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Lactose Intolerance in Foals

The condition of primary lactase deficiency, commonly referred to as lactose intolerance, in foals is rare and arises from the failure of the intestines to secrete the enzyme lactase. Lactase enzymes are made in the small intestine by absorptive cells located on finger-like projections of mucosa on the area known as the brush border. Lactase enzymes are essential to breaking down the lactose in the dam's milk. Lactose is an essential sugar present in milk and contains both glucose and galactose units. When lactose cannot be broken down, the undigested and unabsorbed nutrients enter the lower bowel. The additional sugar causes an increase in bacterial fermentation and the retention of excess water and electrolytes. Lactose intolerance often manifests with symptoms of persistent diarrhea, bloating, weight loss, colic behaviors (looking at or biting sides, rolling, depression, etc.), and general failure to thrive.

Unlike primary lactase deficiency, secondary lactase deficiency is not uncommon and occurs when the cells of the mucosal brush border in the small intestines are damaged from bacterial infections such as Rotavirus or Clostridium or a high parasitic load in the intestines – specifically from threadworms, large roundworms or Giardia.

Diagnosis of lactose intolerance can be made based on clinical signs or, more definitively, by administering an oral lactose tolerance test to assess whether lactase activity is present or not. This test requires a lengthy fast, followed by blood samples being taken at 30-minute intervals for 3-4 hours after dosing. Another option is to orally administer an exogenous lactase enzyme and observe if the foal’s symptoms lessen.

Exogenous lactase enzymes are synthesized from fungi or yeasts and are available in various forms of gels, liquids, capsules, or tablets. Tablets can be crushed, mixed with yogurt, and syringed into the foal – this is usually accepted well by the foal. In many cases, when exogenous lactase enzymes are administered orally, the foal’s condition can improve dramatically, especially if the lactase deficiency was triggered by a viral, bacterial, or protozoal diarrheal disease that is also being treated. In rare cases feeding a soy-based, non-lactose-containing foal milk replacer, along with early weaning and the introduction of small amounts of high-quality hay or grain to help meet energy needs, may be necessary for foals that do not respond favorably to other treatments.


Foal Diarrhea: Established and Postulated Causes, Prevention, Diagnostics, and Treatments. Olimpo Oliver-Espinosa, DVM, MSc, DVSc. 2018.

Foal diarrhea: causes, diagnosis and treatment (Proceedings). Allison Stewart, BVSc (hons), MS, DACVIM-LAIM, DACVECC. August 1, 2011.

Lactose Intolerance in Foals. M. M. Sloet van Oldruitenborgh‐Oosterbaan. 05 January 2010

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