Could storage mites in pet food lead to false-positive food allergy diagnoses?

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Canine atopic dermatitis is an inflammatory skin condition associated with the production of immunoglobulin E (IgE) specific for a substance that is either ingested or inhaled. Typical signs include pruritus, erythema, lichenification, seborrhea, and lesions from chronic inflammation, licking, chewing, and scratching. House dust mites (HDM) of the Dermatophagoides genus are the most common allergens recognized by the circulating IgE of atopic dogs. In laboratory experiments, dogs that are sensitized to HDM experience atopic flares when exposed to various species of storage mites. Because storage mites commonly grow on cereals used in commercial pet foods, researchers conducted a literature review to determine whether allergen cross-reactivity could lead to a false-positive diagnosis of food allergy. The results of their study were published in BMC Veterinary Research.¹

The investigators searched two databases for detailed studies using a series of key words. Of 87 identified articles, 10 were selected that were common to both database searches. Five articles reported results from laboratory studies, and five reported on field studies.

Laboratory studies

The survival and multiplication of Dermatophagoides HDM was first reported on dry dog food in 1972.² Nearly 40 years later, Tyrophagus putrescentiae storage mites were shown to survive and grow on three commercial dry dog foods.³ Tyrophagus mites grew on all three dog foods, with the highest numbers of mites found whenever molds had been allowed to grow on the kibble.

In 2015, samples of dog foods stored in nine different sealed plastic bags and in a lidded cup were evaluated to see whether T. putrescentiae could infest and proliferate them.⁴ After 3 months, Tyrophagus storage mites were discovered in 55% of the bags. Mites had not made holes in the packaging itself but had entered the bags via faulty seals. Lidded cups were not contaminated. The same year, investigators evaluated whether T. putrescentiae storage mites preferred to grow on protein-, fat- or carbohydrate-rich diets and found they grew best on the dog foods richer in proteins and fat.⁵ 

Finally, in 2016, several experiments were conducted to evaluate the growth of T. putrescentiae in different conditions.⁶ In the first experiment, mites were found to grow better on green and brown rather than white and red colored kibbles. (Whether the different-colored kibbles had different nutrients was not explained.) The second study found higher mite growth when kibbles were crushed rather than intact, and when the initial mite population density was highest. The third showed that four different strains of Tyrophagus mites grew better on crushed dog food than a commercial mite diet. The final experiment confirmed that a high initial mite inoculum leads to a higher final mite count.

Field studies

In the first field study, 30 purchased and 50 pet owner–obtained commercial dry dog food samples tested with an enzyme-linked immunosorbent assay for HDM were all negative.⁷ In another study, 23 bags of commercial dry dog foods were tested for mite contamination by microscopy.⁸ Even though the bags were opened and closed twice daily over a 6-week period during this study, storage mites were not discovered.

The influence of different storage conditions on the contamination of dry dog foods with storage mites was first reported in 2008.⁹ Dry dog food bags were kept for 6 weeks in either laboratory storage with low average temperature and humidity or a ventilated garage with high temperature and humidity. More mites were detected in the bags stored in high temperature and humidity conditions.

In 2011, identical bags of a single commercial dry dog food were stored in 10 different households.¹⁰ Each bad was divided equally between the original sack with its reusable seal, a paper bag whose top was rolled for closure, and a plastic box with a sealed lid. These containers were stored next to each other, and the food was sampled every month for storage mite detection. After 3 months, mite numbers were significantly higher in the food samples stored in paper bags (60% of bags had evidence of mites) compared with baseline.

In the last field study, dog owners in eastern Australia provided 20 samples of commercial dry dog foods stored in open bags or storage boxes in home environments.¹¹ The food samples were examined for the presence of storage mites, and a small portion was kept for 2 additional months before their incubation under higher humidity and temperature conditions. Finally, nine new bags of commercial dog foods were purchased and tested as above. Mites were undetectable in any specimens after any of the incubation times. Similarly, storage mites were not observed when opening newly purchased bags or after storing the foods for 6 weeks at room temperature. In contrast, when incubating samples of these foods at high temperature (26 °C [78.8 °F]) and humidity levels (80%), storage mites were present as early as 3 weeks after beginning the experiment and in 78% of foods after 6 weeks of incubation.

Conclusions and take-home

Several clear conclusions can be drawn from this literature review. The research shows that storage mites, especially for the most commonly found and studied species T. putrescentiae, can grow in commercial dog kibbles, particularly protein- and fat-rich foods. They proliferate best when the initial contaminating mite density is high, when kibbles are crushed, or when mold overgrowth has occurred due to suboptimal storage. Typically, storage mite contamination is low to undetectable in newly purchased commercial dog food. If bags are stored indoors in temperate conditions, little to no overgrowth occurs. In contrast, when bags are stored in situations with high temperature and humidity, mites can enter through faulty seals and proliferate in the packages.

The allergenic cross-reactivity between the Tyrophagus storage mites and the Dermatophagoides HDM is known to be very high. Tyrophagus mites have been shown to trigger flares of allergy in laboratory beagles experimentally sensitized with the Dermatophagoides HDM. Although this cross-reactivity has only been demonstrated in a laboratory setting, atopic dogs with high-levels of Dermatophagoides farinae HDM–specific IgE are likely to have signs flaring after eating a food contaminated with storage mites. Thus, when performing food trials for the diagnosis of food allergies in dogs (and probably in cats), it makes sense to minimize factors that encourage storage mite proliferation. Using food from newly purchased, sealed bags that have been stored indoors in temperate conditions appears to decrease the risk of mite contamination. A diet trial with home cooked ingredients unlikely to have mite contamination is another option.

References

1. Olivry T, Mueller RS. Critically appraised topic on adverse food reactions of companion animals (8): storage mites in commercial pet foods. BMC Vet Res. 2019;15(1):385. doi: 10.1186/s12917-019-2102-7

2. Sinha RN, Paul TC. Survival and multiplication of two stored-product mites on cereals and processed foods. J Econ Entomol. 1972;65:1301-1303. doi: 10.1093/jee/65.5.1301.

3. Canfield MS, Wrenn WJ. Tyrophagus putrescentiae mites grown in dog food cultures and the effect mould growth has on mite survival and reproduction. Vet Dermatol. 2010;21:58-63. doi: 10.1111/j.1365-3164.2009.00778.x.

4. Hubert J, Nesvorna M, Volek V. Stored product mites (Acari: Astigmata) infesting food in various types of packaging. Exp Appl Acarol. 2015;65:237-242. doi: 10.1007/s10493-014-9864-1.

5. Erban T, Rybanska D, Hubert J. Population growth of the generalist mite Tyrophagus putrescentiae (Acari: Acaridida) following adaptation to high- or low-fat and high- or low-protein diets and the effect of dietary switch. Environ Entomol. 2015;44:1599-1604. doi: 10.1093/ee/nvv129.

6. Rybanska D, Hubert J, Markovic M, Erban T. Dry dog food integrity and mite strain influence the density-dependent growth of the stored-product mite Tyrophagus putrescentiae (Acari: Acaridida). J Econ Entomol. 2016;109(1):454-460. doi: 10.1093/jee/tov298

7. DeBoer DJ, Schreiner TA. Commercial dry dog food in the north central United States is not contaminated by Dermatophagoides house dust mites. Vet Dermatol. 2001;12(4):183-187. doi: 10.1046/j.0959-4493.2001.00248.x

8. Henneveld K, Beck W, Müller R. Evaluation of storage mites in commercial dry dog food and in the environment as well as their importance in veterinary medicine [in German]. Tierarztl Prax. 2007;35:325-332. doi: 10.1055/s-0038-1622643

9. Brazis P, Serra M, Sellés A, et al. Evaluation of storage mite contamination of commercial dry dog food. Vet Dermatol. 2008;19(4):209-214. doi: 10.1111/j.1365-3164.2008.00676.x

10. Gill C, McEwan N, McGarry J, et al. House dust and storage mite contamination of dry dog food stored in open bags and sealed boxes in 10 domestic households. Vet Dermatol. 2011;22(2):162-172. doi: 10.1111/j.1365-3164.2010.00931.x

11. Hibberson CE, Vogelnest LJ. Storage mite contamination of commercial dry dog food in south-eastern Australia. Aust Vet J. 2014;92(6):219-224. doi: 10.1111/avj.12185