The role of cervids in the transmission of Blastocystis spp. to livestock and humans: a comprehensive review.

Blastocystis spp. is a common intestinal parasite with debated pathogenicity in animals but frequently linked to human disease in clinical studies, particularly ST1. Cervids, uniquely coexisting in wild and captive populations, may bridge transmission to both livestock and humans. This review synthesizes 44 studies (n = 6,137 cervids) to assess their potential transmission role by biological basis, host characteristics, and ecological factors. For biological basis, cervids carry livestock-associated subtypes (ST10/14 for cattle, ST5 for pigs) and human-pathogenic ST1 (124 cases, 91% in captive Chinese sika deer). For host characteristics, overall infection rate was 24.65% with marked variation; highly susceptible species including Odocoileus virginianus, Rusa unicolor, Hydropotes inermis, and Elaphurus davidianus represent priority sources with higher pathogen shedding, yet absence of reported clinical cases suggests cervids may serve as undetected carriers. For ecological factors, wild cervids harbor 19 subtypes including regionally restricted ST31 (North America) and ST43/44/49 (Europe) as evolutionary reservoirs, captive cervids show marked ST1 enrichment while maintaining livestock-associated subtypes, positioning them as zoonotic amplifiers and livestock sources. China represents a priority region for surveillance due to converging ST1 rich captive deer, intensive livestock operations, and dense human populations. Cervids have the biological basis and ecological conditions to transmit Blastocystis spp. to livestock and humans, but direct molecular evidence is lacking. Priority research includes whole-genome sequencing tracing at cervid-livestock-human interfaces.