Disease Alert Soursobs

The growth of soursobs this year is unprecedented. (Fig 1-2) It is a very toxic plant to horses, and you need to be aware of the consequences of allowing your horse to graze or reside in paddocks with this plant. If you think your horse won’t eat it, then he or she is in the 25% category. We did a taste test and 75%of horse offered the plant ate it. (Fig 3)


Figure 1 Soursob (Oxalis pes caprae)


Figure 2 Overgrowth of soursob just out of Gawler South Australia


Figure 3 Yum

Why oxalates are bad

Oxalates tie up calcium. The result is low blood calcium which is called hypocalcemia. Horses need to maintain a certain calcium level in the blood. In the acute form of hypocalcemia the blood calcium gets so low that the muscles start to become hyper excitable and the horses twitch and shake.  (see the video)  In the chronic form of toxicity, from eating plants with oxalates over a long period of time, the parathyroid gland and sends parathyroid hormone (PTH)to increase calium levels in the blood. PTH robs the calcium reserves in the bones to maintain blood levels, and the bones are weakened. In response the bones are filled with fibrous tissue that enlarges and disfigures the bones. This is especially true in the head, and is called bighead. Feeding excessive bran can also cause this. Bran is very high in phosphorous which also ties up calcium. Horses on soursob and bran are really at risk.


Figure 4 The mandible of a pony with soursob toxicity (Note the narrowing of the intermandibular space (arrow) and the grossly thickened bones of the mandible)


Figure 5 Facial bumps


Symptoms can include:

Hyper motile colic
Excessive mouth sucking and chewing noises and activity
Shifting leg lameness
Unexplained weight loss
Thickening of the mandible at the junction (rami) near the incisor teeth. (Fig4)
Small bumps on the maxilla.  (Fig 5)

More severe symptoms may not be noticed until it is too late including:
Spontaneous fractures (Fig 6)
Ataxia and the inability to stand due to fractured vertebrae
Synchronous diaphragmatic flutter
or thumps where and heart and respiration are the same.
Muscular excitation with tremors
Renal damage
Unusual unexplained death


Figure 6 Left Pony with severe osteoporosis and a spontaneous fracture of the calcaneus Right normal hock

Treatment and prevention

Get them off the plant.
Calcium oral supplement

Please be aware that this is a problem that is currently only in the early stages.  The growing season will end in late spring when the temperatures rise and the rain stops. It takes a few months of ingestion before it really hits so start therapy now. Most of our cases were in August and September, but we are already seeing cases this year.
Call the clinic to discuss the treatment and diagnostic options.
It is not an expensive disease to treat.  Prevention is the best way to stop the disease.


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