Common causes and signs of food intolerance in veterinary patients


Although many ingredients affect dogs and cats adversely, associated syndromes are often poorly understood and lack specific diagnostic tests.

Monika Wisniewska / stock.adobe.comOur current understanding of adverse food reactions in dogs and cats frequently relies on human-based methods. Adverse food reactions, which are generally classified either as food allergy or food intolerance, cause wide-ranging symptoms in people, and several diseases have known dietary causes. While the prevalence of adverse food reaction in companion animal species is unknown, food intolerance and dietary indiscretion are generally considered more common than food allergy.1

In a recent review, Mark Craig, BVSc, CertSAD, MRCVS, describes the leading causes of food intolerance in dogs and cats, including specific ingredients, additives used during food manufacturing, and the effects of microbial spoilage. He explains that foodborne illness is often difficult to diagnose in veterinary medicine due to the lack of available diagnostic tests and the dose- and species-dependent nature of clinical signs. Initial signs may occur hours to days after food consumption and can persist for several days, further complicating the diagnostic process.

Microbial contamination

Bacterial spoilage rarely causes foodborne illness in otherwise healthy dogs and cats, as they are considered relatively resistant to pathogenic effects from Salmonella, Campylobacter and Escherichia coli.2 Dogs and cats are also more resistant than are humans to bacterial toxins produced by Clostridium botulinum, Staphylococcus aureus and Bacillus cereus. However, Dr. Craig notes that companion animals are highly susceptible to aflatoxicosis due to Aspergillus contamination of grains and legumes, and dogs are also particularly prone to seizures after ingesting moldy foods containing Penicillium.

Bacterial contamination of oily fish, such as anchovies, tuna and sardines, can cause histamine release from the spoiled meat. When ingested, heavy accumulations of histamine can cause scombrotoxin poisoning, which is characterized by severe gastrointestinal (GI) signs occurring within 30 minutes of ingestion.3 Both dogs and cats are susceptible to scombroid, which is also the most common cause of seafood poisoning in humans.

Plant- and metal-derived toxins

Many plants, including spinach, rhubarb and beetroot, contain high levels of oxalic acid, which binds minerals and can cause formation of calcium oxalate uroliths. In dogs, zinc-responsive dermatosis may occur from feeding of poor-quality diets containing cereals, legumes or nuts that are rich in phytic acid.4

Metal toxicosis most frequently occurs in dogs and cats after ingestion of lead, cadmium, arsenic or zinc. Dr. Craig notes that associated signs vary depending on age, amount ingested and duration of exposure, as metals can accumulate in tissues over time. For example, acute cases of lead toxicity frequently include central nervous system signs, whereas chronic or low-level exposure may cause GI signs.

Additives and supplements

Few studies have addressed the toxic effects to pets from additives introduced during food manufacturing.5 Although the preservative benzoic acid is linked to atopic dermatitis in humans, its effects have not been documented in dogs or cats. Another additive, propylene glycol, may cause Heinz body formation and reduced erythrocyte survival in cats.


Dr. Craig advises cautionary use of dietary supplements in dogs and cats, as safe upper limits have not been established for many nutrients. Adverse effects have been reported after excessive administration of dietary vitamins A and D in cats and dogs, respectively.


Methylxanthine toxicity is well documented in dogs and cats from ingestion of caffeine or theobromine. While caffeine is found in tea, coffee, soft drinks and some human medications, Dr. Craig emphasizes that pets may encounter theobromine in both likely (e.g., chocolate) and surprising (e.g., garden mulch) sources. Because theobromine is eliminated slowly, clinical signs can persist even after ingestion of small doses. Methylxanthine toxicity is associated with restlessness, polydipsia and GI signs that may progress to serious neurologic signs, cardiac arrhythmia and respiratory failure.


Xylitol, a common artificial sweetener in many human foods as well as in medical and dental products, is highly toxic to dogs. This sugar alcohol stimulates insulin release, which can result in severe hypoglycemia, seizures and liver failure.6

Lactose intolerance is common in dogs and cats. Lactase activity in the small intestine naturally decreases in puppies and kittens after weaning, so these patients become partially or completely unable to digest this sugar. Lactose-intolerant patients typically experience bloating and diarrhea. Alternatively, pets that cannot tolerate milk may be unable to digest the main protein, casein. Diarrhea has also been linked to ingestion of several other carbohydrates found in human foods, including the popular artificial sweetener sorbitol and the main carbohydrate found in beans, raffinose.

Miscellaneous foods

Dr. Craig mentions several other human foods that can adversely affect pets, including allium species, grapes and hops. Organosulfoxides contained in alliums such as onions, garlic, leeks and chives are toxic to dogs and cats in both cooked and uncooked forms. Affected patients typically exhibit GI signs progressing to weakness, pale mucous membranes, jaundice, dark urine, tachycardia and tachypnea.

Hops, an ingredient used for brewing beer, can cause hyperthermia, tachycardia, panting, vomiting and seizures in dogs and cats. Ingestion of grapes, including dried forms such as raisins and sultanas, can cause GI signs and renal failure in dogs.

Dysmotility, dysbiosis and physical damage

Prolonged gastric retention and vomiting are common in dogs fed viscous diets and those high in fats and starches. Dysmotility can also result from methane production in the colon by foods containing fermentable fibers.

Abrasive food components, such as bone and hair from ingestion of a carcass, can physically damage the GI tract, while excessive fat may cause gastric fermentation. Ingestion of sand can cause small intestinal obstruction, ulceration, hemorrhage and dysmotility.

Dr. Craig also addresses the role of the GI microbiome in digestion and patient immunity. Many foods are known to significantly affect GI microbiota populations; however, the specific role of the microbiome in canine and feline food-related illnesses remains unknown. Thus, species-specific research is needed to determine how diet composition influences the development of dysbiosis.


Day MJ. The canine model of dietary hypersensitivity. Proc Nutr Soc 2005;64:458-464.

Miller EP, Cullor JS. Food safety. In: Small Animal Clinical Nutrition, 4th ed. Ed. Hand MS, Thatcher CD, Remillard RL, et al. St. Louis, MO: Walsworth Publishing;2000:183-194.

Roudebush P. Adverse reactions to foods: allergies versus intolerance. In: Textbook of Veterinary Internal Medicine, 7th ed, Vol. 1. Ed. Ettinger SJ, Feldman EC. St. Louis, MO: Saunders Elsevier; 2010:673.

Watson TDG. Diet and skin disease in dogs and cats. J Nutr 1998;128:2783s-2789s.

Roudebush P. Pet food additives. J Am Vet Med Assoc 1993;203:1667-1670.

Murphy LA, Coleman AE. Xylitol toxicosis in dogs. Vet Clin North Am Small Anim Pract 2012;42:307-312.


Link to study:

Craig JM. Food intolerance in dogs and cats. J Small Anim Pract 2019;60(2):77-85.


Dr. Natalie Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.


Related Videos
© 2023 MJH Life Sciences

All rights reserved.