Don'y overlook the very distinct possibility of Clostridium Perfringens -- various types. At about a month old, that is when you will start to see some signs of it if you have a problem.
Sy,ptoms vary to some degree -- some calves will be fine at noon, playing and be dead at 4 PM. Some will just look bloated all the time, will slosh when they walk by you. Others will have what you describe too -- bloody stool is a sign of intestinal distress -- as Clostridium will affect the intestines and EAT them up from the inside out.
On a calf you lose for no apparent reason, do a necropsy on them and check the intestines. The linings will be VERy read and inflamed -- a dead give away for Clostridial infections.
Also, although it is a little early in life for this -- you may look into the chance of having some Coccidiosis present. Do you have a serious vird problem - especially around the watering systems?? Thingsto look into -- I always use Alpha-7 Clostridial vaccine for the newborn, this gives you a 30 day window early -- then I booster them all with Vision 7 or 8 at about that month age.
Good luck -- watch the calf -- it is sneaky !!
Terry
Up late tonight watching the blizzard we now have in full force -- with an overdue Hfr in the calving pen due to Tycoon -- could not be worse timeing -=- we don't have aa barn for them -- not even a shed!
