Pyelonephritis in Horses: Kidney Infection Signs and Treatment
- Pyelonephritis is a kidney infection, usually caused by bacteria that travel up the urinary tract into one or both kidneys.
- Common signs include fever, weight loss, dull attitude, pain over the kidney area, increased drinking, and increased urination.
- See your vet promptly if your horse has fever, flank pain, urine changes, or unexplained weight loss. Kidney infections can become chronic and may lead to kidney damage.
- Diagnosis often includes a physical exam, bloodwork, urinalysis, urine culture, and kidney ultrasound. Some horses also need imaging to look for stones or obstruction.
- Treatment usually involves a longer course of antibiotics chosen from culture results, plus fluids and treatment of any underlying problem such as stones, bladder dysfunction, or poor urine flow.
What Is Pyelonephritis in Horses?
Pyelonephritis is inflammation and infection of the kidney and renal pelvis. In horses, it is usually a bacterial kidney infection that starts lower in the urinary tract and then moves upward into the kidneys. Less often, infection may spread through the bloodstream or develop when urine flow is blocked by a stone or another structural problem.
Healthy horses are fairly resistant to urinary tract infections, so pyelonephritis is not one of the most common equine diagnoses. When it does happen, though, it matters. Kidney tissue can be damaged over time, and some horses become chronically ill before the problem is recognized.
Signs can be subtle at first. A horse may seem off, lose weight, drink more, urinate more, or act painful through the flank and back. Others develop fever and look obviously sick. Because long-standing cases may have fewer clear lab changes, your vet may need imaging and urine culture to confirm what is going on.
The good news is that many horses improve with timely treatment. The best plan depends on how sick the horse is, whether one or both kidneys are affected, and whether there is an underlying issue such as stones, bladder paralysis, or another urinary tract problem.
Symptoms of Pyelonephritis in Horses
- Fever
- Pain in the flank or kidney area
- Weight loss
- Depressed attitude or poor performance
- Increased drinking
- Increased urination
- Urine dribbling, straining, or other lower urinary signs
- Blood-tinged, cloudy, or foul-smelling urine
- Reduced appetite
- Signs of kidney failure such as marked lethargy or dehydration
See your vet immediately if your horse has fever with flank pain, is straining to urinate, seems dehydrated, or is suddenly drinking and urinating much more than usual. Those signs can point to a urinary emergency, kidney involvement, or an obstruction.
Call your vet soon even for milder changes like gradual weight loss, poor performance, or wetter bedding. Pyelonephritis can be easy to miss early, and chronic cases may cause ongoing kidney damage before the cause becomes obvious.
What Causes Pyelonephritis in Horses?
In horses, pyelonephritis is usually caused by bacteria ascending the urinary tract. That means bacteria enter through the urethra, move into the bladder, and then continue up the ureters into the kidneys. Merck notes that the risk factors for pyelonephritis are similar to those for bacterial cystitis, and that stones in the kidney or ureter are a common cause when they interfere with normal urine flow.
Urine that does not flow normally gives bacteria more opportunity to multiply. Problems such as bladder paralysis, urethral damage, urinary stones, or incomplete bladder emptying can all raise risk. Chronic inflammation of nearby tissues may also contribute in some horses.
Less commonly, infection may spread through the bloodstream. Horses with other illnesses, weakened immune defenses, or long-standing urinary tract disease may be more vulnerable. Reported equine urinary pathogens include E. coli, Streptococcus, and Enterococcus species.
Your vet will also think about underlying causes, not only the infection itself. If the reason bacteria reached the kidney is not addressed, the infection can recur even after an initial response to antibiotics.
How Is Pyelonephritis in Horses Diagnosed?
Diagnosis starts with a careful history and exam. Your vet will ask about fever, urine changes, water intake, weight loss, performance decline, and any prior urinary problems. On exam, some horses are painful over the loin or flank area, while others mainly show vague signs like poor appetite or dullness.
Most horses need bloodwork and urinalysis. Blood tests help assess inflammation, hydration, and kidney values. Urinalysis can look for white blood cells, blood, protein, bacteria, and urine concentration. A urine culture with susceptibility testing is especially helpful because it identifies the organism and helps your vet choose an antibiotic that is more likely to work.
Imaging is often important, especially if the case seems chronic or if your vet suspects stones or obstruction. Merck notes that long-term cases may not show clear abnormalities on blood or urine testing, so ultrasound or radiographs may be needed. Ultrasound can help evaluate kidney size, shape, dilation, debris, and whether one kidney looks more affected than the other.
Some horses need additional testing if the picture is complicated. That may include repeat cultures, catheterized urine collection, endoscopy of the lower urinary tract, or referral imaging. The goal is not only to confirm infection, but also to find the reason it happened and estimate how much kidney function may be affected.
Treatment Options for Pyelonephritis in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm call or clinic exam
- CBC/chemistry panel and basic urinalysis
- Urine culture when feasible
- Oral or injectable antibiotics selected by your vet
- Monitoring hydration, appetite, temperature, and urine output at home
- Recheck exam and repeat labwork if the horse is stable
Recommended Standard Treatment
- Complete exam and pain assessment
- CBC, chemistry, fibrinogen or inflammatory markers
- Urinalysis plus urine culture and susceptibility testing
- Kidney and urinary tract ultrasound
- Targeted antibiotic plan, often for several weeks
- IV or oral fluids depending on hydration and kidney status
- Treatment of contributing issues such as cystitis or bladder dysfunction
- Scheduled rechecks with repeat bloodwork and urinalysis
Advanced / Critical Care
- Hospitalization and close monitoring
- Serial bloodwork, urinalysis, and repeat cultures
- IV fluids and intensive supportive care
- Advanced imaging or referral evaluation for stones, obstruction, or severe unilateral disease
- Procedures to address obstructive stones when available
- Surgical management in selected cases, including nephrectomy of a severely damaged nonfunctional kidney
- Management of complications such as sepsis, azotemia, or dehydration
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Pyelonephritis in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Do my horse's signs fit a kidney infection, a bladder infection, or another urinary problem?
- What tests do you recommend first, and which ones are most important if I need to watch the cost range?
- Was a urine culture collected, and when should we expect results?
- Do you suspect stones, obstruction, or bladder emptying problems that could have caused this infection?
- Is my horse stable for home care, or do you recommend hospitalization?
- How long will antibiotic treatment likely need to continue, and what side effects should I watch for?
- What signs would mean the kidneys are worsening or that I should call right away?
- When should we repeat bloodwork, urinalysis, or ultrasound to make sure treatment is working?
How to Prevent Pyelonephritis in Horses
Not every case can be prevented, but lowering the risk usually means protecting normal urine flow and catching lower urinary tract problems early. If your horse has urine dribbling, straining, blood in the urine, repeated wet bedding, or signs of bladder dysfunction, schedule a veterinary exam rather than waiting to see if it passes.
Make sure your horse has reliable access to clean water and a management routine that supports regular drinking and urination. Good hydration helps the urinary tract do its job. Horses with a history of stones, neurologic problems, or prior urinary infections may need closer monitoring and earlier rechecks.
If your horse has a diagnosed bladder infection or another urinary issue, follow the treatment plan all the way through and keep recheck appointments. Stopping antibiotics early or skipping follow-up can allow infection to persist and move upward.
Prevention also means looking for the reason an infection started. Your vet may recommend imaging, repeat urinalysis, or culture after treatment, especially in horses with recurrent signs. That extra step can help prevent a short-term infection from becoming a chronic kidney problem.