I couldn't find anything on Google about chicken snakes being poisonous to dogs but if you do kill one (otherwise known as a 'witch snake') you will incur some serious bad luck apparently
http://www.tsha.utexas.edu/handbook/...s/FF/ldf2.html
And I wouldn't worry too much. In my experience of growing up with animals in Africa, they tend to have a natural instinct to stay away from certain things like snakes and scorpions and so on