Outbreaks of white spot disease (WSD) caused by the parasite Ichthyophthirius multifiliis results in high mortality and morbidity leading to significant economic losses for both the aquaculture and ornamental industries. Severe cases of WSD have been reported in various freshwater fish species including rainbow trout, carp and channel catfish. In contrast, zebrafish ( Danio rerio) exhibit a high level of natural resistance to the parasite. The mechanisms underlying this increased protection in zebrafish are currently unknown. However, identifying the genes responsible for this resistance could be highly beneficial for mitigating the disease and for understanding protective immunity .
To investigate this, we conducted a comparative transcriptomic analysis between naturally resistant zebrafish and susceptible rainbow trout (Oncorhynchus mykiss). Both fish species were exposed to an equal number of parasites per unit surface area, and gills were sampled at 2, 24, 48 and 72 hours post infection (hpi). The differential gene expression analysis revealed an immediate response in zebrafish that peaked at around 24 hpi and normalized at 72 hpi (Table 1). Additionally, there was a continuous decline in transcripts mapping to the I. multifiliis genome, indicating that the infection was being cleared (Figure 1). In contrast, the rainbow trout elicited minimal response (Table 1), with a continuous increase in transcription (Figure 1) suggesting ongoing growth and establishment of the parasite.
We observed an increase in the transcription of genes in zebrafish associated with the Toll-like receptor signaling pathway, MAPK signaling pathway and a cellular response. The implications of these findings are discussed and candidate genes responsible for resistance have been identified. To assess the functionality of these candidate genes, a knockout study using the CRISPR/Cas technology was performed. The knockout fish were then exposed to I. multifiliis to evaluate susceptibility.