Abstract
Background:
Bovine Respiratory Syncytial Virus (BRSV) is a major viral pathogen involved in the bovine respiratory disease complex (BRD), causing substantial economic losses in cattle production worldwide. Despite its recognized importance, information on BRSV circulation and associated serological responses in Moroccan cattle remains limited.

Aim:
This study aimed to investigate the serological evidence of BRSV exposure and to identify epidemiological factors associated with BRSV seropositivity in clinically affected cattle from two major livestock-producing regions of Morocco.

Methods:
A cross-sectional sero-epidemiological study was conducted between October 2020 and July 2021 in 16 commercial cattle farms located in the Casablanca–Settat and Rabat–Salé–Kénitra regions. A total of 94 serum samples were collected from cattle exhibiting clinical signs consistent with bovine respiratory disease. Anti-BRSV antibodies were detected using a commercial enzyme-linked immunosorbent assay (ELISA). Associations between seropositivity and potential risk factors, including age, sex, breed, and geographical origin, were evaluated using Fisher’s exact test and odds ratio analysis.

Results:
Seventy-four of the 94 tested animals were seropositive for BRSV antibodies, corresponding to an overall seropositivity rate of 78.7% (95% CI: 69.3–86.2), indicating widespread exposure among symptomatic cattle. No significant associations were found between seropositivity and age (p = 0.29), sex (p = 0.51), or sampling region (p = 0.10). In contrast, purebred cattle showed significantly higher seropositivity than crossbred animals (87.5% vs. 65.8%; OR = 3.64, 95% CI: 1.27–10.43; p = 0.016). Because sampling was restricted to clinically affected animals, the findings reflect exposure among symptomatic cattle rather than population-level seroprevalence.

Conclusion:
The study demonstrates widespread serological evidence of BRSV exposure among clinically affected cattle in Morocco, supporting the endemic circulation of the virus in the investigated herds. The high antibody detection rate highlights the need for strengthened surveillance programs, improved biosecurity measures, and the implementation of effective vaccination strategies to reduce the impact of respiratory disease in cattle production systems. Further studies combining serological and molecular approaches are warranted to better characterize BRSV epidemiology and transmission dynamics in Moroccan cattle populations.

Key words: Bovine respiratory syncytial virus; Cattle; Humoral immune response; Morocco; Seroepidemiology.