If you feed your boa in it's tank....Which is a good idea in my book.....Some snakes can have that response. Although, out of all my snakes.....I have never had one do it.
If you are going to either handle the snake or change something around in the tank try rubbing your snake with your snake hook. (You own one right?
If not, you should.) This will condition your snake into knowing when it is touched by the hook there will not be any feeding occuring. This worked great when I owned retics and burms. I never had a problem when I used this technique.
Another problem could be that your boa is feeling threatened by your presence inside the tank. ie, it's territory. I would use the hook technique in the exact same way. If it is a fearful strike and not a predatory one........You might have to hold your snake back with the hook until you get them water changed.
Either that, or just take the darn thing out. If it is causing a stressful situation to you and the snake...Just take him/her out when you are cleaning things up in there. Heck, it's just another excuse to hold your boa for a little bit.
Hope this helps,
John
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"To be the best..........You must lose your mind."