One of the main things with an emaciated snake is to prevent further dehydration. If they are in such a shape that they won't eat on their own, they likely won't drink on their own either. Here's what I would do:
1. Get the snake hydrated. Soak him for an hour or so in clean water, and watch him and see if he tries to drink while he's soaking. If he doesn't drink, then you need to get some water in his stomach. Don't overdo it. If you can get a couple of CCs of water into him every 12 hours or so, that will help immensely. If you have a 2cc syringe that's perfect. You will also need some sort of tube to attach to the syringe. A red rubber French Catheter is best, but small diameter aquarium tubing will work in a pinch if the snake isn't too small. I have had some success just letting the snake drink from the tip of the syringe, too. If you end up using a tube, then make real sure you don't insert the tube into the snake's glottis (the tube in the bottom of hs mouth where his tongue is).
If he has severe folds of skin on his tail, that indicates an extreme dehydration and loss of body weight. When a snake loses about 20% of his body weight due to dehydration, he is in real danger of renal failure. A snake does not eliminate uric acid in the same way mammals do. Mammals eliminate uric acid in a soluble form, so our urine comes out as a liquid. With snakes, it comes out in the form of uric acid crystals (the white chalky stuff you find in the bottom of the cage). Their kidneys need plenty of water to accomplish that. When they dehydrate, they can't eliminate the uric acid and it starts to build up in their kidneys forming uric acid crystals in the tissue. Once that happens, generally the snake does not recover.
It would be a real good idea to give him some fluid subcutaneously. You will need syringes and needles and either some sterile saline or Lactated Ringer's. Those things you will need to get from a veterinarian. I usually give about 2-5% of the body weight every 12 hours.
You don't want to feed him until you get him hydrated. When you feed a dehydrated snake, you trigger the production of more uric acid, which he already can't eliminate effectively and you just compound the problem.
Once you get him hydrated again, then you can make a slurry to feed him by intubation. You will need the syringe and French Catheter for this. Here's how to make a good slurry:
You will need a food processor for this and a fine mesh strainer.
1 egg
6 frozen/thawed pinkies
2 tablespoons Jump Start (see Colchicine's post)
3 or 4 tablespoons Pedialyte
(optionally you can add some B-complex vitamin supplement to stimulate appetite)
Put that all in the food processor and process it until it is completely liquefied. Then press it through the strainer to remove the bone bits and pieces that won't pass through the syringe and tube.
When you tube the snake, don't feed too much at one time. After you withdraw the tube, hold the snake with it's head elevated for a few minutes. This will keep the slurry from running back out of his mouth after you release him into his cage. You can keep the remaining slurry in a small tupperware container for 4 or 5 days in the refrigerator. I'd feed him a small amount (I don't know how big the snake is so I can't give you realistic amounts) every 2 days for 3 feedings, then increase the amount and decrease the frequency so that you are feeding him larger amount every 5 days.
Alternately, you can use a Pinkie Pump if you have one. They are relatively expensive and most people can't justify purchasing one because they just won't use it that much. If you use a Pinkie Pump, use extreme caution, they can build up fairly intense pressure that can be released suddenly causing damage to the snake's stomach and esophagus.
You will want to continue the hydration supportive therapy while you are tube-feeding him also, especially soaking and subcutaneous fluids.
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We do not inherit the Earth from our ancestors; we borrow it from our children. Ralph Waldo Emerson