Other Neglected Mosquito-Borne Flaviviruses: Ilhéus, Bussuquara, Rocio, Kokobera, Stratford, and Wesselsbron Viruses

History of ILHV

In 1944, ILHV was isolated from Aedes and Psorophora species mosquitoes in Bahia, on the eastern coast of Brazil. Its identification was later confirmed by viral isolation from female Aedes scapularis in the wetlands of the Pantanal in center-west Brazil. It is also found in Culex , Sabethes , Haemagogus , and Trichoprosopon mosquitoes as well as some bird species in Latin America. Neutralizing antibodies against ILHV may be found in sentinel monkeys, birds, and horses. In a recent serosurvey, 10.3%–10.7% of the tested horses from two different subregions of the Pantanal wetlands ( n = 760) were seropositive for ILHV, 7.8% for St. Louis encephalitis virus (SLEV), and 3.2% for West Nile virus (WNV). No tested sheep ( n = 283) were infected with ILHV.

ILHV-Association Disease

ILHV only sporadically infects humans and has not caused any epidemics. Consequences of infection range from asymptomatic to a severe febrile, encephalitis-like syndrome. Mild cases involve gastrointestinal or respiratory symptoms of approximately 1-week duration. The central nervous system (CNS) and the cardiovascular system may be involved in severe cases, but no long-term sequelae or deaths have been reported in humans.

Typically, patients with ILHV develop mild febrile illness with malaise; headache; muscle, bone, and joint pain; and photophobia. Infection by ILHV may be mistaken for infection by other flaviviruses that are associated with severe disease, including dengue virus (DENV), SLEV, and yellow fever virus (YFV), with which it cross-reacts in serological assays. Other symptoms of ILHV infection of humans include vesicular rash, sore throat, retroocular pain, conjunctival injection, facial swelling, earache, nausea and vomiting, jaundice, and abdominal pain. In the case of a 15-year-old farmer from northern Bolivia, no cardiac, neurologic, or renal pathology was present and he fully recovered soon afterward. The region where the disease was contracted is near the border of Brazil and is surrounded by rivers and chestnut fields. Fishing and agriculture are the primary sources of income in the area.

Ilhéus and Bussuquara Viruses

Based upon a 600-nucleotide sequence of the NS5 gene, ILHV has been classified as a member of the Japanese encephalitis virus (JEV) complex of mosquito-borne flaviviruses, which include Bussuquara, Rocio, Cacipacore, and Iguape viruses. Several of these have only been detected in South America. ILHV is most closely related to Rocio virus (ROCV). While Ilhéus, Rocio, and Bussuquara viruses are the causative agents of acute febrile illness in humans, the other above listed viruses are not currently known to cause human disease.

Bussuquara virus-infected murine peritoneal macrophages develop a flattened shape with a high number of large spikes of cytoplasm prolongations within 3 days of infection. This is also found in YFV-, ROCV-, and SLEV-infected cells. Enlarged, hypertrophic rough and smooth endoplasmic reticula are abundant as well as many free, cytoplasmic ribosomes. Spherical objects are present in the cytoplasm, usually found inside vesicles. These structures may represent viral particles.

ILHV is primarily maintained in a sylvatic, enzootic cycle between birds and mosquitoes, especially Psorophora ferox mosquitoes, but has also been reported in Psorophora lutzii , Psorophora varipes , Sabethes chloropterus , Haemagogus spegazzinii , and Trichoprosopon mosquitoes. Two strains of ILHV have been isolated from the double-collared seedeater and the shiny cowbird in Brazil as well. Several other bird species are seropositive for ILHV, including the ruddy ground dove, the diamond dove, and the saffron finch.

Several tested species of nonhuman primates in Northeast Brazil produce neutralizing antibodies against ILHV, including marmosets ( Callithrix jacchus and Callithrix penicillata ), coati ( Nasua nasua ), and black-striped ( Sapajus libidinosus ) and blond ( Sapajus flavius ) capuchin monkeys. In addition to potential public health concerns, the latter findings have implication for animal conservation, since S . flavius is a critically endangered species and the area of collection was near the epicenter of the concurrent Zika virus (ZIKV) outbreak in mid-2015–January 2016. A 2017 report detected low incidence of ILHV- and virus-specific neutralizing antibodies in black howler monkeys ( Alouatta caraya ) in Argentina, 1 of 108 tested monkeys. Bussuquara virus-specific antibodies were detected in 1 of 189 Argentine military recruits. ILHV- and Bussuquara-specific hemagglutination-inhibiting antibodies have also been in 7.4% and 0.3% of water buffaloes ( Bubalus bubalis ), respectively, in the Brazilian Amazon ( n = 654).

Vaccination—ILHV

Intraperitoneal infection of 6-week-old C57BL/6 mice with ILHV induces protective cross-reactive neutralizing antibodies against challenge with a lethal dose of ROCV with a 100% survival rate. Prior infection of the mice with SLEV also produces cross-protective immunity, although it is less effective than that induced by ILHV. Other Brazilian flaviviruses, including all four serotypes of DENV, YFV, and ZIKV, do not protect against ROCV. ROCV is more closely related to ILHV and SLEV than other flaviviruses, having high amino acid sequence similarities. Since ROCV can cause severe encephalitis in humans, immunization with attenuated ILHV has potential for vaccination against ROCV.

History of Rocio Virus

From 1975 to 1980, ROCV caused severe outbreaks of encephalitis in at least 23 municipalities in the Ribeira Valley along the southeastern coast of Brazil. Retrospective serological studies indicate that ROCV encephalitis had previously been present in seven febrile patients during 1973 and 1974 as well. Presently, a total of over 1000 ROCV encephalitis cases have been reported and are known to have resulted in approximately 100 deaths. Over 200 additional people developed serious sequelae. A large epidemic occurred in 1975–76 in São Paulo, the current Brazilian capital, affecting 465 people with 61 deaths (fatality rate of 13%). ROCV was isolated from 10 of the patients, but only from those who died within 5 days of disease onset. The greatest numbers of cases were in those aged 15–30 years. Young males were also disproportionally affected. No further cases of ROCV infection have been reported since 1980, despite the presence of seropositive humans and animals in different regions of Brazil, including eight people in Bahia who had either IgG or IgM antibodies and from two children in the Ribeira Valley. The presence of IgM is indicative of recent infection. Since Bahia is 1200 km from São Paulo, ROCV has a large range and has been proposed to be spread by migratory birds. Further outbreaks may occur in humans due to environmental modifications, such as conversion of forest into agricultural areas, which bring humans into greater contact with the mosquito vector and reservoir hosts.