Parasitic Infections as Chronic Drivers of Reduced Biological Efficiency in Sheep and Goats: Mechanisms, Consequences, and Mitigation Strategies

Parasitic infections are major health and production challenges in sheep and goats. Their effects extend beyond acute clinical disease, as chronic and subclinical infections can cause persistent physiological disturbances that impair host function and reduce production performance. Although individual parasites and their associated diseases have been widely studied, how parasites with different life cycles, tissue tropisms, and pathogenic strategies ultimately converge on shared host responses remains less well integrated. This review examines major gastrointestinal, hepatobiliary and visceral, blood- and tissue-associated, and ectoparasitic infections of sheep and goats and the mechanisms linking parasitic infection to impaired host function. Distinct pathogenic processes include intestinal dysfunction, hematological impairment, organ and tissue dysfunction, nutritional inefficiency, and immune activation with metabolic trade-offs. Despite their different origins, these processes may be accompanied by overlapping downstream host responses, particularly oxidative stress, gut microbiota dysbiosis, and metabolic alterations, although the strength of evidence varies among parasite–host systems. Together, these changes may constrain biological efficiency—the capacity to acquire, utilize, and allocate resources toward physiological maintenance and productive functions—and thereby impair production performance. The consequences extend from individual animals to populations and production systems, including impaired growth and reproductive performance, reduced product quality, increased disease management costs, and less readily detected production losses that may accompany subclinical infection. Mitigation should remain grounded in established parasite-targeted control, complemented where appropriate by nutritional support and enhancement of host resistance or resilience. Microbiota-targeted interventions and technology-enabled precision management represent promising additional approaches, but their parasite-specific efficacy and practical applicability require further validation. By linking parasite-specific pathogenic processes with convergent host responses and production outcomes, this review provides an integrated framework for understanding parasite-associated losses and highlights the importance of combining parasite control with strategies that maintain or restore host function.