Renal function is an important component of overall health in any species. The kidneys perform several important functions, including removing waste, maintaining electrolyte balance and blood pressure, supplying calcium for bone health, and producing factors for red blood cell stimulation, to name a few. In addition, the kidneys receive substantial blood flow—approximately 25% of the cardiac output. Therefore, changes in blood flow, either increased or decreased, can have a significant impact on renal health.
Equine necropsy cases—including fetuses, foals, and adults—submitted to the University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) over an eight-year period (2010-2018) were queried for diagnoses related to renal pathology.
Of the 10,541 submissions, 3.6% (386) had some type of renal pathology. Of those diagnoses, renal lesions were determined to be primary in 38% of cases (148), secondary to another process in 55% of cases (211), or incidental in 7% of cases (27).
Significant processes associated with primary lesions included:
- Inflammatory/infectious (95 cases);
- Congenital (21 cases);
- Neoplastic renal carcinoma/adenocarcinoma (10 cases);
- Nephroliths (10 cases);
- Toxic (four cases);
- Trauma (one case); and
- Other miscellaneous (i.e., chronic renal failure, protein losing nephropathy, etc.) conditions (seven cases).
Within the infectious/inflammatory group, the most common cause was a bacterial pathogen resulting in nephritis. The largest proportion of cases was due to Leptospira interrogans infections in fetuses. Leptospirosis is a cause of abortion that often localizes to the kidney. Actinobacillus equuli was another common bacterial isolate, most often seen in neonatal foals and occasionally adults.
In foals, infection with A. equuli is colloquially termed “sleepy foal disease.” Routes of infection for A. equuli include a contaminated umbilicus, inhalation, or ingestion.
Several cases of bacterial pyelonephritis—inflammation of the renal pelvis that most often results from an infection that extends up to the kidneys from the lower urinary tract (i.e., urinary bladder)—were identified. The most common bacterial isolates identified from cases at the UKVDL included Streptococcus zooepidemicus, Escherichia coli, and Enterococcus sp.
Lesions that comprised the congenital category included renal dysplasia, renal agenesis, congenital renal cysts, or congenital hydronephrosis/hydroureter. While there have been proposed associations of hereditary or nutritional components for developmental renal abnormalities in some species (i.e., dogs and pigs), in horses the pathogenesis of these congenital lesions remains to be elucidated.
Significant nephroliths (kidney stones) were identified in 10 horses. The stones were composed of calcium carbonate or a mixture of calcium carbonate with other minerals. Typically, horses with nephroliths remain asymptomatic until the stone results in obstruction. Development of nephroliths has been associated with any nidus of renal disease including cysts, papillary necrosis, pyelonephritis or neoplasia.
Papillary (medullary crest) necrosis due to suspected long-term non-steroidal anti-inflammatory drug (NSAID) use was identified in three of the four cases in the toxic category. NSAIDs (i.e., phenylbutazone, or Bute, and flunixin meglumine, or Banamine) are routinely used in horses for pain management. These drugs work by inhibiting a specific group of enzymes, cyclooxygenase (COX), which subsequently reduces inflammation. Unfortunately, other downstream effects of this pathway include decreased production of prostaglandins. Specific cells located in the renal medulla produce prostaglandins that are mediators of blood flow to the tissue. Therefore, decreased prostaglandin production results in impaired blood flow to the medulla and subsequent necrosis. Other compounding factors include dehydration and use of multiple NSAIDs. Luckily, renal complications due to NSAIDs are well-described with the result that there is judicious use of these medications in practice.
The equine kidney has a vital role in overall health. A variety of disease processes can impair renal function. Awareness of these diseases is important for equine health.
CONTACT: Jennifer Janes, DVM, PhD—firstname.lastname@example.org—859/257-8283—University of Kentucky Veterinary Diagnostic Laboratory, Lexington