Chloroxylenol and benzethonium chloride exposure alters spinal neuron development and behavior in developing zebrafish

Chloroxylenol (CHX) and benzethonium chloride (BEC) are replacement compounds being used after the ban (U.S. FDA) of triclosan and triclocarban in 2016 from personal care products. These two compounds are also recommended by the World Health Organization (WHO) as disinfectants against COVID-19. Toxicity data for both CHX and BEC are available, however, neurotoxicity studies for both compounds are still limited. Here, we determined the consequences of CHX and BEC exposure in a static exposure paradigm during embryogenesis on neurodevelopment using the zebrafish model (Danio rerio). CHX exposure (1-5 mg/L) dramatically impacted nervous system development without increasing mortality. The exposure altered embryonic motor output, primary motoneuron cell size, spinal interneuron cell size, primary motoneuron (PMN) axon pathfinding and secondary motoneuron (SMN) axon pathfinding. CHX exposure also altered slow muscle fiber development. Changes in neural activity as revealed by the induced changes in embryonic motor output (spontaneous coiling) may underlie the errors in PMN axon pathfinding as well as the changes in spinal interneuron morphology. Errors in SMN axon pathfinding resulting from CHX exposure were directly linked to errors in PMN axon pathfinding. Similar to CHX, BEC exposure (1-5 mg/L) altered embryonic motor output, spinal interneuron development, and slow muscle fiber development. In contrast to CHX, BEC exposure did not alter PMN or SMN axon pathfinding. Moreover, we found that BEC exposure (5 mg/L) from 5 to 96 hpf was lethal, resulting in almost 100 % mortality. Thus, these two triclosan replacement compounds exhibited markedly different modes of toxicity.