We feed a few steers out for ourselves and to sell. We had one that went off feed, and his belly swelled up after a few days. After he died, I cut him open, and his belly was full of fluid that smelled like urine. What could have happened to him?
I cannot say for sure, but I would bet this calf developed bladder stones. Urinary tract stones are made up of minerals that precipitate out of the urine. While kidney stones are more common in people, bladder stones are more common in livestock, dogs and cats.
If these stones are large enough, they can block the urethra, and the animal cannot urinate. This is more common in males because the urethra is smaller. In cattle, sheep and goats, the urethra has an "S"-shaped bend called the "sigmoid flexure" that straightens out when the animal has an erection. This bend makes passing stones even more difficult, and that sigmoid flexure is the most common place where we see cattle block.
When an animal is blocked and cannot urinate, he or she may strain, walk with a humped-up appearance and gradually become more depressed as toxins build up in the system. Without correction, the urethra or bladder eventually ruptures. If the urethra ruptures, there will be swelling around the rupture. If the bladder ruptures, the abdomen swells like it did in the case you describe.
If it's caught early, your veterinarian can do a salvage procedure where the urethra is opened up above the blockage. When successful, this allows the animal to make it to slaughter.
The most significant risk factor for bladder stones comes from the diet—especially grains high in certain minerals, such as struvite (magnesium ammonium phosphate). Forages don't eliminate the risk. Legumes like alfalfa and clover are very high in calcium, and they often lead to calcium oxalate stones.
Animals most at risk for bladder stones are cattle on feed, small ruminants eating grain and show cattle. While a balanced, professionally developed nutritional plan is always recommended in livestock, it is especially important in those animals considered higher risk.
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