UK DIAGNOSTIC ROUNDS (CONT.)
At necropsy, vitreous humor from both cows was submitted for ocular fluid chemistry (Table 2). Postmortem samples of CSF that test below 1 mg/dL of magnesium or samples of vitreous humor below 1.34 mg/dL Mg are reliable indicators of grass tetany if collected within the first 48 hours after death. Of note, low calcium in vitreous humor (Ca2+ < 4.01 mg/dL) is also a common finding in grass tetany cases. Ocular fluid results suggested possible hypomagnesemia; urine from the black cow was submitted for urine creatinine and mag- nesium in order to calculate the creatinine corrected urinary Mg concentration (CUM), the most practical and sensitive guide to Mg status. Te black cow had a 0.7 mg/dl urine Mg and 83.0 mg/dl urine creatinine. Of note, the Red cow’s urine was a “gel” and could not be processed as a fluid. Conversion to mmol/L: Magnesium mg/dl X .4114= mmol/L [Urine Mg= 0.7 x 0.4114= 0.288 mmol/L] Creatinine mg/dl X .0884=mmol/L [Urine Cr= 83.0 x 0.0884=7.337 mmol/L] • CUM=urinary Mg (mmol/L) /urinary creatinine (mmol/L) • CUM= 0.288/7.337 = 0.039 mmol/L • CUM < 1 mmol/L is deficient; <0.4 is clinical (Black Cow =0.039)
Serum chemistries were performed on the six serum samples from affected cattle (Table 3). Serum is not always an accurate sample as muscle damage may cause leakage of Mg2+ ions from within cells into the bloodstream, causing an artificially high Mg result. Samples #1 and #5 had serum Mg concentrations of 1.1 mg/dL (Clinical hypomagnesemia < 1.2 mg/dL serum Mg) and serum Ca concentrations of 6.5 and 7.9 mg/dL respectively (Clinical hypocalcemia < 7.6 mg/dL serum Ca). Serum calcium is often < 8.0 mg/dL in grass tetany cows because hypomagnesemia impairs the release of parathyroid hormone (PTH), increasing the risk of concurrent hypocalcemia. A presumptive diagnosis of hypomagnesemia was communicated to the veterinarian and treatment administered with subcutaneous CMPK and oral Mg gel. A total of 7 mature, lactating cows died within 24 hours.
Table 3: Serum Chemistry Panel Results Serum Units #1
#2 5.1 #3 4.9 #4
Sodium 132-152 mmol/L H 159 H 169 H 174 138 Potassium 3.7-6 mmol/L 4.7 Chloride 91-111 mmol/L 104
5.8 Calcium 8-11.5 mg/dL L 6.5 8.4
Phospho- rus
Magne- sium
108 H 118 97 L 7.7 9.6
3.5-8 mg/dL H 16.7 H 10.1 H 11.0 4.6 1.7-2.9 mg/dL L 1.1 1.9
2.1 2.1
BUN 5 - 27 mg/dL H 86 H 50 H 82 14 Creatinine 0.7-2 mg/dL H 4.1 H 2.5 H 6.5 1.5 Outcome
Died Tx:CMPK Mg Gel pre-tx
lived tx
died trauma tx #5 105 #6
147 H 160 5.2
5 109 L 7.9 8.5
H 10.3 H 9.5 L 1.1 1.7 20
1.9 lived
lived tx
H 60 H 2.9 lived tx
Te complicating factor in this case was the extreme level of dehydration in several of the affected cattle, resulting in hemoconcentration in serum samples 1, 2, 3 and 6 and a diagnosis of sodium ion toxicosis/water deprivation in the necropsied red cow. Hypernatremia is defined as serum sodium concentrations of 160 mEq/L or greater; ocular fluid sodium is reportedly 95% of the serum sodium value. Te brain tissue sodium from the black cow was within normal limits at 1380 ppm while the red cow was extremely elevated at 2170 ppm sodium (>1800 ppm sodium is indicative of sodium ion toxicosis/water deprivation). Why the cattle did not go to available water is unknown, but it is possible the hypomagnesemia and hypocalcemia prevented them from reaching the water source.
Cattle exhibiting symptoms of grass tetany need immediate veterinary treatment; preferably 1.5-2.25 grams of magnesium administered slowly by intravenous injection for an adult cow. Most commercially avail-
38 KVMA News Continued on pg. 39
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