Vet clinic The toxic dan­gers of acorns that horse own­ers need to be aware of dur­ing au­tumn

As au­tumn rolls on, con­firmed cases of acorn poi­son­ing in­crease. Sarah Smith MRCVS out­lines the dan­gers for horses graz­ing in the vicin­ity of oak trees from her work on the toxic ef­fects of acorn in­ges­tion

Horse & Hound - - Contents -

OAK trees are an in­te­gral part of the land­scape in the UK, pro­vid­ing shade in pad­docks but also drop­ping acorns in the au­tumn.

It has long been thought that acorns may be toxic for horses and cat­tle. For this rea­son, there is a cen­turies-old tra­di­tion in the New For­est called pan­nage. Pigs are re­leased into the for­est in the au­tumn with two pur­poses: first, the pigs gain weight from eating the acorns and se­cond, by eating the acorns, pro­tect the New For­est ponies from any tox­i­c­ity associated with acorn in­ges­tion.

The num­ber of acorns pro­duced by trees varies from year to year. De­pend­ing on the acorns pro­duced, the verder­ers (those who ad­min­is­ter the law con­cern­ing the New For­est) re­lease an ap­pro­pri­ate num­ber of pigs to eat the acorns and pro­tect the ponies.

The num­ber of cases of acorn tox­i­c­ity ad­mit­ted to equine hos­pi­tals also varies from year to year, with peaks seen re­cently in 2011 and 2013. It is likely that these cor­re­spond to years of un­usu­ally high acorn pro­duc­tion.

Un­til re­cently, there has been lit­tle sci­en­tific ev­i­dence avail­able about the ef­fect of acorns on horses and ponies. The re­cent spikes in cases of acorn tox­i­c­ity led my­self and a group of vets in the south of Eng­land to de­scribe the se­ri­ous toxic ef­fects of acorn in­ges­tion. Our work has made vets and own­ers aware that clin­i­cal signs of acorn tox­i­c­ity should be treated ur­gently — and that pre­ven­tion is the best pos­si­ble course of ac­tion.


ACORN tox­i­c­ity is usu­ally seen in the au­tumn, when acorns fall to the ground.

Al­though oak leaves are toxic to cat­tle, their tox­i­c­ity is not well re­ported in horses — prob­a­bly be­cause the leaves are not palat­able. Usu­ally only one or two horses will fall ill among a larger group of horses that are

all ex­posed to the same oak tree.

There are also re­ports of horses that have eaten large num­bers of acorns for years with­out fall­ing ill.

Acorns con­tain a high con­cen­tra­tion of tan­nins, which are nat­u­ral com­pounds found in plants and trees and which we in­gest in tea and wine. They have a dry taste, which it is thought may pro­tect plants from be­ing eaten by an­i­mals.

When acorns con­tain­ing tan­nins are in­gested by horses, the tan­nins are bro­ken down into more toxic el­e­ments. These toxic com­po­nents bind to pro­teins within the body, caus­ing dam­age to the gut wall and kid­neys. The tox­ins can also bind to bac­te­ria, al­ter­ing the mi­cro­bial flora (“good bac­te­ria”) within the gut.

The ac­tions of the tan­nins lead to colic (ab­dom­i­nal pain) or diar­rhoea. The signs of acorn tox­i­c­ity in horses can vary from mod­er­ate to se­vere signs of colic, to bloody diar­rhoea and kid­ney fail­ure.

At the present time, it is not known whether in­di­vid­ual horses are par­tic­u­larly sus­cep­ti­ble or whether some acorns are par­tic­u­larly toxic. Pro­tec­tion from acorn tox­i­c­ity in other species, such as pigs, comes from the pro­duc­tion of tan­nin-bind­ing sali­vary pro­teins. These pro­teins are not nor­mally pro­duced by horses, mak­ing them sus­cep­ti­ble to acorn tox­i­c­ity.

It is pos­si­ble, how­ever, that this varies be­tween horses, there­fore pro­vid­ing some with pro­tec­tion. Equally, it is pos­si­ble that some acorns con­tain more toxin than oth­ers, or that the tox­i­c­ity varies be­tween trees. We don’t yet know, there­fore, how many acorns a horse needs to eat to show signs of tox­i­c­ity.


CLIN­I­CAL signs of acorn tox­i­c­ity de­velop ex­tremely rapidly. A horse can progress from nor­mal to se­verely af­fected in less than 12 hours, with death oc­cur­ring within a fur­ther 12-24 hours.

There is no de­fin­i­tive di­ag­nos­tic test cur­rently avail­able in the UK, as this re­quires iden­ti­fi­ca­tion of tan­nin me­tab­o­lites in the urine. Di­ag­no­sis is there­fore made

‘A horse can be se­verely af­fected within 12 hours, with death oc­cur­ring 12-24 hours later ’

on the ba­sis of clin­i­cal signs of mod­er­ate to se­vere colic or bloody diar­rhoea, along with a his­tory of acorn in­ges­tion or the pres­ence of acorn husks in fae­ces.

In horses that are show­ing signs of colic associated with acorn in­ges­tion, with­out diar­rhoea or kid­ney dam­age, the chances of sur­vival are good. The colic is caused by gaseous dis­ten­sion of the large colon which can re­solve with time, al­though it oc­ca­sion­ally re­quires ab­dom­i­nal surgery.

Once bloody diar­rhoea de­vel­ops, how­ever, the chance of sur­vival is low. The diar­rhoea is of­ten ac­com­pa­nied by kid­ney fail­ure and can progress to mul­ti­or­gan fail­ure and death.

As the toxic ef­fects of acorn in­ges­tion can be se­vere, prompt and in­ten­sive treat­ment is re­quired. There is no spe­cific treat­ment or an­ti­dote avail­able for acorn tox­i­c­ity. Se­vere cases re­quire hos­pi­tal­i­sa­tion for in­ten­sive care in­clud­ing sup­port­ive IV (in­tra­venous fluid) ther­apy, painkillers, anti-in­flam­ma­tory med­i­ca­tion and gas­tro­pro­tec­tants. Horses that sur­vive may re­quire ad­di­tional sup­port to pre­vent or con­trol the de­vel­op­ment of sec­ondary com­pli­ca­tions such as lamini­tis.

As there is no cur­rent an­ti­dote for acorn tox­i­c­ity and the con­se­quences can be lifethreat­en­ing, pre­ven­tion is the best safety pre­cau­tion. Un­less you have a help­ful herd of pigs to eat the acorns, oak trees should be fenced off in the au­tumn to pre­vent horses from ac­cess­ing the acorns. Any acorns should be col­lected and re­moved be­fore horses are re­turned to the pas­ture.

If it is not pos­si­ble to pre­vent ac­cess to the acorns then sup­ple­men­tary feed­ing of hay has been shown to be pro­tec­tive in cat­tle and may be ben­e­fi­cial to horses, prob­a­bly by re­duc­ing the num­ber of acorns they eat. If a horse falls ill hav­ing grazed pas­ture with acorns on, con­tact your vet to ar­range emer­gency treat­ment.

Un­less you have a help­ful herd of pigs to eat the acorns — a method used in the new For­est — oak trees should be fenced off in au­tumn to pre­vent horses ac­cess­ing the acorns, as the con­se­quences can be fa­tal

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