# Virus-like particle (VLP)-based indirect ELISA (iELISA) for the detection of beak and feather disease virus (BFDV) antibodies

## TL;DR

A new iELISA test using virus-like particles was developed to detect antibodies against BFDV in birds, offering a reliable and scalable diagnostic tool.

## Contribution

Development of a recombinant capsid protein-based iELISA using virus-like particles for BFDV antibody detection in psittacine birds.

## Key findings

- The iELISA demonstrated high sensitivity (96.5%) and strong agreement with the HI assay (Gwet’s Agreement Coefficient 1=0.843).
- The diagnostic cut-off was optimized using TG-ROC analysis, achieving a Youden’s index of 0.74.
- The assay is adaptable and suitable for large-scale epidemiological studies and conservation efforts.

## Abstract

Beak and feather disease virus (BFDV) poses a significant threat to avian biodiversity and global aviculture. Reliable serodiagnostic tools are critical for assessing host immune status and guiding disease management. The haemagglutination inhibition (HI) assay, although historically regarded as the gold standard, is limited by technical complexity and its reliance on seropositive Galah erythrocytes, restricting broader application. This study describes the development and validation of a recombinant BFDV capsid protein-based indirect enzyme-linked immunosorbent assay (iELISA) for detecting anti-BFDV antibodies using dried blood spots from multiple psittacine species. The assay demonstrated high sensitivity and strong analytical performance, employing virus-like particles (VLPs) recombinantly expressed in Escherichia coli. Optimisation of the diagnostic cut-off by two-graph receiver operating characteristic (TG-ROC) analysis established an OD threshold of 1.73, achieving 96.5% sensitivity with a Youden’s index of 0.74. Discriminative capacity was further supported by receiver operating characteristic (ROC) analysis, yielding an area under the curve (AUC) of 0.896. Agreement with the HI assay was very strong (Gwet’s Agreement Coefficient 1= 0.843). This iELISA represents a scalable and universal serodiagnostic tool, supporting clinical diagnosis, enabling large-scale epidemiological investigations, and advancing conservation-focused BFDV surveillance.

Efficient detection of anti-BFDV antibodies in psittacines using recombinant capsid protein as coating antigen.Use of TG-ROC curve to determine cut-off value supported by Gwet’s Agreement Coefficient 1.Easily adaptable method with very high sensitivity in detecting anti-BFDV antibodies.

Efficient detection of anti-BFDV antibodies in psittacines using recombinant capsid protein as coating antigen.

Use of TG-ROC curve to determine cut-off value supported by Gwet’s Agreement Coefficient 1.

Easily adaptable method with very high sensitivity in detecting anti-BFDV antibodies.

The online version contains supplementary material available at 10.1007/s00253-025-13683-z.