The Impact of Pseudoloma neurophilia Infection on Body Condition of Zebrafish

Review of Pseudoloma neurophilia (Microsporidia): A common neural parasite of laboratory zebrafish (Danio rerio)

Zebrafish (Danio rerio) is now the second most used animal model in biomedical research. As with other vertebrate models, underlying diseases and infections often impact research. Beyond mortality and morbidity, these conditions can compromise research end points by producing nonprotocol induced variation within experiments. Pseudoloma neurophilia, a microsporidium that targets the central nervous system, is the most frequently diagnosed pathogen in zebrafish facilities. The parasite undergoes direct, horizontal transmission within populations, and is also maternally transmitted with spores in ovarian fluid and occasionally within eggs. This transmission explains the wide distribution among research laboratories as new lines are generally introduced as embryos. The infection is chronic, and fish apparently never recover following the initial infection. However, most fish do not exhibit outward clinical signs. Histologically, the parasite occurs as aggregates of spores throughout the midbrain and spinal cord and extends to nerve roots. It often elicits meninxitis, myositis, and myodegeneration when it infects the muscle. There are currently no described therapies for the parasite, thus the infection is best avoided by screening with PCR-based tests and removal of infected fish from a facility. Examples of research impacts include reduced fecundity, behavioral changes, transcriptome alterations, and autofluorescent lesions.

Tenth Aquatic Models of Human Disease Conference 2022 Workshop Report: Aquatics Nutrition and Reference Diet Development

Progress in biomedical research requires rigorous studies and reproducible outcomes. However, despite recent achievements, standard reference diets (SRDs) for aquatic model organisms, vital for supporting scientific rigor and reproducibility, are yet to be adopted. At this workshop, we presented findings from a 7-month diet test study, tightly coordinated and conducted across three aquatic research facilities: Zebrafish International Resource Center (ZIRC), Kent and Sharpton laboratories (Oregon State University), and Xiphophorus Genetic Stock Center (XGSC, Texas State University). We compared the impact of two commercial diets and a suggested zebrafish SRD on general fish husbandry, microbiome composition, and health in three fish species (zebrafish, Xiphophorus, and Medaka), and three zebrafish wild-type strains. We reported outcomes, gathered community feedback, and addressed the aquatic research community's need for SRD development. Discussions underscored the influence of diet on aquatic research variability, emphasizing the need for SRDs to control cross-experiment and cross-laboratory reproducibility. Species-specific reference diets are essential for model organism health and consistent research outcomes.

Assessment of Oral Albendazole and Fumagillin in the Treatment of Pseudoloma neurophilia in Adult Zebrafish

Pseudoloma neurophilia ( Pn ), the causative agent of the most commonly reported disease of zebrafish, is a microsporidian parasite that confounds research by inducing behavioral and physiologic changes in zebrafish. Although a treatment for P. neurophilia has not been documented in zebrafish, albendazole (ALB) and fumagillin (FUM) have been used to treat microsporidian infections of other fish species. To investigate the efficacy of oral ALB and FUM in the treatment of Pn, we performed a pilot study that demonstrated the safety and palatability of novel gel-based diets containing FUM or ALB in adult AB zebrafish. In a subsequent study, approximately 250 adult AB zebrafish (previously infected with Pn ) were treated with these medicated diets for 4 wk. At 4 different time points (weeks 0, 5, 10, and 16 of the study), fish were euthanized and whole-body qPCR was performed to assess Pn prevalence across treatment and control groups. There was no statistically significant association between treatment group and Pn prevalence at any time point, although potential biologically relevant reductions in Pn prevalence occurred in the combination therapy group at weeks 5 and 16 and in the ALB group at week 5. Based on high-performance liquid chromatography analyses, the medicated diets contained less ALB and more FUM than expected, highlighting the importance of validating medicated feed concentrations to ensure safety, efficacy, and consistency. While Pn remains challenging to eradicate and control, results of this study warrant further investigation into the utility of ALB and FUM as potential treatments for this pathogen.

Advanced Techniques for the Intelligent Diagnosis of Fish Diseases: A Review

Aquatic products, as essential sources of protein, have attracted considerable concern by producers and consumers. Precise fish disease prevention and treatment may provide not only healthy fish protein but also ecological and economic benefits. However, unlike intelligent two-dimensional diagnoses of plants and crops, one of the most serious challenges confronted in intelligent aquaculture diagnosis is its three-dimensional space. Expert systems have been applied to diagnose fish diseases in recent decades, allowing for restricted diagnosis of certain aquaculture. However, this method needs aquaculture professionals and specialists. In addition, diagnosis speed and efficiency are limited. Therefore, developing a new quick, automatic, and real-time diagnosis approach is very critical. The integration of image-processing and computer vision technology intelligently allows the diagnosis of fish diseases. This study comprehensively reviews image-processing technology and image-based fish disease detection methods, and analyzes the benefits and drawbacks of each diagnostic approach in different environments. Although it is widely acknowledged that there are many approaches for disease diagnosis and pathogen identification, some improvements in detection accuracy and speed are still needed. Constructing AR 3D images of fish diseases, standard and shared datasets, deep learning, and data fusion techniques will be helpful in improving the accuracy and speed of fish disease diagnosis.

Progression of infection and detection of Pseudoloma neurophilia in zebrafish Danio rerio Hamilton by PCR and histology

Pseudoloma neurophilia is a critical threat to the zebrafish (Danio rerio) model, as it is the most common infectious agent found in research facilities. In this study, our objectives were two-fold: (1) compare the application of diagnostic tools for P. neurophilia and (2) track the progression of infection using PCR and histology. The first experiment showed that whole-body analysis by qPCR (WB-qPCR) can be a standardized process, providing a streamlined diagnostic protocol, without the need for extraction of specific tissues. Evaluating the course of infection in experimentally infected fish, we showed key dynamics in infection. Starting with a low dose exposure of 8000 spores/fish, the prevalence remained low until 92 days post-exposure (dpe), followed by a 30%-40% prevalence by histology or 40%-90% by PCR until the end of the experiment at 334 dpe. WB-qPCR positively detected infection in more fish than histology throughout the study, as WB-qPCR detected the parasite as early as 4 dpe, whereas it was undetected by histology until 92 dpe. We also added a second slide for histologic analyses, showing an increase in detection rate from 24% to 26% when we combined all data from our experiments, but this increase was not statistically significant.

FELASA-AALAS Recommendations for Monitoring and Reporting of Laboratory Fish Diseases and Health Status, with an Emphasis on Zebrafish (Danio Rerio)

The exchange of fish for research may expose an aquatic laboratory to pathogen contamination as incoming fish can introduce bacteria, fungi, parasites, and viruses capable of affecting both experimental results and fish and personnel health and welfare. To develop risk mitigation strategies, FELASA and AALAS established a joint working group to recommend good practices for health monitoring of laboratory fish. The recommendations address all fish species used for research, with a particular focus on zebrafish (Danio rerio). First, the background of the working group and key definitions are provided. Next, fish diseases of high impact are described. Third, recommendations are made for health monitoring of laboratory fishes. The recommendations emphasize the importance of daily observation of the fish and strategies to determine fish colony health status. Finally, report templates are proposed for historical screening data and aquatic facility description to facilitate biohazard risk assessment when exchanging fish.

Recent Advances with Fish Microsporidia

There have been several significant new findings regarding Microsporidia of fishes over the last decade. Here we provide an update on new taxa, new hosts and new diseases in captive and wild fishes since 2013. The importance of microsporidiosis continues to increase with the rapid growth of finfish aquaculture and the dramatic increase in the use of zebrafish as a model in biomedical research. In addition to reviewing new taxa and microsporidian diseases, we include discussions on advances with diagnostic methods, impacts of microsporidia on fish beyond morbidity and mortality, novel findings with transmission and invertebrate hosts, and a summary of the phylogenetics of fish microsporidia.

Behavioral and Reproductive Effects of Environmental Enrichment and Pseudoloma neurophilia infection on Adult Zebrafish (Danio rerio)

Recent studies have shown beneficial effects of environmental enrichment (EE) for zebrafish, while infection of zebrafish with the common pathogen Pseudoloma neurophilia has negative effects. This study investigates the effects of P. neurophilia infection and EE in housing and breeding tanks on measures of behavior, growth, and reproduction. Zebrafish were socially housed and were either infected, P. neurophilia-infected (PNI) (n = 12 tanks), or SPF for P. neurophilia (SPF) (n = 24 tanks). Fish were housed with or without EE, which consisted of placing plastic plants in the tanks; sprigs from plants were placed in half of the breeding tanks for half of breedings, alternating breeding tanks without EE weekly. Behavioral testing included the Novel Tank Diving Test (NTT) and Light/Dark Preference Test (LDT) conducted prior to breeding. At the end of the study, biometric data were collected. Histopathology and molecular analysis for common diseases in fish confirmed that SPF fish remained SPF and that fish from all PNI tanks were infected. PNI fish produced significantly fewer eggs and had lower body weights and lengths than did SPF fish. Fish with EE had longer body lengths, than did fish without EE, and male fish had longer body lengths than female fish. The biometric results and reproductive measures show that SPF fish exhibited better growth and suggest that EE in housing tanks could improve fish growth. The behavioral test results were inconclusive regard- ing whether infection status or EE altered anxiety-like behavior. Our results support other recent studies showing negative effects of P. neurophilia infection on zebrafish.

Effects of Pseudoloma neurophilia infection on the brain transcriptome in zebrafish (Danio rerio)

Laboratory zebrafish are commonly infected with the intracellular, brain-infecting microsporidian parasite Pseudoloma neurophilia. Chronic P. neurophilia infections induce inflammation in meninges, brain and spinal cord, and have been suggested to affect neural functions since parasite clusters reside inside neurons. However, underlying neural and immunological mechanisms associated with infection have not been explored. Utilizing RNA-sequencing analysis, we found that P. neurophilia infection upregulated 175 and downregulated 45 genes in the zebrafish brain, compared to uninfected controls. Four biological pathways were enriched by the parasite, all of which were associated with immune function. In addition, 14 gene ontology (GO) terms were enriched, eight of which were associated with immune responses and five with circadian rhythm. Surprisingly, no differentially expressed genes or enriched pathways were specific for nervous system function. Upregulated immune-related genes indicate that the host generally show a pro-inflammatory immune response to infection. On the other hand, we found a general downregulation of immune response genes associated with anti-pathogen functions, suggesting an immune evasion strategy by the parasite. The results reported here provide important information on host-parasite interaction and highlight possible pathways for complex effects of parasite infections on zebrafish phenotypes.

Behavioural effects of the common brain-infecting parasite Pseudoloma neurophilia in laboratory zebrafish (Danio rerio)

Research conducted on model organisms may be biased due to undetected pathogen infections. Recently, screening studies discovered high prevalence of the microsporidium Pseudoloma neurophilia in zebrafish (Danio rerio) facilities. This spore-forming unicellular parasite aggregates in brain regions associated with motor function and anxiety, and despite its high occurrence little is known about how sub-clinical infection affects behaviour. Here, we assessed how P. neurophilia infection alters the zebrafish´s response to four commonly used neurobehavioral tests, namely: mirror biting, open field, light/dark preference and social preference, used to quantify aggression, exploration, anxiety, and sociability. Although sociability and aggression remained unaltered, infected hosts exhibited reduced activity, elevated rates of freezing behaviour, and sex-specific effects on exploration. These results indicate that caution is warranted in the interpretation of zebrafish behaviour, particularly since in most cases infection status is unknown. This highlights the importance of comprehensive monitoring procedures to detect sub-clinical infections in laboratory animals.