I use to have the same problem. Now I rarely see them anymore.They come in from outside so it is rather difficult to completely eradicate them. This is what I did.

Proper Ventilation of the cages dries out waste and reduces ammonia concentration in the drawers which there attracted by. This is probably one of the biggest and most easiest problems to remedy.
I changed my waterbowls to the disposable wax soup bowls, it seems the gnats can't climb on the wax well and that halps eliminate there water source.I just throw them away instead of filling them back up..cleaner that way as well!!
Also since hogs like it rather dry put more ventilation holes in there cages combined with a few strong occilating fans. This will help dry out the waste which helps elimante there food source and breeding areas(also helps balance out temps). Daily spot cleaning is also essential.
Sani chips seem to be the best for soaking and clumping up waste , but it has some definete drawbacks especially when it comes top feeding and burrowing. Pehaps use that for a few weeks with the other methods to get a handle on them.
Hope this helps!!
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Justin Mitcham
ExtremeHogs.com

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