Acute Protein-Losing Enteropathy in a Horse

History. You are called to a home a few miles from your clinic by parents who are concerned about their daughters 4-year-old quarterhorse mare. They report that the horse is listless and has had diarrhea for 2 days.

Clinical Examination. You arrive at the client’s home to find that the horse is stabled in a small barn with no access to pasture. Poor-quality grass hay is stacked in the barn. On physical examination, you find the horse to be somewhat emaciated, with dry mucous membranes, a foul-smelling diarrhea, and a fast heart rate (tachycardia). When you pinch up a section of the horses skin, it falls back to the normal position slowly, which indicates dehydration.

You take a blood sample and then begin an intravenous administration of polyionic fluid (lactated Ringers solution). You tell the clients that you will return later. Analysis of the blood sample indicates a hematocrit of 55% (normal range for the horse, 35%-45%) and a plasma protein concentration of 4.5 g/dL (normal range, 5.9-7.8 g∕dL). You become con­cerned that the administration of fluids, without replacement of plasma proteins, will exacerbate the horse’s hypoprotein­emia, so you arrange to obtain plasma from a donor horse. You return to examine the sick horse and find it is still listless. Edema is now evident along the ventral abdomen and in the limbs.

Comment. Acute enteropathy (intestinal disorder) often causes diarrhea. The loss of water and solutes leads to dehydra­tion. Before treatment, blood volume and interstitial fluid volume are both reduced. The hematocrit (fraction of cells in blood) is typically elevated because fluid is removed from the bloodstream but blood cells are not. In some forms of enter­opathy (called protein-losing enteropathy) the capillaries in the intestine become leaky to plasma proteins. Albumin, in par­ticular, moves from the bloodstream into the intestinal lumen and is eliminated in the feces.

This horse has a severe shortage of plasma proteins. The shortage of plasma proteins probably resulted from a com­bination of poor nutrition (which depresses the production of plasma proteins by the liver) and the protein-losing enter­opathy. The deficit of plasma proteins in this horse is even more severe than might be suspected on the basis of the plasma protein concentration of 4.5 g/dL, because this value is the net result of two opposing processes. The loss of protein in the diarrhea lowered the plasma protein concentration, but the loss of water (dehydration) decreased plasma volume and therefore increased the plasma protein concentration.

The administration of intravenous fluids adds water and electrolytes to the circulating blood volume, but the plasma proteins remaining in the bloodstream are further diluted. As a result, plasma oncotic pressure decreases even further, and this leads to excess filtration of water out of capillaries and into the interstitial space. The result is edema, especially in the dependent regions of the body (ventral abdomen and legs). Restoration of a normal plasma protein concentration would reverse the edema.

Treatment. The enteropathy in cases such as this one is often self-limiting. Therefore the aim of treatment should be to remedy the dehydration, the electrolyte loss, and the plasma protein deficit. Intravenous administration of both polyionic fluids and plasma is usually effective. Better nutrition, regular deworming, and improved stable management would be important steps for long-term health.