Epidemiological Factors Associated with Ligula sp. (Cestoda: Diphyllobothriidae) Plerocercoid Infection in Freshwater Fish from Romania

Ligula spp. are widespread cestodes, but their epidemiology remains insufficiently characterized across Romanian freshwater ecosystems. This study investigated factors associated with Ligula sp. plerocercoid infection in fish. A total of 8547 fish representing 36 species were collected from lentic and lotic ecosystems between February 2025 and September 2026 and examined by necropsy. Ligula sp. infection was detected in 311 fish from five cyprinid and leuciscid species, with an overall prevalence of 3.64%, mean intensity of 1.81 plerocercoids per infected fish, and mean abundance of 0.066 per examined fish. Prevalence was significantly associated with fish length class, aquatic ecosystem type, season, and host species (all p < 0.001), and was highest in the 5–9.9 cm length class (11.3%), lentic ecosystems (7.4%), and summer (11.2%). Greater body length was associated with lower odds of infection (OR = 0.773 per 1-cm increase, 95% CI: 0.748–0.798), while lotic ecosystems were associated with lower odds than lentic ecosystems (OR = 0.0057, 95% CI: 0.0014–0.0230). Higher Fulton’s condition factor was also associated with lower odds of infection (OR = 0.228, 95% CI: 0.152–0.342). These findings provide epidemiological data on Ligula sp. infection in freshwater fish and highlight variation according to host size, season, and ecosystem type, which may inform targeted surveillance and aquaculture biosecurity.