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Who, me?

A ‘Fussy Eater’, or Possibly Rancid Wildlife Formula?


Shirley Casey and Mackenzie Goldthwait, DVM



Animals must eat to live – and like young of many species, juvenile wild animals are especially eager to eat. There are times, however, when wild animals in rehabilitation may refuse food – and then they may be described as ‘fussy eaters.’ In such instances, it is up to the rehabilitator to identify the cause for the reluctance to eat and hopefully address it promptly. This can be challenging since there can be many reasons that the wild animal might not eat.  The animal could be: (1) responding to a new, unfamiliar and stressful captive environment, (2) adjusting to new feeding methods and foods, (3) injured, in pain, ill, or teething, (4) reacting to a medication that affects appetite, or (5) not hungry due to overfeeding or feedings that are too frequent.  Or it could be some combination of those and possibly even other factors.  


While the items listed above are more typical considerations, rancidity is another possibility that is rarely considered since it is more uncommon. Unfortunately, there have been recent cases when some wild animals in rehabilitation have been unintentionally fed food and formula that were rancid. Since rancidity has been given little attention in the broader rehabilitation community or smaller social media groups, the following presents a quick review of rancidity, its causes, and health conditions that may occur in wildlife that is fed rancid food.





When people hear the term “rancidity,” many know it somehow relates to bad food, but that’s about it. Some may know that it can cause a temporary stomach upset if eaten. Many suspect that rancid food would be very obvious to detect and easy to avoid. Rehabilitators certainly do not expect commercial foods produced and sold for domestic pets or wildlife in captivity to be rancid, especially milk replacement formula. However, some milk replacement powders (even from brand new, unexpired, and unopened cans) intended to make formula for wild mammal babies have been found to be rancid in tests recently conducted by an independent laboratory.   


The term “rancid” refers to foods with fats or oils that have ‘gone bad’ or spoiled. Any foods containing fats can become rancid - including nuts, seeds, grains and rice, dry animal food (including dog food and rodent chow), and more. While improved manufacturing and storage methods, as well as expanded use of preservatives (including natural antioxidants), have substantially reduced the onset or development of rancidity in foods containing fats since the 1960’s, rancidity still occurs. The higher the fat content, the more likely for rancidity to be a possibility.


When people are offered food that is rancid, they may notice an ‘off’ odor (often described as similar to soap, crayons, wet cardboard, metallic, paint, or wood varnish).  Or they may notice a ‘bad’ taste. In situations where people do not immediately recognize a food item as rancid and then eat that food, they may initially become nauseated, vomit, or lose their appetite.  They may then develop bloat, diarrhea and painful cramping. Since rancidity is uncommon in human foods, people are unlikely to consider rancidity when they try to determine what they may have eaten that caused such an uncomfortable and sometimes debilitating experience. If they identify which food was likely rancid, they would most certainly stop eating it. 



Why do foods become rancid? 


Rancidity is the natural process of decomposition (degradation) of fats or oils by oxidation (a process in which the composition of a chemical substance, like oil, changes because of the addition of oxygen). Rancidity can cause a change in odor, palatability, and nutritional quality. These fats and oils are composed of lipids that degrade to the point of becoming either unpalatable or unhealthy to ingest. Ingestion of rancid lipids has been linked to the development or exacerbation of many diseases.


Oxidative rancidity, known as autoxidation, occurs when oxygen is absorbed from the environment. In the presence of oxygen and/or excess sunlight, most lipids will break down and degrade. Since oxygen is eight times more soluble in fats than it is in water, it is this exposure that is the main cause of the autoxidation process, increasing the oxygen saturation of the oil. Time and heat can significantly speed up the rancidity process. 


Vegetable oils (unsaturated fats) tend to be less stable and turn rancid more quickly than animal fats. They can also become much more rancid than animal fats, even before the human sense of smell can detect it. They are more susceptible to oxidation than saturated fats, meaning the more polyunsaturated a fat is, the faster it can become rancid. 

What happens when a rancid food is eaten?


﷯Various clinical signs and symptoms develop after a rancid food is eaten. When a healthy human adult eats a single meal of a tiny amount of slightly rancid food, there maybe be very minor and temporary gastrointestinal symptoms. However, when rancid food is consumed in a larger quantity by a younger and/or compromised wild animal for a longer period (especially as the sole food source), the health impacts can be more severe – and possibly fatal. If a wild animal is reluctant to eat on its own, that could provide a clue to help identify the cause as possible rancidity. 

Stages of Health Problems Wildlife Milk

Refusal is not an option when an animal is being gavaged (i.e., tube fed), so that clue would not be evident. Since many of the signs and symptoms resulting from consuming rancid food are similar to other (and often more common) causes, the rehabilitator and the consulting veterinarian need to understand and assess a range of possible factors – including the possibility of a rancid food product.  

The information described below (and shown above) is compiled from a variety of sources, including from people who accidentally consumed rancid food, as well as from published medical and scientific literature. Further information has been incorporated that has been provided on cases reported by veterinarians and animal caregivers, animal research conducted on animals, necropsies and scientific papers. The stages and time frames described below and in the chart at right are approximate and vary based on:  the animal’s age (very young or aged); condition of the animal (healthy vs. severely compromised); the degree of rancidity in the food (e.g., low or high); the amount and time span of rancid food consumed; and other factors. Any combination of these factors can contribute to the severity and extent of the problems – and the ease and likelihood of recovery or progression to a more critical medical condition. In addition, many molds and bacteria reproduce well in spoiled or rancid food (especially in a moist environment), can further complicate health problems and increase mortality.



Progression of health conditions after eating rancid foods:


- Initial, early stage (days 1-2) – recovery likely if they stop eating the rancid food

May refuse food (likely due to bad odor or poor palatability). 

Wild animals, even the young, are extremely sensitive to odors and taste (essential for survival). It may be impossible to assess willingness to eat if the animal is gavaged (i.e., tube fed).

May vomit or regurgitate the food.


- Mid-stages (days 3 to 9) – recovery possible, but increasingly difficult 

• People report feeling nauseated or uncomfortable after eating rancid food – and avoid further eating. Same seems to occur in wild animals.

• Bloat and abdominal distension.

• Cramping and abdominal pain. Acts restless and increasingly uncomfortable.

• Develops soft stool, which progresses to involuntary diarrhea.

       - If diarrhea is severe, dehydration is likely, with electrolyte imbalance. 

       - Nutrition is compromised since animal is unable to obtain adequate nutrition to effectively function, grow or develop.

• May be ravenous – or totally lack appetite.

• As diarrhea and lack of nutrition continue, dehydration can worsen. 

• Slow to poor digestion/absorption/utilization of food.

• General weakness, lethargy, apathy.

• Little to no weight gain.

• Slow, reduced growth.  

• Poor coat or skin condition.


- Late stages (days 10+) – recovery is very difficult and uncertain

Weight loss, severe malnutrition or emaciation.

General weakness, collapse.

Secondary infections may develop (gastrointestinal and/or respiratory).

Organ damage - including liver, heart, spleen and reproductive organs. Some organs may enlarge.

Possible causes of fatalities:

Severe dehydration with electrolyte imbalance.

Weakened immune system.

Secondary infections may become more severe.

Organ failure.

Malnutrition, starvation.

Respiratory distress.



How is rancidity detected in food?


Rancidity is usually detected by taste or smell, such as the ‘off’ odor or taste described previously. Rehabilitators should sniff any food product at frequent intervals prior to preparing diets for the animals in care to become familiar with its normal odor and assess whether it has an ‘off’ odor. Wild animals, with their heightened olfactory senses, can definitely notice problems with food, including rancidity. If a wild animal is hungry and offered an appropriate food for the species, age, and health, it is likely to eat it.  When it refuses it, it’s time for some problem-solving. It is especially critical that the rehabilitator become adept at detecting rancidity for all animals being gavaged (i.e., tube fed), since the animal has no ability to refuse forced feeding. 


If the animal has been willing to eat and then stops, especially if this immediately follows the introduction of a new food or formula prepared from a different container, consider the possibility of rancidity. Take a quick sniff of the food. Quickly adjust and observe whether the animal’s willingness to eat changes if the animal is presented with another food source. If necessary, continue to  consider other factors.


A definitive way to detect rancidity is by performing a chemical test. One of the primary tests used to measure lipid oxidation is the Peroxide Value (PV) test. It is generally accepted that the first compounds formed by oxidation of an oil are hydroperoxides. PV is a measurement of the quantity of peroxide oxygen present in an oil. Generally speaking, oils with a PV higher than 10 are not considered acceptable. 



Avoid causing rancidity 


Store foods (including milk replacement powders) in a cool dry place. Avoid allowing the foods to reach or stay at room temperature for a prolonged period of time, or at higher temperatures for hours. Consider freezing the foods (opened and unopened containers) to further reduce the chance of spoilage. Most importantly, minimize the product’s exposure to oxygen – such as by not sifting or repackaging with large amounts of air. Also avoid exposure to sunlight. 





While it is rare for rancidity to occur in foods fed to wildlife in rehabilitation, it is possible and has happened as recently as this year (2020). As a result, it is incumbent on the wildlife rehabilitator to be aware of this possibility, work to recognize the signs in the product, and take steps to avoid it. 


If the rehabilitator suspects rancidity might be causing health problems, the animal’s diet should be immediately changed to stop feeding the product in question. Document the product information (name, lot number and date). The producer, manufacturer and distributor should be notified – and they may offer to replace the product (and test it). In some cases, the rehabilitator may arrange to have a laboratory test the product for rancidity.





Shirley Casey, co-founder of WildAgain Wildlife Rehabilitation, Inc., has been a licensed rehabilitator since 1986. She conducts research on wildlife topics as well as conducts training and publishes on a wide range of rehabilitation topics, including on


Mackenzie Goldthwait, DVM, graduated from Tufts University School of Veterinary Medicine in 1989 with a special interest in wildlife medicine. She was the veterinarian at Cape Wildlife Center in Massachusetts for 18 months. She is a veterinarian in Highlands Ranch, Colorado. Dr. Goldthwait, a Master Birder, also monitors bird populations in the US and internationally.





Addis, Mekonnen, and Desta Sisay. 2015. A review on major food borne bacterial illnesses. Medicine.


Fekete, S. G, E. Andrasofsky, and R. Glavits. 2009. Pathological chances induced by rancid feed in rats and effects on growth and protein utilization. Acta Vet Hung. V. 57(2). Pp. 247-61. 


Greenberg,  Samuel M.  A. C. Frazer,  B. Roberts. 1953. Some Factors Affecting the Growth and Development of Rats Fed Rancid Fat. The Journal of Nutrition. V. 50 (4) pp. 421-440.


Lupcho, T. , Harrist, A. and C. Van Houten. 2016. Gastrointestinal Illness Associated with Rancid Tortilla Chips at a Correctional Facility – Wyoming. Morbidity Mortality Weekly Report. V. 65. Pp. 1170-1173. icon. 


Mathur, Anukriti, et al. 2019. A multicomponent toxin from Bacillus cereus incites inflammation and shapes host outcome via NLRP3 inflammasome. Nature Microbiology. V. 4, pp. 362-74. 


Tawde, S. and N.G. Magar. 1957. Effect of feeding rancid ghee (clarified butter) to rats. Indian Journal of Dairy Science. V. 10, pp. 73-78.

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