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Revista da Sociedade Brasileira de Medicina Tropical

Print version ISSN 0037-8682

Rev. Soc. Bras. Med. Trop. vol.47 no.3 Uberaba May/June 2014 

Major Articles

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

Walfrido Kühl Svoboda[1] 

Lívia Carício Martins[2] 

Luciano de Souza Malanski[3] 

Marcos Massaaki Shiozawa[4] 

Kledir Anderson Hofstaetter Spohr[5] 

Carmen Lúcia Scortecci Hilst[6] 

Lucas M. Aguiar[7] 

Gabriela Ludwig[8] 

Fernando de Camargo Passos[9] 

Lineu Roberto da Silva[10] 

Selwyn Arlington Headley[11] 

Italmar Teodorico Navarro[11] 

[1]Universidade Federal da Integração Latino-Americana (UNILA), Instituto Latino-Americano de Ciências da Vida e da Natureza (ILACVN), Foz do Iguaçu, PR, Brasil

[2]Departamento de Arbovirologia e Febres Hemorrágicas, Instituto Evandro Chagas, Ananindeua, PA

[3]Instituto Chico Mendes de Conservação da Biodiversidade, Porto Velho, RO

[4]Escola de Medicina Veterinária, Universidade Norte do Paraná, Arapongas, PR

[5]Escola de Medicina Veterinária, Universidade de Cuiabá, Cuiabá, MT

[6]Departamento de Clínica Veterinária, Universidade Estadual de Londrina, Londrina, PR

[7]Universidade Federal da Integração Latino-Americana, Foz do Iguaçu, PR

[8]Centro Nacional de Pesquisa e Conservação de Primatas Brasileiros, Instituto Chico Mendes de Conservação da Biodiversidade, João Pessoa, PB

[9]Departamento de Zoologia, Universidade Federal do Paraná, Curitiba, PR

[10]Secretaria da Saúde do Paraná, Curitiba, PR

[11]Departamento de Medicina Veterinária Preventiva, Universidade Estadual de Londrina, Londrina, PR



Saint Louis encephalitis virus (SLEV) primarily occurs in the Americas and produces disease predominantly in humans. This study investigated the serological presence of SLEV in nonhuman primates and horses from southern Brazil.


From June 2004 to December 2005, sera from 133 monkeys (Alouatta caraya, n=43; Sapajus nigritus, n=64; Sapajus cay, n=26) trap-captured at the Paraná River basin region and 23 blood samples from farm horses were obtained and used for the serological detection of a panel of 19 arboviruses. All samples were analyzed in a hemagglutination inhibition (HI) assay; positive monkey samples were confirmed in a mouse neutralization test (MNT). Additionally, all blood samples were inoculated into C6/36 cell culture for viral isolation.


Positive seroreactivity was only observed for SLEV. A prevalence of SLEV antibodies in sera was detected in Alouatta caraya (11.6%; 5/43), Sapajus nigritus (12.5%; 8/64), and S. cay (30.8%; 8/26) monkeys with the HI assay. Of the monkeys, 2.3% (1/42) of A. caraya, 6.3% 94/64) of S. nigritus, and 15.4% (4/26) of S. cay were positive for SLEV in the MNT. Additionally, SLEV antibodies were detected by HI in 39.1% (9/23) of the horses evaluated in this study. Arboviruses were not isolated from any blood sample.


These results confirmed the presence of SLEV in nonhuman primates and horses from southern Brazil. These findings most likely represent the first detection of this virus in nonhuman primates beyond the Amazon region. The detection of SLEV in animals within a geographical region distant from the Amazon basin suggests that there may be widespread and undiagnosed dissemination of this disease in Brazil.

Key words: Saint Louis encephalitis; Serology; New World monkeys; Horses; Arboviruses


Saint Louis encephalitis virus (SLEV) belongs to the genus Flavivirus, family Flaviviridae, which consists of approximately 70 virus species and subspecies distributed worldwide1. Most flaviviruses are transmitted between susceptible vertebrates by hematophagous arthropods, in particular mosquitos1,2. Flaviviruses are the most import causes of infectious diseases in humans from Brazil3; these viruses include Bussuquara, Cacipacore, dengue (serotypes 1, 2, 3, and 4), Iguape, Ilhéus, Rocio, Saint Louis encephalitis, and yellow fever3,4.

SLEV may have initially originated in Central America5, but it has now disseminated throughout the Americas, with reports of its presence in the USA6,7, Canada7, Argentina8,9, Uruguay10, and Trinidad11. In Brazil, SLEV was likely first isolated in the 1960s from a pool of Sabethes belisarioi mosquitoes captured on the Belém-Brasília Highway12. Since then, this virus has been identified predominantly in humans1315 and horses1618 from Brazil.

Serological confirmation of SLEV in wildlife is very rare; it has been described in the white-tailed deer in the USA19, while seropositivity has been demonstrated in wild and sentinel animals and arthropods from both the Amazon region20,21 and the State of São Paulo22, Brazil. Moreover, at least three epizootic incidents of SLEV have occurred in nonhuman primates, including sentinel Cebus monkeys in the Brazilian Amazon region21. Additionally, a serological survey conducted in French Guiana detected low levels of SLEV antibodies in free-ranging primates23.

This study presents the findings of a serological investigation of SLEV in free-ranging New World monkeys and farm horses from southern Brazil.


Study location

All monkeys used in this study were trap-captured within the Porto Rico County region, located between the northwestern region of the State of Paraná and the southeast region of the State of Mato Grosso do Sul, on the upper Paraná River in Brazil (Figure 1). This region consists of islands and sub-tropical forest reserves (where the animals were captured) that are protected by the Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis (IBAMA). The riparian forests of the islands of Mutum, Porto Rico, Gaivota, and Japonesa and the forests on the opposite shore of the Paraná River were included in this study. Porto Rico County is located in the northwestern region of the State of Paraná (22°46′20″S latitude and 53°16′01″W-GR longitude). This sub-tropical region has an average annual rainfall of 1,200-1,300mm and temperatures ranging from 16 to 29°C.

FIGURE 1 -  Google earth images illustrating the region where the research was conducted. The yellow pins indicate the coordinates where monkeys seropositive for Saint Louis encephalitis virus were trap-captured.  

Nonhuman primates

One hundred thirty-three nonhuman primates [43 Alouatta caraya (black and golden howler monkey), 64 Sapajus nigritus, and 26 S. cay (both called capuchin monkeys)] were trap-captured as previously described24. The biological data for the captured nonhuman primates are given in Table 1; all primates were captured between June 2004 and December 2005 by a team of biologists and veterinarians with permission from IBAMA (license number 104/04). All primates were anesthetized25, after which blood samples were obtained by jugular or brachial venipuncture, and sera were centrifuged (±1,000g) and stored at -196°C until used. All trapped animals were released after complete recovery at the capture location.

TABLE 1 - The species, sex, and age distributions of free-ranging New World monkeys captured within the Porto Rico County region, Southern Brazil. 

Monkey species Female (F) Male (M) Total (F, M)
infant juvenile sub-adult adult infant juvenile sub-adult Adult
Alouatta caraya 1 3 16 5 6 12 43 (F=20, M=23)
Sapajus nigritus 4 2 13 5 16 3 21 64 (F=19, M=45)
Sapajus cay 5 2 4 1 2 2 10 26 (F=11, M=15)
Total 1 12 4 33 6 23 11 43 133

All captured primates were strict forest inhabitants. However, a number of S. nigritus specimens (n=13) were trapped in a fo rest reserve (within the Paraná River basin) close to a farm whose owner reportedly had frequent contact with monkeys.


Blood samples were obtained by jugular venipuncture from 23 mixed-breed adult horses (16 males and 7 females) located in proximity to where the S. cay monkeys were captured. All sampled horses were located on the same property and were representative of the total horse population maintained at this holding. Serum samples were obtained and stored at -196°C until they were used.

Serological assays

All serological assays were performed at the Evandro Chagas Institute (IEC-PA), Department of Arbovirology and Hemorrhagic Fevers, Belém, PA, Brazil. All samples were stored on dry ice and then air-shipped to Belém.

Hemagglutination inhibition test

All samples were initially subjected to a microplate hemagglutination inhibition (HI) test26 against a panel of standardized antigens for 19 arboviruses, including four from the genus Alphavirus (eastern equine encephalomyelitis, western equine encephalomyelitis, Mayaro, and Mucambo), six Flavivirus (yellow fever, Saint Louis encephalitis, Rocio, Ilhéus, Cacipacoré and Bussuquara), eight Orthobunyavirus (Oropouche, Caraparu, Catú, Guaroa, Maguari, Tacaiuma, Utinga and Belém) and one from the genus Phlebovirus (Icoaraci). Animals were considered seropositive when a titer of ≥ 20 was observed with the HI assay.

Mouse neutralization test

All positive monkey sera with HI titers >20 were subjected to a mouse neutralization test (MNT), a confirmatory assay that was used to characterize the virus associated with the infection and performed according to a previously described protocol26. The results were calculated using the neutralization logarithmic index (NLI)27. Sera from horses were not evaluated by MNT.

Virus isolation

All blood samples were inoculated into Aedes albopictus cell culture (clone C6/36); immunofluorescence was used for viral identification28.

Statistical analysis

Statistical significance was analyzed using the Chi-square test (Yates corrected) to establish differences between the characteristics evaluated (species, gender, age, and the presence of horses). Associations between variables and positivity were determined by odds ratios (ORs) with 95% confidence levels. The results were considered statistically significant when the ρ-value was 5%.

Ethical considerations

This study was approved by the Animal Experimental Ethics Committee, Universidade Estadual de Londrina (Protocol number 34/05).


Serological assay and virus isolation from nonhuman primates

According to the HI assay, SLEV antibodies were present in primates, specifically in 11.6% (5/43) of A. caraya, 12.5% (8/64) of S. nigritus, and 30.8% (8/26) of S. cay animals (Table 2). However, confirmation of SLEV infection by MNT was obtained for only 2.3% (1/43) of A. caraya, 6.3% (4/64) of S. nigritus and 15.4% (4/26) of S. cay primates (Table 2). Arboviruses were not isolated from any of the 133 primate-derived blood samples, although SLEV antibodies were detected by HI in 15.8% (21/133) of serum samples, with HI titers to SLEV ranging from 20 to 640 (Table 2 and Figure 2). Negative seroreactivity was observed for all other arboviruses.

TABLE 2 - Results of hemagglutination inhibition assays and mouse neutralization tests for free-ranging New World monkeys captured within the Porto Rico County region, Southern Brazil. 

HI result*(antibody titer) Alouatta caraya Sapajus nigritus Sapajus cay Total (male/female)
male female male female male female
Negative 21 17 40 16 10 8 112 (71/41)
20 2 2 1 2 2 9 (4/5)
40 2 1(2.7) 1 2(1.8/2.7) 6 (5/1)
80 1(1.9) 1(2.1) 1(2.7) 3 (1/2)
160 1(2.8) 1 (—/1)
640 1(3.5) 1(3.1) 2 (2/—)
Total 23 20 45 19 15 11 133 (83/50)

HI: hemagglutination inhibition.

*HI test results: positive HI ≥ 20;

(n)neutralization logarithmic index (NLI): positive NLI ≥ 1.8.

FIGURE 2 -  Relationships between the total numbers of captured monkeys (by species) and positive results in the hemagglutination inhibition (HI) and mouse neutralization test (MNT) assays.  

The association of the results obtained based on NMT analysis of characteristics evaluated (species, sex, age, and the presence of horses within the same habitat) with the neutralization logarithm index (NLI) is summarized in Table 3. The prevalence of anti-SLEV antibodies was elevated in S. cay (15.4%; 4/26) relative to S. nigritus (6.3%; 4/64) and A. caraya (2.3%; 1/43) primates. Interestingly, negative seroreactivity of HI to SLEV antibodies was observed in infant and juvenile primates, while SLEV antibody serum prevalence was identified in sub-adult (13.3%; 2/15) and adult (9.2%; 7/76) animals, most likely due to their more extensive contact with this pathogen. Additionally, primates that live within proximity to farm horses demonstrated elevated seroreactivity to SLEV antibodies (15.4%; 4/26) compared to those living without potential contact with horses (4.8%; 5/107). However, no significant differences were observed based on species (ρ=0.1090), sex (ρ=0.7281), age (ρ=0.1876), and the presence of horses within the same habitat (ρ=0.0727).

TABLE 3 - Associations between the characteristics studied (species, sex, age, and the presence of horses within the same habitat) and the presence of anti-Saint Louis encephalitis virus antibodies (neutralization logarithm index) in serum samples of free-ranging New World Monkeys. 

Variables Species Neutralization logarithmic index test ORa ρ-values
positive negative total
n % n % n %
Alouatta caraya 1 2.32 42 97.68 43 32.33 Ac x Sn 0.36 (0.01-3.6) 0.63
Sapajus nigritus 4 6.25 60 93.75 64 48.12 Ac x Sc 0.13 (0.01-1.38) 0.12
Sapajus cay 4 15.38 22 84.61 26 19.55 Sn x Sc 0.37 (0.07-1.95) 0.38
 male 5 6.02 78 93.98 83 62.41 0.74 0.72c
 female 4 8.00 46 92.00 50 37.59 0.16 < OR < 3.48
 infant 7 100.00 7 5.27
 juvenile 35 100.00 35 26.31 NC 0.18b
 sub-adult 2 13.33 13 86.67 15 11.28
 adult 7 9.21 69 90.79 76 57.14
Presence of horses in the same habitat
 yes 4 15.38 22 84.62 26 19.55 3.71 0.07c
 no 5 4.67 102 95.33 107 80.45 0.76 < OR < 17.79
 Total (%) 9 6.77 124 93.23 133 100.00

Ac: Alouatta caraya; Sn: Sapajus nigritus; Sc: Sapajus cay; NC: not calculated; ORa: odds ratio (inferior and superior limits);


cFisher's exact test.

Serological assay and virus isolation from horses

The overall seroprevalence of SLEV antibodies in horses was 39.1% (9/23) and was elevated in females (57.1%; 4/7) relative to male horses (31.3%; 5/16). SLEV antibody titers in seropositive horses were as follows: 160 (three males; two females), 320 (one male and female) and 640 (one male and female). However, significant differences were not observed based on sex [ρ=0.3630; OR = 0.34 (0.04 < OR < 2.91)], and these results were not confirmed with the MNT assay. Additionally, arboviruses were not isolated from any of the 23 horse-derived blood samples, but as previously observed in primates, specific seropositivity only to SLEV antibodies was observed with the HI assay.


The results of this study reveal that we have identified the presence of SLEV in several species of nonhuman primates and horses from southern Brazil. The classical HI and MNT methodologies used during this investigation are based on the conventional serologic diagnosis of flavivirus due to the presence of virus-specific antibodies in the serum. Similar serological strategies have been used to detect SLEV in primates23, white-tailed deer19,29, livestock11, mules19, and horses8,10,11,17,18. 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 region4,2022. However, SLEV was recently isolated from a horse in the State of Minas Gerais16, and SLEV has been serologically identified in both horses from Corumba, Central West Brazil17 and in patients from the State of São Paulo14. These results suggest a southern drift of SLEV from the Amazon region to other parts of continental Brazil, most likely attributable to migratory birds16,21,22.

In the USA, SLEV causes encephalitis in approximately 100 human cases annually30 and is one of the most common causes of arbovirus-induced disease31, with sporadic epidemics6,30,31. However, this arboviral disease has different epidemiological features in Brazil, largely in terms of the relatively few cases of SLEV-induced encephalitis identified in humans20,22. Additionally, most flaviviruses that occur in Brazil, with the exception of dengue virus, are predominantly maintained as sylvatic zoonotic diseases that occasionally produce infections in humans and domestic animals that have entered the ecosystems where these viruses occur4. Consequently, it can be speculated that this epidemiological difference might possibly be attributable to specific environmental and biological conditions within Brazil that alter the virulence or pathogenicity of SLEV in humans. Although the factors that are actively or otherwise associated with this phenomenon have not been fully elucidated, at least two theories should be considered. First, the elevated endemicity of other closely related flaviviruses 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 confirmed the presence of SLEV in eight of them14. Secondly, the difficulty in efficiently 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 asymptomatic4,16 or present with flu-like disease syndromes that can progress to acute or subacute meningeal and focal neurological manifestations32. Moreover, three patients from an outbreak of SLEV in northwestern São Paulo demonstrated hemorrhagic manifestations typical of dengue fever virus33. Taken together, the confirmation of SLEV in several states in Brazil13,14,16 in addition to the Amazon region may suggest dissemination due to migratory birds16,22, as previously postulated. Furthermore, Culex declarator and Culex coronator are known vectors, while monkeys, sloths, armadillos, and marsupials are reservoirs of this virus in Brazil4. Therefore, the seropositivity of nonhuman primates and horses to SLEV demonstrated in this study in a region geographically distant from the Amazon region is of great concern, and additional investigation must be conducted to understand the dynamics associated with this virus in a new environment but in conventional hosts.

During this investigation, the majority (57.1%; 12/21) of seropositive monkeys demonstrated titers of SLEV according to HI that were ≥ 40, suggesting that these antibodies are attributable to the presence of SLEV and not cross-reactivity from contact with similar related viruses. Alternatively, in a seroepidemiological survey conducted in French Guiana, primates had low antibody titers to SLEV (HI <40) but elevated titers to yellow fever (HI >320), and the authors suggested that that SLEV antibodies identified could have been due to cross-reactions with yellow fever virus23. Low titers of SLEV were also identified in horses, livestock, and wildlife from the island of Trinidad11. However, during this investigation, seropositivity was only identified for SLEV, and negative results were obtained with all other viruses by HI, indicating that there was no cross-reaction with any similarly related virus.

In Argentina, SLEV studies have focused primarily on populations of mosquitoes34,35, humans36, and horses8, but the importance of monkeys in the sylvatic SLEV cycle has not been investigated. However, during this study, serological results confirmed the participation of nonhuman primates in the maintenance cycle of SLEV in southern Brazil. Consequently, these results most likely represent the first identification of SLEV in nonhuman primates beyond the Amazon region of Brazil.

Horses are commonly found in the region where sample collection was performed, largely on farms that are close to forested areas, and the relationships contributing to the presence of SLEV antibodies in these animals should be considered. Elevated prevalence rates of SLEV antibodies in horses from the Amazon18 and Pantanal17,18 regions have been described, but this report is the first description of SLEV in southern Brazil within an ecosystem that is very distant and quite different from the Amazon and Pantanal regions. Therefore, these results suggest that horses may participate as vertebrate hosts in the dissemination of SLEV and most likely should be considered amplifying sources of SLEV for primates, or vice versa. However, additional investigations must be performed to confirm this theory.

This report is the first describing the seroprevalence of specific SLEV antibodies in free-ranging monkeys within the State of Paraná, southern Brazil, and implicates nonhuman primates in the natural maintenance cycle of SLEV in the Southern Cone region, where an SLEV encephalitis outbreak was identified in northern Argentina35. Future research will aim to identify local mosquito and human populations to establish their roles in the SLEV life cycle.


We are grateful to Mr. Basílio S. Buna, Geraldo M. da Silva, and Luiz R.O. Costa of the Evandro Chagas Institute (IEC-PA), Department of Arbovirology and Hemorrhagic Fevers, for performing all serological assays. Our gratitude is also extended to the Entomology Team of SESA-PR (Porto Rico-PR) for technical support during sample collection and to the Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis (IBAMA) for permission to capture primates for this experiment. We are also grateful to Dr. João Luis Garcia, Universidade Estadual de Londrina, for assistance with the statistical analysis.


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Secretaria Estadual de Saúde do Paraná (SESA-PR), Secretaria de Estado da Ciência, Tecnologia e Ensino Superior (SETI-PR), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Gabriela Ludwig has a temporary scholarship from Centro Nacional de Pesquisa e Conservação de Primatas Brasileiros, Instituto Chico Mendes de Conservação da Biodiversidade, Projeto Nacional de Ações Integradas Público-Privadas para Biodiversidade – Fundação de Empreendimentos Pesquisa e Desenvolvimento.

Received: April 18, 2014; Accepted: June 30, 2014

Address to: Dr. Selwyn Arlington Headley. Depto de Medicina Veterinária Preventiva/UEL. Rodovia Celso Garcia Cid, PR 445, km 380, Campus Universitário, Caixa Postal 10.001, 86057-970 Londrina, PR, Brasil. Phone/Fax: 55 43 3371-4485. e-mail:

CONFLICT OF INTEREST The authors declare that there is no conflict of interest.

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