Knowledge is key to safety; Plants that poison horses - DVM
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Knowledge is key to safety; Plants that poison horses


Toxicity of the various plants is related both to concentration and type of PA. Concentration can vary between plants and between years. For example, PA concentrations from the same plants and location have varied from none to nearly 20 percent in sequential years, making it difficult to predict toxicity or evaluate risk.

PA-induced disease is determined by both the amount eaten (dose) and the exposure time (duration).

Photo 2: Cynoglossum officinale (houndstongue) is native to Euroasia, but has spread throughout North America. It is a noxious weed that invades pastures, rangelands and cultivated fields. A biennial, it forms a leafy rosette the first year, followed by flowering (A) and seed production in the second year (B). The plant is erect, hispid, usually 30 cm to 120 cm tall with large basal or cauline oblong-lanceolate leaves. The common name, houndstongue, comes from the tongue-like appearance of houndstongue's large, hairy leaves. The flowers are terminal and usually blue or purple. Later they form prickly nutlets (3mm-5mm) which entangle hair or clothing. It is commonly found as dense stands in moist or shady areas in fields and pastures.
High PA doses cause sudden liver failure, but because most poisonings are from contaminated feed, doses usually are low and exposures are long. Clinical liver disease in these animals may not develop until months after exposure. Young animals are most sensitive and there are several reports of fetal and neonatal toxicity without evidence of maternal toxicity.

Signs of poisoning are related to liver failure and include weight loss, weakness, sleepiness, yawning, incoordination, neurologic derangement, icterus, photosensitivity, aimless walking, chewing motions and head pressing.

Diagnosis is made by identifying exposure. As most poisonings are caused by contaminated feed, documenting exposure can be difficult. Hay contamination is patchy; often only a few bales from an entire crop may be bad. Finding the bad bales is difficult, so it often is better to inspect the field.

Though non-specific, good clinical work-ups and post-mortem examinations are helpful to narrow the diagnosis. Classical histologic changes of hepatocellular necrosis, fibrosis, biliary hyperplasia and hepatocyte megalocytosis are suggestive of PA intoxication. However, similar changes may be caused by aflatoxins and other alkalating agents.

In some cases, PA metabolites can be extracted and identified from animal tissues. Current research seeks to improve diagnostics to better identify and predict the outcome to poisoned animals.

As most cases result in liver failure, supportive care is the only treatment. As both contamination rates and PA concentrations within the plant can vary, assessing risk is impossible. Avoiding exposure to any of these plants is recommended:

Photo 3: Trifolium hybridum (alsike clover), a perennial legume, is a short-lived forage plant that is included in many pasture-seed mixes. It often escapes cultivation and persists; it can spread in clusters to dominate pastures. The erect or ascending stems are 15 cm-70 cm tall and the leaves are oval and flat (~0.4 cm X 0.3 cm) in leaflets of three. It has many flowers (0.25 cm) with white to pink petals that fade to red-brown. It is most common in low, moist meadows and pastures.
Trifolium hybridum (aslike clover – Photo 3.

Alsike clover is one of about 300 Trifolium species that have been associated with phytoestrogenism, slobbers, liver disease and photosensitivity. It is an introduced European plant that is included in some pasture mixes.

Several toxins have been suspected, but none has been proven. Toxicity may be related to environmental conditions and mold or aflatoxin production. Exposures of weeks to months generally are required before animals develop disease.

Three syndromes

Horses are the only species known to be susceptible to poisoning. Three syndromes have been identified.

The first, called "dew poisoning", is characterized by photosensitivity (sunburn), colic and diarrhea, depression or excitation.


Source: DVM360 MAGAZINE,
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