I contacted John Forsythe about your problem, since he was the one who took photo 1571 in Ceph Base and wrote the caption. He has worked with briareus and is the very best person to help you.
I'm reproducing his reply, since there is a lot here that will help all briareus owners -even all octo owners, so all should read this:
"As for the briareus skin infection photo, that sort of lesioning was brought on by trying to culture juveniles together. This is a very aggressive species, they constantly grab onto one another and this grabbing ultimately results in bacterial infections of the skin. As this is not a gregarious species, in fact a highly cannibalistic species, there was also almost certainly a stress factor also involved in having animals together. So I'm wondering if this person is trying to keep more than one animal together. Otherwise, something else in the husbandry practices of this person are out of line to cause this sort of infection in a single animal: inadequate water quality, poor diet or too much light with nowhere to hide. O. briareus is an obligate nocturnal species. It is NEVER seen in nature in daylight, only on night dives. Not dusk, not sunrise, full darkness. Forcing a nocturnal species to be day active can be a real stressor.
If the animal is still eating, I'd recommend giving the octo pieces of shrimp meat that have been injected with an antibiotic. Now, this may be the problem, getting the drug to use. It is possible that a Vet will sell them something. I'd recommend Chloramphenicol(Chloromycetin) if they can get it. Not widely available anymore but it really clobbers the gramf-neg Vibrio's that are the most common pathogens involved in skin infections. More importantly it remains active in the highly ionic environment of seawater and the bloodstream of cephs. Nitrofurazone is also quite effective against Vibrio's but only in bath treatment. Now there are other more modern drugs that might also work. Basically, anything reported to be effective in treating marine fish with gram negative vibrio/pseudomonas type infections should be effective, and safe fish dosages for say tropical fish (not catfish and the like which you can't kill with a 2 x 4) should also be safe for cephs.
Finally, oral administration is more effective for ceph infections than baths, because if you have an external skin infections, there is a 90% probability that you also have systemic infections going on in the digestive gland or other organs also. So oral admin attacks both. You need to continue the drug dosages for 10 days, don't stop early or the problem might just flair back up.
For Chloramphenical, we used a dosage equivalent to 100mgChloram/kg of octo. So need to do a little math to figure out how much of the liquid form of the drug to inject into the shrimp assuming the octo will eat the entire amount in one meal. This is crucial. Also, use the smallest size Insulin syringe for the injections, as they are subdivided into small enough units. You actually inject a very small amount of fluid into the shrimp piece.
Having said all of this, if the octopus is not eating, the only options are bath treatments or intramuscular injections in the hope of knocking down the infection and getting the animal to regain appetite. If the octo is tiny, then injections won't work and you're left with bath treatment. The dosages in a several hour bath exposure to Chloram is 12.5 micrograms/ml or for Nitrofurazone, 25micrograms/ml in the treatment bath.
This is a lot to digest and this person needs to have some resources available to pull it off but this is what we do when we have problems, though I'll bet we haven't had to revert to antibiotic therapies for over 5yrs now as we really have a handle on the health of our populations.
Hope this is useful. It is all published. At the NRCC publications page, go to book chapters and the reference to look for is:
Forsythe, Hanlon and Lee, 1990: "A formulary for treating cephalopod mollusc diseases."
So I think we might all want to read that article. I've thanked John by email, but would also like to thank him here for the quick reply and very helpful information.