Exogenous proteases are well established for hydrolysing proteins and improving the digestibility of amino acids, which, in animal production, enhances growth performance and diet formulation flexibility. Attention has now turned to the wider, or ‘extra-proteinaceous’, benefits of proteases. Both host and (gut) microbiome utilise amino acids for diverse functions beyond just cellular growth and proliferation. For example, the host produces a diverse array of proteinaceous secretions that seek to modulate the activity of the intestinal microbiome and opportunities to interact with host (e.g., epithelial) receptors. Reciprocally, the microbiome utilises various protein-based components to perform individual lifestyle traits, such as colonising gut niches, production of virulence traits, etc., which can be influenced by protease activity and amino acid availability. In addition, microbial fermentation of proteins and amino acids produces metabolites that may be considered undesirable (e.g., toxic) for the host’s cells or, for example, can act as signalling molecules, shaping the secretion of gut hormones and function. Parallels may also exist between various antibiotic growth promoters and proteases, in that both can target/impact microbial proteins (synthesis and degradation, respectively). There are many aspects to consider, such as determining optimal intestinal levels and spectra of proteolytic activity under various scenarios, but great potential exists.

Coccidiosis remains a major disease and economic challenge for the global poultry industry. Coccidiosis in chickens is caused by seven Eimeria species that target specific regions of the gastrointestinal tract and cause malabsorptive or haemorrhagic disease. These Eimeria species infect segment-specific epithelial cells and thus need to navigate the host’s indigenous microbiome and intestinal defences to establish an infection and cause disease. Good husbandry practices, prophylactic use of anticoccidial drugs and/or live parasite vaccination have been the primary control measures employed but there are challenges with vaccination and growing constraints on anticoccidial drug use. This review, therefore, considers available information on the key steps of the infection process, notable microbiome- or host-related changes occurring, and the (potential) influence of dietary ‘alternatives’ to anticoccidial drugs. There is good available evidence to indicate that some phytogenics, prebiotics, probiotics, betaine, n-3 fatty acids, as well as carbohydrase enzymes and anti-IL-10 antibodies, can (beneficially) modulate at least some of these features in coccidiosis-specific challenge studies. As a minimum, these anticoccidial drug ‘alternatives’ could support the establishment of a desirable host microbiome and optimum immune system development. It is important to better understand the potential of these ‘alternatives’ in commercial production and how they can complement, or reduce, the use of anticoccidial drugs.