Hard masses of necrotic fat are occasionally identified in the peritoneal cavity of mature cattle, especially the Channel Island breeds, Japanese Black cattle, and beef cattle grazing fescue for long periods. Abdominal fat necrosis has also been seen in goats and some species of deer maintained on pastures consisting primarily of tall fescue. These masses are commonly mistaken for a developing fetus on palpation per rectum because they feel like “floating corks,” similar to cotyledons. Clinical signs are uncommon; however, in cases in which many masses are present an extraluminal intestinal obstruction can result in episodes of moderate abdominal pain, distention of the proximal intestine, and decreased fecal production.
The composition of the deposits in cattle with fat necrosis is identical to that of fat in healthy cows. The abnormal fat deposition is confined to the abdominal cavity. Current understanding suggests that the etiology of abdominal fat necroses is differs from that for fat deposits elsewhere in the body. Fat necrosis has historically been termed lipomatosis; however, this term is no longer appropriate because the masses are neither neoplastic nor hyperplastic.
The specific etiology is unknown; one proposed cause is consumption of feeds containing high concentrations of long-chain, saturated fatty acids. Fat necrosis is most commonly seen in beef cattle ≥2 years old after prolonged grazing of tall fescue infected with the endophyte Neotyphodium (Acremonium) coenophialum; fat necrosis tends to be associated with pastures with endophyte infection rates of ≥65%. Fat necrosis occurs throughout the southeastern US, where tall fescue is the primary pasture plant for grazing. Overconditioning, genetic factors, and lack of exercise may be associated with fat necrosis in cattle.
Hard masses of necrotic fat form in the omentum, mesentery, and perirenal fat. The masses are usually flat and rarely pedunculated and may cause clinical signs when they compress the abomasum, small intestine, and spiral colon; obstruct the birth canal; or more rarely compress the ureters.
Palpation per rectum is useful in diagnosis and in determining prevalence in a cattle herd. Advanced cases in aged dairy cows may be detected by abdominal ballottement with the identification of large firm masses in the abdomen. Ultrasonographic examination of the abdomen reveals the presence of hyperechoic masses of variable size in the omentum, with free or localized masses appearing to float in excessive peritoneal fluid. Hyperechoic masses adjacent intestines may be associated with luminal constriction. A presumptive diagnosis of abdominal fat necrosis can be made by means of ultrasound-guided biopsy of the echogenic masses or by direct biopsy during right flank exploratory laparotomy. Less commonly, isolated fat masses may be found freely floating in the peritoneal fluid at surgery.
The size of the masses usually slowly increases; however, spontaneous resolution can occur. Removal of cattle from fescue pastures or dilution of fescue intake by supplying legume or other grass pastures can produce slow decrease in the size of masses. Isoprothiolone (50 mg/kg per day, orally, for 8 weeks) increases lipolysis of adipose tissues and inhibits lipid deposition into adipocytes, reportedly decreasing the size of fat necrosis lesions in Japanese Black cattle.
In affected deer herds, 90% of females may be affected with fat necrosis. Clinical signs include gradual onset of anorexia, depression, and uremia due to large masses of necrotic abdominal fat obstructing the ureters; the latter causing hydroureter and hydronephrosis.
A second form of abdominal fat necrosis in domestic animals, less well defined, appears to be related to pancreatitis. Although not associated with specific clinical signs, the lesions (discrete or confluent masses of necrotic adipose tissue) are usually confined to peripancreatic fat. However, fat necrosis lesions may also be found throughout the abdomen.
A third form, a focal necrosis of abdominal and retroperitoneal fat (steatitis or yellow-fat disease), occurs most often in sheep but also affects pigs, horses, cats, and other species. Little information is available about the condition in these species; however, abdominal radiography or transabdominal ultrasonography may help identify focal necrosis in cats.