It should be mentioned that not only chews can be a source of microbiological threats to dogs. Maintenance foods can also be dangerous, not only in terms of the presence of bacteria. More and more research is being conducted on the presence of mycotoxins in the foods of domestic animals. As reported by Pigłowski, mycotoxins were the most frequently reported hazard category in RASFF in 1981–2017. The largest number of notifications was related to aflatoxin B1 [52].

Mycotoxins are harmful metabolites of mold fungi that are found in various foods. Most mycotoxins belong to three types of fungi—Aspergillus, Penicillium, and Fusarium. The most disturbing substances are aflatoxins, vomotoxins, ochratoxins, zearaleon, fumonisin [53]. The large group of EU recommendations on safe levels in animal products only apply to three mycotoxins (Table 1).

Mycotoxins are produced in cereal grains, as well as in fodder before, during and after harvesting in various environmental conditions [55]. In the case of cereals, it has been shown that most of the impurities are close to the surface of the grains. By removing only part of the outer layers of the grains, microbial contamination can be significantly reduced [56]. Research by Oliveira et al. [57] showed that metabolites of lactic acid bacteria (LAB) are a reliable alternative for reducing fungal infections before and after harvest.

The presence of mycotoxins in feed may reduce feed consumption and adversely affect the health of the animals [58,59,60]. Their presence can also cause inhibition of the total weight gain [61]. In addition, the possible presence of toxic residues in edible animal products such as milk, meat and eggs can have a detrimental effect on human health [62,63]. Contamination with fungi and their metabolites in the form of mycotoxins affects both the organoleptic properties and the nutritional value of the feed and carries the risk of poisoning. Studies have shown that a high percentage of feed samples are contaminated with more than one mycotoxin [64]. The effects of consumption of foods contaminated with mycotoxins depend on the amount of toxins present and the duration of exposure, as well as the individual sensitivity of the animals. Mycotoxins have a variety of chemical structures responsible for their various biological effects. Depending on their exact nature, these toxins can be carcinogenic, teratogenic, mutagenic, immunosuppressive, trembling, hemorrhagic, hepatotoxic, nephrotoxic and neurotoxic [65,66]. For example, aflatoxin B1 has a strong hepatotoxic effect, however, it has been shown that it can be minimized by supplementing the feed with curcumin [67]. Controlling the growth of mold fungi and the production of mycotoxins is very important for the feed producer and for the animals [68]. The study by Singh and Chuturgoon [69] aimed to compare the microbiological quality of standard feeds with premium feeds. These studies showed that regardless of the brand, all foods samples were contaminated with fungi (mainly Aspergillus flavus, Aspergillus fumigatus and Aspergillus parasiticus) and mycotoxins (most often aflatoxins and fumonisins). The obtained results suggest that more expensive dog foods do not provide the highest quality, nor do they guarantee microbiological purity. On the other hand, research by Leiva et al. [70] has shown that in the case of dry feed, the extrusion process may be helpful in reducing the pathogenicity of microorganisms and does not affect the digestibility of the feed.

In another studies, an alarming presence of fumonisins in animal feed was found [71]. The simultaneous contamination of the feed with fumonisin fractions was also quite common. It should be kept in mind that toxins show a synergistic effect, especially since the coexistence of fumonisin with other Fusarium sp. toxins is a real possibility.

Studies by Witaszak et al. [72] confirmed the presence of five types of fungi-producing mycotoxins in the amounts permitted by EU regulations. However, the low level of mycotoxins in dog foods does not eliminate the risk and caution should be exercised because long-term daily consumption of even small amounts of mycotoxins can lead to slow damage to the animal’s body and the development of many diseases, including cancer. The presence of mycotoxins in foods was confirmed by a study by Shao et al. [73]. Only one of 32 samples was free from mycotoxin contamination. Moreover, all other samples were contaminated with at least three different types of mycotoxins.

Analyses by Tegzes et al. [74] aimed to compare cereal and cereal-free dog foods in terms of their mycotoxin content. The test results confirmed the presence of mycotoxins in dry cereal dog foods, while in cereal-free foods they were not found. This study suggests that the risk of exposure to mycotoxins is higher with dry dog foods containing cereals. To minimize the risk, dog food manufacturers should choose grain types that are less susceptible to the presence of mycotoxins.

Cereals are usually the main component of veterinary foods, intended for dogs with various diseases that require safe and wholesome nutrition. Cereal grains can often be contaminated with Fusarium fungi, which can produce mycotoxins. In studies by Witaszak et al. [75], samples of veterinary feeds were examined for the presence of mold species and mycotoxins. Only 9.5% of the samples were free from mycotoxins produced by Fusarium, however, none of the tested samples exceeded the permissible limits of mycotoxin content in feed, as defined by EU regulations. This means that it is necessary to systematically test both domestic animal and veterinary feeds in terms of the content of harmful microorganisms and their metabolites, because especially veterinary feeds should be characterized by the highest level of safety for animals.

Studies by Macías-Montes et al. [76] showed that the presence of mycotoxins is quite common in dry dog foods. However, the concentrations of most of them are among the lowest reported so far. It was proven that the mycotoxin content was not influenced by the feed quality. Chronic exposure to mycotoxins and their hidden forms may prove problematic.

The results of Okuma et al. [77] research revealed a low incidence of aflatoxin and ochratoxin in commercial pet foods. Although deoxynivalenol has been detected in many trials, its levels were well below those that can cause acute toxic effects.

A study by Gazzotti et al. [78] showed that all samples of extruded complete dog foods were compliant with current European legislation on mycotoxin contamination. However, these results revealed the need for further research into the potential risks of chronic low-dose exposure to various types of mycotoxins to which pet species are currently exposed.

 

3. Conclusion 

In summary, despite the many methods of detecting and preventing food contamination, there are still cases of bacterial contamination of both pet foods and treats, while in terms of the overall mycotoxin content, these products may appear safe. In order to minimalize the risk to the health of pets, the priority should be prevention, i.e. systematic testing of the raw materials and feeds, in terms of the content of harmful microorganisms and their metabolites. One form of simple risk reduction is, for example, leak testing of treat packages.