Development of piscirickettsiosis in Atlantic salmon (Salmo salar L.) smolts after intraperitoneal and cohabitant challenge using an EM90‐like isolate: A comparative study

Abstract

Piscirickettsiosis, caused by the intracellular Gram‐negative bacteria Piscirickettsia salmonis, is at present the most devastating disease in the Chilean salmon industry. The aim of this study was to analyse disease development after challenge with a P. salmonis strain (EM90‐like) under a controlled environment by comparing intraperitoneal challenge with cohabitation challenge. The P. salmonis EM90‐like isolate was cultured in a liquid medium for the challenge of 400 Atlantic salmon (Salmo salar) smolts. Cumulative mortality was registered, necropsy was performed, and bacterial distribution in the tissues and histopathological changes were analysed. The results revealed a similar progression of the disease for the two different challenge models. Pathological and histopathological changes became more visible during the development of the clinical phase of the disease. Bacterial DNA was identified in all the analysed tissues indicating a systemic infection. Bacterial tropism to visceral organs was demonstrated by real‐time quantitative PCR and immunohistochemistry. Better knowledge of disease development during P. salmonis infection may contribute to further development of challenge models that mimic the field situation during piscirickettsiosis outbreaks. The models can be used to develop and test future preventive measures against the disease.

Comparative evaluation of experimental challenge by intraperitoneal injection and cohabitation of Atlantic salmon (Salmo salar L) after vaccination against Piscirickettsia salmonis (EM90‐like)

“…Previously reported challenge studies with P . salmonis show variation in onset of mortality and disease development compared with the present study, which could be related to differences in water temperature and salinity between trials (Meza et al, ; Rozas‐Serri et al, ; Vásquez, ). The aim of the present study was to compare the disease development of two P .…”

“…The bacterial inoculum dose for i.p. injected fish was obtained from a previous study elaborated by Meza et al (), where a theoretical dose of 1.0 × 10 5 cfu/mL was used. The fish were kept in brackish water with a salinity of 25‰ at 15°C and 24 hr of light.…”

“…Primer and probe sequences targeting P . salmonis were obtained from Meza et al (). Primer sequences were 5′‐AGG GAG ACT GCC GGT GAT/A3′ for forward primer and 5′‐ACT ACG AGG CGC TTT CTC A‐3′ for the reverse primer.…”

“…injected fish was obtained from a previous study elaborated by Meza et al (2019), where a theoretical dose of 1.0 × 10 5 cfu/mL was used.…”

“…To determine the optimal antigen concentration for bead conjuga- with the present study, which could be related to differences in water temperature and salinity between trials (Meza et al, 2019;Rozas-Serri et al, 2017;Vásquez, 2012). The aim of the present study was to compare the disease development of two P. salmonis challenge models, i.p.…”