The purpose of this study is to describe a polycystic kidney disease (PKD) in adult captive pygmy hippopotamuses (Choeropsis liberiensis liberiensis) from the Smithsonian National Zoological Park (SNZP). A total of six pygmy hippopotamuses were diagnosed at necropsy with PKD. Four cases had moderate to severe renal lesions that contributed substantially to their death and two hippopotamuses had earlier stages of disease development but died from complications associated with unrelated problems. All pygmy hippopotamuses were female in the late second to early fourth decade of life. Two pygmy hippopotamuses were wild caught and four were captive born at the SNZP. Two of the pygmy hippopotamuses were siblings from the same wild caught dam which also had PKD. The different sires of these two sibling pygmy hippopotamuses also had one offspring each with PKD. One pygmy hippopotamus with PKD was unrelated to the other five pygmy hippopotamuses and was wild caught.
Clinical signs exhibited by pygmy hippopotamuses with PKD were anorexia (four animals), lethargy (three animals), weight loss (one animal), polydipsia (one animal), and polyuria (one animal). One animal had no premonitory clinical signs prior to death. Two pygmy hippopotamuses had BUN values of 300 mg/dl and 250 mg/dl respectively at time of death; one of these animals also had a creatinine value of 9.8 mg/dl.
In all animals, reniculated kidneys contained few to numerous, variably sized cysts that partially to completely effaced the cortices and medullae. Cysts contained clear, light yellow to dark brown fluid. A few cysts contained small dark red-brown calculi. Two pygmy hippopotamuses had inanition. There were no other gross lesions associated with the renal disease. Histologically, the kidneys had marked interstitial fibrosis primarily within the medullae. Many collecting ducts and renal tubules were ectatic and filled with hyaline material. Some ectatic renal tubules were lined by hyperplastic and dysplastic epithelium. In addition, there was membranous glomerulonephropathy and renal arteriosclerosis.
This renal disease has histologic features of an acquired polycystic kidney disease, but the pedigree suggests a familial pattern. Complications associated with this renal disease have been a significant cause of death of pygmy hippopotamuses at the SNZP.