Ciliated parasites as the primary agents for early epizootic ulcerative syndrome (EUS) in yellow catfish, Tachysurus fulvifraco

Abstract

Epizootic ulcerative syndrome (EUS) is an emerging disease that accounts for the high morbility and mortality in yellow catfish, Tachysurus fulvifraco. Though the EUS of T. fulvifraco was most frequently supposed to be associated with bacterial infection, the causative agent for the disease is still not completely determined. To address this, an epidemiological investigation on EUS was carried out from 2019 to 2022 in Hubei province, one of the main culture areas of T. fulvifraco in China. It showed the outbreaks of EUS were mainly occurred in March and April, during which the water temperature ranged from 13 °C to 20 °C. At the early stage of EUS, the diseased T. fulvifraco typically aggregrated, with circular-hole shaped ulcers on skin. Gross examinations on the T. fulvifraco of early EUS revealed that the disease was commonly accompanied by ciliates infections. To further illustrate, recent diagnoses on T. fulvifraco of early EUS from Wuhan city were recorded in details. Critically, 78.1% of the specimens were heavily infected with Ichthyophthirius multifiliis and/or Epistylis horizontalis. Histopathological analyses revealed that these ciliated parasites infection caused severe damages on T. fulvifraco skin and gills. In comparison, only 12.5% of the T. fulvifraco specimens were infected with the bacterium, Aeromonas versonii. Taken together, our observations indicated that the ciliated parasites were the primary agents for early EUS, that contribute to the formation of ulcers on T. fulvifraco. These data facilitate us to understand the epidemic of ESU in farmed T. fulvifraco, and further explore the effective treatments to the disease.