Dr. David Newman, DVM, ADDL Pathologist
Hypomagnesemia (often referred to as grass tetany, grass staggers, spring tetany, or lactation tetany) is magnesium (Mg) deficiency that occurs most often in the spring or fall when pastures are growing fastest that are high in potassium and nitrogen and low in Mg and sodium. It is most common in grazing lactating ruminants because milk production removes Mg uptake. Hypomagnesemia is a complex metabolic disturbance involving decreased blood and cerebrospinal fluid Mg levels that if untreated can lead to hyperexcitability, muscle spasms, convulsions, and death.
In the live animal diagnosis is usually confirmed by response, followed by confirmation of hypomagnesemia in blood samples collected prior to treatment. Tetany usually occurs when plasma total Mg is <1.2 mg/dL (0.5 mmol/L) in cattle and <0.5 mg/dL (0.2 mmol/L) in sheep. Mg concentrations <1.8 mg/dL (0.75 mmol/L) in the vitreous humor of the eye removed from animals within 24 hours after death are indicative of hypomagnesemia. Common differentials for hypomagnesemia include but are not limited to: hypocalcemia, encephalitis, listeriosis, and ketosis. Analysis of blood collected can be done at most veterinary clinical pathology laboratories. If analysis of a dead animal for hypomagnesemia is desired, remove the entire eye and freeze it, followed by submission to the ADDL using appropriate shipping procedures. Testing the vitreous humor for magnesium levels can be arranged at a referral laboratory.
Alternatively, an entire dead animal can be submitted to the ADDL for necropsy ($90 plus $8 accession fee) to determine the cause of death. Often, hypomagnesemia is a diagnosis of exclusion as postmortem changes are usually unremarkable and nonspecific.