The Role of Viral Pathogens in Horse Respiratory Diseases: A Cytological and Molecular Approach Using Next-Generation Sequencing

Respiratory problems may be associated with pathogens among which viruses may play an important role in causing or promoting clinical signs. However, traditional diagnostic methods for equine infectious diseases, such as PCR, are limited to known pathogens and often miss rare or novel viruses. Metagenomic next-generation sequencing is a transformative method that allows the sequencing of all genomes present in a sample. Recent studies have used next- generation sequences to identify viral sequences in horses with unexplained respiratory symptoms, revealing potential links between viruses and respiratory problems. The aim of the study was to verify a relationship between respiratory diseases and the presence of viral agents through molecular analyses, applied to bronchoalveolar lavage fluid obtained from horses with history or presence of clinical signs of respiratory diseases. (2) Methods: A cohort of 14 horses was enrolled. All the subjects were submitted to a clinical assessment of the respiratory tract, thoracic ultrasound evaluation, respiratory tract endoscopy, and bronchoalveolar lavage. The latter one was used for cytological analysis, DNA and RNA extraction, and molecular biology analysis. (3) Results: No positive results were obtained in the molecular studies except for a sequence of 753 bp obtained by next-generation sequences, with complete homology to Equid gammaherpesvirus 2 strains. The samples were taken from a thoroughbred female horse aged 2 years old and referred for poor performance and sporadic cough at the beginning of the training session. (4) Conclusions: viral involvement may be not common in horses with respiratory diseases. We cannot exclu