Chickens Dying (?) Help!

[Chickenllady]

Yesterday we had a snake attack on our flock. We believe it was a rat snake, and it was about five feet long. I found it curled up in one of our nesting boxes and there were four dead chickens, all of which had dark blue-purple combs. They had no puncture wounds.
We got rid of the snake and I assumed the snake may have attacked them, but this morning there were more dead chickens- three, to be exact, all with the same symptoms of the others. I found one that was still alive and carried him out of the coop to try and get him to feel better, but at the time his comb was also dark blue, wings and tail were low, and he was sneezing.
I have no idea what is going on, and i've tried searching it up, but to no avail. Yesterday the chickens that died showed no signs of injury or ailment, and the rooster I was talking about above was doing perfectly fine, as were the other two. Please help!
We have about 60 chickens (it's probably down to fifty now) and three or four different coops which are quite large. We keep them as clean as possible and I check them about 3-4 times a day. I spend a lot of time with them so I would immediately notice any symptoms, but so far I haven't noticed any at all. Any thoughts on what might be happening, and how we could prevent what it is?
We do not have the option to contact a vet- no vets around our area for about 3 hours will treat chickens, so this is our last hope, unfortunately. I love my birds and I would hate to see any more pass.

 

[Nambroth]

Have you brought in any new birds in the last six months or so, or have you been to any sites where others keep poultry? You mentioned sneezing; was he the only one sneezing? Any discharge from the eyes/nose/mouth, foamy eyes, or breathing sounds (rasping, gurgling, coughing, etc) from any of your birds?

 

[Chickenllady]

Have you brought in any new birds in the last six months or so, or have you been to any sites where others keep poultry? You mentioned sneezing; was he the only one sneezing? Any discharge from the eyes/nose/mouth, foamy eyes, or breathing sounds (rasping, gurgling, coughing, etc) from any of your birds?

A while ago we brought back a rooster that had breathing issues - raspy breathing, gurgling and he had an abscess beneath his eye, I believe. After that we've had a lot of chickens getting sick and passing away, but I have no idea what he could have had. He passed away only three days after we got him.
There's a new rooster we got a bit ago who's comb is turning purple and he's acting sluggish. I'm really hoping I can figure out what this is before it's too late.

 

[Nambroth]

This is not a diagnosis, and trying to figure things out through an internet forum is tricky at best (dangerous at worst if you get bad info!), but based on this it sounds as if you inadvertently brought in a disease, possibly a respiratory disease, with your gurgling rooster. Without knowing more, there isn't much I can suggest for treatment, because it's hard to know what disease you are dealing with. I strongly recommend researching to see what services are available to you in your state (if you are in the US) for doing diagnostics on a deceased bird, and if you lose another, send it off for necropsy so that you can get an answer as to what might be making your flock sick/killing them. Here's a list to labs to get you started: http://www.metzerfarms.com/PoultryLabs.cfm
How to send a bird for necropsy: https://www.backyardchickens.com/articles/how-to-send-a-bird-for-a-necropsy.73562/