Tribulus, or Coonabarabran, staggers have been reported around the Coonabarabran district since the 1940’s. The occurrence of the stagger has a long association with drought, where summer storms have occurred on bare paddocks and the subsequent predominating pasture was Tribulus terrestris.
In December of 2018 reports of Coonabarabran staggers were received after parts of the district had experienced a small summer rainfall event, resulting in the growth of weeds. Several farms which experienced an outbreak had Tribulus terrestris germinate on fallow paddocks, and farmers starved for feed and caught up with daily feeding allowed it to grow and subsequently allowed sheep to graze it.
A mob of 1000 three to five year old merino ewes reportedly exhibiting ataxia were investigated. The farmer had noted the development of lameness in the mob when he mustered them for weaning. No abnormalities were noted when the sheep were brought in for marking one month prior. Lambs did not appear affected.
Basic neurological examination of a severely affected (stage 2) ewe was performed and a unilateral hind limb weakness, exacerbated by stress was identified, proprioception and deep pain responses were normal, as was mentation, palpebral and menace reflexes.
The ewe was euthanised via exsanguination and a post-mortem performed. Grossly, the only abnormality detected was decreased muscle mass on the affected hind limb. Histology was performed on the brain, spinal cord, affected and unaffected muscle, sciatic nerve, liver and kidney. The only abnormality detected was mild Wallerian degeneration of the spinal cord and sciatic nerve.
A diagnosis was made based the presence of the stagger as described by C.A. Bourke (1984);
"Stage 1 corresponded to a mild hindquarter disturbance. Affected sheep frequently stood with hind feet wide apart, and when they walked, their hip action was clearly awkward.
Stage 2 followed as a natural progression from stage 1. At rest, the rump dropped, the stifle and hock joints were flexed and the hindquarters were observed to be leaning to a particular side. The hind leg on the side which the hindquarters leaned was placed under the midline of the body. The hind leg on the other side was held in the abducted position. When running, affected sheep generally moved their hindquarters sideways, consequently the body progressed diagonally.
Stage 3 was characterized by the development of a foreleg weakness; affected sheep were unable to stand up or when assisted to their feet, to remain standing."
It was estimated that 2% of the flock was affected with variations of stages one and two. One animal was identified in stage three.
C.A. Bourke suggested that molybdenum deficiency may play a role in the development of Tribulus staggers. T. terrestris contains a purine called xanthosine which is broken down by an enzyme xanthine-oxidase-dehydrogenase. Molybdenum is required in the synthesis of xanthine-oxidase dehydrogenase.
How is this significant to the development of a stagger that only occur in one area? It is proposed that volcanic soils surrounding Coonabarabran are low in molybdenum, and furthermore acidic soils make molybdenum unavailable to the plant.
One study undertaken by C.A. Bourke (2012) demonstrated a correlation between molybdenum deficiency and high xanthosine and the development of a stagger similar to that seen with Tribulus staggers. This data however wasn’t statistically verified. Similar histological lesions were present in these animals as other animals naturally affected by Tribulus staggers.
Furthermore, sheep have a xanthine scavenging mechanism which means that no conversion of xanthosine occurs within the blood resulting in accumulated levels in the brain. Cattle and horses do not have the same mechanism and as such it does not accumulate in the brain and cause a stagger as it does in sheep.
Interestingly it has never been reported in dorpers. Previous evidence suggests increased occurrence in British breeds however this year it has been identified in many different breeds.
Huge Thanks To Judy Ellem and Callen Thompson for their help throughout this case.