Unusual fluorescent granulomas and myonecrosis in Danio rerio infected by the microsporidian pathogen Pseudoloma neurophilia

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.

Detection of the parasitic nematode, Pseudocapillaria tomentosa, in zebrafish tissues and environmental DNA in research aquaria

Although zebrafish continue to increase in popularity as a vertebrate animal model for biomedical research, chronic infectious diseases in laboratory populations remain prevalent. The presence of pathogens such as Pseudocapillaria tomentosa, a parasitic nematode found in the intestine of infected zebrafish, can significantly influence experimental endpoints and negatively impact reproducibility of research findings. Thus, there is a need for screening tests for zebrafish with the sensitivity to detect even low levels of pathogens present in tissues. Assays based on the detection of DNA are commonly used for such screening tests. Newer technologies such as digital PCR provide an opportunity to improve the sensitivity and precision of these assays, so they can be reliably used to detect pathogen DNA in water, reducing the need for lethal testing. We have designed a qPCR-based assay with the sensitivity to detect less than 5 copies of the P. tomentosa SSU-rDNA gene in tissues of infected zebrafish and environmental DNA from aquarium water housing infected fish. In addition, we adapted this test to a dPCR platform to provide a precise quantification of P. tomentosa DNA and demonstrate the resistance of this assay to inhibitors commonly found in freshwater aquaria.

The Impact of Pseudoloma neurophilia Infection on Body Condition of Zebrafish

The zebrafish is a widely used animal model in biomedical research. Despite this, pathogens continue to be common in laboratory zebrafish. It is important to determine and describe the pathophysiology of cryptic infections on zebrafish to elucidate the impacts on experimental endpoints. Body condition is a basic measurement used experimentally and in health monitoring of animals. We exposed three wild-type zebrafish strains: AB, WIK, and 5D to Pseudoloma neurophilia. After 8 weeks postexposure, we individually imaged and processed fish for histology. Morphometric analysis was performed on images and an index of body condition was calculated based on the ratio of length/width from the dorsal aspect. Histological sections from each fish were examined to establish sex, severity of infection, and tissue distribution. We observed a significant decrease in body condition in female fish that was not observed in males. In addition, we observed a negative correlation between the total gonadal area of P. neurophilia exposed females and unexposed controls. These results illustrate the sex-specific impacts of a common chronic pathogen on zebrafish health and a commonly used experimental endpoint, further supporting the establishment of rigorous health monitoring programs in laboratory zebrafish colonies that include screening for chronic infectious diseases.

Comparison of Antemortem and Environmental Samples for Zebrafish Health Monitoring and Quarantine

Molecular diagnostic assays offer both exquisite sensitivity and the ability to test a wide variety of sample types. Various types of environmental sample, such as detritus and concentrated water, might provide a useful adjunct to sentinels in routine zebrafish health monitoring. Similarly, antemortem sampling would be advantageous for expediting zebrafish quarantine, without euthanasia of valuable fish. We evaluated the detection of Mycobacterium chelonae, M. fortuitum, M. peregrinum, Pseudocapillaria tomentosa, and Pseudoloma neurophilia in zebrafish, detritus, pooled feces, and filter membranes after filtration of 1000-, 500-, and 150-mL water samples by real-time PCR analysis. Sensitivity varied according to sample type and pathogen, and environmental sampling was significantly more sensitive than zebrafish sampling for detecting Mycobacterium spp. but not for Pseudocapillaria neurophilia or Pseudoloma tomentosa. The results of these experiments provide strong evidence of the utility of multiple sample types for detecting pathogens according to each pathogen's life cycle and ecological niche within zebrafish systems. In a separate experiment, zebrafish subclinically infected with M. chelonae, M. marinum, Pleistophora hyphessobryconis, Pseudocapillaria tomentosa, or Pseudoloma neurophilia were pair-spawned and individually tested with subsets of embryos from each clutch that received no rinse, a fluidizing rinse, or were surface-disinfected with sodium hypochlorite. Frequently, one or both parents were subclinically infected with pathogen(s) that were not detected in any embryo subset. Therefore, negative results from embryo samples may not reflect the health status of the parent zebrafish.

Recommendations for Health Monitoring and Reporting for Zebrafish Research Facilities

The presence of subclinical infection or clinical disease in laboratory zebrafish may have a significant impact on research results, animal health and welfare, and transfer of animals between institutions. As use of zebrafish as a model of disease increases, a harmonized method for monitoring and reporting the health status of animals will facilitate the transfer of animals, allow institutions to exclude diseases that may negatively impact their research programs, and improve animal health and welfare. All zebrafish facilities should implement a health monitoring program. In this study, we review important aspects of a health monitoring program, including choice of agents, samples for testing, available testing methodologies, housing and husbandry, cost, test subjects, and a harmonized method for reporting results. Facilities may use these recommendations to implement their own health monitoring program.

Biosecurity and Health Monitoring at the Zebrafish International Resource Center

The Zebrafish International Resource Center (ZIRC) is a repository and distribution center for mutant, transgenic, and wild-type zebrafish. In recent years annual imports of new zebrafish lines to ZIRC have increased tremendously. In addition, after 15 years of research, we have identified some of the most virulent pathogens affecting zebrafish that should be avoided in large production facilities, such as ZIRC. Therefore, while importing a high volume of new lines we prioritize safeguarding the health of our in-house fish colony. Here, we describe the biosecurity and health-monitoring program implemented at ZIRC. This strategy was designed to prevent introduction of new zebrafish pathogens, minimize pathogens already present in the facility, and ensure a healthy zebrafish colony for in-house uses and shipment to customers.