THE NA­TURE OF THE THREAT

EQUUS - - Eq Tack & Gear -

The dan­ger posed by wilted or dried red maple leaves has long been rec­og­nized: Horses who con­sume them may sicken or die within hours or days. Tox­ins in the plant dam­age the he­mo­glo­bin in the horse’s red blood cells, so they can no longer carry oxy­gen. Af­fected red cells may rup­ture, clog­ging the kid­neys with waste prod­ucts, and the liver and spleen will re­move dam­aged cells from the blood­stream faster than they can be re­placed by the bone mar­row, which re­sults in se­vere ane­mia.

Starved of oxy­gen, other tis­sues and or­gans be­gin to fail. Out­ward signs of red maple poi­son­ing in­clude lethargy, poor ap­petite, colic, pale yel­low­ish gums that progress to dark muddy brown, in­creased heart rate, faster res­pi­ra­tion and dis­tinc­tive dark red to black-col­ored urine.

The speed and sever­ity of red maple tox­i­co­sis de­pends on how many wilted leaves have been eaten rel­a­tive to the horse’s body weight. As lit­tle as half a pound of wilted leaves can kill a small pony or minia­ture don­key; a pound or two can be a fa­tal dose for an av­er­age adult horse.

“If you see your horse eat­ing fallen red maple leaves, first re­move the horse from the source of the maple leaves, and call your vet­eri­nar­ian im­me­di­ately,” says Knight. Do not wait for signs to ap­pear---your horse’s best chance of sur­vival de­pends on

The speed and sever­ity of red maple tox­i­co­sis de­pends on how many wilted leaves have been eaten rel­a­tive to the horse’s body weight.

The study re­vealed

another piece of the puz­zle: The equine

di­ges­tive tract con­tains mi­crobes that can turn gal­lic acid into an even more dam­ag­ing sub­stance

called py­ro­gal­lol.

the ear­li­est pos­si­ble in­ter­ven­tion.

There is no spe­cific an­ti­dote to red maple tox­i­co­sis, but sup­port­ive treat­ment in­clud­ing in­tra­venous flu­ids and pos­si­bly blood trans­fu­sions may help a horse sur­vive long enough for the tox­ins to clear his sys­tem. “Some stud­ies have shown that large amounts of vi­ta­min C [a po­tent an­tiox­i­dant] can counter the ox­ida­tive ef­fect of the tox­ins,” says Knight, “but that treat­ment has to be given very early to be ef­fec­tive.”

Once signs ap­pear, a horse’s odds of sur­vival drop sig­nif­i­cantly, even with hos­pi­tal­iza­tion. One 2006 study from North Carolina State Univer­sity re­viewed the cases of 32 horses with red maple poi­son­ing ad­mit­ted to re­fer­ral hos­pi­tals in the South­east. Of those cases, 29 horses were ane­mic, 24 had se­ri­ous sys­temic in­flam­ma­tion, 12 had kid­ney dys­func­tion, nine had lamini­tis and 13 had colic---19 (59 per­cent) died.

The ex­act mech­a­nism of poi­son­ing has not been fully doc­u­mented. Stud­ies from Cor­nell Univer­sity have un­cov­ered sig­nif­i­cant clues. Com­pounds ex­tracted from red, sugar, sil­ver and Nor­way maple leaves in­cu­bated with equine blood sam­ples caused ox­i­da­tion, he­mol­y­sis (the break­down of red blood cells) or other types of dam­age to the cells. Sev­eral chem­i­cals were iden­ti­fied as toxic to the equine red blood cells, most no­tably gal­lic acid. “While gal­lic acid was a ma­jor player, most likely it is not the only player,” says Jeanelle Boyer, PhD, who con­ducted the study. “Quite likely there is a syn­er­gis­tic ef­fect in com­bi­na­tion with other chem­i­cals.”

In the next phase of the re­search, Boyer an­a­lyzed ex­tracts from sil­ver maple, sugar maple and Nor­way maple leaves and found that all con­tained some level of gal­lic acid. The ex­tracts de­rived from the sil­ver and sugar maples did less dam­age to the equine red cells than did the red maple ex­tracts, but the changes were sig­nif­i­cant enough to be po­ten­tially harm­ful to a horse. The ex­tracts drawn from the Nor­way maple were far less toxic. In fact, Boyer spec­u­lated that horses would most likely not be able to in­gest enough Nor­way maple leaves to cause them­selves any se­ri­ous harm.

In a sep­a­rate study, Cor­nell re­searchers Karyn Bischoff and Karan Agrawal in­ves­ti­gated the ef­fects of gal­lic acid and tan­nins from red maple leaves that were in­cu­bated with sam­ples of di­ges­tive flu­ids drawn from the equine ileum, the low­est por­tion of the small in­tes­tine. Their find­ings re­vealed another piece of the puz­zle: The equine di­ges­tive tract con­tains mi­crobes that can turn gal­lic acid into an even more dam­ag­ing sub­stance called py­ro­gal­lol.

Maple tox­i­co­sis is unique to horses and other equids---it does not oc­cur in sheep, cat­tle or other farm an­i­mals. The rea­son, says Knight, is that “horses do not have a di­ges­tive sys­tem like that of ru­mi­nants [such as cat­tle, sheep and goats] that can break down gal­lic acid into harm­less com­po­nents.”

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