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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.

Toxic threat

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.

Rapid decline

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 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 multi-organ 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 life-threatening, 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.

Ref Horse & Hound; 9 November 2017