# Temporal serum neurofilament light chain concentrations in sheep inoculated with the agent of classical scrapie

## TL;DR

This study tested if a protein called Nf-L in sheep blood could detect scrapie disease before symptoms appear, but found it only rises when symptoms are already present.

## Contribution

The study is the first to evaluate serum Nf-L as a pre-symptomatic biomarker for scrapie in sheep using longitudinal measurements.

## Key findings

- Serum Nf-L concentrations increased only during the final 10% of the incubation period when neurologic signs were present.
- Nf-L was not elevated during the pre-symptomatic phase of scrapie in sheep.
- RT-QuIC results were inconsistent antemortem, but all sheep had at least one positive test.

## Abstract

Neurofilament light chain (Nf-L) has been used to detect neuroaxonal damage in the brain caused by physical injury or disease. The purpose of this study was to determine if serum Nf-L could be used as a biomarker for pre-symptomatic detection of scrapie in sheep.

Four sheep with prion protein genotype AVQQ were intranasally inoculated with the classical scrapie strain x124. Blood was collected every 4 weeks until 44 weeks post-inoculation, at which point weekly collection commenced. Serum was analyzed using single molecule array (Quanterix SR-X) to evaluate Nf-L concentrations.

Scrapie was confirmed in each sheep by testing homogenized brainstem at the level of the obex with a commercially available enzyme immunoassay. Increased serum Nf-L concentrations were identified above the determined cutoff during the last tenth of the respective incubation period for each sheep. Throughout the time course study, PrPSc accumulation was not detected antemortem by immunohistochemistry in rectal tissue at any timepoint for any sheep. RT-QuIC results were inconsistently positive throughout the timepoints tested for each sheep; however, each sheep had at least one timepoint detected positive. When assessing serum Nf-L utility using receiver operator characteristic curves against different clinical parameters, such as asymptomatic and symptomatic (pruritus or neurologic signs), results showed that Nf-L was most useful at being an indicator of disease only late in disease progression when neurologic signs were present.

Serum Nf-L concentrations in the cohort of sheep increased as disease progressed; however, serum Nf-L did not increase during the presymptomatic window. The levels increased substantially throughout the final 10% of the animals’ scrapie incubation period when other clinical signs were present. Serum Nf-L is not a reliable biomarker for pre-clinical detection of scrapie.