probably still won't change my shoes when I leave the house. But I will definitely follow feed store rules better and be sure not to go visit other farms with fowl.
Also, once a bird leaves my property, it cannot return unless processed already.
My chick that did catch Marek's was quarantined away from the flock so the viral load wasn't going back into the flock. And I will likely NOT vaccinate for Marek's in the future... because, it's 90% effective IF given properly. And vaccinating just means your birds MAY develop LESS symptoms. This link gives me enough reason to consider not vaccinating...
http://www.pbs.org/newshour/updates/tthis-chicken-vaccine-makes-virus-dangerous/
A good looong and informative read, if you didn't already find it...
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
Once I realized I had Marek's on my hands (and not a vitamin deficiency) I started handling that bird last to help prevent spread to the flock.
I pick up poo from my pasture daily.. or we would be in the mountains! Currently 72 birds. I clean my droppings boards weekly. I scrub my water dishes with a brush when they get green. I refresh nests if I notice poo on eggs (broody hens and chicks). I bring in fresh sand to the coop maybe twice a year.
I am taking a hen to the vet to check for mites today. If confirmed I will clean out all the hay from lay boxes and what got pushed out to the ground when I treat. And consider doing that twice per year maybe after my heaviest parasite load season or something like that. It's an ongoing learning process. And holy cow, can things ever go bad fast when you have a large flock!
I hope to get to the point of recognizing tumors during processing!
What I wonder is why even call Marek's birds safe to eat if they are likely to have tumors (as indicated by my research tumor pressing on nerves causing paralysis) and it's illegal to eat birds with tumors.
I actually like selling birds from my flock, because I consider them to be good birds that are well cared for and I love what I'm doing, it makes room for different birds. But I am considering only selling chicks/eggs that have not been out of the brooder in the future, since I can't know everything and consider my word to be important as well as having HEALTHY birds. But if I achieve my breeding goals for FBCM then my birds will be exceptional quality!
Simple fact is my birds are free range. I live in the PNW where wildlife IS the norm which makes it way more challenging than when I lived in the city in So Cal. Plus many of my neighbors have chickens and I have no control over what they bring to their property. Back when I decided to get chickens, I knew I have sooo many plants considered toxic that I cannot possibly get every one out. I had to make the decision to worry all the time or do my best and forget the rest. Well, I've not YET lost a bird to plant poisoning. Nor do I stress every time I see a plant that doesn't belong. I do still work diligently to get them out if possible though. And that is going to have to be the stance I take on Marek's I think. Now I'm still worried about it sweeping through my whole flock in the next few weeks!!! Anyways, I'm gonna do my best and forget the rest. Yet be more prepared to act fast in the future.
I just didn't *think* parasites or disease was going to happen since I take "good" care of my birds. So first suspicion was vitamin deficiency. How foolish for me to think my birds won't succumb to environmental factors. But now I know instead of just looking for indicators.. I must be proactive. Which will be easier with practice and learning what to look for. Experience is the mother of ALL teachers!
Regarding tracking things on your shoes... I brought my pup in to get vaccines at the vet and let my dog walk in to the facility. The staff said pick the pup up! My response was "oh, I thought my shoes were tracking it anyways." They told me a small track on your shoes is much different than the pup being down their sniffing it directly up. Not exactly the same, but maybe still a little relevant.
However, when I volunteered at the zoo rehab center.. we had shoe sanitizing stations that you just dip your shoes and keep going, if it was important to you.
Hang in there. 
