Genomic Characterization and Seroprevalence Studies on Alphaviruses in Uruguay

Alphaviruses (Togaviridae) are arthropod-borne viruses, comprising 31 viral species and at least eight serogroups.1,2 They are enveloped, single-stranded positive RNA viruses. Their genome is 11–12 kilobases in size and encodes five structural (C, E3, E2, 6K, and E1) and four nonstructural proteins (nsP1, nsP2, nsP3, and nsP4). Alphaviruses are maintained in nature in enzootic or epizootic cycles between mosquitoes as vectors and vertebrates as amplifying hosts. Humans and other animals are usually dead-end hosts, where infections may lead to disease. Enzootic and epizootic cycles differ in the vectors and/or the vertebrate amplifying hosts involved; besides, enzootic strains may become epizootic, which is likely mediated through mutation and subsequent adaptation.3,4 In humans, most of the infections are asymptomatic and disease severity may range from a febrile illness to a more severe syndrome such as arthritis or encephalitis. Ecological, geographical, and climate changes, coupled with their intrinsic genetic variability and adaptability, often affect the ecological niche of alphaviruses, making them ideal emerging or reemerging viruses. Recent examples are the emergence of chikungunya and Mayaro viruses (MAYV) in Central and South America or the reemergence of Madariaga virus (MADV) in Central America.5–9 The earliest report on alphavirus circulation in Uruguay dates from 1970. In that study, a serological survey performed by complement fixation and hemagglutination inhibition tests for eastern equine encephalitis virus (EEEV), western equine encephalitis virus (WEEV), MAYV, and Mucambo virus (MUCV) showed antibodies only for WEEV and EEEV, ranging between 3.3–6% in children and 0.9–2% in adults.10 In 1972–1973, Argentina and Uruguay experienced equine epizooties caused by WEEV and a virus isolation from a sick horse was reported.11 Since then, no study was carried out until 2011 in Uruguay, when the first human case of a fatal encephalitis caused by WEEV was described.12 To gain insight into the actual prevalence of alphaviruses in Uruguay, we investigated the presence of alphavirus genome in mosquitoes and performed a serological survey in horses.