Systematic Literature Review on Keratin Digestion in Animals with Special Reference to Phthiraptera

Keratin is structurally robust protein stabilized by disulfide bonds. It is the primary component of feathers, hair, wool, and stratum corneum. While keratinolytic capabilities are well-characterized in environmental microorganisms and certain specialized insects, the digestive physiology of Phthiraptera (chewing lice) obligate ectoparasites that consume keratinous material is poorly understood. This systematic review synthesizes all available evidence regarding keratin digestion mechanisms across the animal kingdom, with focused analysis on Phthiraptera, to evaluate whether chewing lice possess endogenous or symbiont-mediated keratinolytic capacity. This review systematically searched PubMed, Web of Science, Scopus, CAB Abstracts, and grey literature sources from inception to December 2024. Studies reporting digestive mechanisms in keratinophagous animals were included. From 1,247 initial records, 34 studies met inclusion criteria: vertebrate keratinophagy (n=14), non-phthirapteran insects (n=12), and Phthiraptera (n=8). Validated keratinolytic mechanisms include extreme gastric acidity (snakes), disulfide bond reduction via thiols (Tineola larvae), alkaline serine proteases (dermestids), and bacterial symbiosis (coleopteran models). In Phthiraptera, only one biochemical study exists, reporting lipase activity but no protease activity. The remaining seven studies (1934?1989) describe morphologically intact keratinous fragments in gut contents, with no evidence of enzymatic degradation. The assertion that chewing lice digest keratin lacks empirical support. Available evidence indicates Phthiraptera ingest keratinous material but likely extract surface lipids rather than degrade the keratin polymer. We propose a revised nutritional model wherein lice subsist on sebaceous secretions and epidermal debris, with keratin passing intestinally intact. Urgent research priorities include metatranscriptomic analysis, symbiont functional characterization, and redox potential measurements.