The oak genus (Quercus) comprises several hundred species and hybrids. Oaks are naturally found in the Northern Hemisphere of both the Old and New Worlds; however, they also occur worldwide as ornamental trees. Approximately 90 native species of oak are found in the US and 30 in Europe. Oaks range in size from small shrublike plants, especially in drier climates, to large majestic trees. The majority of oak species are deciduous and are recognized by the presence of multiple buds at twig ends and characteristic acorns, partially enclosed by scaly caps at their base. Oak leaves vary in shape, texture, and color.
Etiology and Pathogenesis of Oak Toxicosis
Oak foliage and acorns are consumed as food by many livestock and wildlife species. Several oak species have been specifically implicated in causing intoxication in the US:
Q gambelii or Gambel oak (see images of Gambel oak and )
Q havardii or shinnery oak (see images of shinnery oak , , and )
Q turbinella or shrub live oak
Q stellata or post oak
All species of oak should be considered potentially toxic. In addition to causing intoxication, the tannins in oaks can affect nutrition because they form complexes with nutrients, thereby limiting their bioavailability, and, in ruminants, alter ruminal microbiota.
Several Gambel oaks along a narrow wash. Typically, they are 15–30 feet tall but can reach up to 65 feet and often grow as a large shrub or a multitrunked tree with crooked branches. The drought-tolerant, deciduous trees are native to the western US.
Courtesy of Janet Fox at SEINet Arizona-New Mexico Chapter. Used under CC BY-NC 3.0.
Leaves are deciduous, 3–7 inches long, with 5 to 11 deep lobes; glossy green above, paler beneath, turning yellow, orange, or red in autumn.
Courtesy of Paul Rothrock at SEINet Arizona-New Mexico Chapter. Used under CC BY-SA 4.0.
Leaves are gray-green to olive green. They have a lustrous upper surface and are whitish and densely hairy below.
Courtesy of Max Licher at SEINet New Mexico Arizona Chapter. The image is dedicated to the public domain under CC0 1.0.
Acorns develop in one year, maturing in the autumn, and occur alone or in clusters of 2 or 3.
Courtesy of David Thornburg at SEINet New Mexico Arizona Chapter. The image is dedicated to the public domain under CC0 1.0.
Shinnery oak is a deciduous, low-growing, thicket-forming shrub widely distributed in the southern Great Plains of North America.
Courtesy of David Thornburg at SEINet New Mexico Arizona Chapter. The image is dedicated to the public domain under CC0 1.0.
Although the likelihood of oak intoxication is dependent on multiple factors, risk increases when animals are forced to subsist on buds, leaves, or acorns for a period of time because of the lack of other suitable feedstuffs. Ultimately, the risk of intoxication is greater with increasing levels of consumption. A general guideline is that animals must eat ≥ 75% of their diet from oaks to become intoxicated (1).
Other factors contributing to the risk of intoxication include time of year, animal species, animal age, growth patterns of certain oak species, toxin concentrations (eg, which vary between oak species or by leaf age), and variable acorn production (by precipitation). One of the largest reported incidences of cattle intoxication occurred across 60 ranches in Northern California; over 2,700 cattle died when they were forced to eat oak buds because of unseasonable snow and freezing weather (2).
Most animals are susceptible to oak poisoning; however, cattle and sheep are most commonly affected. Toxicosis has occurred in horses, llamas, exotic ruminants, moose, dogs, and rabbits. Susceptibility varies, with species such as goats and mule deer relatively resistant to tannins. Resistance is due to proline-rich proteins (PRPs) in saliva and inducible tannase enzymatic activity in the rumen and ruminal mucosa. These adaptations allow goats and mule deer to tolerate higher tannin doses than cattle and sheep.
Historically, a congenital chondrodystrophy in beef calves was called “acorn calf disease” because the condition was believed to be caused by acorn ingestion during pregnancy (3). The abnormality is now hypothesized to be likely related to a nutritional deficiency and not related to acorn consumption (3).
The toxic effects following oak consumption are due to the actions of tannins. Tannins are polyphenolic compounds that can complex with proteins, starch, cellulose, and minerals. Tannins are categorized as either condensed or hydrolyzable according to their chemical structure. The hydrolyzable tannins are responsible for intoxication. They are hydrolyzed in the GI tract to their constituent phenolics and sugars. For example, gallotannins are hydrolyzed to gallic acid, which is further metabolized to pyrogallol and other phenolics and ultimately glucuronidated in the liver for elimination. The other common hydrolyzable tannin group is called ellagitannins.
The precise pathophysiological effects of hydrolyzable tannins and their metabolites are not fully understood. However, tannins bind to and denature proteins lining the GI tract, causing erosions, hemorrhage, and ulceration. Damage to the mucosa of the GI tract results in increased absorption of phenolics and further tissue damage in organs with higher toxin concentrations (eg, kidney and liver). In most species, the GI tract and the kidneys are the primary target organs; however, hepatic damage can occur as well, particularly with higher dosages (4). Oxidative damage from free radical formation and inhibition of mitochondrial respiration are other potential mechanisms for cell damage. Renal tubular damage could be the result of direct toxic vascular damage exacerbated by renal ischemia or reperfusion injury.
Clinical Signs of Oak Toxicosis
Clinical signs of oak toxicosis in ruminants typically begin after animals consume large amounts of buds and leaves or acorns for 2 to 3 days or longer. Initial clinical signs are nonspecific and include anorexia, listlessness, rumen stasis and constipation. Clinical signs progress to diarrhea (often bloody), dehydration, colic, and subcutaneous edema of the neck, brisket, abdomen, and perineum; these are often associated with concurrent renal failure and metabolic acidosis.
Renal failure is characterized by increased BUN and creatinine concentrations, proteinuria, glucosuria, hyperbilirubinuria, hyperphosphatemia, hypocalcemia, and urine with a low specific gravity. In sheep and goats, edema is less commonly reported. In horses, diarrhea, colic, and tenesmus are more common and severe. Clinical signs consistent with circulatory shock are reported in horses as well. Liver dysfunction characterized by increased liver enzyme activity can occur; however, liver damage is less frequent than GI and renal damage.
Lesions in Oak Toxicosis
Gross postmortem lesions include pale, swollen kidneys, perirenal edema, subcutaneous edema, ascites, and hydrothorax. In ruminants, edema and subserosal petechial or ecchymotic hemorrhage of the intestinal mucosa and ulceration of the esophagus and rumen can also occur. In horses, lesions of the GI tract are often severe and involve substantial edema of the cecal and colonic mucosa with ulcerative and hemorrhagic gastroenteritis noted. Oak leaves, leaf fragments, and acorns are often present in the rumen.
Diffuse renal tubule degeneration, necrosis of cortical tubular epithelial cells, and dilated tubules with flattened cells or with basement membranes without cells can be observed microscopically. Dilated tubules contain hyaline, granular or cellular casts of epithelial cells, neutrophils, and cellular debris. Diffuse, severe, necrohemorrhagic and ulcerative typhlocolitis, enteritis, and submucosal edema have been noted in horses.
Diagnosis of Oak Toxicosis
Diagnosis of oak toxicosis is based on compatible history and clinical and autopsy findings. In one case series in horses (5), a diagnosis of oak intoxication was made if three of the following criteria were present:
history of exposure to oak leaves/buds or acorns within the previous 7 days
clinical and laboratory data supporting alimentary or renal disease
identification of oak leaves or acorns in the feces or GI tract at postmortem
gross and histopathological findings consistent with oak or acorn toxicosis in nonsurvivors
Exposure, but not intoxication, can be confirmed by detecting tannin metabolites (eg, gallic acid or pyrogallol) in urine, serum, or GI content samples. However, relatively few diagnostic laboratories offer this testing.
Pearls & Pitfalls
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Differential diagnoses of oak poisoning include the following:
pigweed (Amaranthus spp) toxicosis
heavy metal exposure
ochratoxicosis
Treatment of Oak Toxicosis
No specific treatment exists for oak toxicosis. The primary goal is supportive treatment for dehydration, circulatory shock, metabolic acidosis, electrolyte abnormalities, and renal failure. Treatment needs to consider logistical (ie, ruminants on range pasture or accessible in confinement) and economic variables. In the case of horses, hospitalization and more intensive treatment might be possible. The first step in any case is to prevent further exposure to leaves, buds, and acorns.
Historically, calcium hydroxide (slaked or hydrated lime), laxatives or cathartics (ie, mineral oil, sodium sulfate, or magnesium sulfate), and polyethylene glycol (PEG) given early in the course of disease have been suggested for treating intoxicated ruminants (4). The main goal of these treatments is to bind tannins and metabolites in the GI tract and thereby decrease systemic absorption. If binding compounds is the goal, the administration of one or more doses of activated charcoal is preferable. Note that calcium hydroxide cannot be administered directly to animals because of its alkaline nature; it must be diluted in feed to a concentration of no more than 10–15%. Dosing an anorectic animal using feed or water is problematic.
In ruminants, transplantation of ruminal microbiota might be beneficial; unfortunately, there is limited evidence for the efficacy of such treatments for intoxicated ruminants. In affected horses, fluid therapy, pain management, and antimicrobial administration need to be considered and tailored to the specific case.
Although dogs can develop renal failure and a hepatopathy after acorn ingestion, clinical signs might be related only to a physical intestinal blockage from the acorns.
The prognosis is variable. In one case series in horses, mortality was over 50% and was positively associated with older age, signs of shock (heart rate, hematocrit, blood lactates, blood creatinine), hemorrhagic diarrhea, ileus, and increased colon wall thickness noted by transabdominal ultrasonography (6). In ruminants (and other animals), a decrease in BUN and creatinine has been associated with increased survival (2). Recovery can take several weeks, so economic considerations might be a factor. In cattle, considerable weight and production losses can occur during the illness; however, compensatory weight gain after recovery is possible.
Prevention of Oak Toxicosis
Several preventive steps are recommended for oak toxicosis, and the following are the most straightforward approaches:
keeping animals out of areas with substantial quantities of oak
decreasing grazing pressure to allow greater forage availability during oak budding and leaf growth periods
providing adequate feed alternatives
Consumption of a pelleted ration supplement containing 10–15% calcium hydroxide plus access to more palatable feeds may be used as a preventive measure if exposure to acorns or oak leaves cannot be avoided. Removal of fallen acorns from the grazing area might also be helpful if logistically possible. Interestingly, PEG has been used to allow for enhanced consumption of tannin-containing plants by ruminants in regions where such plants (ie, Quercus spp) are used as a feed source (4, 7).
For More Information
Mendieta-Calle J, Hughes KL, Howerth EW, Schaffer PA. Histologic and ultrastructural findings in oak (Quercus spp.) toxicity on 2 beef cattle farms in Colorado. J Vet Diagn Invest. 2025;37(1):135-140.
References
Vermeire LT, Wester, DB. Shinnery oak poisoning of rangeland cattle: causes, effects & solutions. Rangelands. 2001;23(2):19-21. doi:10.2458/azu_rangelands_v23i2_vermeire
Spier SJ, Smith BP, Seawright AA, Norman BB, Ostrowski SR, Oliver MN. Oak toxicosis in cattle in northern California: clinical and pathologic findings. J Am Vet Med Assoc. 1987;191(8):958-964. doi:10.2460/javma.1987.191.08.958
White PJ, Windsor PA. Congenital chondrodystrophy of unknown origin in beef herds. Vet J. 2012;193(2):336-343. doi:10.1016/j.tvjl.2012.04.016
Burrows GE, Tyrl RJ. Quercus. Toxic Plants of North America. 2nd ed. Wiley-Blackwell; 2012:677-689.
Smith S, Naylor RJ, Knowles EJ, et al. Suspected acorn toxicity in nine horses. Equine Vet J. 2015;47(5):568-572. doi:10.1111/evj.12306
Hermange T, Runault B, Couroucé A. Retrospective study of 25 cases of acorn intoxication colitis in horses between 2011 and 2018 and factors associated with non-survival. Animals. 2024;14(4):599. doi:10.3390/ani14040599
Rogosic J, Estell RE, Ivankovic S, Kezic J, Razov J. Potential mechanisms to increase shrub intake and performance of small ruminants in Mediterranean shrubby ecosystems. Small Rumin Res. 2008;74:1-15. doi:10.1016/j.smallrumres.2007.07.006
