Trichostrongylus colubriformis is a species of gastrointestinal nematode that typically infects the duodenum (beginning of the small intestine) of the gastrointestinal tract (GIT) of sheep (Amarante, 2015). In severe stages of infection, this nematode could damage the intestinal mucosal, resulting in reductions in nutrient absorption. Absorption is compromised due to physiological modification of the GIT. Changes caused by the infestation include intestinal mucosa villus atrophy, and crypt depth reduction (Cardia et al., 2011); formation of tunnels that compromise luminal function; mucosal epithelium erosion; and serum protein exudation into the intestinal lumen (Taylor et al., 2010).

The total mixed ration could be described as the combination of different nutrients to attend animal requirements, and therefore, these nutrients are added to animals’ diets to participate in chemical and physiological functions necessary to keep the animal healthy (Khol-Parisini et al., 2015). Among the minerals introduced, dietary phosphorus (P) is added to guarantee energy for vital functions (ATP), enzymatic P dependent activities avoid bones devices among many other benefits (Humer and Zebeli, 2015). However, despite the essentiality of P for animal life, the physiological parameters, such as availability and absorption, of this mineral could be impaired by the T. colubriformis infection. Once the duodenum is the main absorptive site of P, and the infection could damage the duodenal mucosa, reducing the absorptive surface, and consequently, reducing the nutrient absorption (Cardia et al., 2011).

Not only the absorption could be compromised, but also the GIT digesta flow. Passage rate, is the fractional amount of digesta leaving the compartment per unit of time (Grovum and Williams, 1973), and retention time is the average period of time that digesta spends in the compartment. Some authors studying T. colubriformis infection in lambs observed lower reticulo-ruminal passage rates (Dias e Silva et al., 2019). Slower passage from the reticulo-rumen generally results in greater retention times leading to greater nutrient degradation and digestion, more microbial metabolism, and consequently, greater production of short-chain fatty acids (ruminant alternative energy source) and methane (Huhtanen et al., 2006).

Therefore, infection with the parasite could modify GIT physiological and histological parameters to improve resilience between the endoparasite and the host, however, these modifications could waste or redirect energy that was targeted to growth and development. Also, infected animals can produce more methane, that contributes to the global warning, a worldwide problem, and increase losses of P that makes the sheep production more expensive and contaminate the groundwater. Therefore, this study aimed to evaluate gastrointestinal tract histological and physiological parameters in Santa Ines lambs infected with T. colubriformis and fed different levels of phosphorus.