Liver disease in the horse: diagnostic aids and differential diagnosis

Liver disease in the horse: diagnostic aids and differential diagnosis

Jul 01, 2007

The most common clinical signs of hepatic insufficiency in horses are weight loss, hepatic encephalopathy, icterus and colic.

Figure 1: Hepatic ultrasonography can provide helpful information on the size of the liver. In this patient, the liver appears rounded and swollen and actually is fractured (arrow) from excessive deposition of fat.
Because of the lack of specificity in clinical signs, liver-disease identification relies heavily on serum biochemical analysis. Although increases in sorbitol dehydro genase (SDH), gamma-glutamyl transpeptidase (GGT) activity and serum bile-acids concentration (SBA) are highly specific for liver disease in the horse, they are not specific for the type of disease.

Figure 2: Ultrasonography can help detect abnormal architecture. Here, a neoplasic lesion generated a focal hypoechoic lesion (arrows) that was distinct from the surrounding parenchyma.
Therefore, to further identify the cause of liver disease, diagnostic imaging and histopathology often are necessary. This second article will review liver ultrasonography, biopsy and differential diagnosis.

Diagnostic imaging of the liver

Figure 3: Biliary ducts (arrow) normally are not visible, but are in this horse with biliary obstruction. A short segment of a portal vein, characterized by short, hyperechoic parallel lines is visible (arrowhead).
Transabdominal ultrasonography is useful for obtaining information on the general size of the liver (Figure 1); changes in the hepatic parenchyma, including abscesses, cysts and neoplastic masses (Figure 2); and detecting dilated bile ducts (Figure 3) or obstruction with choleliths (Figure 4).

Figure 4: This patient has obstructive cholelithiasis that is characterized by the presence of a hyperechoic focus (arrow) that generates an acoustic shadow within a dilated bile duct (arrowhead).
Ultrasonography should be used to assess both the right and left sides of the liver, though the liver is most reliably imaged on the right side from approximately the 9th to the 14th inter costal spaces. The edges of the liver should appear crisply sharp.

The architecture of the normal equine liver should be relatively uniform and appear less echogenic than the spleen. The walls of portal veins are more echogenic than hepatic veins and often are seen as short, white parallel lines in the parenchyma (Figure 3).

The biliary ducts are not normally seen in healthy horses, and the common bile duct cannot be imaged transabdominally in adult horses.

Lack of notable size or architectural ultrasonographic changes in the liver do not rule out the possibility of significant disease. If the liver is small or there are focal ultrasonographic changes, ultrasonography is useful for guiding biopsy instruments into the liver.

Liver biopsy

A liver biopsy often provides the most specific information for an etio logic diagnosis. The procedure is performed in the standing horse at the right 12th to 14th intercostal space, at the intersection of a line drawn from the tuber coxae to a point midway between the elbow and the point of the shoulder. Sedation may be needed.

Although a liver biopsy can be obtained solely by using the previously mentioned anatomical landmarks, an ultrasound-guided biopsy often significantly increases the chances of obtaining a useful sample.

The skin overlying the area to be sampled should be clipped, aseptically prepared and a local-acting anesthetic injected subcutaneously. A stab incision is then made with a No. 15 scalpel blade. A Tru-Cut (Baxter-Travenol, St. Louis) biopsy instrument is inserted and directed craniad and ventrad through the diaphragm into the liver.

Semi-automatic biospy instruments and automatic biopsy guns are helpful for a quick and accurate liver biopsy.

Samples should immediately be placed in formalin for histopathologic evaluation and in transport media for culture.

Precautions to consider prior to performing the procedure include the risk of hemorr hage, pneumothorax and peritonitis from bile leakage and colon or abscess puncture, and spread from infectious hepatitis.