Isolation of Saint Louis Encephalitis Virus from a Horse with Neurological Disease in Brazil

Abstract

St. Louis encephalitis virus (SLEV) is a causative agent of encephalitis in humans in the Western hemisphere. SLEV is a positive-sense RNA virus that belongs to the Flavivirus genus, which includes West Nile encephalitis virus, Japanese encephalitis virus, Dengue virus and other medically important viruses. Recently, we isolated a SLEV strain from the brain of a horse with neurological signs in the countryside of Minas Gerais, Brazil. The SLEV isolation was confirmed by reverse-transcription RT-PCR and sequencing of the E protein gene. Virus identity was also confirmed by indirect immunofluorescence using commercial antibodies against SLEV. To characterize this newly isolated strain in vivo, serial passages in newborn mice were performed and led to hemorrhagic manifestations associated with recruitment of inflammatory cells into the central nervous system of newborns. In summary this is the first isolation of SLEV from a horse with neurological signs in Brazil.

Serological evidence for Saint Louis encephalitis virus in free-ranging New World monkeys and horses within the upper Paraná River basin region, Southern Brazil

“…Moreover, three patients from an outbreak of SLEV in northwestern São Paulo demonstrated hemorrhagic manifestations typical of dengue fever virus 33 . Taken together, the confi rmation of SLEV in several states in Brazil 13,14,16 in addition to the Amazon region may suggest dissemination due to migratory birds 16,22 , as previously postulated. Furthermore, Culex declarator and Culex coronator are known vectors, while monkeys, sloths, armadillos, and marsupials are reservoirs of this virus in Brazil 4 .…”

“…The importance of these results lies primarily in the observation of SLEV circulation in a completely new habitat within Brazil, considering that most cases of SLEV have been described within the Amazon region 4,[20][21][22] . However, SLEV was recently isolated from a horse in the State of Minas Gerais 16 , and SLEV has been serologically identifi ed in both horses from Corumba, Central West Brazil 17 and in patients from the State of São Paulo 14 . These results suggest a southern drift of SLEV from the Amazon region to other parts of continental Brazil, most likely attributable to migratory birds 16,21,22 .…”

“…First, the elevated endemicity of other closely related fl aviviruses that elicit cross-protection in humans, such as that occurring with dengue and yellow fever immunization, might easily result in the underdiagnosis of SLEV; this occurred in a study of 519 patients who were initially diagnosed as having dengue fever, but later molecular investigation confi rmed the presence of SLEV in eight of them 14 . Secondly, the diffi culty in effi ciently recognizing and/or diagnosing SLEV encephalitis in patients at both public and private health services might also contribute to the reduced number of cases in Brazil, considering that patients may either be asymptomatic 4,16 or present with fl u-like disease syndromes that can progress to acute or subacute meningeal and focal neurological manifestations 32 . Moreover, three patients from an outbreak of SLEV in northwestern São Paulo demonstrated hemorrhagic manifestations typical of dengue fever virus 33 .…”

“…In Brazil, SLEV was likely fi rst isolated in the 1960s from a pool of Sabethes belisarioi mosquitoes captured on the Belém-Brasília Highway 12 . Since then, this virus has been identifi ed predominantly in humans [13][14][15] and horses [16][17][18] from Brazil. Serological confi rmation of SLEV in wildlife is very rare; it has been described in the white-tailed deer in the USA 19 , while seropositivity has been demonstrated in wild and sentinel animals and arthropods from both the Amazon region 20,21 and the State of São Paulo 22 , Brazil.…”

A Saint Louis encephalitis and Rocio virus serosurvey in Brazilian horses

“…Interestingly, SLEV circulation may have coincided with human outbreaks of dengue-like diseases in São José do Rio Preto 9-10 and with outbreaks in horses from Minas Gerais state 13 . Furthermore, a case was reported recently in Ribeirão Preto City, São Paulo 23 .…”