Hypomagnesaemic tetany (grass tetany) is most common in late pregnant and lactating cows. Clinical signs develop when there are low magnesium levels in the cerebrospinal fluid and neurological signs and death are seen. Low magnesium levels in the cerebrospinal fluid result from serum magnesium levels falling below a critical level. (Radostits et al 2007)
Affected animals may appear excited, develop muscular spasms and convulsions and can die of respiratory failure. Often animals are found dead with no clinical signs observed beforehand. There may be evidence of rubbed ground surrounding the dead cattle indicating paddling and excitement prior to death.
Most cows in the New England calve in the June-August period, which coincides with cold and often wet conditions. Despite this, hypomagnesaemia is not commonly diagnosed in the New England except occasionally in specific localities. When grass tetany occurs the usual disease course follows with clinical signs (and death) in a number of animals, over several weeks and is usually associated with heavy cows at peak lactation on grass dominant pastures.
A mob of mature aged Poll Hereford cows (5-7 year old) were joined to a Shorthorn bull and had begun calving four weeks earlier. The cows were grazing improved pasture and were being supplemented with a proprietary dry mix at 250g/head/day (recommended 100-500g/head/day) containing a mixture of minerals and trace elements.
Four cows died suddenly over one week. All cows were apparently in good condition and had four-week-old calves at foot. Other cow mobs hadn't suffered any losses.
The cow (Figure 1) was in very good condition and presented with left side up. There was evidence of recent predation of the left eye. There was some bruising on the dental pad of the maxilla. Mammary tissue appeared normal. No abnormalities were detected in the abdominal cavity. Aside from serosanguinous pericardial fluid, there were no other abnormalities detected in the thoracic cavity.
Aqueous humor was collected from the right eye facing down (protected from predation) via vacutainer and needle, to measure magnesium levels.
The magnesium level in the aqueous humor was 0.54 mmol/L (normal range: 0.48-1.5 mmol/L). Despite the result being in normal range, the laboratory notes 'interpretation of Ca/Mg results for eye fluids is dependent on time post-mortem prior to sampling, with both concentrations increasing with time.' Given that the cow had likely been dead for several hours, it is presumed that magnesium levels at the time of death were lower than what was reported.
As hypomagnesaemia was suspected, the owner was advised to move the cattle to a paddock with higher clover content and different pasture.
Advice also included the addition of a magnesium supplement to the diet at a level above that of the existing supplementation. The loose lick supplement being provided contained 10g of Mg/kg. The rate of 250g of loose lick/head/day equated to an average of 2.5g Mg/head/day. This was thought to be insufficient and an addition of 50g Mg per/head/day was advised.
Following the paddock change and increase in magnesium supplementation no further deaths occurred.
Grass tetany is diagnosed by a combination of history, clinical signs and interpretation of laboratory results.
Low serum magnesium levels can be measured in live cattle by taking a blood sample for laboratory analysis. Post-mortem magnesium levels can be determined using a sample of aqueous humor. It may be difficult to confirm a diagnosis of hypomagnesaemia post-mortem as fluid magnesium levels change (generally rising) over time. In this case, as this cow had likely been dead for several hours, it is presumed that magnesium levels at the time of death were deficient. As the history and clinical signs are consistent with hypomagnesaemia, it was considered the most likely diagnosis.
Grass tetany is prevented by maintaining adequate levels of blood magnesium levels, which is often achieved by magnesium feed supplementation. Magnesium oxide 'Causmag' is a routinely recommended and commonly used feed supplement in the New England area. Causmag is often mixed with hay and fed daily to susceptible cattle at the rate of 60g/head/day. (Elliot 2009)
With clinically affected cases, it is possible to provide treatment in the form of a magnesium/calcium combined injection, which can return an animal from near death. Despite treatment being effective, the underlying cause of hypomagnesaemia needs to be addressed to avoid further relapses or other individuals in the same herd suffering the same fate.
The cause of hypomagnesaemia which results in grass tetany is complex and is often multi-factorial. The following is a list of factors that can contribute to low blood (and hence cerebrospinal) magnesium levels which cause grass tetany. (Elliot 2009)
The diagnosis of grass tetany was made on the basis of a history of older, fat, lactating cows grazing grass dominant, improved pasture. In this case, initial supplementation of magnesium was also inadequate. Other mobs grazing similar pasture did not experience grass tetany possibly because they were younger or at a different stage in pregnancy/lactation.