I don't think there's always diarrhea, but this is what the problem usually is, or at least most often related to this. (This was written by Pamela Alley).
You can't really administer anything orally to a cold or unresponsive rabbit, but there would be an extremely slim chance that an infusion of fluids and some antibiotics and probiotics would help.
Enterotoxemia
Enterotoxemia. is a disease usually of the young animal, and is caused by the iota toxin released by a bacterium called Clostridium spiroforme.
Also called 'carbohydrate overload of the hindgut', enterotoxemia is a quite fast disease, and often results in death due to the systematic effects of the toxin.
C. spiroforme is a normal inhabitant of the cecum, a member of the cecal flora, and it is well-regulated by the growth of other normal flora until a larger amount of nutrients unused by other bacteria becomes available.
When this occurs, C. spiroforme initiates a population explosion, and, in the presence of glucose, begins to release its deadly chemical toxin.
The first sign the breeder will see is a sudden, messy diarrhea which may be so severe that fluid fecal material simply oozes or flows from the animal. The smell. is sour, unlike normal feces. A fecal flotation will reveal few or no coccidial oocytes (eggs).
The rabbit will be quite depressed, which makes sense ... they feel lousy, after all, and their belly may be very painful upon palpation. They will also become dehydrated rapidly, and this is the major cause of death in these young animals.
This disease is not limited to young stock; older animals may also contract it and die in large numbers.
Administration of antibiotics to reduce the population of C. spiroforme and electrolyte-containing fluids by mouth as well as given by injection are the main medical treatments for this disease at this time.
Competitive inhibition of C. spiroforme might be achieved by the use of probiotics such as live culture yogurt or commercial probiotic products. An immediate increase in the amount of crude fiber in the diet may also be of help. The outlook is grim, but recovery is possible.
As the main cause of the actual disease is a toxin, an antitoxin would be nice to have, as there is for tetanus. It is unfortunate that no such product is available.
Since we have no antitoxin, our best hope is to give the animals enough time to recover a balance of normal flora in the gut as well as removal of toxin from the system via normal metabolic processes.
Enterotoxemia is not a transmissible disease, unlike coccidiosis. As few as one, or as many as dozens of animals in the barn may be affected, old and young, dependent on just how high the carbohydrate might be in the diet.
Again, the outlook is grim, but be persistent and patient, and it will run its course.