Pyelonephritis

Pyelonephritis is an infection that reaches the kidneys themselves, rather than remaining in the bladder or lower urinary tract. It tends to occur when bacteria travel upward through the ureters, or occasionally when infection spreads through the bloodstream. The kidney tissue becomes inflamed, and in some cases small abscesses can form. Owners often notice signs that overlap with simpler bladder infections—frequent urination, straining, or blood in the urine—but the animal may also seem generally unwell, with reduced appetite, lethargy, or visible discomfort in the back or flanks. Fever can be present, though not always. Some dogs and cats show very few outward signs early on, and the condition is identified during investigation of subtle or recurring urinary symptoms. This page explores the signals that may point towards kidney involvement, what happens in the tissue during infection, how pyelonephritis is investigated through imaging and laboratory work, and the range of approaches used to address it. The condition varies in severity, and the course it takes depends on factors including how quickly it is identified, the type of bacteria involved, and the overall health of the kidneys.