Plasma l-indospicine and 3-nitropropionic acid in ponies fed creeping indigo: Comparison with results from an episode of presumptive creeping indigo toxicosis.
Authors: Darby Shannon, Sanchez L Chris, Mallicote Martha F, House Amanda M, Plummer Caryn E, Nadruz Veridiana, Benmoha Rachel H, Roberts Stephen M, Derendorf Hartmut, Silva-Sanchez Cecilia, Claire Jami, MacKay Robert J
Journal: Equine veterinary journal
Summary
# Editorial Summary: Creeping Indigo Toxicosis — Identifying the Culprit Toxin Creeping indigo toxicosis is an emerging disease affecting horses in Florida and neighbouring states, yet the specific toxins responsible remain incompletely understood. Researchers fed five ponies dried creeping indigo containing 1 mg/kg/day of l-indospicine (IND) with trace nitropropionic acid (NPA) for five days, then monitored them for 28 days whilst measuring plasma toxin concentrations; they also analysed blood samples from horses involved in a presumptive toxicosis incident for comparison. Although the experimental ponies showed only modest clinical changes — notably 6% median weight loss and elevated plasma protein — plasma IND rose to 3.9 mg/L by day 6 and demonstrated a prolonged elimination half-life of 25 days, suggesting significant tissue accumulation; critically, sick horses from the field outbreak had approximately double the plasma IND concentration of clinically normal pasturemates, whereas plasma NPA remained negligible (<0.05 mg/L) across all samples. These findings establish IND as a detectable and accumulating toxin in affected horses, supporting its role in the syndrome, though the contribution of NPA to the neurological manifestations remains unclear and warrants further investigation. For practitioners managing horses with suspected creeping indigo exposure, plasma IND measurement offers a diagnostic marker, whilst recognition of slow toxin clearance (25-day half-life) indicates prolonged risk even after pasture removal.
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Practical Takeaways
- •Indospicine accumulates slowly in equine blood with a 25-day half-life; monitor plasma IND levels in horses with suspected creeping indigo exposure, particularly those showing neurological signs
- •Sick horses show approximately double the plasma IND concentration compared to asymptomatic herd mates, suggesting toxin level correlates with disease severity and can help confirm diagnosis
- •The role of 3-nitropropionic acid in creeping indigo toxicosis remains unclear; further investigation needed to identify other putative toxins responsible for neurological manifestations
Key Findings
- •Indospicine (IND) concentrations in creeping indigo ranged from 0.4–3.5 mg/g dry matter, while 3-nitropropionic acid (NPA) was <0.01–0.03 mg/g
- •Plasma IND reached 3.9 ± 0.52 mg/L on day 6 of feeding trial with estimated elimination half-life of 25 days and steady-state concentration of 22 mg/L
- •Sick horses during toxicosis episodes had plasma IND concentrations approximately twice that of clinically normal pasture mates
- •Weight loss (median 6%) and increased plasma protein concentration occurred during feeding trial, but no definitive clinical neurological signs were reproduced