Scientists have confirmed what is believed to be the first known case of neurological disease in a horse caused by the mosquito-borne St Louis encephalitis virus (SLEV).
The researchers isolated the virus from the brain of a horse in Brazil.
St Louis encephalitis virus is considered endemic in the Americas, with cases being diagnosed from Canada to Argentina. There is no vaccine or treatment available.
An average of about 128 SLEV cases in people are reported annually in the United States. Cases in temperate areas of the US occur mostly in the late summer or early fall. In the southern US, where the climate is milder, they can occur year round.
The name of the virus goes back to 1933 when, within five weeks, an explosive epidemic centered on St Louis, Missouri, resulted in more than 1000 human cases being reported to authorities.
The virus belongs to the Flavivirus genus, which includes the West Nile encephalitis virus, Japanese encephalitis virus, Dengue virus, Yellow Fever and other medically important viruses.
The virus is carried by birds, with mosquitoes acting as the vector infecting humans.
The Brazilian researchers, writing in the open-access journal, PLoS Neglected Tropical Diseases, said they had isolated an SLEV strain from the brain of a horse with neurological signs in the countryside of Minas Gerais. They described it as a newly isolated strain.
Human infections with SLEV are mostly symptom-free. However, infected individuals can have mild malaise or flu-like symptoms, especially young or middle-aged patients. Severe cases can involve high fever, neurological problems, altered consciousness, and headache; which are accompanied by brain swelling or meningoencephalitis
It can be lethal in up to 30 per cent of cases, associated with direct damage to the central nervous system.
SLEV has been detected in Brazil for over 40 years.
The researchers described the horse case as a significant event.
They said their work confirmed that SLEV was the agent that caused disease and, ultimately, the death of the horse in this case.
“To our knowledge, this is the first observation that SLEV can cause disease in wild or domestic animals, which indicates that some aspects of the SLEV viral cycle and its ability to cause disease need further studies.”
They said the SLEV strain they isolated from the horse was able to induce systemic and neurological signs in mice.
Evidence based on known genotype information suggested that the sample in question likely originated from the Brazilian Amazon Region.
“The circulation of SLEV from the Amazon Region in the southeast region of Brazil suggests a possible involvement of migratory birds in disseminating the virus, since SLEV has been detected in 49 species of wild birds in Brazil, many of which are migratory.”
The researchers were Erica Azevedo Costa, Rafael Elias Marques, Taismara Simas Oliveira, Ronaldo Furtini, Maria Rosa Quaresma Bomfim, Mauro Martins Teixeira, Tatiane Alves Paixão and Renato Lima Santos.
Rosa R, Costa EA, Marques RE, Oliveira TS, Furtini R, et al. (2013) Isolation of Saint Louis Encephalitis Virus from a Horse with Neurological Disease in Brazil. PLoS Negl Trop Dis 7(11): e2537. doi:10.1371/journal.pntd.0002537
The full study can be read here.