Hyperlipidemia is a general term for increased levels of any or all of the lipids in plasma. In veterinary medicine, hyperlipidemia usually refers to elevated concentrations of cholesterol and/or triglycerides, since these are the lipids that are most commonly measured.

The most common cause of hyperlipidemia in small animals is postprandial sampling, typically detected as a hypercholesterolemia on a biochemical panel. Sample collection following 12 hours of fasting will help determine if pathologic hyperlipidemia is present. In veterinary medicine, pathologic hyperlipidemia is usually secondary to an underlying disease process rather than a primary disorder of lipid metabolism.

Primary hyperlipidemia

Primary hyperlipidemias have been reported in ponies and donkeys, although hyperlipidemia is most often a secondary condition in equines. Affected animals have decreased insulin sensitivity, and stress and obesity are predisposing factors.

In dogs, primary idiopathic hyperlipidemia is uncommon but has been described most completely in Miniature Schnauzer dogs from the United States. Affected dogs have fasting hypertriglyceridemia, and may have hypercholesterolemia, hyperchylomicronemia, and elevations of other lipoproteins. The frequency and severity of hypertriglyceridemia increase with age, and, although dogs are often asymptomatic for long periods of time, they may develop pancreatitis, hepatobiliary disease, ocular disease, seizures, and other complications. The cause of the disease is unknown, but a hereditary cause is suspected. It is recommended that all Miniature Schnauzer dogs in the United States be evaluated for hypertriglyceridemia while clinically healthy.

Primary hyperlipidemias consisting of hypercholesterolemia with or without concurrent hypertriglyceridemia have been described in a few other dog breeds, including the Briard and Rough Collie.

Primary hyperchylomicronemia has been documented in domestic breed cats and is due to a defect in lipoprotein lipase activity. Affected cats have marked hypertriglyceridemia and are prone to developing lipid deposits in the skin and retinas.

Secondary hyperlipidemia

Secondary hyperlipidemia occurs in horses due to anorexia, although pregnancy, lactation, obesity, pituitary pars intermedia dysfunction (PPID), and any illness that causes a negative energy balance can aggravate the problem. Anorexia results in mobilization of LCFAs from body fat stores. Horses do not form ketone bodies as readily as other species, such as cattle (see discussion of ketosis below), thus increased LCFAs in plasma tend to result in the formation of triglycerides in the liver which are then stored in the liver or released as very low-density lipoproteins. If the level of very low-density lipoproteins is sufficiently high in the plasma, visible lipemia occurs. This is a normal physiological response, and in most horses there are no long term effects of the condition. However, in some horses, and in miniature horses, ponies, and donkeys, the response may be excessive, resulting in marked hypertriglyceridemia, severe hepatic and renal lipidosis, and increased hepatic enzyme activities, sometimes culminating in death. The association of hypertriglyceridemia, obesity, insulin resistance, and hyperinsulinemia with laminitis is characteristic of equine metabolic syndrome (see Chapter 10: Endocrine System).

In small animals, many endocrinopathies, as well as cholestasis, pancreatitis, and protein-losing nephropathy, predispose to secondary hyperlipidemia. With hypothyroidism, hyperadrenocorticism, cholestasis, and protein-losing nephropathy, the fasting hyperlipidemia is primarily due to hypercholesterolemia. Therefore, lipemic plasma is not seen. Acute pancreatitis and diabetes mellitus in dogs are often associated with hypertriglyceridemia, hypercholesterolemia, or both. The mechanisms of hyperlipidemia in these conditions are likely to be multifactorial and are not fully elucidated. Obese dogs or those on a high fat diet may also have secondary hyperlipidemia.