Through pelleting, it has been possible to mold a mixture of ingredients, which once compacted in a cylindrical or spherical form are called pellets (Loor, 2016). The use of pellets has become a key element in the commercial manufacture of equine feed since their granular form simplifies their handling (Muhammad, 2024). In addition, they can be produced using agricultural by-products, which represents an option for their use (López, 2017). Among the advantages of using pellets in equines are that they help improve the digestibility of nutrients, reduce energy during feed consumption, prevent nutrient selection, reduce waste in feeders and improve economic reward and productive parameters (Loor, 2016). Soto and Rojas (2016) evaluated the potential replacement of balanced feed with Stylosanthes multilinea pellets in equines, using incremental inclusion levels of 0, 15, 30, and 45%. The results indicated that replacing up to 45% of the balanced feed with pellets did not produce significant effects on apparent digestibility, dry matter content, or crude protein concentration of the total ration. In a study conducted by Christ et al. (2020), the use of wood pellets as bedding material in individual horse stalls was evaluated and compared to wheat straw. The results indicated that, from an economic perspective, the use of wood pellets reduces costs. Additionally, it was observed that horses housed on this type of bedding spent less time searching for food. Pellets used in equine nutrition are highly palatable, which is one of their key advantages. Ryon et al. (2023) evaluated the palatability of pellets formulated with Cannabis sativa, soybean meal, beet pulp, and rice bran. Their findings indicated that all formulations were well accepted by horses, with those containing Cannabis sativa exhibiting the highest palatability compared to the other formulations. Symoens et al. (2024) recently compared the effectiveness of steamed hay and alfalfa pellets in enhancing pulmonary function and reducing inflammation in horses with severe asthma. Their findings showed that alfalfa pellets reduced the weighted clinical scores from 13 to 2, whereas steamed hay just lowered the scores from 10 to 6.

Soybean flour exhibits absorbent properties due to its chemical composition, which includes a variety of functional groups and components that facilitate the adsorption of various substances. Among its main absorbent properties, it stands out for its ability to adsorb liquids, particularly water, due to the presence of polysaccharides such as cellulose and hemicellulose, which form bonds with water molecules, thus enabling moisture absorption. Within this liquid adsorption property, its ability to absorb oils and fats is also notable, owing to its high concentration of proteins and fiber (Huang et al., 2024).

Additionally, soybean flour exhibits absorption properties mediated by functional groups such as amino (-NH2) and carboxyl (-COOH), which can interact with various substances through electrostatic interactions. These characteristics give soybean flour the ability to: (i) absorb and retain essential nutrients, such as amino acids, proteins, and minerals, along the digestive tract, and (ii) absorb toxic compounds or contaminants, such as certain heavy metals, through an ion-exchange mechanism (Daneshvar et al., 2002). These properties make soybean flour a promising material for antibiotic adsorption, particularly due to its low cost and widespread use in animal feed.

The equine is a monogastric herbivore with a relatively small stomach, however, it has a well-developed and functional cecum and colon. Its digestive process is fundamentally enzymatic from the mouth to the terminal part of the ileum and to a lesser degree in the cecum and colon where the fermentation of crude fiber and other nutrients takes place with 70% of the efficiency of ruminants (Arrieta et al., 2007). They have developed their cecum where a large number of microorganisms are housed, which break down the cellulose that cannot be digested in the upper digestive tract. Volatile fatty acids are produced in the cecum and absorbed as a source of energy (Martínez Marín, 2008).

Soybean meal is one of the primary protein sources due to its high lysine content. This essential amino acid plays a crucial role in foal growth, as horses cannot synthesize it on their own. Lysine is vital for tissue generation and repair, making it particularly important for growing foals, which require more protein than mature horses (Bockisch et al., 2023). When horses lack access to pasture and their diet primarily consists of cereals, lysine deficiency becomes more pronounced, reducing the overall quality of the ingested protein (Hoyos et al., 2022).

Clinically used quinolones have a two-ring structure, with nitrogen at the 1-position, a carbonyl group at the 4-position and a carboxyl group at the 3-position (Millanao et al., 2021). The potency and spectrum are significantly increased when a fluorine atom is attached at the 6-position, possibly because of improved tissue penetration and binding to bacterial topoisomerases (Alós, 2009), see Figure 2 . Antibiotics of the fluoroquinolone class show excellent activity against Gram-negative aerobes including Enterobacteria, Pseudomonas aeruginosa, Mycoplasma spp., Rikettsia spp. and Escherichia spp. (de Jesús, 2007). It should be noted that Gram-negative bacteria possess an outer membrane that is often a permeability barrier to an antibiotic molecule as it surrounds the peptidoglycan layer of the cell wall and as a consequence, Gram-negative bacteria generally have reduced susceptibility to antibiotics compared to Gram-positive bacteria (Kuriyama et al., 2014). Compounds that carry a double ring derived from the pyrrolidone ring in position 7 increase their activity against Gram-positive bacteria; if they carry a methoxy group in position 8, their activity against anaerobes improves, which is the case of moxifloxacin and gatifloxacin, fourth-generation fluoroquinolones (Alós, 2009).

Moxifloxacin, like other quinolones, inhibits the bacterial enzymes topoisomerase II (DNA gyrase) and topoisomerase IV, which are required for the replication, transcription, repair, and recombination of DNA and RNA in bacterial cells (Carrillo et al., 2018). When DNA gyrase is exposed to a quinolone, the drug interacts with the surface of the alpha-helical domain of the enzyme, compromising DNA anchoring and repair. Toxic effects occur through the formation of an irreversible complex formed by quinolone, gyrase, and DNA. This complex prevents replication fork progression and transcription complexes, leading to chromosome fragmentation and cell death (Stroman et al., 2005; Marín, 2008) (see Figure 3 ).

Moxifloxacin has a slightly different chemical structure from older veterinary fluoroquinolones by the 8-methoxy substitution. As a result of this modification, this new generation of drugs has broad-spectrum antimicrobial activity against Gram-positive bacteria and anaerobes (Papich, 2020). The fourth-generation quinolones (moxifloxacin, trovafloxacin), retain activity against microorganisms especially A. aureus and Enterococcus, S. pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Legionella pneumophila, Moraxella catarrhalis and to a lesser extent Chlamydia pneumoniae and Mycoplasma pneumoniae (Balfour and Wiseman, 1999; Constantinou et al., 2007) being at least 2 to 8 times more potent than the first-generation quinolones (ofloxacin and ciprofloxacin) in their in vitro activity (Rolston et al., 2006). As stated by de Jesús (2007) and Valdéz-Cruz et al. (2013), Table 1 lists the majority of bacteria responsible for common diseases in equines, highlighting the in vitro activity of Moxifloxacin against them.

Survival curves show that when the concentration of fluoroquinolones is close to the minimum inhibitory concentration (MIC) of the bacteria, the drug stops bacterial growth, however, when the concentration is increased in relation to the bacterial MICs, cell death increases up to a certain concentration of the drug (optimal bactericidal concentration), but an increase above this optimum can lead to a decrease in the bactericidal effect (Marín, 2008).

In respect to Table 1 , it is shown that moxifloxacin presented improved activity against penicillin-sensitive and -resistant pneumococci, a comparatively high activity against Gram-positive cocci and bacilli; likewise, better activity compared to ciprofloxacin and levofloxacin, reaching inhibition of more than 70% at concentrations of 1.0 μg/mL; it had similar activity against gram-negative organisms compared to the second and third generation of fluoroquinolones; it had a rapid elimination rate of fluoroquinolone-resistant organisms. On the other hand, although MICs give information about a specific antibiotic concentration that inhibits the growth of a bacterial strain, it does not distinguish between bactericidal or bacteriostatic effects, nor does in vitro activity imply in vivo efficacy, so the mutant prevention concentration (MPC) will be 8 to 10 times higher than the MIC, where the range of concentrations between MIC and MPC is defined as the mutant selection window. The MPC is an important concept to prevent the growth of organisms that have some level of drug resistance prior to therapy (Pong et al., 1999; Schedletzky et al., 1999; Blondeau et al., 2000; Hoogkamp and Roelofs, 2000; Blondeau and Hansen, 2001; Rolston et al., 2003; Stroman et al., 2005; Kim et al., 2006; Odenholt and Cars, 2006; Rolston et al., 2006; Constantinou et al., 2007; Singh et al., 2009; Gao et al., 2019).

There are few studies addressing the adverse effects of moxifloxacin use in horses. In a study by Gardner et al. (2004), the efficacy of this antibiotic was evaluated against bacterial infections in horses, with an emphasis on its application in the treatment of pneumonia. The results showed that moxifloxacin has potent activity against key respiratory pathogens in horses. Despite its favorable characteristics, such as high systemic availability and a prolonged half-life, its use is not recommended for the treatment of equine bacterial pneumonia due to associated gastrointestinal side effects. In this study, four mares developed mild intermittent diarrhea, which began approximately 8 hours after the administration of the first dose of moxifloxacin. Additionally, one of the mares experienced a brief episode of colic 12 hours after the third dose. Toxins A and/or B from Clostridium difficile were isolated from the feces of the mare that exhibited the most severe diarrhea. These gastrointestinal side effects highlight the concerns associated with the use of moxifloxacin in the treatment of bacterial infections in horse.

In a study conducted by Sumano López et al. (2020) on adverse drug reactions in horses, it is noted that fluoroquinolones can induce joint damage in developing or immature animals. This finding suggests that these drugs should not be administered to young animals still in critical stages of bone development, such as growing horses and pregnant mares. These results highlight the importance of considering the age and developmental stage of the animal when evaluating the use of these medications to prevent significant adverse effects.

Some factors that can influence digestion in equines are animal individuality, chemical composition of the feed, feeding capacity, type of work, physical form of the feed, physiological stage, water content of the feed, speed of transit of the feed in the digestive tract (48–72 hours) and amount of fiber in the ration (Van Weyenberg et al., 2006). Equines present digestive physiology with determining characteristics such as efficient mastication, rapid gastric passage rate, intense enzymatic digestion in the small intestine, and prolonged microbial action in the large intestine (Arrieta et al., 2007). The digestive system of horses has adapted to be able to ingest large quantities of grass very evenly distributed throughout the day and to obtain energy efficiently from it. The enzymatic digestion of the feed releases glucose, amino acids and fatty acids for absorption. Additionally, the well-developed large intestine enables the acquisition of supplementary energy in the form of volatile fatty acids through the microbial fermentation of fiber and the fraction of the feed that is not enzymatically digested (Martínez Marín, 2008).

Fluoroquinolones are well absorbed after oral administration in animals with a single-chamber stomach and the newer ones tend to have a longer half-life. Most fluoroquinolones are metabolized in the liver; they are eliminated mainly through the kidneys, although some are excreted in bile (Marín, 2008; Carrillo et al., 2018) and the elimination pathways of moxifloxacin are balanced, not depending exclusively on renal or hepatic function. Moxifloxacin has a low plasma protein binding, approximately 40–42%, which is important since only the free drug is active against the bacteria and can penetrate the target tissues. The plasma protein to which it mainly binds is albumin (Carretero Colomer, 2001). Its bioavailability is excellent and allows oral administration in most situations; this bioavailability is approximately 100%, it does not decrease with concomitant food intake, but gastrointestinal undesirable effects are the most frequent. They occur in 2–20% of cases, depending on the molecules and situations, and constitute 50% of the side effects of fluoroquinolones (Revest and Tattevin, 2014) ( Figure 3 ).

It is suggested to perform in vitro and in vivo tests to test the efficacy of this product to know its antibiotic stability, controlled release, precise dosage, and animal acceptance.

Antibiotic stability: Ensuring that the antibiotic maintains its potency over the shelf life of the pellet is a crucial challenge (Barrueco et al., 2013).

Controlled release: Designing pellets that release the antibiotic in a controlled and effective manner in the body can be complex. It is important that the drug is released at the right time and place to maximize its effectiveness and minimize side effects (Thapa et al., 2018).

Precise dosage: Ensuring that each pellet contains a precise and consistent dose of the antibiotic is essential to ensure the effectiveness of the treatment and avoid problems of bacterial resistance (Sumano López et al., 2020).

Acceptance by the animal: Pellets must be palatable to the animals that will consume them. If the antibiotic or ingredients in the pellet alter the taste or texture, the animals may reject it, which could compromise the treatment (Ryon et al., 2023).

Regulation and security: Complying with safety standards and regulations for the use of antibiotics in animals is essential. This includes ensuring that the antibiotic levels in the pellet are safe and do not cause adverse effects (Loor, 2016; Sumano López et al., 2020).