A majestic row of ancient oak trees alongside rolling green pastures is often a feature of expensive equestrian estates in England, the US and parts of the Western Cape.
Only a few horses develop a taste for the acorns, leaves and bark of these trees, and the signs of chronic poisoning are not always obvious, although they can be life-threatening.
Oak poisoning often occurs when a horse is moved for the first time into a paddock where there are oak trees, when there is overgrazing, or when drought decreases the quantity and palatability of the grass. Not all horses will eat acorns, but it is possible for some of them to become addicted to acorns.
Green acorns and young leaves are more toxic than the more mature ones, so signs of poisoning may occur seasonally in spring, particularly after heavy rain. Fatal acorn poisoning in free-ranging pigs has been recognised for centuries, and has also been described in cattle and dogs.
Signs and symptoms
The main cause of oak poisoning is suspected to be high levels of tannic acid, although it is likely that other chemicals in green leaves and acorns also play a role.
Riders may notice that their fit competition horses tire easily, kick at their stomachs, lie down, and have painful joints or mild signs of colic. A showjumping horse starts knocking down poles or ducks out of a jump; a dressage horse loses concentration and rhythm.
You may notice tearing and a watery discharge from the nostrils. The rectal temperature is normal, while pale gums and a decreased heart rate are fairly typical signs of acorn poisoning. Constipation is frequent and there may be blood in the faeces; other signs include dehydration and increased thirst.
Acorn poisoning is not always easy to diagnose, but acorn husks can sometimes be seen in the faeces, and affected horses will improve rapidly when moved out of paddocks containing oak trees.
Over time, some horses develop mouth ulcers, a decreased appetite, poor hair coat and weight loss. Blood tests may indicate kidney and sometimes liver failure. In fatal cases, the post-mortem may reveal signs of haemorrhagic shock, kidney damage, and faecal impaction.
It is difficult to make a specific causal diagnosis, so the history of where the horse was grazing and whether oak trees were present is very important.
Horse owners need to be aware of the risk of oak poisoning, and acorns should be regularly removed from grazing paddocks. Grooms can prune back the tree branches so that they are too high for horses to reach.
If you notice that the oak bark is being chewed off the trunks, you can put a steel fence around each tree.
You may notice steel fencing around oak tree trunks in photographs of upmarket equestrian properties where racehorses and showjumpers are kept in the US and UK; oak poisoning is a frequent occurrence here.
Treatment is symptomatic, which means your vet will treat the symptoms displayed, often without making a diagnosis. Your horse will likely receive intravenous fluids for dehydration, and specific treatment based on blood tests.
Medications for kidney and liver support may be required. Your vet is also likely to consider tubing your horse, using mineral oil, activated charcoal or Epsom salts, while the injection of painkillers may be needed to treat colic.
Dr Mac is an academic, a practising equine veterinarian, and a stud owner.