Incidence, gross features, and histologic characterization of feline fibrocystic hepatobiliary lesions and extrahepatic biliary malformations.

Feline fibrocystic hepatobiliary lesions (FHLs) and extrahepatic biliary malformations (EBMs) are supposedly common, but prevalence studies and detailed gross and histologic descriptions within a large cohort are lacking. Our aims were to identify the prevalence of FHLs and EBMs within a large feline population, characterize gross and histologic features, and report comorbidities. Of 140 cats, 20 (14.3%) had either FHLs and/or EBMs. FHLs were most common (15/20, 75%) and included cystic biliary lesions (CBLs, 14/20, 70%) and congenital hepatic fibrosis (CHF, 1/20, 5%). CBLs had gross variability but histologically comprised multiloculated cysts lined by biliary epithelium overlying variably hyalinized collagenous trabeculae. Gross features of CHF included severe hepatic fibrosis with embedded biliary cysts, parenchymal collapse, and nodular regeneration. Histologically, there was loss of lobular architecture, proliferation of small irregular to cystic bile ducts, bridging fibrosis, inapparent portal veins, and scattered hepatocellular necrosis. Gallbladder malformations were present in 5/20 cats (25%) and were classified as duplex (2/5) or bilobed (3/5). Two of 20 cats (10%) had anomalous, blind-ended, extrahepatic bile ducts extending from the gallbladder into the pancreas/peripancreatic mesentery. FHLs and EBMs were rarely concurrent (1/20 cats, 5%). CHF contributed to significant morbidity, but other FHLs/EBMs were incidental. Concurrent pancreatic/renal cysts were uncommon (2/20, 10%) and were seen in association with a CBL and CHF. Common comorbidities included cardiovascular and/or renal disease and neoplasia. We provide evidence that the majority of feline FHLs represent ductal plate malformations, with CBLs having morphologic features compatible with von Meyenburg complexes (VMCs)/biliary hamartomas.