Creeping Indigo (CI) is a non-native flowering ground cover plant that was introduced to Florida in the mid-1900’s. Ironically, it was brought in by universities to see if it could be used as livestock forage. This didn’t work out, as animals began to show signs of toxicity and death when fed a diet of Indigofera over a period of a few weeks. There are over 750 different species of Indigofera; the toxic species in Florida is I. hendecaphylla. There are two toxins that affect livestock: 3-nitropropionic acid (3-NPA) and Indospicine. 3-NPA causes a majority of the neurologic effects seen in animals, while Indospicine mainly effects the eyes and mucous membranes. Only Indospicine can be found in the serum of affected animals.
So, what does this plant look like? Creeping Indigo leaves are similar to clover. It runs low to the ground, with branched runners fanning out in all directions from the center. The root is a white, slender, tapering taproot that is hard to pull up and can reach almost 3 feet underground. The stems are pale-green to yellow, tough, and thickly set with alternating, pinnate, clover-like leaflets that are 1-5 cm long. The slender, tubular ﬂowers are brick-red to pink to white. The most characteristic and identiﬁable feature are the needle-like, stiff, sharp-tipped seed pods, 1-3 cm long, that are found under the leaves in dense, downward-pointing clusters (see Orange arrow below). These seeds disperse when you mow the yard or field, and can hitch a ride on the mower to new areas. The plant is killed back in winter in central and north-central Florida but sprouts from the root in the spring.1
How much does a horse have to eat to start showing signs of toxicity? Based on current research, an average-sized horse only needs to consume 10 pounds daily for 2 weeks to become symptomatic. Foals can also show signs from drinking contaminated milk from their mother. A majority of horses show neurologic signs. This can range from a change in demeanor (more calm or less energetic) in the early stages, progressing to low head-carriage and episodes of standing narcolepsy, head-pressing into corners, or compulsive walking around the inside of a stall or paddock.1 Some affected horses show signs of vestibular disease, such as tilting their heads to one side and their necks and bodies twisted in the same direction. These signs may be accompanied by rhythmic blinking and jerking eye movements (nystagmus). The blink response to hand gestures toward the eyes (menace response) is frequently absent or reduced, although constriction of the pupils to bright light is usually unchanged.1 There can be flaccid (drooping) paralysis of the muzzle and lips. In retrospect, owners note an abnormal gait that has been developing over the preceding several days, characterized by incoordination and weakness in all limbs, with unpredictable crossing of pairs of limbs, interference between hooves, buckling of joints during weight-bearing, a “crab-like” gait and abnormal posturing at rest1. Some affected horses develop a bizarre “goose-stepping” gait in their front legs. If the horse continues to eat CI, they eventually become recumbent and unable to rise. Once the horse is down, they either become unconscious or develop convulsions, which can become generalized and severe before death or euthanasia. This can all progress in a matter of days once a toxic level is reached.
There are also non-neurologic signs of CI toxicity. These can include weight loss, inappetence, high heart rate/respiratory rate, labored breathing, excessive salivation or foaming at the mouth, dehydration, pale mucous membranes, bad breath, dropping hay balls (quidding) due to paralysis, and ulceration of the tongue and gums. The toxins can also affect the eyes, causing excessive tearing, corneal opacity or ulceration, and squinting.
There is no treatment for CI toxicity. If you catch the signs of toxicity early enough and remove the horse from the contaminated pasture, some of the effects are reversible. However, the gait abnormalities can persist. Management of affected horses should include removal from the source, conﬁnement to prevent any injuries, and non-speciﬁc supportive therapy.1 The University of Florida specifies only two herbicides containing aminopyralid that kill CI: Milestone (Dow AgroChemicals) at 5 ﬂ oz per acre or GrazonNext HL(Dow AgroChemical) at 24 ﬂ oz per acre.1 You will have to stay of top of it and likely retreat the next year. Dead plants retain toxicity and must be removed and disposed of. Grass clippings and manure from animals that graze herbicide-treated pastures should not be composted.1 The plant thrives in just about any environment, but overgrazed pastures are especially at risk.
Written by: Morgan Bosch, DVM
Source: Creeping Indigo Toxicity by: Rob MacKay (UF-LAH) https://largeanimal.vethospitals.ufl.edu/hospital-services/internal-medicine/creeping-indigo-toxicity/