The prevention and treatment of Birdsville disease of horses

Plasma l-indospicine and 3-nitropropionic acid in ponies fed creeping indigo: Comparison with results from an episode of presumptive creeping indigo toxicosis

BACKGROUND

Creeping indigo (Indigofera spicata) toxicosis is an emerging problem among horses in Florida and bordering states.

OBJECTIVES

To quantify the putative toxins l-indospicine (IND) and 3-nitropropionic acid (NPA) in creeping indigo collected from multiple sites and to measure plasma toxin concentrations in ponies fed creeping indigo and horses with presumptive creeping indigo toxicosis.

STUDY DESIGN

Experimental descriptive study with descriptive observational field investigation.

METHODS

Air-dried creeping indigo was assayed for IND and NPA content. Five ponies were fed chopped creeping indigo containing 1 mg/kg/day of IND and trace amounts of NPA for 5 days, then observed for 28 days. Blood samples from these ponies and from horses involved in a presumptive creeping indigo toxicosis were assayed for IND and NPA.

RESULTS

IND in creeping indigo plants was 0.4-3.5 mg/g dry matter whereas NPA was <0.01 to 0.03 mg/g. During creeping indigo feeding, clinical and laboratory signs were unchanged except for significant weight loss (median 6%, range 2%-9%; p = .04) and significant increase from baseline plasma protein concentration (median 16 g/L, range 8-25 g/L; p < .001). These changes could not definitively be ascribed to creeping indigo ingestion. Plasma IND rose to 3.9 ± 0.52 mg/L on day 6. Pharmacokinetic modelling indicated an elimination half-life of 25 days and a steady state plasma concentration of 22 mg/L. Plasma IND concentration in sick horses during an incident of creeping indigo toxicosis was approximately twice that of clinically normal pasture mates. Plasma NPA was <0.05 mg/L in all samples.

MAIN LIMITATIONS

Creeping indigo used in the feeding trial may not be representative of plants involved in creeping indigo toxicosis. There was no control group without creeping indigo in the feeding trial.

CONCLUSIONS

Indospicine can be detected in blood of horses consuming creeping indigo and the toxin accumulates in tissues and clears slowly. The role of NPA in the neurological signs of this syndrome is unclear.

Effects of the naturally occurring arginine analogues indospicine and canavanine on nitric oxide mediated functions in aortic endothelium and peritoneal macrophages

The ability of the naturally occurring non-protein toxic amino acids indospicine and canavanine to inhibit nitric oxide synthesis was tested in isolated rat aorta and cultured rat peritoneal macrophages. Both compounds inhibited acetylcholine induced relaxation of rat aorta contracted with noradrenaline, a process mediated by nitric oxide generated in vascular endothelium. Nitric oxide is generated in vascular endothelium from arginine by a constitutive nitric oxide synthase. Indospicine and canavanine also increased superoxide mediated reduction of cytochrome c by phorbol myristate acetate stimulated rat peritoneal macrophages. The increase in superoxide under these conditions was due to decreased nitric oxide synthesis. Macrophage synthesis of nitric oxide is mediated by an inducible form of nitric oxide synthase. It is concluded that indospicine and canavanine are inhibitors of constitutive and inducible nitric oxide synthases and it is suggested that the toxicity associated with these compounds could be related to this activity.

Dealing with plant poisoning of livestock: the challenge in Queensland

The current and possible future situation of diagnosis, prevention and treatment of plant poisoning of livestock in Queensland is reviewed. Topics discussed are livestock producers' perceptions, field investigation, plant identification, veterinary laboratory techniques and prevention through knowledge of poisonous species and the circumstances of poisoning. Also considered are management strategies for prevention including the manipulation of rumen flora and epidemiological approaches, therapies including immunisation, plant control and sources of information on poisonous plants and poisoning.