UK DIAGNOSTIC ROUNDS (CONT.)
Magnesium is an essential mineral as its presence is vital for many enzymes of major metabolic pathways, in normal nerve conduction and muscle contraction, and in bone mineral formation. Approximately 60-70% of total magnesium in the bovine is stored in bones. “Grass tetany” occurs when the magnesium level in blood decreases rapidly, resulting in a decrease in the Mg concentration in the cerebrospinal fluid. With- out sufficient Mg present in spinal fluid, there is uncontrolled activation of the nerves resulting in constant overstimulation and contraction of muscles, appearing first as nervousness then muscle stiffness and rigidity (“tetany”), that can progress to convulsions then death.
Table 1: Antemortem Diagnostic Test Interpretation Mg Status
Plasma mg/dL
Normal Suspect
Subclinical
Plasma mmol/L
Symptomatic < 1.70
2.19-2.92 0.9-1.2 1.95-2.19 0.8-0.9 1.70-1.95 0.7-0.8 <0.7
Serum mg/dL
1.9-2.4 1.6-1.8 1.2-1.5 <1.2
Urine mg/dL
>10
2.4-10 1.7 < 1
Urine
mmol/L >4.4
1-4.4 0.7
< 0.4 <1
Low blood magnesium concentrations are not always associated with convulsions and tetany (Table 1). Tere is a close, fine-tuned relationship between Mg absorption and renal secretion in order to maintain Mg homeostasis in the extracellular fluid and the CSF over a wide range of blood Mg concentrations. Urine Mg concentration is considered a more sensitive indicator of Mg availability and can be used as a diagnostic test because urine Mg concentrations decrease rapidly as animals become hypomagnesemic. Te issue with urinary Mg concentration is that it varies with changes in urine concentration since renal regulation of water balance is independent of Mg status. Terefore, a correction calculation such as the “creatinine corrected urinary Mg concentration” (CUM) is necessary if not collecting urine over a 24-hour period.
In a recent case presented to the UKVDL, a beef producer delivered 2 cows (one black cow and one red cow) for necropsy and 6 serum samples from additional suspected cases. As of that morning, he had 5 dead adult cows within the previous 15 hours; all 5 were lactating cows with small calves at their sides (1 month±). Te owner described some affected cows as exhibiting rapid weight loss, others looked very dehydrated, and a couple had “thick snot” coming from the nostrils. Tere was no change in diet; the cows were on grass hay, water was gravity-fed from a spring into a trough, trace mineral was consistently available, and cattle were on the same pasture. No junk piles, no burn piles, no weed trimmings, and the woods were fenced off. No cattle had been added to the herd. He said the sick and dead cows were found around the hay and mineral feeders, about 300 yards away from the water. Te unaffected portion of the herd was on the other side of the field, near the water supply. He did not know why or how the herd got split up and there was nothing preventing the affected portion of the herd from getting to water. No calves were affected.
Table 2: Ocular Fluid Chemistry Panel Results Case
Sample Units
Phosphorus Magnesium BUN
Creatinine
mmol/L mmol/L mg/dL mg/dL mg/dL mg/dL mg/dL
Sodium mmol/L 133 Potassium Chloride Calcium
1.5
1.8 L 32 H 1.1 H
Black Cow Vitreous
6.4 H 113 5
Red Cow Vitreous H 169 H 9.2 H 139 5.7
H 4.2 H 2.7 H 120 H 2.5
*Ocular Ca is 65% of serum. < 4mg/dL in vitreous suggests hypocalcemia ** Clinical hypomagnesemia < 1.34 mg/dL in vitreous (48 hrs max)
Normal-UK Vitreous 128-146 3-6
107-133 *
0.3-1.7 2-2.6** 4-17
0.3-0.9
Continued on pg. 38 Winter 2023 37
CSF
mg/dL 2.4
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