Horse fatalities have been confirmed in Hawaii from eating a highly palatable invasive creeping plant in pastures.
Researchers Mohsen Mohamad Ramadan and Devon Dailey, writing in the journal Veterinary Sciences, have provided the first account of fatal toxicity in horses on the island of Oahu resulting from ingesting creeping indigo, Indigofera spicata.
Creeping indigo is a perennial herb native to tropical Africa, Madagascar, and the Mascarene Islands. It is now widespread in grazing pastures throughout tropical Asia and Australia. In Hawaii, it was introduced before 1929 as a pasture legume, and soon appeared to be poisonous to livestock.
Creeping indigo is a low-growing aggressive, drought-tolerant and relatively palatable. It is adapted to low elevations, dry, and disturbed areas on all major Hawaiian islands.
It has become invasive in many regions where it was introduced as a forage or cover crop, such as in Puerto Rico and southern Florida.
Because it is highly appealing to livestock, horses may be poisoned after eating large amounts of the legume. It carries a high level of toxic alkaloids that can cause miscarriage in pregnant cattle, among other effects.
Creeping indigo grows close to the ground, which makes it difficult to find in pastures. Besides growing from seeds, it spreads from its long taproots, which makes it hard to destroy with a single application of chemicals.
Two toxins found in creeping indigo are 3-nitropropionic acid (3-NPA) and indospicine. 3-NPA is a potent and irreversible inhibitor of a key enzyme that transforms glucose and oxygen into useable energy. It likely accounts for the early and prominent neurologic signs of toxicity seen in affected animals.
Because 3-NPA is metabolized quickly, it is unlikely to be found when testing the serum of affected animals. It is associated with motor disorders in livestock and humans when ingested.
Indospicine is a non-protein amino acid that is toxic to the liver. One of its principal actions is inhibition of nitric oxide synthase – an action linked to the development of ulceration of mucous membranes.
Although horses are relatively resistant to the liver-damaging effects of indospicine, it accumulates in the tissues of horses dying from the disease. Indospicine can be detected in the serum of affected animals.
Indospicine is highly toxic to livestock in small doses, the researchers said, causing loss of vitality, liver degeneration, and abortion in cattle and goats. It is particularly dangerous to horses, who relish plants containing it.
Horses, they said, need to eat around 4.5kg of creeping indigo daily for about two weeks to develop signs of toxicity. Symptoms vary but can include runny squinting eyes, sleepiness, involuntary rapid and repetitive movements of the eyes, an abnormal gait, and mild colic-like signs. Special tests on postmortem tissues can identify the toxin.
A survey in the town of Waialua, on the north shore of Oahu island, indicated that creeping indigo is common and abundant on grazing lands during the rainy season and requires intensive chemical and physical control measures.
Four pastures were surveyed where ranchers reported the deaths of more than 17 horses since 2020. Serum chemistry reports from horses in the area pointed to the toxic effects arising from creeping indigo ingestion.
Ramadan and Dailey said they documented these incidents to alert state animal and livestock support officials, groups monitoring invasive species, and horse owners regarding the problem of this noxious weed and to support breeders with information to confront its invasiveness.
Herbicide treatment is not economically feasible, and breeders opted to physically uproot the plants from the paddocks and restrain horses to clear pastures while they were eliminating the plants.
Ramadan and Dailey said their report indicates a recent increase in toxicity and fatalities in horses on the four ranches.
“The plant is widespread on pasture lands of windward Oahu during the rainy season. Chemical control with broad-spectrum herbicides appears ineffective, and manually pulling the plants is not sufficient for managing this weed.
“In general, a single herbicide application will not suppress creeping indigo permanently; research on chemical control is essential for new selective herbicides that kill leaves and taproots and do not injure useful pastures.”
The pair urged authorities to fund research on long-term solutions to decrease the creeping indigo population.
“Creeping indigo is very palatable to horses, and it is important for horse owners to recognize the plant and know the signs of toxicity and ways to prevent it from being consumed.
“Statewide surveys and outreach programs are needed to accurately determine the infestation levels of I. spicata and unreported livestock toxicity.
“This weed should be elevated to the top list of noxious weeds targeted for management in Hawaii.”
The Hawaii Department of Agriculture may participate in a subsidy program to help ranchers in chemical control to reduce the problem until biological control agents are found, the pair added.
They urged state officials to introduce a long-term control strategy to reduce the problems associated with the weed.
Ramadan, M.M.; Dailey, D. Trouble for Horses in Paradise: Toxicity and Fatality Resulting from the Consumption of Indigofera spicata (Fabaceae) on Oahu Island. Vet. Sci. 2022, 9, 271. https://doi.org/10.3390/vetsci9060271