Comparison of histologic methods for the detection of Desmozoon lepeophtherii spores in the gills of Atlantic salmon

Desmozoon lepeophtherii is a microsporidian associated with gill disease in farmed Atlantic salmon (Salmo salar). Detection of the parasite in histologic tissue sections is challenging using common histochemical stains given that the small, widely distributed parasite spores typically occur individually or in small clusters. We compared the ability of 4 histologic methods to detect D. lepeophtherii spores in serial sections of Atlantic salmon gill tissue: hematoxylin and eosin (H&E), Gram–Twort (GT), calcofluor white (CW), and immunohistochemistry (IHC). Using CW as a benchmark to calculate a relative ratio, IHC consistently detected more spores than CW (median: 1.3), followed by GT (median: 0.2) and H&E (median: 0.1). IHC detected significantly more spores than GT (p < 0.05) and H&E (p < 0.05), and GT more than H&E (p < 0.05). We found significant underestimation of numbers of microsporidia spores in gill disease in Atlantic salmon using conventional histochemical stains and recommend the use of CW or IHC to detect the parasite in tissue sections.