A-Z Index × Submit A-Z Index × Submit A-Z Index Search Dropdown × Submit Facebook Twitter LinkedIn Syndicate Emerging Infectious Disease journal ISSN: 1080-6059 Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.
During November 2023–April 2024, a total of 1,543 horses with neurologic disease from rural areas in 17 provinces of Argentina were reported to national animal health authorities. Nested reverse transcription PCR, quantitative real-time reverse transcription PCR, and sequencing of equine brain necropsies confirmed western equine encephalitis virus (WEEV) infection in 23 horses and 1 sheep. Phylogenetic analyses identified a lineage previously detected in Argentina in 1957, overlapping areas affected during the 1980s epizootics. Overall, we aspirated 1,362 female mosquitoes (12 species) from 1 of the most affected areas; Aedes (Ochlerotatus) albifasciatus (70.1%) and Ae. ( Ochlerotatus) scapularis (26.8%) mosquitoes were the most abundant species. Three of 27 mosquito pools (n = 1,089 mosquitoes) were identified as WEEV positive (2 Ae. albifasciatus , 1 Ae. scapularis ). Increased rainfall, agricultural expansion, vector proliferation, and low vaccination coverage were likely key factors contributing to the reemergence of WEEV.
Western equine encephalitis virus (WEEV) (species Alphavirus western ) was first isolated in 1930 on the West Coast of the United States ( 1 ). The virus caused equine epizootics and human epidemics in the western, central, and southern United States. WEEV is pathogenic to humans and equids, causing a febrile illness that can progress to severe neurologic disease and death ( 2 ).
WEEV activity has been reported across North America (Canada, Mexico, and the United States), Central America, and South America (Argentina, Brazil, Guyana, Paraguay, and Uruguay). Its maintenance cycle is well characterized in North America and involves passerine birds (e.g., house sparrows [ Passer domesticus ] and house finches [ Hemorhous mexicanus ]) as enzootic hosts and Culex tarsalis mosquitoes as the primary enzootic vector. WEEV can also infect various mammals, initiating an epizootic mammal–mosquito cycle ( 2 ).
During 1996–2013, enzootic and epizootic WEEV activity decreased drastically in the Americas. For example, surveillance data from California showed a decline in WEEV-positive mosquito pools and seropositive birds ( 3 ). Both human and equine cases have declined substantially over time. In North America, the last reported human case occurred in 1999; however, a single fatal case was documented in Uruguay in 2009 in an otherwise healthy 14-year-old boy ( 4 , 5 ). Equine cases followed a similar trend; the last major outbreak was reported in 1975, and only sporadic, smaller outbreaks continued into the 1990s ( 6 , 7 ).
In Argentina, WEEV was first isolated in 1933 from a sick equine in Monte Veloz, Buenos Aires Province (MV strain) ( 8 ). Subsequently, several epizootics of varying magnitudes were reported in the temperate region of the country (Buenos Aires, Córdoba, Santa Fe, Entre Ríos, and La Pampa Provinces) during 1957–1958, 1972–1973, 1982–1983, and 1988–1989 ( 8 , 9 ). A few human cases were reported only in the southern part of the epizootic area, in Viedma, Río Negro Province ( 9 ). Despite the historical effects of this virus in Argentina, its ecology remains only partially char...