Alphavirus madariaga-associated encephalomyelitis in equines from the Brazilian Pantanal reveals arbovirus circulation in a peri-urban environment.

Overview

• This research article reports the detection and genetic analysis of Alphavirus madariaga (MADV), an emerging arbovirus, in horses showing neurological disease in the Brazilian Pantanal region.
• The study highlights the circulation of MADV in a peri-urban environment near human populations, underlining the risk of transmission and the importance of horses as indicators of neurotropic arbovirus activity.

Introduction to Alphavirus madariaga (MADV)

• MADV is a member of the eastern equine encephalitis virus complex, known for causing encephalomyelitis in equines and potentially affecting humans.
• It is considered an emerging arbovirus in South America, with relatively limited data on its presence and molecular characterization in horse populations.
• Prior to this study, MADV’s epidemiology in Brazil and its genetic diversity had not been well documented.

Study Context and Purpose

• The research was conducted in the state of Mato Grosso, specifically near Santo Antônio do Leverger, a peri-urban area about 30 km from Cuiabá, Brazil.
• A key objective was to confirm the presence of MADV in horses displaying acute neurological symptoms and to genetically characterize the detected virus strains.
• The study provides insight into the arbovirus circulation in a region representing an interface between sylvatic (wild) and human-modified environments, which may enhance disease transmission risk.

Methods

• Two horses with acute neurological symptoms and a rapid clinical progression (~2 days) were evaluated.
• Histopathological examination was performed, revealing nonsuppurative meningoencephalomyelitis, indicative of viral inflammation affecting the brain and spinal cord.
• Molecular screening was conducted using TaqMan RT-qPCR assays targeting multiple neurotropic viruses including rabies virus, equine herpesvirus-1 (EHV-1), western equine encephalitis virus (WEEV), Venezuelan equine encephalitis virus (VEEV), West Nile virus (WNV), and flaviviruses.
• All viruses tested were negative except for MADV, which showed mid-range Ct values (27.9 and 31.4), indicating viral RNA presence at moderate levels.
• Partial gene fragments of the NS1 and E1 genes of the virus were amplified and sequenced to characterize the strain.

Results

• Phylogenetic analysis of both NS1 and E1 gene sequences placed the detected virus strain, named “Santo Antonio,” firmly within the MADV genotype III clade.
• The E1 gene analysis provided strong support for this classification, highlighting the genetic relationship with known MADV strains.
• This confirmed the local circulation of MADV in central-western Brazil and its association with equine disease.
• The detection in a peri-urban area near human settlements emphasizes the potential for interaction between wild and domestic cycles of the virus, possibly increasing risk for humans and animals in the region.

Significance and Implications

• The study enhances understanding of MADV distribution, demonstrating its presence beyond previously recognized locations and its ability to cause neurological disease in horses in Brazil.
• By identifying horses as sentinels—early warning indicators—for MADV and other neurotropic arboviruses, the findings support equine surveillance as a critical tool in monitoring arbovirus threats before spillover into human populations.
• Recognizing peri-urban zones as interfaces where arbovirus transmission may intensify can inform public health strategies aiming to mitigate emerging infectious diseases.
• Overall, the research underscores the need for continuous molecular surveillance, improved diagnostic capacities, and ecological studies to better understand arbovirus dynamics and prevent outbreaks.