I understand that older slaves were used in house labor and other tasks less physically demanding than the field labor. But once the person was too old to do even that, or perhaps contracted a communicable disease that could cause other slaves to die, was it common practice to "put the slave down" like we do with old house pets today? What about slaves born with birth defects or severe intellectual disabilities?

Not sure if this would have been documented, so it may not be answerable.