The dual role of intestinal parasites in shaping human gut microbiota: distinguishing helminthic and protozoan dynamics

The human gut microbiota is essential for supporting the host's health and immune system. Imbalances in this microbiota, known as dysbiosis, are associated with numerous inflammatory and autoimmune diseases. Despite extensive research on bacterial components, the significant modulatory role of eukaryotic gut inhabitants, particularly intestinal parasites, remains largely overlooked. While traditionally viewed as pathogens, emerging evidence highlights a profound duality in their role, suggesting they are integral and active modulators of the gut microbiota and challenging the conventional view of parasitism. For instance, helminths often induce microbial diversity and promote the expansion of anti-inflammatory microbes. Conversely, pathogenic protozoa are generally associated with reduced microbial diversity, fostering the growth of pathobionts and leading to significant gut dysbiosis. However, these interactions are highly context-dependent, and a comprehensive understanding is hindered by varied findings and limited data, particularly concerning protozoan infections. Furthermore, while mechanistic evidence is robust, human intervention studies remain limited. This narrative review synthesises the complex interrelationship between intestinal parasites and the human gut microbiota, distinguishing the distinct dynamics of helminthic and protozoan infections. Specifically, this review examines the inherent duality of intestinal parasites by investigating parasite-specific factors, including species, parasite burden, and coinfections, which can impact gut microbial composition and function. Ultimately, this review provides a comprehensive framework for understanding the profound influence of the gut ecosystem, shifting the paradigm beyond solely pathogenic views.