Randy May of NeoScales has tentatively identified a disease-causing organism found among captive crested geckos. This is particularly bad news because so far these little geckos had seemed fairly resistant and didn't generally have the high parasite load of some captive species, such as leopard geckos. He's posted about it on Kingsnake, but I wanted to quote the most significant portion of his post to get the word out here, so here it is:

Signs & Symptoms
• Rapid weight loss
• Lethargy
• Swollen Vent (late stage)
• This is contagious though contact with the feces of ailing geckos

Treatment
• Highly susceptible to Flagyl (Metronidazole)
• 250mg/kg for 3 treatments is probably enough
• You must keep the cage clean and bleached during treatment to prevent re-infection.
• You should also treat all other geckos that have had any contact with the affected one. Treating the entire colony (could easily done by medicating their food) may be needed as a precaution if food/water dishes and cages are moved from cage to cage.

Take home message: If noticed early on, this is an easy and highly treatable disease!!!

There are other causes of weight loss (like intestinal blockage), but amebiasis should be part of the treatment plan straight away - if you want the best chance to save your ailing gecko.

Feel free to comment and thanks for the long read (JFYI - I'm a reasearch scientist by profession),
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-Randy May
www.neoscales.com