These plants all contain soluble oxalates that are metabolic toxins that also cause renal disease (nephrosis) and hypocalcemia.
The oxalates are oral irritants, so most horses avoid eating them. Though poisoning in horses is infrequent and poorly described,
clinical signs in sheep and cattle begin as depression, colic, weakness, irregular gait that can progress to lateral recumbency,
coma and convulsions. Clinical chemistry changes begin as a hypocalcemia that progresses to azotemia and hypercreatinemia.
Treatment, though it may not be effective once clinical signs have developed, should include aggressive fluid therapy (diuresis).
Cestrum diurnum (day-blooming jasmine or wild jasmine — Photo 3).
Photo 3: Cestrum diurnum (day-blooming jasmine or wild jasmine) is an introduced tropical plant commonly found in Florida,
Texas, California and Hawaii. It grows 4m to 5 m high. The leaves are elliptic, with a dark-green, glossy upper surface. Flowers
are white and clustered on the axillary peduncles. They form green berries that ripen black.
Cestrum diurnum is found on disturbed soil in Florida, Texas, California, Mexico and Hawaii. Similar to Solanum malacoxylon (the cause of enzootic calcification in South America), jasmine contains a steroidal glycoside of 1,25-(OH)2 cholecalciferol
(a vitamin D analog). This induces increased intestinal absorption of Ca with sustained secretion of calcitonin and decreased
parathyroid activity. All these contribute to hypercalcemia and the deposition of calcium in many organs and tissues.
Clinical signs of poisoning include weight loss, lameness and a humped-up appearance with choppy gait. In the serum there
is a moderate hypercalcemia with normal phosphorus levels. Altered calcium metabolism causes mineral deposition or dystrophic
calcification of elastic tissues, including tendons, ligaments, major arteries, heart and kidney.
Although there is no proven treatment, most lesions slowly resolve after exposure is discontinued.
Gastrointestinal toxic plants
Mechanical damage — foxtail, bristlegrass, sandbur, cheatgrass.
These weedy grasses are poor-quality forages and their grass awns can become embedded in the mucous membranes of the gums
and tongue, causing ulcers and abscesses. Affected horses salivate excessively and have difficulty eating normally. The more
invasive species often displace good forage and contaminate forages.
The economic impact is high, because extensive care is required to extract the foreign plant material, drain and clean wounds
and maintain appropriate antibiotic therapy. Larger plant awns and burrs also can cause gastrointestinal obstruction and colic.
Avoiding contaminated hay and maintaining pastures to avoid exposure is recommended.
Ranunculaceae (buttercup family).
Buttercup contains the toxin ranunculin that is converted to protoanemonin, a mucosal irritant. Clinical signs of poisoning
include blistered lips, stomatitis, gastroenteritis, increased salivation, abdominal pain and diarrhea. Most poisonings occur
in sheep because buttercup is not very palatable to horses. Dried plants appear to be non-toxic.
Nightshades (Solanum spp), S. rostratum (buffalo bur — hay contaminant), S. ptycanthum (black nightshade), S. dulcamara (bittersweet), S. elaeagnifolium (silverleaf nightshade), S. carolinense (Carolina horse nettle), S. dimidiatum (western horse nettle), S. triflorum (cutleaf nightshade — Photo 4).
Photo 4: Solanaceae (nightshade family) include over 2,000 species that commonly poison livestock. S. triflorum (cutleaf nightshade)
is an annual that branches from the base and grows decumbent in the late summer and fall. Leaves are deeply pinnate and the
white flower ripens into a green, 1-cm berry. It commonly grows in disturbed areas.
Nightshade poisoning has been associated with several syndromes because these plants contain various toxins, such as steroidal
glycoalkaloids that cause severe gastroenteritis and cholinesterase inhibitors that cause neurologic disease. The most common
alkaloid is solanine, a potent mucosal irritant that causes stomatitis.