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Disease Investigation 2

INVESTIGATION OF DISORDERS OF LIPID METABOLISM IN SMALL ANIMALS AND EQUINES

Lipaemia is a common finding in blood samples submitted to the laboratory from small animals, particularly dogs. Whilst this is usually a post-prandial effect, a proportion of these cases represent pathological lipaemia. Hyperlipaemia in equines is a lipid metabolism disorder affecting ponies, donkeys and miniature horse breeds. Laboratory investigation of the condition is necessary to confirm a clinical diagnosis and to assess the degree of organ failure.

DOGS

As in other species, lipids are transported in the blood in lipoprotein complexes. The presence of excess chylomicrons and/or very low density lipoproteins in the blood leads to gross lipaemia.

Investigation of a lipaemic sample involves first assessing whether the lipaemia is a post-prandial effect. Resampling after a 16 hour fast is recommended. If this fasted sample is lipaemic or if the plasma triglycerides are >2.25 mmol/l then the lipaemia is likely to be of pathological significance. Cholesterol levels should also be checked at this point along with amylase and lipase to assess possible pancreatic damage.

A pathological lipaemia may be due to an inherited defect in lipid metabolism. This is rare in the dog, would most commonly be seen in a young puppy and is recognised as idiopathic hyperlipoproteinaemia in Miniature Schnauzers. More commonly the lipaemia is secondary to diabetes mellitus, hypothyroidism, hyperadrenocorticism or nephrotic syndrome. Identification of the underlying cause requires appropriate screening tests.

Free T4 and cholesterol (FT4C) or the TRH Stimulation Test (TRHS) are recommended for screening for hypothyroidism. A fasting plasma glucose (GLU), serum fructosamine (FRUC) and urinalysis (UA) are recommended for the diagnosis of diabetes mellitus. Hyperadrenocorticism may be diagnosed using the ACTH Stimulation Test (ACTH) or Dexamethasone (Low Dose) Screening Test (DSCR). A Proteinuria Investigation (PI) is appropriate for the diagnosis of nephrotic syndrome (proteinuria, hypoproteinaemia, hypercholesterolaemia and peripheral oedema) and for assessment of the degree of the proteinuria.

Treatment of the underlying cause will usually result in resolution of the lipaemia. Where no underlying cause is identified, the lipaemia should be considered idiopathic and a low fat diet should be prescribed. Plasma triglycerides, amylase and lipase should be monitored on a regular basis as dogs with severe and persistent lipaemia are at risk of developing acute pancreatitis.

CATS

Lipaemia is much less common in blood samples submitted to the laboratory from cats than it is in samples from dogs. Again investigation of a lipaemic sample involves resampling after a 10 to 16 hour fast. If the fasted sample is grossly lipaemic, if plasma triglycerides are >1.5 mmol/L or if plasma cholesterol is >6.5 mmol/L, then the lipaemia is likely to be of pathological significance. (Plasma cholesterol levels up to 14 mmol/L and triglyceride levels up to 2.5 mmol/L may be seen in healthy kittens up to 4 months of age). In a cat of less than one year inherited hyperchylomicronaemia should be considered, particularly if the blood sample forms a creamy layer after overnight refrigeration (due to excess chylomicrons) and if plasma triglycerides are >10 mmol/L. Otherwise a differential diagnosis of lipaemia secondary to diabetes mellitus, glomerular disease, cholestatic disease, hepatic lipidosis, hyperadrenocorticism, acromegaly, obesity or antithyroid therapy should be considered.

A fasting plasma glucose (GLU), serum fructosamine (FRUC) and urinalysis (UA) are recommended to screen for diabetes mellitus. A Proteinuria Investigation (PI) may be used to investigate glomerular disease, whilst a Liver Profile (LP) would assist in investigating hepatic lipidosis or cholestatic disease. Hyperadrenocorticism may be diagnosed using the ACTH Stimulation Test (Feline) (ACTF) whilst IGF1 assay (GH) my assist in the diagnosis of acromegaly.

Treatment of the underlying condition should lead to resolution of the lipaemia. Rarely, no underlying cause will be identified; the lipaemia should then be considered idiopathic and a low fat diet should be prescribed. Plasma triglycerides and cholesterol may again be monitored on a regular basis. Unlike the dog, the association between pathological lipaemia and pancreatitis is not clearly defined. Also plasma lipase and amylase are not reliable indicators of pancreatitis in cats so their monitoring cannot be recommended.

EQUINES

Hyperlipaemia in equines is associated with pony breeds usually older than 18 months, is more common in mares than stallions or geldings and is usually associated with pregnancy. The majority of affected animals are overweight and stressed, malnutrition and concurrent disease are predisposing factors. Clinical signs are variable, reflecting the presence of hepatic or renal dysfunction, but include inappetence and dullness initially leading through incoordination, weakness and nervous symptoms to recumbency and convulsions. Case mortality rates (including euthanasia) range from 60% to >85%.

A diagnosis of hyperlipaemia is confirmed by a plasma triglyceride concentration >5mmol/L. Clinical biochemistry is required to assess hepatic and renal damage/function (EEP) and the metabolic status of the animal prior to and during treatment (EELP).

Test Code	Test Name	Sample
LIPP	Lipid Profile	Serum + OXF
PP	Pancreatic Profile	Serum or Heparin Plasma
ELIP	Equine Lipid Profile	Serum + OXF
EEP	Equine Extra Profile	Serum
EELP	Equine Electrolyte Profile	Serum or Heparin Plasma