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Creeping Indigo Toxicity

By Rob MacKay, BVSc (Dist), PhD, Dip. ACVIM, Professor, Large Animal Medicine

History of Creeping Indigo Toxicity in Florida

In southern Dade County, a mysterious and often fatal condition of adult horses and livestock had begun to be recognized by the early 1980s. This was known locally as “Grove Poisoning”, reflecting the belief that that toxic chemicals sprayed on trees in local fruit groves were the cause. Affected horses were dull, apathetic and incoordinated, some had convulsions, ulcers of the tongue, and whitening or streaking of the corneas of the eyes. Most of them died within weeks. The problem was seen most commonly in the late summer or fall.

Beginning in the 1970s, a University of Miami botanist, Julia Morton, began to walk the properties on which these deaths were occurring. She noticed an abundance of wild Guatemalan Indigo plants (Indigofera suffruticosa) and initially supposed that these were the culprits. By the 1980s, however, Dr. Morton had investigated many more incidents and had become convinced that the condition was actually caused by a different indigo herb, creeping indigo (also known as trailing indigo; initially identified as Indigofera spicata), that had widely invaded local pastures and was being enthusiastically eaten by horses. She compared Grove Poisoning with reports of Birdsville Indigo Poisoning, an almost identical nervous system disease of horses in the north of Australia grazing another Indigofera species, I. linnaei.

By 1987, it was estimated that at least 300 horses in the south Dade county area had died of Grove poisoning so, at the urging of Todd Hardwick from the University of Florida’s Cooperative Extension Service, Dr. Morton decided to present her findings at a local meeting of concerned equine veterinarians and horse owners. She explained that creeping indigo had first been imported into the United States in 1925 and established at the Agricultural Experimental Stations of the universities of Florida and Hawaii, so that its supposed utility as livestock forage and ground cover could be studied. When used as a dietary supplement, creeping indigo was found by these early investigators to be highly palatable and nutritious (roughly equivalent to peanut hay) but when livestock, rabbits, and guinea pigs were fed CI as their primary diet, they sickened within weeks and usually died if feeding continued. After some initial skepticism, the causal role of CI was accepted by local veterinarians, although the condition was unknown elsewhere in the state.

Investigation of CI at UF had stopped by 1936; however, by the mid 1950s the now naturalized plant was identified at multiple sites around Alachua County (the home of UF). Simultaneously, almost certainly as the result of a separate CI introduction of unknown source, the plant had naturalized and was spreading in Dade County. Speculation as to the origin of this Dade county introduction have included fill imported into South Florida for land reclamation, seed from Cuba or Puerto Rico, or importation by renowned botanist David Fairchild into the Tropical Plant Introduction Garden at Chapman Field in Miami. The first identification of the plant in Dade County was in 1940 in the lawn of a Homestead resident. Following decades of spread from at least these two foci (Alachua and Dade counties), CI had advanced throughout the entire state by 2016  (Figure 1) and is now found in the adjacent counties of neighboring states.

Beginning in about 2000, individual cases and small outbreaks of the disease in equids began to be recognized in central and north-central parts of the state, especially in areas north of Tampa and around Brooksville. Since then, CI cases have continued unabated along the I-4 corridor and sporadically in the northern counties of the state and in Alabama and Georgia. This extension in geographic range of the syndrome appears to correspond to increasing abundance of creeping indigo in the central and north central parts of the state. Creeping indigo now has become a very common plant around Alachua and Seminole counties and can be found growing in any disturbed ground, especially along roadsides and the edges of footpaths, including many at the University of Florida. Even more threateningly, it has become the dominant herb in some pastures in the Ocala area.

Creeping Indigo  

The Indigofereae are a small tribe of often very attractive, pink to red-flowered shrubs and herbs. The largest genus is Indigofera which is pantropical, with 75% of the ~750 species restricted to Africa–Madagascar, and the rest in the Sino-Himalayan region, Indonesia, and Brazil. Because of their complex chemistry, Indigofera contain many toxic and medicinally used species. These toxic chemicals have evolved as a defensive response to predation by herbivores and pathogens. Other Indigofera are economically important indigo dye-producing and pasture legume species that occupy an extremely wide range of different habitats. Indigofera spicata is native to East Africa Madagascar, and Indonesia. It was valued as a cover crop in coffee estates in Africa and was introduced into India, Java, Malaya, and the Philippines both as ornamental ground cover and a cover crop for tea, rubber, oil palm, and sisal plantations. It has also been introduced into Australia, several pacific islands, central America, and the Caribbean.

At the time Dr. Morton published her work, I. spicata/I. hendecaphylla were considered synonymous and referred to as I. spicata complex. In 1992, however, it was discovered that they were actually different species and review of voucher specimens at the major herbaria in Florida indicates that creeping indigo in Florida is I. hendecaphylla. Both I. hendecaphylla and I. spicata have been connected with equine and livestock deaths, although such an association remains to be confirmed for authenticated I. hendecaphylla in Florida and thus it will be important to collect plants from all future outbreaks and have them identified by the UF Herbarium.

i spicata FL map

Figure 1. Digitized voucher specimens from the four major herbaria in Florida showing location and year of collection

Creeping indigo is prostrate (low profile) to sub-erect with branched runners fanning out in all directions from the crown of a white, slender, tapering taproot firmly set in the earth and descending nearly a meter (Figures 2-4). The stems are pale-green to yellow, tough, and thickly set with alternate, pinnate, clover-like leaflets that are 1-5 cm long. The slender, tubular flowers are brick-red to pink (mostly pink around Gainesville). The most characteristic and identifiable feature, the needle-like, stiff, sharp-tipped seed pods, 1-3 cm long, are borne profusely in dense, downward-pointing clusters. The plant, perennial in tropical and subtropical climates, is killed back in winter in central and north-central Florida but sprouts from the root in the spring.

Both plants are prostrate to sub-erect with branched runners fanning out in all directions from the crown of a white, slender, tapering taproot firmly set in the earth and descending nearly a meter (Figures 2-4).  The stems are pale-green to yellow, tough, and thickly set with alternate, pinnate, clover-like leaves that are 1-5 cm long. The slender, tubular flowers are brick-red to pink (mostly pink around Gainesville). The most characteristic and identifiable feature, the needle-like, stiff, sharp-tipped seed pods, 1-3 cm long, are borne profusely in dense, downward-pointing clusters.  The plant, perennial in tropical and subtropical climates, is killed back in winter in central and north-central Florida but sprouts from the root in spring.

Signs of Creeping Indigo Toxicity

Consumption of 10 pounds I. linnaei daily for 2 weeks is sufficient to cause disease. Suckling foals may be affected by ingestion of toxins in their dams’ milk. Both neurologic and non-neurologic signs are seen.

  • Non-neurological signs. There may be weight loss, inappetence, high heart and respiratory rates, labored breathing, high temperature (a rare finding), profuse salivation (ptyalism) or foaming from the mouth, dehydration, pale mucous membranes, feed retention in the cheeks (quidding), halitosis, watery discharge from the eyes (epiphora; Figure 5), squinting (especially in bright light), corneal opacity, corneal ulceration and neovascularization, severe ulceration of the tongue and gums (Figure 6), and prominent digital pulses without other signs of laminitis.
  • Neurological Signs Often an early sign is a change in personality – affected horses at first seem quieter and less energetic than usual. Degrees of obtundation (sleepiness) ranging from mild lethargy to recumbency and loss of consciousness may be seen as the condition progresses over days to weeks. Head carriage is low and there may be episodes of standing sleep-like activity (narcolepsy), head-pressing into corners, or compulsive walking around the inside of a stall or paddock. Some affected horses have been seen with their heads tilted to one side and their necks and bodies twisted in the same direction indicating involvement of the balance centers (vestibular system) of the brain. 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 retained. The muzzle and lips may hang flaccidly. In retrospect, it is often clear that an abnormal gait has been seen 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 rest. Some affected horses develop a bizarre goose-stepping gait in their front legs. Most horses that continue to consume the plant eventually become cast on their sides and are unable to rise. They either become unconscious or develop convulsions which may become generalized and severe before death or euthanasia after days in recumbency.
  • Laboratory Findings Abnormal findings on routine hemograms and plasma chemistry panels are mild, non-specific, and unhelpful in making the diagnosis. Typically, there is low-normal or low total white blood cell count with lymphopenia and mild electrolyte derangements that may include hyponatremia, hypophosphatemia, hypomagnesemia and metabolic acidosis. Aspartate aminotransferase and creatine kinase activities often are slightly to moderately above reference range.
  •  Necropsy Findings Both gross and histologic examinations of horses poisoned by creeping indigo are most notable for the lack of diagnostic findings. It has been shown in experimental animals that subacute and chronic toxicity by 3-NPA, the creeping indigo toxin that causes neurologic signs, does not cause changes evident on light microscopy, although Brazilian investigators documented changes by electron microscopy of the mitochondria of horses that had consumed I. lespedezioides. Horses may show mild liver pathology, evident histologically as periacinar necrosis, infiltration of lymphocytes and haemosiderin-laden macrophages in periacinar regions and vacuolation and swelling of surviving hepatocytes.

Toxins

Two important toxins account for the signs of creeping indigo toxicosis – namely, 3-nitropropionic acid (3-NPA) and indospicine. There is evidence that 3-NPA causes the largely irreversible neurologic signs described above while indospicine causes the corneal edema, ulcerations, and other non-neurologic signs. Each of these toxins is described briefly:

  • 3-nitropropionic acid 3-NPA is a highly toxic compound produced by the plant primarily as defense against destruction by herbivores. This nitrotoxin is by no means unique to Indigofera but is produced as anti-herbivore defense by many plants and fungi. Poisoning associated with 3-NPA therefore occurs quite commonly in a variety of settings and the mechanism is well understood. The toxin is a potent and irreversible inhibitor of mitochondrial succinate dehydrogenase, a key enzyme in transforming glucose and oxygen into useable energy. Nerve cells are extremely vulnerable to energy deprivation, thus accounting for the early and prominent neurologic signs seen with all types of 3-NPA toxicity. 3-NPA accounts for up to 0.27% of the dry matter of creeping indigo. Because it is metabolized quickly, it is unlikely to be found in the serum of sick inappetent animals.
  • Indospicine is a non-protein amino acid. It is toxic to the liver because of antagonism to the essential amino acid arginine, with which it competes. One of its principal toxic actions is inhibition of nitric oxide synthase, an action likely associated with the development of corneal edema and ulceration of mucous membranes. Although horses are relatively resistant to the liver damaging effects of this toxin, it persists in the tissues of horses dying or killed with the disease and these tissues are potentially toxic if fed to dogs. Indospicine accounts for 0.05 to 0.15% of the dry matter of creeping indigo and it can be detected in the serum of affected animals.

Treatment and prevention

Horses that are quickly removed from the offending plants often recover completely, but gait abnormalities sometimes persist. There is no effective treatment. Early investigations into the prevention and treatment of Birdsville disease in Australia proposed the use of arginine-rich protein sources such as peanut meal (4.3% arginine) and gelatin (8.0% arginine). The lack of significant liver lesions in horses because of their relative resistance to indospicine, together with the likelihood that the neurological disease results from 3-NPA poisoning, suggests that arginine alone would have little benefit in the treatment of nervous signs although nonneurologic signs may respond. Thiamine was also suggested as an effective treatment for nitrotoxicity in ruminants but others showed this treatment to be ineffective. The fact that 3-NPA neurodegeneration is used as an induction model for Huntington’s disease research is testament to the current futility of all treatments. Management of affected horses should include their removal from the source, confinement to prevent any injuries and non-specific supportive therapy. It was previously suggested that livestock poisonings by I. spicata can be prevented by keeping the proportion of this plant below 20% of the total forage available but recent evidence does not support this view. The best means for preventing poisoning is to stop access by horses to paddocks where creeping indigo is present or to remove plants by physical means or herbicide application.

Although there is no herbicide commercially recommended for eradication of creeping indigo, the University of Florida Pasture Weed Identification and Control page a the IFAS Extension EDIS website (https://edis.ifas.ufl.edu/topic_pasture_weeds), suggests spraying either of two herbicides containing aminopyralid: Milestone (Dow AgroChemicals) at 5 fl oz per acre or GrazonNext HL(Dow AgroChemical) at 24 fl oz per acre. Retreatment the following year will likely be necessary. Dead plants retain toxicity and must be removed and disposed of. Manure from animals that graze herbicide-treated pastures should not be composted. Also, any grass clippings removed from these treated pastures should not be composted. Be sure to follow all label directions when using pesticides!

Additional information can be found in the EDIS publication Creeping Indigo, A Poisonous Plant of Concern in Florida Pastures at: http://edis.ifas.ufl.edu/ag399