Bladder Stones and Grit
A guinea pig that is hunched up with fur sticking out at right angles to its body, not moving and grinding its teeth, may have a urinary tract stone blocking the exit from the bladder. This is a clinical emergency and you must get the guinea pig to a vet immediately.
Stones (uroliths) or gravel in the urine (urolithiasis) can be a result of an imbalance in the relationship between calcium, phosphorous and other minerals. High calcium and in particular high oxalate levels are implicated.
Increasingly, medical research into this area indicates that low levels of the enzyme oxalobacter formigenes, which breaks down oxalates, have a role to play in many cases of stone formation. I have also seen a pattern emerging amongst people I have spoken to where there is a connection between the water supply and stone formation. Initially I had thought that was to do with the calcium levels in the water but fluoride may also be implicated. See links below.
The veterinary drug Alopurinol, used for a similar condition in cats (and gout in humans) may also be worth trying. It is described as 'for urate stones'.
I would be interested to know whether anyone has any experience of using the human product Rowatinex on their animals. The ingredients are volatile oils including Pinene, Campheme, Cineol, Fenchone, Borneol and Anethol which may be harmful to rabbits. I haven't had time to dig into this yet so please remember this is has to be fully researched before even trying it. If anyone has the time to do so, please contact me with the results.
The herb of choice for urinary calculus is Chanca Piedra (Phyllanthus niruri/amarus), literally translated as 'Stone Breaker'. In Indian Ayurvedic medicine it is known as Bhumi Amla. Phyllanthus niruri/amarus acts to weaken the mucus bond between the mineral particles in certain types of stone, but not all.
Raintree's PDF on Chanca Piedra.
These studies on Pub Med indicate that ingestion of excess fluoride facilities calcium oxalate crystalluria and promotes the formation of bladder stones in rats, under the experimental conditions used The Fluoride Connection
Hyperoxaluria (excessive urinary oxalate) is a common abnormal finding in patients with calcium oxalate kidney stones. Of all patients with recurrent calcium oxalate stones, 20-30% demonstrate some degree of excessive urinary oxalate. Very useful article from eMedicine
Developing therapies for enteric hyperoxaluria secondary to Crohn's Disease and other malabsorption diseases, and recurrent kidney stones, and a number of other calcium oxalate-related conditions. Ixion Bio-Tech Products
Oxalate degradation by the anaerobic bacterium Oxalobacter formigenes is important for human health, helping to prevent hyperoxaluria and disorders such as the development of kidney stones. Article by Sylvia H. Duncan, Anthony J. Richardson, Poonam Kaul, Ross P. Holmes, Milton J. Allison, and Colin S. Stewart. Oxalobacter formigenes and its potential role in human health PDF
Abstracts only by various authors but you can purchase articles. Oxalobacter formigenes (Pub Med)
DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, Braunschweig, Germany sell Oxalobacter formigenes. Purchase Oxalobacter formigenes from DSMZ