CONDITION
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.
Why this matters now
Pyelonephritis can occur at any age, though middle-aged to older dogs and cats may be more susceptible, particularly if they have pre-existing conditions that affect the urinary tract or immune system. Female dogs tend to develop urinary tract infections more frequently than males, and in some cases these can ascend to the kidneys. Certain breeds with anatomical variations in the urinary tract, or those prone to forming stones, may face a higher likelihood of bacterial ascent. The condition can also emerge in younger animals if there is an underlying structural abnormality or immune compromise.
The infection may develop over days to weeks, depending on how quickly bacteria colonise the kidney tissue and how effectively the body's defences respond. Some animals show clear signs early, while others present with subtle or intermittent symptoms that only become more obvious as inflammation spreads within the kidney. If the infection is addressed early, the kidney tissue may recover with minimal lasting change. In cases where the infection persists or recurs, scarring of the kidney tissue can develop, and this may gradually affect kidney function over months or years.
Signals & patterns
Early signals
Frequent urination in small amounts
The animal may ask to go outside more often, or visit the litter tray repeatedly, but produce only small volumes each time. This pattern often reflects irritation in the urinary tract, which can accompany the early stages of ascending infection.
Reduced appetite or interest in food
Many animals with pyelonephritis begin to eat less, or show less enthusiasm at mealtimes. This change can be subtle at first and may be attributed to other causes, but it often reflects the body's systemic response to infection and inflammation.
Lethargy or quieter behaviour
The animal may seem less playful, sleep more, or show less interest in usual activities. This general malaise can be mild and easy to overlook, particularly in cats, but it often signals that the infection is affecting the animal more widely than the urinary tract alone.
Discomfort when the back is touched
Some dogs and cats flinch, tense, or move away when the area over the kidneys is touched or pressed gently. This reaction can indicate pain or tenderness in the tissues around the inflamed kidney.
Blood-tinged or cloudy urine
Urine may appear darker, pink, or murky, reflecting the presence of red blood cells, white blood cells, or bacteria. This visible change often prompts closer attention, though it can also occur with bladder inflammation alone.
Later signals
Persistent or worsening lower back pain
The animal may adopt a hunched posture, be reluctant to jump or climb, or show clear discomfort when moving. This pattern can indicate deeper inflammation or abscess formation within the kidney tissue.
Increased thirst and urination volume
If kidney function begins to decline, the animal may drink more and pass larger volumes of dilute urine. This shift reflects the kidney's reduced ability to concentrate urine, a sign that damage to the tissue may be progressing.
Weight loss or poor coat condition
Over time, chronic infection and reduced kidney function can lead to gradual weight loss and a dull, unkempt coat. These changes tend to emerge when the condition has been present for weeks or longer without resolution.
Intermittent fever or shivering
Some animals develop episodes of fever, which may come and go. Owners may notice the animal feeling warm to the touch, shivering, or seeking out warm places more than usual.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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