# Immunological insights into the occurrence of Lawsonia intracellularis in horses from southern Brazil using flow cytometry

## TL;DR

This study investigates the presence of a bacteria called Lawsonia intracellularis in horses from southern Brazil and evaluates an experimental vaccine's immune response using a new diagnostic test.

## Contribution

The study introduces and validates a flow cytometry antibody test (FCAT) for detecting anti-Lawsonia intracellularis antibodies in horses.

## Key findings

- FCAT demonstrated high sensitivity (100%) and specificity (92%) for detecting anti-L. intracellularis antibodies.
- 26% of horses in southern Brazil tested positive for anti-L. intracellularis antibodies, with higher rates in females and older horses.

## Abstract

Lawsonia intracellularis is an obligatory intracellular bacterium associated with equine proliferative enteropathy (EPE), which significantly impacts equine health. Despite its clinical relevance, epidemiological and diagnostic approaches for this infection in horses have remained underexplored. This study aimed to evaluate the humoral immune response in horses immunized with an experimental vaccine for L. intracellularis and to determine the occurrence of anti-L. intracellularis antibodies in horses from southern Brazil using the flow cytometry antibody test (FCAT).

A total of 12 horses were immunized with an experimental vaccine containing inactivated L. intracellularis and adjuvants. Blood samples were collected on day 0 (D0) and every 7 days until day 35 (D35) to monitor the antibody response using FCAT. In addition, serum samples from 79 horses from the North and Northwest regions of Rio Grande do Sul were tested to determine the occurrence of anti-L. intracellularis antibodies. The FCAT protocol was optimized for equine samples, and a cut-off fluorescence threshold was determined using Receiver Operating Characteristic curve analysis.

FCAT demonstrated high accuracy, with a sensitivity of 100% and specificity of 92% at a fluorescence cut-off of ≥15%. Immunization triggered a robust humoral response, with a significant increase in fluorescence from day 7 to day 35. In the occurrence analysis, 26% of the horses tested positive for anti-L. intracellularis antibodies. The highest occurrence was observed in females (35.8%) and horses aged 16–22 years (50%).

This study provided evidence that L. intracellularis infection is relatively common among horses in southern Brazil. FCAT was a sensitive and specific diagnostic tool for detecting anti-L. intracellularis antibodies in horses. The findings highlight the need for better diagnostic and preventive measures to control EPE in equine populations.