WHY ?
Reptile's self-defense system against internal parasites is rather low.
Those animals own internal parasite at an endemic state, which means that in a way it belongs to their nature to have some. All wild reptiles are pretty infested, and that's one of their major health problems.
In front of a reptile of any kind (but specially wild caught snakes) the question about internal parasites is not "is he infested?" but more surrely "at what level is he infested?".

Whatever the level is, an infested animal is weaker than a clean one.
At an average or high level the food feeds more parasites than the reptile himself. It means that the growth is much lower (1/3 or more) for a bad infested animal than for a perfectly clean one.
At the extreme stage some kind of worms (nematods) can destroy the intestine or migrate to the lumbs or in the muscles, where they enkist, and at this point the life of the animal turns in real danger.

WHAT?
Unfortunatly there are several kinds of internal parasites. None product can strike against all and only a combination of several ones perfectly deworms.
There's no significant move about these kind of products for more than 20 years and it's always the same old names...

1) Flagyl at 25-50mg/kg
against amibians and protozoans mainly

2) Panacur at 50mg/kg
against nematods mainly

3) Telmin KH at 25mg/kg
against ascarids and tenias mainly

Repeat treatment twice within a week or two.

Note about needed quantity :
Whatever could be the kind (dewormer, antibiotics, etc) or even the name, and due to their cold blood metabolism reptiles need a stronger dose of medecine (around double) to get the same effect compared to hot blood animals.
But in case of doubt it's anyway obviouly better to give not enought (doesn' hurt) than too much (could kill) and to repeat the treatment a week after.

WHERE?
Flagyl and Panacur
Available on some online shops like "herpsupplies.com" for example, or in drugstores.

Telmin KH
Lab is JANSSEN-CILAG, box of 10 pills at 100mg only. Cost around $8.00/box and one single box can afford treatment for one animal for years.
Exist in France but I don't know if it exist also exactly under this name in other countries. If it doesn't exist in your country use something similar (name of the active molecule is "mebendazole".

HOW?
It's always better to avoid as much as possible needle-feeding of any other enforce way to swallow which stress strongly the animal.
As the dewormer has to be diggested to be efficient, the best way is to use a prey to carry it in the stomach.
It's much better to deworm and feed at the same time. It allows to administrate the dewormer by a clever way than enforce swallow and it helps the product to be efficient because the diggest process is more active with a prey in the belly compared to what it is when empty.
The strenght of the treatment is not affected using this process and it's softer for both the animal and the snake-keeper.

1) If the animal eats dead prey

If the product is under liquid form inject the dewormer under the skin of the prey or drop it directly on the fur and then feed the snake with this prey. He will diggest both and doesn't even notice anything.

If the product is under pill form break it in little pieces and drop the needed quantity in the mouth or the hear (may be weird but efficient!) of the prey before feeding. Here also nobody notice the product and everything goes right into the stomach without problem.

If the product is under powder form wet a bit the fur of the pray, give it to the snake and when he has ingested around half of the prey drop the powder on the wet part of the fur. It stays like glued on and here also everything goes right in the stomach. In this case the quantity can be a little bit increased because a little part of it will fall from the prey when the snake shake and move it during swallowing. Don't forget to clean the substrat just after if some product has falled on it!

2) If the snake eats live preys

Idem for liquid or powder.

Necessary to use an as gentle as possible kind of enforced swallow for pill.
Hold the snake behind the head. Tickle softly his nose with a pen or something similar. The animal will open wide his mouth himself. When the mouth is wide open move quickly the pen across the jaws to keep them open. Drop pieces or entire pill (depending on size of both the mouth and the pill pill and quantity needed) in the mouth, just before the pen (check that the pill is not just at the entrance of the hole of the trachea).
Shake just a little bit and softly the pen up and down, and the snake will probably open wide again to try to escape this. Use this instant to move in a glance the pen before the pill. Then push back (as this movment goes in the way of teeth's curve, there's no risk to break some) gently to bring the pill at the entrance of the throat. Don't try to push the pill deep inside, just in front of the throat is enought!
Pull sidely the pen out of the mouth, free the animal and let him quiet for 15 minutes.
Then feed him. The pill will be pushed by the prey into the stomach during swallowing process.

3) If the snake is in a non-feeding cycle
No escape to needle-feeding.

WHEN?
If the animal is older than six months and has never been dewormed (which is obviously the case for a recently wild caughted animal), don’t ask you any question, it's 99% sure that he needs to be. Do it.

After that there's two way to know when the animal needs deworming or not.

1) Fece's analysis
We bring feces to the vet and ask for analysis. Supposed to be absolutly unmistakable if both are serious, but sometimes nematods, if enkysted in muscles or located in the lumbs, are not found in feces, even if the animal is deeply infested.
Anyway obviously it cost the vet and the analysis.

2) Weighting
Let's assume that we're not "chain feeding" (we wait the animal to defece his precedent meal before feeding him again, which is by far the best way to feed anyway).

Let's assume too that we have already perfectly deworm the animal recently.

We weight the snake and the prey(s) just before feeding and memory all that. We weight him again just after he defece that meal and we calculate the "profit ratio".
Profit ratio = prey(s)'s weight / (snake's weight just after feeding - snake's weight just before feeding)
There we've got the profit ratio for a clean animal.

This ratio depends on the kind of animal (snake, iguana, turtle, etc) , on the specie, on the kind of food and also on the age of the animal. It could depend too of the heating. If the animal lays most of the time upon a heat mat (they usually love that) , diggest process is more efficient and the ratio increase a little bit.
This is not a graved in the stone number, but for a same animal in a same enclosure and a same kind of food we can assume this number to be correct for a around a year.

We weight like that each time before feeding and after defecing. As the growth for an infested animal is much lower than for a clean one, when the profit ratio has dropped SIGNIFICANTLY we know that the animal needs deworming.

For my experience a correct ratio for boids (feeded with live rats inside a well general heated enclosure and with a permanent heat mat on) is around 50-60% for youngs and 40-50% for adults. Around 30% or less the snake starts to be ready for deworming.
It's a little under if feeded with mices (a bit less nutritive), and pretty under with poultry (pretty less nutritive).
Please don't ask me about hamsters, gerbils, rabbits and so on, I don't know!

All that cost nothing, except the price of the weight scale at the beginning but who could pretend to be a good herp without already owning one?

FINAL
I'm using all that for around 20 years without any problem, getting fast growing, very healthy and unstressed animals, so I suppose that interested people by deworming question will get the same.
As I don't pretend to know everything in anything, if you have any suggest to add, welcome.

Pascal
Paris-France