UK DIAGNOSTIC ROUNDS Hypomagnesemia, Hypocalcemia and
Sodium Ion Toxicosis - Pick your Poison By Michelle Arnold, DVM, MPH, DABVP
Hypomagnesemic tetany is a rapidly progressing and potentially fatal disorder classically seen in older, lactating beef cows when grazing young, succulent grass in early spring, particularly during cool and rainy weather. Other common
(Food Animal); Ruminant Extension Veterinarian, Associate Professor. University of Kentucky Veterinary Diagnostic Laboratory (UKVDL)
names for this disorder, including spring tetany, grass staggers, wheat pasture poisoning, and lactation tetany, reflect the season of the year, symptoms seen, types of forage, or physiology of the animals most often affected. However, hypomagnesemia may also occur in unexpected circumstances, including post-drought.
Maintenance of normal blood magnesium (Mg) depends on daily absorption of sufficient Mg from the rumen to meet the amount lost to milk production, soft tissue and bone growth, fetal development during pregnancy, and the small amount in feces. Any excess dietary Mg is quickly excreted by the kidneys in the urine. How- ever, cattle have no effective tissue Mg res- ervoir so a shortage cannot be compensat- ed for by increasing Mg2+ ion absorption from other sites in the body.
In addition,
Mg is not under direct hormonal control to keep it in balance, as occurs with other major minerals. Although a simple lack of Mg intake in the diet can happen as in cas- es of starvation or if off feed, deficiencies are most often due to interference with Mg absorption in the rumen. Absorption basi- cally depends on 1) the amount of soluble Mg2+ ions available (“in solution”) in the rumen fluid and 2) the performance of the transport mechanisms that move Mg2+ ions across the rumen wall to the blood- stream. High K+ in rumen fluid is consis- tently cited as the most important factor in development of hypomagnesemia. Te movement of magnesium across the rumen wall depends on an active transport mech- anism (or “pump”) driven by an electrical potential created at the cell membrane. High potassium along with low sodium conditions alters the ion gradient required for active transport. When a drought breaks, new plant growth rapidly absorbs excess potassium in the soil, causing the first new plant shoots to contain high lev- els of potassium (K+) that can trigger grass tetany.
Continued on pg. 37 36 KVMA News
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