[adamanteus]

Believe it or not, they don't want to be captive...... Watch the increase in their rate of breathing when you approach. Garters survive well in captivity, but they don't choose it. They are adaptable and (for a snake) intelligent, and they soon learn to associate you with feeding, but given they slightest opportunity, they escape.

Also, in the wild parasites are frequently passed in the faeces and the snakes move away leaving the parasites behind. In captivity the snake remains in close proximity to it's own faeces and the parasites can re-enter the snakes body, so the parasitic load can easily become higher in captivity.