The parasitic amoeba, Neoparamoeba perurans is the causative agent of Amoebic Gill Disease in salmonids. The parasite has previously been reported to lose virulence during prolonged in vitro maintenance. In this study, the impact of prolonged culture on N. perurans virulence and its proteome was investigated. Three isolates of N. perurans maintained in culture for varying durations were compared. Two isolates, attenuated and virulent, had their virulence assessed in an experimental trial using Atlantic salmon smolts and their bacterial community composition was evaluated by 16S rRNA Illumina MiSeq sequencing. Soluble proteins were isolated from a newly acquired, virulent and attenuated N. perurans culture and were analysed using two-dimensional electrophoresis (2D PAGE) coupled with liquid chromatography tandem mass spectrometry (LC-MS/MS). An experimental challenge trial using Atlantic salmon smolts confirmed a loss in virulence in an N. perurans culture that was maintained in vitro for 3 years. A greater diversity of bacterial communities was found in the microbiome of the virulent isolate harbouring predominant genera belonging to Pseudoaltermonas spp, Vibrio spp and Fluviicola spp. Microbial community richness was reduced in the attenuated microbiome, with a singular species, Thalassopira xiamenensis, representing a large proportion of its microbiome. A collated proteome database of N. perurans, Amoebozoa and four bacterial genera resulted in 24 proteins differentially expressed between the three cultures. The present LC-MS/MS results indicate protein synthesis, oxidative stress and the plausible occurrence of immunomodulation are ultimately upregulated in a newly acquired N. perurans culture and future studies may exploit these protein identifications for therapeutic purposes in infected farmed fish.