48. BLACK LOCUST

Robinia pseudo-acacia

(pea family)

TOXICITY RATING: High to moderate.

ANIMALS AFFECTED: Horses are particularly at risk, but all animals ingesting the plant may be poisoned.

DANGEROUS PARTS OF PLANT: Leaves, especially wilted leaves, young shoots, pods, seeds, inner bark.

CLASS OF SIGNS: Depression, poor appetite, weakness, paralysis, abdominal pain, diarrhea (which may be bloody) and abnormalities in the heart rate and/or rhythm. Death is possible.

PLANT DESCRIPTION: These moderate-sized trees with rough bark often bear two short spines at the base of each leafstalk (easiest to see on young leaves). Leaves are alternate and pinnately compound with oval, entire leaflets (fig. 48). The fragrant flowers are creamy white, sweet-pea-like, and arranged in long drooping clusters. The fruit is a flat brown pod which contains kidney-shaped beans (fig. 48A). Black locusts are common in well-drained woods, thickets, and waste areas, especially in the southeastern part of the state. They are often planted along highways and fencerows as ornamentals and for erosion control.

SIGNS: This discussion will center on the effects in horses, the species most likely to be poisoned by black locust. Horses may ingest the bark or leaves when hungry and no other forage is available, or if they are confined or bored in the vicinity of the tree.

There are several toxic components in black locust including the toxic protein robin, the glycoside robitin, and the alkaloid robinine. The toxins affect the gastrointestinal tract as well as the nervous system. Clinical signs can manifest as soon as one hour after consumption and can include depression, poor appetite, generalized weakness to paralysis, abdominal pain, diarrhea (which may be bloody) and abnormalities in the heart rate and/or rhythm. With sufficient amounts ingested, death may occur within a few days, although black locust is not always lethal. Some animals recover despite showing clinical signs, an indication of the dose-dependent nature of the toxin.

Honey locust (Gleditsia triacanthos, pea family) has been implicated in causing similar toxic signs, but the information on this is not clear. Prickly ash (Zanthoxylum americanum, citrus family) superficially resembles black locust in vegetative aspect and has been blamed for loss of sheep.

FIRST AID: If horses are observed eating black locust, contact a veterinarian immediately, since emergency measures to rid the gastrointestinal tract of toxin may be implemented. Beyond this, therapy is aimed at preventing further exposure and keeping other animals away from the trees, and treating clinical signs symptomatically. Recovery may take days to weeks. Be extra cautious around affected horses to prevent human injury, and these horses should not be ridden until all clinical signs have resolved.

SAFETY IN PREPARED FEEDS: Reports are not clear on this matter, but given the potentially toxic nature of black locust, it should never be allowed to contaminate feeds, especially those destined for horses.

PREVENTION: Do not confine horses in an area where black locust grows. If this is unavoidable, provide enough palatable feed so that the horses leave the trees alone. Some horses are wood and bark chewers, however, and for these horses is may be necessary to fence off the trees or utilize a different pasture to prevent toxicosis. Paints and sprays to prevent wood chewing may be tried, but long-term success with these treatments may be difficult.