'Tetany' caused by Cape-Weed
- Jenny Paterson
- Oct 29
- 3 min read
These are the signs to look out for in case this happens to a horse near you!
Cape-weed, especially when it has been mowed, can cause ‘neurological’ issues in horses.
Sometimes signs are only mild as in being spooky but occasionally they are far more serious as in the case of the pony in the photo below.
Numerous horses grazing cool season grasses show signs of ‘walking stiffly’ (can be first sign of impending laminitis for instance) or being ‘bad with needles’.
The pony in the photo had spent less than an hour grazing the recently mown grass riddled with capeweed. Soon after, he was observed walking stiffly with his head arched back, locking up and tipping over on his side. He managed to get himself up again but it would take very little stimulation to set him off again.
The vet was called, bloods were drawn and results were negative for various mosquito-born viruses.
However, the blood tests did reveal LOW blood calcium levels* (while magnesium levels were good).
Having seen these same kinds of ‘Grass Tetany’ type symptoms’ multiple times over the years we knew what to do.
The hallmark signs that this was a ‘tetanic’ episode being:
spasmodic bouts of rigidity, ‘locking up’ and tipping over (as in the photo)
highly adrenalin reactive. The pony was very easily startled which would instantly trigger an ‘episode’.
extremely averse to needles!
bloodshot eyes
the positive response to the appropriate mix of minerals/vitamins his owner syringed into him multiple times per day over the next week.
I have to say this pony’s owner was amazing, despite, as you can imagine, this not being an easy task with the pony so easily triggered.
The amounts were adjusted according to the improvement observed while this regime was kept up until he was no longer having episodes.
By the time bloods were re-run he was absolutely calm and fine about needles!
His blood calcium level had come back up to where it should be.
We also took a sample of the offending ‘grass/cape-weed’ and had it sent away for analysis:
Results were not unexpected:
Potassium 3.75% (far too high)
Crude protein 30%(far too high)
Nitrates 2890 mgs/kg (far too high for equines)
DCAD …………700 (through the roof – needs to be closer to100 for horses)
ALL of these serious imbalances of nutrients collectively caused the low blood calcium.
The very high potassium being of major concern.
From Andre Voisin “Grass Tetany” :
*Tetany, therefore, is not a disease but a symptom that may be the result of several causes, one of the best known being disturbed calcium metabolism. the animal ‘will walk very stiffly, their hind legs having lost flexibility’.
(Acute Grass Tetany occurs more commonly in cattle under certain conditions, where they can succumb and die very quickly).
Broadleaf plants like cape-weed, cat’s ear (abundant here in NZ atm), clover, trefoil and plantain pose no problem to horses if they only eat a few plants here and there. The trouble arises when the conditions favour their proliferation to where they out-compete the grass. Ideally steps are taken to make sure this doesn’t happen especially where horses are grazed.
Be aware these plants become far more potent after being mowed!
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