The 3 main allergy categories in dogs and cats are allergies to environmental substances, fleas, and food. Food allergies are the least common, diagnosed in about 15% to 20% of dogs and cats with itchy skin and less than 1% of dogs and cats with any disease.[1] Many animals with allergies are allergic to multiple substances in different categories.
 
Causes
An allergy develops after the immune system has become sensitized to the trigger substance. This means that food allergies take time to develop. Any food ingredient that a dog or cat has been eating for a while could potentially cause a food allergy. A new food that an animal just started eating wouldn’t cause allergy symptoms.
 
The most commonly reported food allergy triggers are ingredients that are most common in dog and cat diets; examples are beef, chicken, dairy products, wheat, and fish. But anything an animal has eaten could theoretically be responsible, and studies have reported individual dogs and cats becoming allergic to foods as diverse as kidney bean, barley, and tomato.[2]
 
Signs
Itching is the most common sign of any type of allergy in dogs and cats. Itching caused by a food allergy is typically nonseasonal, but because animals with a food allergy can also be allergic to environmental substances, their itching might be worse at certain times of the year.
 
Food allergies cause the same skin problems as other allergies. Repeated skin and ear infections are common. Food allergies can also cause digestive tract symptoms. These are some of the signs:

• Itching, especially of the face, ears, feet, belly, or around the anus
• Reddened skin, bumps, scabs, oozing lesions, hair loss
• Head shaking, ear scratching, yellow or brown waxy discharge in the ears, odor from ear canals
• Diarrhea
• Frequent defecation
• Vomiting

 
Diagnosis
The only way to diagnose food allergy in dogs and cats is with an elimination diet trial. An elimination diet is formulated to eliminate all possible allergy triggers from the animal’s food. Food allergy is diagnosed if the allergy signs improve during the diet trial. Single-ingredient challenge trials are then used to pinpoint the trigger ingredients.
 
An elimination diet trial usually lasts at least 8 weeks. During the trial, the animal can eat nothing except the elimination diet and approved treats: no flavored medications, table food, chew toys made from animal products, and so forth.
 
Two types of prescription diets are used as elimination diets. Limited-ingredient diets have unusual protein and carbohydrate sources the pet is unlikely to have eaten before. Hydrolyzed protein diets have proteins broken down into molecular units too small to trigger an allergic response. Home-cooked elimination diets are appropriate for some pets and are best planned with the input of a veterinary dermatologist or nutritionist.
 
Just changing the brand or flavor of dog or cat food doesn’t work to diagnose a food allergy. Commercial grain-free, raw, and boutique diets also don’t work as elimination diets. Nonprescription dog and cat diets usually contain some of the same ingredients (possible allergy triggers), might contain protein or carbohydrate sources not listed on the label, and can be cross-contaminated with other foods during manufacture.
 
Veterinary dermatologists use skin tests to identify environmental substances that cause an allergic response, but skin tests can’t effectively identify food ingredients that trigger allergies. None of the blood and saliva tests that claim to diagnose food allergy have been found to be reliable.
 
Treatment
Animals with allergies are often allergic to multiple substances, so the more allergy triggers you can eliminate, the more comfortable they’ll be. For pets with food allergies, avoiding trigger ingredients reduces the allergic response. Elimination diets that are nutritionally complete can be fed long term as therapeutic diets. Some pet owners prefer to find a less expensive nonprescription diet that lacks their pet’s trigger ingredients. Effective flea prevention is especially important for animals with allergies to eliminate the possibility that a flea allergy is contributing to the itch.
 
References
1. Olivry T, Mueller RS. Critically appraised topic on adverse food reactions of companion animals (3): prevalence of cutaneous adverse food reactions in dogs and cats. BMC Vet Res. 2016;13(1):51. doi:10.1186/s12917-017-0973-z
2. Mueller RS, Olivry T, Prélaud P. Critically appraised topic on adverse food reactions of companion animals (2): common food allergen sources in dogs and cats. BMC Vet Res. 2016;12:9. doi:10.1186/s12917-016-0633-8

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Cat scratch disease in humans is caused by infection with Bartonella henselae bacteria, which are transmitted by cat fleas. This potentially serious infection is an important reason to use flea control products for all cats all year round.
 
Transmission
Bartonella species are spread by arthropods such as fleas, lice, and sand flies. People can be infected by several species of Bartonella. In the United States, B henselae is the most common cause of disease in humans.
 
Cat fleas shed B henselae in their feces. Cats that harbor fleas carry these bacteria on their claws, on their skin, and in their mouths. Humans are usually infected by cat scratches contaminated with flea feces. People can also be infected by being bitten or having an open wound licked by a cat carrying B henselae.
 
Dogs can also carry B henselae, but they typically have a lower bacterial load than cats. Whether dogs can transmit the infection to humans is not known.
 
Infection in Humans
In humans, B henselae infection can cause fever and swollen lymph nodes. More rarely, it causes inflammation of the heart valves, brain, bone, joints, eyes, or other organs. People with compromised immunity, such as those with HIV infection, are more likely than others to develop serious illness from B henselae infection.
 
Infection in Cats
Cats very commonly carry B henselae. Up to 40% of shelter cats in some geographic locations have B henselae in their bloodstream. Kittens tend to carry more bacteria than adult cats.
 
Cats carrying B henselae usually have no symptoms. Because these bacteria are well adapted to living in cats’ bodies, they very rarely make cats sick. However, some cats develop fever, vomiting, eye inflammation, or swollen lymph nodes.
 
Bartonella infection can be difficult to diagnose in cats. No antibiotic has been shown to completely eliminate the bacteria from carrier cats, and prolonged antibiotic treatment can cause the bacteria to develop antibiotic resistance. For these reasons, testing and treatment are recommended only for cats that have signs of illness caused by Bartonella infection.
 
Prevention
Because flea control prevents B henselae transmission among cats, the American Association of Feline Practitioners recommends that all cats receive flea prevention products year round.
 
The CDC recommends these steps to prevent B henselae infection in people:

• Avoid being scratched, bitten, or licked by cats, especially stray cats and kittens. The risk is highest for people with compromised immunity.
• Wash cat scratch wounds with soap and water.
• People with compromised immunity should wash their hands after handling cats.
• Use flea control products that your veterinarian recommends for your cat.
• Keep cats indoors to reduce the chance of flea exposure.
• People with compromised immunity can keep their cats and do not need to have healthy cats tested or treated for Bartonella.
• People with compromised immunity should avoid adopting cats younger than 1 year, stray cats, sick cats, and cats with fleas.

Sources

• Bartonella henselae infection or cat scratch disease (CSD). Centers for Disease Control and Prevention. Reviewed January 10, 2022. Accessed July 6, 2023. https://www.cdc.gov/bartonella/bartonella-henselae/index.html

• Lappin MR, Elston T, Evans L, et al. 2019 AAFP feline zoonoses guidelines. J Feline Med Surg. 2019;21(11):1008-1021. doi:10.1177/1098612X19880436

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