To my knowledge, cats do not contract cmv. Have you tested for wet form FIP? We hope it's not that, of course, but at least you should know.
Has he been tested for more exotic internal parasites, that perhaps Drontal wouldn't touch? Just a thought. My other thought is perhaps a toxin of some kind, separate from any infection or illness, could cause those symtoms, especially if it is something that he is exposed to over time without anyone knowing.
How is he doing on the Metronidazole?
The 2 cats do fine on the Metronidazole, but the symptoms can recur in 2 months time which has me baffled. I have not tested for wet FIP, but that had crossed my mind. I don't believe it to be a toxin due to the fact that the first time in showed in the first kitten they were living in my barn and the second kitten showed it when living in my house. We did look at a blood smear from the mother cat that died to see if there could be babesia or any foreign signs - clear. The vets here in NH keep telling me that they see a "lot of bloody and just diarrhea" to me this is just not normal.
Thanks for the ideas.
my 18 yr cat had large appetite,then have puddles of diarrhea.yellow turned to brown later on.she have it 3 times a day,months .i took her to the pet hospital they couldn't find nothing wrong with her,,gave her few medicines stopped diarrhea,but it didn't quit then she had some red blood in it.,she got very skinny from diarrhea and died on my couch all four legs spread out ,two loud cries.,mouth been opened.sad too I will miss her,i tried best I could to save her.