Evaluation of in vitro treatments against the causative agent of Diadema antillarum scuticociliatosis (DaSc).

In the 1980s, a mass die-off of the long-spined sea urchin Diadema antillarum occurred on Florida and Caribbean coral reefs. D. antillarum populations largely did not recover, and in 2022, remaining populations experienced another mass mortality event. A ciliate most similar to Philaster apodigitiformis was identified as the causative agent of the 2022 event, which was named D. antillarum scuticociliatosis (DaSc). Here, we investigated possible treatments for this pathogen. We tested the efficacy of 10 compounds at final concentrations of 100, 50, 25, 12.5, 6.25, and 3.13 µM, or a 10-fold serial dilution series, against ciliates cultured from an infected D. antillarum specimen. Of the tested compounds, 8 induced 100% ciliate mortality at some dose after 24 h. The most effective (defined as those requiring the lowest dose to induce 100% ciliate mortality) were quinacrine and tomatine (both effective at 12.5 µM), followed by furaltadone and plumbagin (25 µM), bithionol sulfoxide and 2'4' dihydroxychalcone (50 µM), and oxyclozanide and carnidazole (100 µM). Toltrazuril and a commercially available anticiliate product containing naphthoquinones were not effective at any dose tested. Shortened (15 min) time trials were performed using ciliate cultures reared in natural seawater to better reflect natural environmental conditions, and revealed that 2 of the compounds (quinacrine and tomatine) induced 100% ciliate mortality at 100 µM, with tomatine also effective at 50 µM. This study identified several treatments effective against the causative agent of DaSc in vitro, but their toxicity and utility in vivo remain unknown.