Hi
Have your vets heard of sterile interstitial cystitis, which is similar to feline sterile cystitis (FSC), including treatment with oral glucosamine in milder or cartrofen (or both) in more severe cases which you seem to be unfortunately dealing with?
'Sterile' means non-bacterial; any antibiotic cannot work on it (since it is designed to kill bacteria) or - in milder forms - only suppress it temporarily. Sterile IC is characterised by the absence or low count of bacteria. It can run the whole spectrum from the very mild to the very severe end (which is unfortunately not yet treatable). Research into cartrofen has found that it also works for guinea pigs with medium to more severe sterile IC and we have just had feedback from a severe case where the combination of both cartrofen and glucosamine seems to be working where cartrofen on its own was not enough.
The illness is generally only diagnosed by default after all other potential problems in the urinary tract have been ruled out. It is not much known outside vet circles that see guinea pigs on a very regular basis. Sterile IC seems to affect mostly guinea pigs with a high stress level as their default setting, which happens while still in the womb of a stressed out mother. The current mass production of for sale pets is sadly promoting it and the illness has become a lot more common over the last 15 years.
Links - Interstitial Cystitis - Guinea Lynx Records
Strong Cystitis, Cartrophen and Glucosamine
In view of other potential problems having been ruled out and the antibiotic treatment not working, I would recommend that you try to pursue this avenue before giving up.
The metacam dosage sounds like it is at the upper end for a piggy with a weight that is a bit lower than 1 kg? The next step up would be a stronger non-licensed painkiller like gabapentin, tramadol etc. in combination with the metacam.
I am keeping my fingers firmly crossed!