A recent press release by The Guardian has been making headlines all over the world after a research study was published looking at antibiotic-resistant bacteria within dog foods in Portugal.
“Researchers warned of “an international public health risk” after finding antibiotic-resistant bacteria in a range of different types of raw dog food.
“The trend for feeding dogs raw food may be fuelling the spread of antibiotic resistant bacteria”, the researchers said in a press release for their study, to be presented at the European Congress of Clinical Microbiology & Infectious Diseases.”The Guardian
These statements and others within the article have fueled a lot of passion in the online veterinary community that already have concerns about pathogens within raw dog food. And this makes sense – the public health concern surrounding antibiotic-resistant bacteria should be concerning.
But as always – it’s important not to jump to conclusions over news media titles and instead evaluate the research itself to see what is true, and what isn’t. Often this can lead us to more answers – and of course more questions!
Goal of Study:
“Our aim was to characterize antibiotic resistance profiles and main clinically-relevant features of Enterococcus species among processed (dry/wet/semi-moist) and non-processed (raw-frozen) foods belonging to the main brands commercialized for dogs in Portugal and abroad.”Finisterra et al 2021
Researchers collected 55 samples from 9 different retail stores – 6 local supermarkets, 2 specialty pet stores, and 1 veterinary clinic – all within northern Portugal between September 2019 to January 2020. Brands were mostly international (21), with a small portion of national (4) brands.
Total Samples collected were: 8 dry foods, 22 wet foods, 4 semi-moist foods, 14 raw foods and 7 treats.
|Type of Food||# Brands||# Recipes||# Samples Taken|
What can we say about Diversity and Sample Size in this study…?
This study looked at a total of 55 samples, and 25 brands for this study. The diversity of brands varied significantly between diet types. The wet dog food category has the most diversity of both brands and recipes – with 16 brands and 22 recipes represented in the study. Semi-moist dog food was the least diverse within the study with only two brands, and four recipes represented.
Raw dog food was also significantly less diverse as far as sample size – with only two brands represented, and a total of nine recipes. The raw food category was also the only category where multiple samples were taken from the same recipe & brand at different time periods.
Personally, I would have loved to see more diversity within the brands/recipes represented - especially within both the raw, semi-moist, and kibble categories. And overall larger sample sizes.
There was only one brand (labeled F) that was represented across dry, wet, and treat categories. This brand was negative for antibiotic-resistant bacteria in both wet & dry food categories, however was positive within the treat category. Brand F’s dry and wet foods were obtained from a veterinary clinic, however, the treats were obtained from a local supermarket.
The fact that the same brand has a variation between categories is something that needs further investigation to see where exactly within the chain (ingredient sourcing, processing/manufacturing, packaging) contamination occurred, and why product/brand consistency for pathogens varied.
Proteins presented in study within each diet type tested:
- Wet food: meat and animal subproducts, chicken, beef, fish, quail
- Dry food: poultry, pork, chicken, hydrolyzed salmon, salmon, meat and animal subproducts
- Semi moist: pork, chicken, meat and animal subproducts
- Treats: meat and animal subproducts
- Raw: salmon, veal, chicken, lamb, turkey, stear, deer, goose, white fish.
When considering the diversity in proteins within recipes it’s important to note that the term “meat and animal subproducts” by AAFCO definition can mean a large variety of different unspecified meat products such as beef, pork, lamb, deer, stear, veal. Thus most all food types except for the “semi-moist” and “treat” category contained a combination of poultry, meat and fish protein sources. This may be important when considering where potential contamination of ingredients occurred.
Performing the study:
All samples that were collected were transported to the lab within an hour’s time. However raw dog foods, once they arrived at the facility, were then thawed at room temperature for 6 hours prior to testing.
Did you notice that the raw frozen food was thawed at room temperature for 6 hours? Thawing food at room temperature allows for the rapid multiplication of whatever bacteria is within the food item after about 2 hours according to the USDA. However - it is important to note that as long as the food was thawed within the package - no new bacteria than what was already present within the frozen meat should be multiplying. Meaning thawing at room temperature within the package would amplify the number of individual bacteria present, but it would not introduce new bacteria to the product. Thus not affecting the overall positive vs. negative presence of bacteria within the product. However, as it is actually recommended to thaw raw food in the fridge overnight, then feed - thawing the raw food in this manner would have been more representative of how owners typically feed their pets.
A small amount of each food (25g) was then combined with a solution, mixed well, and placed in an incubator for 2 hours. After mixing and incubator samples were added to a growing solution. Control solutions contained just the “cultivation medium” (Brain-Heart Infusion), and test solutions also contained antibiotics (ampicillin, vancomycin or chloramphenicol). These solutions were then put onto an agar plate to allow for bacterial growth.
After several days of growth three colonies were selected for further analysis. These bacteria within the colonies were then identified by PCR testing and light spectrometry.
After identification bacteria are tested for antibiotic resistance using something called “disk diffusion”.
Basically, how disk diffusion works is you take a sample of bacteria, and cover a pitry disk (agar plate) with the bacteria and allow it to grow until it covers the entire plate. Then you take little pieces of paper that are impregnated with antibiotics on top of the bacteria and see if the bacteria stop growing around the bacteria, or if they continue to grow. Here is a quick video I found on the process if you need a bit more of an explanation.
In the case of this study the bacteria identified were plated and then disks of the following antitbiotics were added to see how they affected the bacterial growth: ampicillin (AMP2), vancomycin (VA5), teicoplanine (TEC30), ciprofloxacin (CIP5), tetracycline (TET30), erythromycin (E15), gentamicin (CN30), streptomycin (S300), linezolid (LZD10), chloramphenicol (C30), quinupristin/dalfopristin (QD15, only for E. faecium), nitrofurantoin (F100, only for E. faecalis) and tigecycline (TGC15)
The following chart represents a summary of the findings of the study.
|Food Type||Enterococcus||1 isolate of AR||MDR|
|Raw||14/14 – 100%||14/14 – 100%||14/14 – 100%|
|Kibble/Dry||7/8 – 88%||6/8 – 75%||0|
|Wet||6/22 – 27%||3/22 – 14%||2/22 – 10%|
|Treats||3/7 – 43%||3/7 – 43%||1/7 – 14%|
- It is important to note that all samples besides the semi-moist diets were positive for Enterococcus species to varying degrees.
- However, not all species of enterococcus found within the diets were antibiotic-resistant. In order to be considered “Multi-drug resistant,” the Enterococcus bacteria must show antibiotic resistance for at least 3 different antibiotics.
- Twenty-six samples (47%; 14 raw, 6 dry, 3 wet, 3 treats) carried isolates resistant to at least one antibiotic and 17 (31%; 14 raw, 2 wet, 1 treat) carried isolates expressing resistance to antibiotics belonging to ≥3 different families, being classified as MDR.
- All diet types except for semi-moist dog foods contained at least one species of antibiotic resistance.
- All raw dog food samples tested positive for MDR bacteria, in comparison to only 10% of wet samples and 14% of treat samples.
“Our results demonstrate that dog food from different international brands is a potential vehicle of clinically-relevant antibiotic resistance and virulence genes, which may constitute a hazard to human health….Independently of the ABR profile, the high incidence of Enterococcus in dog food indicates that temperatures, hygiene practices and selection of raw materials need urgently to be revised in the production of diverse brand dog food products.”Finisterra et al 2021
Why and How are we seeing MDR Bacteria in Dog Food?
As you can see above, all types of dog food (except semi-moist) contained at least one species of antibiotic-resistant bacteria. However, the incidence of antibiotic-resistant bacteria was significantly higher in raw dog food samples, with MDR Bacteria only found in raw, wet and treat samples.
But sample size for all categories was very low and had low diversity, so true incidence and prevalence may not be completely accurate. Regardless – in my opinion – this speaks to a larger issue within the dog food industry as a whole as in my opinion no dog food should contain antibiotic-resistant bacteria. The fact that even our heat-treated foods such as wet and dry food still contained this bacteria is concerning.
Why do we see Antibiotic Resistant Bacteria in Wet and Dry Dog Food?
According to the authors of the study:
“the combination of hygiene and temperature failures during pet food processing, and enterococci tolerance to heat and low moisture/water activity conditions, could have contributed to the common occurrence of enterococci in heat-treated pet food”Finisterra et al 2021
Meaning that enterococci bacteria are partially resistant to heat, thus more likely to survive the process in comparison to other bacteria. This suggests the traditional recommendation for an internal temperature of 165 degrees Fahrenheit is not actually sufficient to kill enterococci bacteria – research actually indicates a kill point- of about 10 degrees higher in traditional cooking.
The paper also suggests that previous studies have found that decreased humidity (moisture) further increases the “kill point” of enterococci bacteria.
“In addition, the thermal resistance of E. faecium increases with decreasing humidity or in low aw matrixes as occurs in dry pet food”Finisterra et al 2021
This means that for those of us that do feed cooked products – testing for Enterococci bacteria of ingredients coming into the facility, and the final product prior to release is important in order to make sure they are free of potentially pathogenic bacteria.
Where does Antibiotic Resistant Bacteria in Pet Food Come from?
According to the CDC sources of antibiotic-resistant bacteria in the human food chain are: meat, chicken, fruits, vegetables, and eggs. Meaning that there is a prevalence of antibiotic-resistant bacteria overall within our food chain from multiple areas of contamination.
The CDC and FDA actually have laws surrounding the use of antibiotics within the food production industry and have bans on several types of antibiotics to be used in the feed industry in order to help prevent/manage the spread of antibiotic resistance.
But what is the incidence of antibiotic resistance within our food chain? How concerned should we be with MDR bacteria?
Incidence of Antibiotic Resistance within Meat Industry
There is consistent monitoring by officials as to the incidence of antibiotic-resistant bacteria, and the types of bacteria within our food chain. Depending on both the type of bacteria and the farming practices, and even the soil or environmental contamination – levels may vary greatly.
But I did want to highlight some of the studies that I found concerning antibiotic-resistant bacteria, in particular two studies which evaluated different “general” farming practices and how it influences antibiotic resistance within our food chain.
Antibiotic Resistance in Poultry – Organic, Conventional, and Raised without Antibiotics
The first study I wanted to mention was one done in 2018 looking at the incidence of antibiotic resistance within the poultry industry. Researchers sampled products from local grocers in Flagstaff Arizona (USA) and in particular looked at e.coli bacteria. (Davis 2018)
In total, E. coli was recovered from 91% of 546 turkey products tested and 88% of 1367 chicken products tested.
Of the turkey (91%) that tested positive for e.coli half of them were positive for multi-drug resistant bacteria. Thus about 45% of conventional raw turkey in the grocery store would contain antibiotic-resistant bacteria. There was however a significant difference between the presence of MDR e-coli bacteria in raw conventionally raised turkey and those that were raised organically. About 25% of raw turkey labeled as organic or “raised without antibiotics” contained MDR bacteria.
This contrasts to chicken which had 88% of the samples that were positive, and only 30% of those contained antibiotic-resistant bacteria. There was no significant difference in MDR bacteria incidence between organic or conventional products. Meaning that about 24% of all raw chicken in the grocery store, regardless of conventional or organic, is contaminated with MDR bacteria.
“The high prevalence of resistance among E. coli isolates from conventionally-raised turkey meat suggests that there is greater antimicrobial use in conventional turkey production as compared to RWA and organic systems. In contrast, there were few differences in antibiotic resistance prevalence among E. coli isolates across categories of chicken production.”Davis et al 2018
Antibiotic Resistance in Beef – Grass Fed vs. Conventional
According to a research study from 2010 that compared the incidence of multi-drug resistant bacteria in grass-fed organic beef vs. conventionally raised beef in Illinois. On average this study found that about half the meat tested from both groups with contaminated with bacteria, and about 90% of those were contaminated with antibiotic-resistant bacteria. (Zhang 2010)
Meaning that when purchasing beef you have a little less than 50% chance of choosing meat that is contaminated by antibiotic-resistant bacteria. There was no significant difference when looking at grass-fed vs. conventionally raised beef. Researchers did note that in the USA facilities that process grass-fed and conventional meats are actually the same – thus cross-contamination is likely between groups despite routine cleaning procedures.
My Take Away from this Research…
When considering this new 2021 research paper looking at the incidence of multidrug-resistant bacteria within dog food – the fact that the raw dog food contained the MDR bacteria is not necessarily surprising…
The individual incidence of MDR bacteria within raw meat products within grocery stores is between 25-50% depending on the type of meat, bacteria in question, and sourcing/type of meat (conventional vs. organic).
But what was surprising was the incidence of MDR bacteria (100%) of raw food samples. As the diversity of brands/samples was low, it is possible that this is not completely representative of the raw dog food industry. However, this also may speak to a lack of quality control within this (and other) manufacturing facilities where raw ingredients may cross-contaminate each other between batches due to insufficient sanitation procedures.
Another surprise of this study was that cooked products also contained this bacteria!
This is concerning as traditionally “heat treatment” is considered one way to kill harmful bacteria. Suggesting that either cross-contamination occurred during the manufacturing process with raw materials, there was a failure in the heat treatment of the diets or these antibiotic-resistant bacteria have become less heat-sensitive.
Personally, I think this issue needs to be addressed and taken on by each company to test ingredients coming into the facility, and the final product to make sure they do not contain these harmful bacteria - this would protect pets from direct exposure to these MDR bacteria and their humans from indirect exposure to them as well. On a larger scale we need to look at environmental factors that can influence the prevalence of these pathogens - water sources, soil, food, and antibiotics in farm animals - and look to reduce antibiotic resistant bacteria that can affect our food chain. This may be adjusting farming practices, antibiotic use in farm animals, water filtration systems, and more.
I hope you found this study overview helpful – I know I enjoyed really piecing apart this study. But I’d love to hear from you – what would you recommend we do to prevent these bacteria from affecting our food chain? How can pet food companies adjust their quality control to prevent spread?
Antibiotic Resistance and Food are Connected. (2021, June 14). Centers for Disease Control and Prevention. https://www.cdc.gov/drugresistance/food.html
Davis, G. S., Waits, K., Nordstrom, L., Grande, H., Weaver, B., Papp, K., Horwinski, J., Koch, B., Hungate, B. A., Liu, C. M., & Price, L. B. (2018). Antibiotic-resistant Escherichia coli from retail poultry meat with different antibiotic use claims. BMC Microbiology, 18(1). https://doi.org/10.1186/s12866-018-1322-5
Finisterra, L., Duarte, B., Peixe, L., Novais, C., & Freitas, A. R. (2021). Industrial dog food is a vehicle of multidrug-resistant enterococci carrying virulence genes often linked to human infections. International Journal of Food Microbiology, 109284. https://doi.org/10.1016/j.ijfoodmicro.2021.109284Zhang, J., Wall, S. K., Xu, L., & Ebner, P. D. (2010). Contamination Rates and Antimicrobial Resistance in Bacteria Isolated from “Grass-Fed” Labeled Beef Products. Foodborne Pathogens and Disease, 7(11), 1331–1336. https://doi.org/10.1089/fpd.2010.0562
About the Author: Nikki is a Registered Veterinary Technician (Veterinary Nurse) and Dog Mom with over a decade of experience with dogs and cats. Since graduation from college (BS Biology, Dip. Animal Nutrition, AS Animal Science) she has adopted two mixed breed dogs – Ranger and Ash, and has focused her time learning about pet food and nutrition.
Nikki shares information on a range of dog nutrition topics: from how to create a homemade complete and balanced dog food recipes, to how to choose a dog food. Nikki strives to give dog parents the information they need in order to make the best nutrition decisions for their pup!
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