Tawni B.R. Firestone | Search USGS

USGS2025

Histological progression and bacterial load dynamics of Renibacterium salmoninarum in Chinook salmon Oncorhynchus tshawytscha

Renibacterium salmoninarum , the cause of bacterial kidney disease (BKD), severely impacts salmonid populations. Much of our understanding of the BKD pathology in salmonids comes from evaluating fatal infections in wild populations or spawning Pacific Northwest salmonids. Our study investigated the histological progression and bacterial load dynamics of R. salmoninarum infection in Chinook salmon following intraperitoneal injection to enhance understanding of the disease’s dynamics. Seventy presmolt salmon were injected with the ATCC-33209 isolate of R. salmoninarum and monitored over 10 wk. Histological signs of disease were observed in spleen and liver tissues at 1 wk post-injection and in kidney tissues at 4 wk post-injection, with signs of disease increasing over time. Additionally, histopathological analysis revealed splenic and hepatic capsulitis (coelomitis), individual hepatocyte necrosis, progressive granulomatous hepatitis, splenitis, and nephritis with necrosis. Gram staining confirmed the presence of Gram-positive bacteria within macrophages and extracellularly in infected tissues. Quantitative PCR revealed significant increases in bacterial loads in both kidney and liver tissues over the study period, with higher bacterial loads observed in the kidney tissue. Bacterial load was strongly correlated with disease stage, with peak bacterial burdens coinciding with the most severe histological changes. Our study provides a comprehensive account of R. salmoninarum infection and disease progression in Chinook salmon after intraperitoneal injection.

Diseases of Aquatic Organisms↗

USGS2025

Leveraging detection uncertainty to estimate Renibacterium salmoninarum infection status among multiple tissues and assays

Effective disease surveillance relies on accurate pathogen testing and robust prevalence estimates. Diagnostic specificity (DSp), the probability that an uninfected animal tests negative, is high when false positives are low. Diagnostic sensitivity (DSe) is the probability an infected animal tests positive; higher DSe means fewer false negatives. However, sensitivity and false negatives are harder to estimate without a "gold standard", an assay that can detect between 90 - 100% of true positive infections. Occupancy estimation of infection prevalence offers one solution by allowing for imperfect detection of the pathogen. Testing potentially infected tissues multiple times allows for the use of a Bayesian multistate occupancy model to estimate the probability of pathogen infection in tissues [Formula: see text] and detection probabilities [Formula: see text] for different assays. Using [Formula: see text] and [Formula: see text] from the posterior distribution, the conditional probability of detecting the pathogen can be modeled, allowing for the calculation of DSe. Renibacterium salmoninarum is a bacterial pathogen causing bacterial kidney disease among salmonid species and was the model pathogen we used to train our model. The current testing standard for salmonids combines initial screening for antibodies using direct fluorescent antibody test (DFAT) with polymerase chain reaction (PCR) confirmation to detect R. salmoninarum. However, detection of R. salmoninarum still varies between species, tissues, and assays. Here, a multi-state occupancy model was used to estimate detection probability among individual and dual kidney/liver infections with DFAT and qPCR in fish with an unknown infection status. Both assays produced false negatives, but qPCR had fewer than DFAT and a higher DSe. Infection state was often misclassified, but multiple surveys per individual or combining tissues for testing improved DSe for both assays.

conterminous United States↗