Bit & Bridle

Equine Kidney Infections: Pyelonephritis

Other Names

  • Pyelonephritis
  • Kidney infection in horses
  • Renal pyelonephritis
  • Upper urinary tract infection

What is Equine Pyelonephritis?

Equine pyelonephritis is a serious bacterial infection of the kidneys, involving inflammation of both the renal pelvis (the collecting area within the kidney) and the renal parenchyma (the functional tissue of the kidney). It represents the most clinically significant form of upper urinary tract infection in horses and can cause progressive, irreversible damage to renal tissue if not treated promptly and effectively.

The condition can arise via two primary routes: ascending infection, in which bacteria travel up from the lower urinary tract (bladder and urethra) to the kidney; or descending (haematogenous) infection, in which bacteria seed the kidney from the bloodstream during bacteraemia. Ascending infection is the more common route in horses, typically following a lower urinary tract infection that has been inadequately treated or that has developed in the context of bladder dysfunction or urolithiasis.

Pyelonephritis may affect one kidney (unilateral) or both (bilateral). Bilateral disease carries a significantly worse prognosis for renal function. Related urinary tract conditions include equine urolithiasis.

Causes

The most common causative organisms in equine pyelonephritis include:

  • Actinobacillus equuli
  • Escherichia coli (E. coli)
  • Proteus spp.
  • Streptococcus equi subspecies zooepidemicus
  • Klebsiella spp.
  • Pseudomonas spp.

Predisposing factors that increase the risk of ascending pyelonephritis include:

  • Lower urinary tract infection (cystitis): Untreated or inadequately treated bladder infection provides bacteria with an established foothold in the urinary tract
  • Urolithiasis: Bladder stones cause mucosal damage and urinary stasis, creating conditions favourable for bacterial growth and ascent
  • Vesicoureteral reflux: Abnormal backward flow of urine from the bladder into the ureters allows bacteria to reach the kidney
  • Urinary catheterisation: Indwelling urinary catheters are a known risk factor for introducing bacteria into the urinary tract
  • Neurological bladder dysfunction: Conditions causing incomplete bladder emptying (equine herpesvirus myelopathy, cauda equina syndrome, or other neurological disease) predispose to urinary stasis and infection
  • Haematogenous spread: Bacteraemia from any source — dental infection, respiratory disease, gastrointestinal infection — can seed bacteria into the renal tissue

Symptoms and Signs

Clinical signs of equine pyelonephritis may develop gradually and are not always immediately specific to the kidney:

  • Fever: Often intermittent, sometimes marked
  • Depression and inappetence
  • Weight loss and progressive poor condition
  • Signs of dysuria: Straining to urinate, frequent small amounts, dribbling, or haematuria (blood in the urine)
  • Polyuria or oliguria: Increased or decreased urine production, depending on the degree and duration of renal involvement
  • Back or flank pain: Some horses show sensitivity or pain on palpation or percussion over the kidney region, though this is not always present
  • Ventral oedema: In chronic cases where significant protein is lost in the urine
  • Colic: May be present, particularly if ureteral involvement causes obstruction or spasm
  • Signs of uraemia in advanced cases: Oral ulceration, lethargy, reduced appetite, and neurological signs if azotaemia (elevated blood urea and creatinine) becomes severe

Diagnosis

  • Urinalysis: Pyuria (white blood cells in urine), haematuria, bacteriuria, elevated protein, and casts (cellular debris from kidney tubules) are all findings consistent with pyelonephritis
  • Urine culture and sensitivity: A midstream urine sample (or catheter sample to avoid perineal contamination) for culture identifies the causative organism and its antibiotic sensitivity — essential for targeted treatment
  • Haematology and biochemistry: Neutrophilia, elevated fibrinogen and serum amyloid A (inflammatory markers), elevated creatinine and urea nitrogen (indicating reduced glomerular filtration rate in established renal disease), hypoproteinaemia in chronic cases
  • Renal ultrasonography: Transrectal ultrasound of the right kidney and transcutaneous ultrasound of the left kidney allows assessment of renal size, echogenicity, pelvic dilatation (hydronephrosis), and the presence of abscesses or mineralisation. This is the most important imaging modality for diagnosing pyelonephritis in horses.
  • Rectal palpation: The right kidney may be palpable per rectum in some horses; enlargement, pain on palpation, or irregular texture may be detectable
  • Renal biopsy: Occasionally performed for histopathological confirmation of inflammatory or fibrotic renal changes, though the risk-benefit must be carefully assessed

Treatment

Successful treatment requires prolonged antibiotic therapy guided by culture and sensitivity results:

  • Long-term antibiotics: Pyelonephritis typically requires 4–8 weeks of antibiotic treatment to fully eradicate infection from the renal tissue. Initial intravenous treatment is often required in hospitalised horses, transitioning to oral or intramuscular therapy as clinical improvement occurs.
  • Antibiotic selection: Based on urine culture results. Trimethoprim-sulphonamide, enrofloxacin, penicillin, and gentamicin are among the agents used depending on sensitivity results. Renal toxicity of certain antibiotics (especially aminoglycosides) must be considered and monitored.
  • Management of predisposing causes: Any concurrent urolithiasis, urinary catheter infection, or neurological bladder dysfunction must be addressed — failure to do so leads to relapse
  • IV fluid support: For horses with azotaemia or dehydration
  • NSAIDs: For fever and pain control, used judiciously given potential nephrotoxicity with prolonged use in horses with compromised renal function
  • Nephrectomy: Surgical removal of a severely infected, non-functional kidney may be considered in horses where unilateral severe disease is causing persistent systemic infection and the contralateral kidney has confirmed adequate function

Prevention

  • Treat lower urinary tract infections promptly and ensure adequate treatment duration to prevent ascent of infection
  • Address urolithiasis without delay — bladder stones are a significant risk factor
  • Use urinary catheters only when necessary, following strict aseptic technique and replacing them on schedule
  • Monitor horses with neurological conditions causing bladder dysfunction closely for urinary tract infection
  • Ensure adequate water intake to maintain dilute urine and good urinary tract flushing

Prognosis

Prognosis depends on whether disease is unilateral or bilateral, the severity of renal damage at the time of diagnosis, the responsible organism, and how quickly treatment is initiated. Horses with unilateral pyelonephritis diagnosed early have a fair to good prognosis for survival and comfortable long-term function, as horses can sustain life on a single functioning kidney. Bilateral disease, chronic disease with significant renal fibrosis, or disease complicated by resistant bacteria carries a guarded to poor prognosis for long-term athletic function and, in severe cases, survival.

When to Call the Vet

  • Any horse with recurring unexplained fever, particularly combined with weight loss or urinary signs
  • Dysuria (straining to urinate), haematuria, or dribbling in a horse — prompt investigation is required to exclude urinary tract disease including pyelonephritis
  • A horse with known urolithiasis or bladder dysfunction that develops fever or appears systemically unwell
  • Any horse showing signs of azotaemia — anorexia, oral ulcers, progressive weight loss, polyuria

Frequently Asked Questions

How long does treatment for pyelonephritis take in horses?

Treatment typically requires 4–8 weeks of antibiotic therapy. This is considerably longer than for lower urinary tract infections because bacteria within the renal tissue are harder to eradicate. Stopping treatment early is a leading cause of relapse. Regular monitoring of clinical signs and urine culture should be performed throughout the treatment period.

Can a horse live normally with one kidney after pyelonephritis?

Yes. Horses have significant renal reserve, and horses with one healthy kidney can maintain normal renal function, live comfortably, and in many cases return to work. If nephrectomy is considered for a severely diseased kidney, confirmation of adequate contralateral kidney function by urinalysis, creatinine clearance, and ultrasound is essential before proceeding.

Is pyelonephritis contagious?

No — pyelonephritis itself is not contagious between horses. It arises from the horse’s own bacterial flora ascending the urinary tract or being seeded haematogenously. Standard hygiene precautions are appropriate when handling urine from an infected horse, but there is no meaningful transmission risk to other horses or humans under normal circumstances.

Can urinary tract infections in horses be prevented with management changes?

Good management reduces risk. Ensuring plentiful clean water intake (which dilutes urine and promotes frequent voiding), maintaining horses in clean conditions that minimise perineal bacterial load, and addressing anatomical or neurological predispositions promptly all contribute to prevention. Horses with recurrent infections should have their urinary tract fully investigated to identify and correct any predisposing anatomical or functional abnormality. Information on urinary health is also relevant in our guide to equine urolithiasis.