Willow is an obese nine-year old spayed female Siamese cat who lives indoors. Her owner has brought her in because he's a little concerned about her. "She's always been a picky eater, but lately she doesn't seem much interested in food; she seems depressed," and "She's normally very vocal, but the past three weeks she hardly makes any noises at all, which is pretty unusual for a Siamese." He reports that she's vomited occasionally without any obvious reason for it. He's tried different and new brands of cat food, without much luck.
You ask if there is any chance Willow could have eaten something toxic, but he says that because the family has a toddler, all the cabinets with anything danreous in them are locked; and that there aren't any houseplants. Further history-taking discloses that Willow had been boarded at a cattery for a week and a half while her owner went away to settle an estate; and that she had never been boarded prior to that experience. The symptoms began to be noticeable about 3 days after Willow came home, and the woner's worried that she has contracted an infection of some kind, despite the cattery's sterling reputation and sparkling cleanliness.
On physical examination you find icterus (seen by yellowing of the sclerae) and a mild hepatomegaly. This is confirmed by radiography and ultrasound, that show slight enlargement of and a hyperechoic liver. You order blood work. This shows an elevated level of ALT (alanine aminotransferase) and AST (aspartate amino transferase).
It's pretty obvious that Willow has a liver disease. Given her history of recent boarding and dietary changes and the absence of other obvious possibilities such as poisoning, you diagnose idiopathic feline hepatic lipidosis. To say that something is "idiopathic" is an admission that you haven't really got any idea what caused it: but this condition is thought to be triggered by stress, dietary change, and environmental alterations. Here you have a fat spoiled kitty whose comfortable life was rudely disrupted when her Human dumped her at a Cat Prison, where they served terrible food and made her associate with other cats whom she intensely disliked.
Steatosis is a form of cellular injury in which triglycerides accumulate in cells, resulting in lipid vacuoles of varous size to infiltrate the cytoplasm. Steatosis can occur in the kidney, skeletal muscle, cardiac muscle, and hepatocytes, with the most common site being hepatocytes. Fatty change in the liver is referred to as hepatic lipidosis.
All the things which cause accumulation of triglycerides in tissues have a single feature in common:they alter the ability of the cell to regulate its uptake of fatty acids, their synthesis into triglycerides, or their ability to transport them out of the cell. In this sense, steatosis is a storage disease of triglycerides. In mild or transient cases, steatosis is reversible and does not have any adverse effects on the organ in question. Even in severe cases there may be no derangement of function, but more often cellular function is deranged due to overcrowding caused by the lipid vacuoles. In severe cases involving the liver, the color and consistency of the organ is affected so greatly that the liver appears yellow and becomes soft and greasy.
In Willow's case, in addition to the indignities and stress of being boarded, she had a lot of excess body fat to begin with, far in excess of the metabolic needs of a cat leading a blameless and indolent life. The liver stores fat within hepatocellular vacuoles, which are later processed, but Willow was "on the edge" of lipidosis anyway, and the boarding and other changes tipped her over it.
This is what Willow's liver would look like if you'd subjected her to a biopsy. The hepatocytes appear irregular and swollen, and well they should because they are filled with fat. The image at left is stained with H&E; the one at right with Oil red O, a stain specific for lipids. below at left is the liver from another case: note the enlargement, the patchy hyperremia, and the yellowish discoloration resulting from blockage of the bile drainage.
On this more magnified view, you can see fatty vacuoles in nearly every cell of the sample. The architecture of the liver has not been deranged too badly by the lipid infiltration - the basic structures of cords and sinuses are still visible. In severe cases, the hepatocytes can be so swollen as to cause functional derangements of the liver simply because the amount of fat stored in the cells affects the normal organelles.
This gross specimen shows how bad it can get: this liver is greatly enlarged and has a yellowish discoloration due to fat and the retention of bile in clogged drainage passageways. Extremely fatty livers can become so swollen that they occlude the flow of blood, leading to regional necrosis; furthermore severe steatosis can occlude the flow of bile, causing cholestasis, severe irritation, and jaundice. of course, there's sup[posed to be some fat in a healthy liver; one of its functions is to process fatty acids for eventual packaging as triglycerides in chylomicrons. A certain degree of fat in the hepatocytes is not only healthy but essential. But when there's more lipid than can readily be processed, liver dysfunction becomes a real possibility. If it goes beyond a certain point, liver damage and liver failure will occur.
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