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
Investigation typically begins with a detailed history of urinary signs, general health changes, and any previous episodes of infection. Physical examination may reveal discomfort when the area over the kidneys is palpated, though some animals show little outward reaction. Laboratory tests and imaging are used to distinguish kidney infection from bladder-only disease, to identify the bacteria involved, and to assess how well the kidneys are functioning.
Urinalysis and urine culture
Blood biochemistry and haematology
Abdominal ultrasonography
Radiography
Advanced imaging
Options & trade-offs
Management is tailored to the severity of the infection, the overall health of the kidneys, and the practical circumstances of the household. Most approaches combine antimicrobial treatment with supportive care, and the balance between these varies depending on how unwell the animal is and what underlying factors are present. What works well for one household or animal may be less feasible or effective for another.
Antimicrobial therapy
Treatment usually involves a course of antimicrobials chosen based on urine culture results, often lasting several weeks to ensure the infection is cleared from the kidney tissue. In some cases, treatment is started before culture results are available, and then adjusted once the specific bacteria and their sensitivities are known. The length and type of antimicrobial can vary depending on the severity of the infection and how the animal responds.
Trade-offs: Long courses increase the likelihood of clearing the infection, but also carry a higher risk of gastrointestinal upset or other side effects. Some bacteria are resistant to commonly used antimicrobials, and selecting an effective drug may depend on culture results. Stopping treatment too early can allow bacteria to persist, but prolonged use is not always well tolerated.
Fluid therapy and supportive care
Intravenous fluids are often used in animals that are dehydrated, vomiting, or not eating well, and can help support kidney function during the acute phase of infection. This approach is more common in hospital settings, particularly when the animal is systemically unwell. Subcutaneous fluids or oral rehydration may be used at home in milder cases.
Trade-offs: Hospitalisation allows closer monitoring and more intensive support, but it is more costly and can be stressful for some animals. At-home fluid support is less invasive and may suit animals that are eating and drinking, but it provides less precise control over hydration status and kidney support.
Addressing underlying factors
If stones, anatomical abnormalities, or other conditions are contributing to recurrent infection, addressing these may reduce the likelihood of future episodes. This can involve dietary changes to dissolve certain types of stones, surgical removal of stones that cannot be dissolved, or management of conditions such as diabetes or Cushing's disease that affect immune function or urine concentration.
Trade-offs: Identifying and addressing underlying factors can reduce recurrence, but not all contributing conditions are easily managed, and some require ongoing treatment or monitoring. Surgical options carry their own risks and recovery time, and may not be suitable for animals with other health concerns.
Monitoring and repeat testing
Follow-up urine cultures are often performed several weeks after the end of antimicrobial treatment to confirm that the infection has been cleared. Blood tests may be repeated to monitor kidney function over time, particularly if there was evidence of reduced function at the outset. Ongoing observation of urinary signs and general health helps identify recurrence early.
Trade-offs: Repeat testing adds to the overall cost and requires further clinic visits, but it provides reassurance that the infection has resolved and can detect subclinical persistence. Some animals find repeated sampling stressful, and in cases where clinical signs have resolved and kidney function is stable, the benefit of additional tests may be less clear.
Common misconceptions
"Once the animal seems better and stops showing urinary signs, the infection must be gone."
Clinical improvement often occurs within days of starting treatment, but bacteria can persist in the kidney tissue for longer than the visible signs last. Stopping antimicrobials early can allow the infection to re-emerge or become more difficult to clear. The duration of treatment is usually guided by the severity of the infection and the type of bacteria involved, rather than by the disappearance of symptoms.
"Pyelonephritis always causes fever and obvious pain."
Some animals with kidney infection show very subtle signs, such as reduced appetite or mild lethargy, without fever or clear discomfort. Others may have intermittent signs that come and go. The absence of fever or dramatic symptoms does not rule out the condition, and in some cases the infection is discovered during investigation of vague or recurrent urinary changes.
"If the urine looks clear, there is no infection."
Visible blood or cloudiness in the urine is common in pyelonephritis, but some animals have infections without obvious changes in urine appearance. Laboratory examination is needed to detect white blood cells, bacteria, and other markers of infection that are not visible to the eye. A normal-looking sample does not exclude the possibility of kidney involvement.
Related conditions
Urinary Tract Infection in Dogs
Urinary tract infections in dogs often begin in the bladder and may progress upward to involve the kidneys, developing into pyelonephritis if bacteria ascend through the ureters. Conversely, dogs with pyelonephritis typically show signs that overlap with simpler bladder infections, making it important to distinguish between the two.
Acute Kidney Injury
Pyelonephritis can be one cause of acute kidney injury, particularly when infection damages kidney tissue extensively or triggers sepsis. Both conditions may present with reduced appetite, lethargy, and changes in urination, though acute kidney injury encompasses a broader range of causes beyond infection.
Otitis Externa in Cats
Leptospirosis is a bacterial infection that can reach the kidneys through the bloodstream and cause inflammation similar to pyelonephritis, sometimes making the two difficult to distinguish without specific testing. Both may present with fever, lethargy, and signs of kidney involvement, though leptospirosis tends to follow environmental exposure to contaminated water or soil.
Urinary Stones (Urolithiasis)
Urinary stones can predispose to pyelonephritis by causing partial obstruction or irritation that allows bacteria to establish themselves more easily in the urinary tract. In some cases, infection and stone formation occur together, each contributing to the persistence of the other.
Discospondylitis
Discospondylitis is a spinal infection that, like pyelonephritis, often arises when bacteria spread through the bloodstream to settle in distant tissue. Both conditions may present with fever, lethargy, and pain in specific body regions—the back in discospondylitis, the flanks in pyelonephritis—and occasionally share a common bacterial source.
Pyelonephritis sits within a broader context of urinary and metabolic health, and its investigation often brings to light other aspects of kidney function, immune response, and anatomical variation. The related content on lower urinary tract infections, kidney function, and the body's handling of bacteria and inflammation may offer useful perspective. Over time, patterns in urinary signs or general health can become clearer, and these are often topics that shape longer-term conversations about an animal's care.