This thesis reveals the metabolic responses to intestinal nematode infections in highperforming chickens and delineates the development of an effective non-invasive diagnostic tool for the detailed characterisation of infections with significant implications for broader impacts on poultry Management High-performing chickens of the Lohmann Brown genotype were experimentally infected with both Ascaridia galli and Heterakis gallinarum, which also led to concurrent histomonosis nfections. The birds were necropsied at specific time points after infections, and the liver and plasma were investigated for the metabolic responses to the infections. Notably, this is the first study providing a detailed, time-dependent analysis of the infection-induced metabolic alteration in nematode infected chickens. The identification of key metabolites associated with different infection stages including larval penetration of the mucosa, patency, and reinfection provides a nuanced understanding of the dynamic host-parasite interplay, paving the way for targeted investigations into specific pathways and their functional consequences. This finding prompts a deeper investigation into the trade-offs between immune defence and Overall performance, with implications for breeding programs and management strategies aimed at optimizing both health and productivity in poultry. The elevated concentrations of amino acids during heightened immune responses at early phase of nematode infection indicated the possible diversion of metabolic resources towards immune response at the expense of performance and productivity. The increased Plasma amino acids were attributed to a potential imbalance in protein secretion and re-absorption in the small intestine. The study further identified altered metabolic pathways, including Arginine proline metabolism which are crucial for collagen synthesis and tissue repair, thus highlighting the importance of amino acids in supporting the defence mechanism during nematode infection. Additionally, the identification of key metabolites like trimethylamine Noxide (TMAO) whose high concentration may indicate the presence of mixed nematode infections, opens up a new perspective for biomarker research. Further exploration into the applicability of TMAO indiverse parasitic situations are encouraged particularly the mechanism of TMAO production and its role in discriminating the status of helminth infections in chicken. The status of nematodes infection is currently achieved through faecal egg counts which may not always be reliable due to variations in worm fecundity amongst other reasons. An alternative method is to measure anti-ascarid antibodies in chicken plasma. However, this practice may not always meet current welfare standards in poultry production, as it involves an invasive technique for blood collection that also requires licensed experts to perform. Therefore, this thesis further developed a new non-invasive nematode diagnosis based on the measure of worm antigens in chicken faeces. The development of this coproantigen ELISA as a non-invasive diagnostic tool presents an opportunity for improved parasite management. This novel approach accurately differentiated infected and non-infected chickens for both nematode species, offering a promising noninvasive alternative to faecal egg counts. While showing high diagnostic accuracy for A. galli, further refinement is needed for accuracy and sensitivity to H. gallinarum. Temporal variations among different diagnostic methods highlighted the need for tool-specific selection. The coproantigen ELISA demonstrated superior performance in overall qualitative diagnostics, surpassing other methods. Concurrently, the antibody ELISA showcased an early detection capability, while faecal egg counting emerged as the method most closely correlated with worm burdens, serving as a reliable measure of infection intensity. In the broader context of existing information, this study advances our understanding of the potential metabolic cost of defence against nematode infections in high-performing chickens. The identified metabolites and pathways not only contribute to the fundamental knowledge of host-parasite interactions but also lay the groundwork for targeted interventions to mitigate the impact of these infections on poultry productivity. The development of innovative noninvasive diagnostic tool adds an applied dimension to the research, providing tangible benefits for poultry producers grappling with the challenges of nematode infections.