Rice farmers use a lot of strychnine mixed with household food scraps to kill rats. You can find piles of the stuff every 50 meters or so in the fields when they get close to harvest. Our dog got killed that way as the farmer did not take the time to tell us he was doing it. It is a long painful death you never want to see. I agree with the other poster that it was likely a snake bite. Condolences as it is never easy to lose a dog.