Epidemic Venezuelan equine encephalitis in North America in 1971: vector studies.
Abstract: A major epidemic of Venezuelan equine encephalitis occurred in south Texas in the summer of 1971. More than 1500 equines died of VEE in Texas, and 110 human cases with no deaths were reported. Vector studies in south Texas and northern Tamaulipas revealed that the overall mosquito infection rates during the peak of the epidemic were about 1:100, one of the highest rates observed for a major epidemic. Mosquito infection rates of this magnitude could easily explain the intensity of VEE outbreaks in both equines and man. A total of 943 VEE virus isolations were made from mosquitoes. Eight of the 12 mosquito species found infected were implicated in the epidemic cycle of VEE for the first time. Sufficient laboratory and field evidence is available to prove that Psorophora confinnis was one of the primary vectors of VEE. The lack of laboratory evidence necessitates the use of the term "probable" primary vectors for other species apparently equally as involved on the basis of field infections; these include Aedes sollicitans, Aedes thelcter and Psorophora discolor. Eight other species from which less than 10 VEE virus isolations were made were considered auxiliary vectors. Mosquitoes of some species were tested individually; such tests showed 2-4% of the probable primary vectors to be infected. The first isolation of VEE virus of the epidemic was made from P. confinnis on June 28, 1971. Highest mosquito infection rates occurred during the week of July 5. Mosquito infection rates declined precipitously in the last 3 weeks of July 1971, signaling the end of the epidemic in the study area. One explanation for the decline was that equines, the principal epidemic hosts, were eliminated as a source of virus by death or by acquisition of natural or induced immunity. Mosquito control appeared to be effective in reducing the infected mosquito population while the immunization of equines with TC 83 VEE vaccine was accomplished. Quarantines appeared to be effective in restricting the VEE virus activity to south Texas. Undoubtedly all of the control measures contributed to stopping the epidemic. Continued VEE surveillance by various government and other agencies failed to reveal any further epidemic VEE activity in the US in 1972. Other arboviruses isolated during the VEE studies in south Texas included St. Louis encephalitis virus, and San Angelo subtype of the California Group. A virus of the Bunyamwera Group was also isolated from Palo Blanco, Tamaulipas.
Publication Date: 1975-01-01 PubMed ID: 235212DOI: 10.1093/oxfordjournals.aje.a112068Google Scholar: Lookup
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- Journal Article
- Arboviruses
- Diagnosis
- Disease control
- Disease Diagnosis
- Disease Etiology
- Disease Management
- Disease Outbreaks
- Disease Prevalence
- Disease Prevention
- Disease Surveillance
- Disease Treatment
- Encephalitis
- Encephalomyelitis
- Epidemiology
- Equine Diseases
- Equine Health
- Horses
- Mosquito-borne Diseases
- Public Health
- Vector-borne disease
- Veterinary Medicine
Summary
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This research examines an epidemic of Venezuelan equine encephalitis (VEE) that occurred in South Texas in 1971, impacting over 1500 equines and 110 humans. The study aims to understand the mosquito vectors involved in this massive outbreak and concludes that Psorophora confinnis was one of the primary vectors.
Context of the Study
- The research was centered around the VEE epidemic that occurred during the summer of 1971, mostly impacting south Texas.
- The epidemic was significant – the primary victims were equines with over 1500 deaths, but there were also 110 reported human cases.
- The key point of interest for this study was the mosquito infection rates which were among the highest ever observed during an epidemic, approximately 1:100.
Vector Identification
- A total of 943 isolations of the VEE virus were made from mosquitoes, implicating particular species in facilitating the epidemic.
- Eight out of the 12 mosquito species found infected were implicated in the epidemic cycle of VEE for the first time.
- Among these, Psorophora confinnis was identified as one of the primary vectors of VEE.
- Other species, including Aedes sollicitans, Aedes thelcter, and Psorophora discolor were deemed “probable” primary vectors based on field infections.
Temporal Dynamics of the Epidemic
- The investigation revealed that the first isolation of the VEE virus of the epidemic was made from Psorophora confinnis on June 28, 1971.
- Highest mosquito infection rates occurred during the week of July 5.
- Interestingly, mosquito infection rates showed a steep decline in the last three weeks of July 1971, signalling the end of the epidemic.
Explanations for Decline and Control Measures
- The authors suggest one possible explanation for the decline in the epidemic could be the result of equines (the primary hosts of this outbreak) either dying off or developing natural or induced immunity to the virus.
- They also found that mosquito control efforts were effective in reducing the infected mosquito population.
- Immunization of equines with a VEE vaccine (TC 83) and the implementation of quarantines contributed to controlling the epidemic effectively.
Post-Epidemic Surveillance
- Despite the end of the epidemic, continued surveillance by various authorities found no more VEE activity in the US the following year.
- However, during the VEE surveys in south Texas, other arboviruses were detected, including St. Louis encephalitis virus, San Angelo subtype of the California Group, and a virus from the Bunyamwera Group in Palo Blanco, Tamaulipas.
Cite This Article
APA
Sudia WD, Newhouse VF, Beadle ID, Miller DL, Johnston JG, Young R, Calisher CH, Maness K.
(1975).
Epidemic Venezuelan equine encephalitis in North America in 1971: vector studies.
Am J Epidemiol, 101(1), 17-35.
https://doi.org/10.1093/oxfordjournals.aje.a112068 Publication
Researcher Affiliations
MeSH Terms
- Animals
- Culicidae / microbiology
- Encephalitis Virus, Venezuelan Equine / immunology
- Encephalitis Virus, Venezuelan Equine / isolation & purification
- Encephalomyelitis, Equine / epidemiology
- Encephalomyelitis, Equine / transmission
- Encephalomyelitis, Equine / veterinary
- Geography
- Horse Diseases / epidemiology
- Horses
- Humans
- Insect Vectors
- Mexico
- Mice
- Mosquito Control
- Serotyping
- Virulence
Citations
This article has been cited 22 times.- Dhawan S, Pan-Ngum W, MacIntyre CR, Blacksell SD. Epidemiological indicators of accidental laboratory-origin outbreaks. Epidemiol Infect 2026 Jan 2;154:e16.
- Dye-Braumuller KC, Prisco RA, Nolan MS. (Re)Emerging Arboviruses of Public Health Significance in the Brazilian Amazon. Microorganisms 2025 Mar 12;13(3).
- Haines CA, Campos RK, Azar SR, Warmbrod KL, Kautz TF, Forrester NL, Rossi SL. Venezuelan Equine Encephalitis Virus V3526 Vaccine RNA-Dependent RNA Polymerase Mutants Increase Vaccine Safety Through Restricted Tissue Tropism in a Murine Model. Zoonoses 2022;2.
- Azar SR, Campos RK, Bergren NA, Camargos VN, Rossi SL. Epidemic Alphaviruses: Ecology, Emergence and Outbreaks. Microorganisms 2020 Aug 1;8(8).
- McLean RG. Letter to the Editor: Venezuelan Equine Encephalitis virus 1B Invasion and Epidemic Control-South Texas, 1971. Trop Med Infect Dis 2020 Jun 22;5(2).
- Dávalos-Becerril E, Correa-Morales F, González-Acosta C, Santos-Luna R, Peralta-Rodríguez J, Pérez-Rentería C, Ordoñez-Álvarez J, Huerta H, Carmona-Perez M, Díaz-Quiñonez JA, Mejía-Guevara MD, Sánchez-Tejeda G, Kuri-Morales P, González-Roldán JF, Moreno-García M. Urban and semi-urban mosquitoes of Mexico City: A risk for endemic mosquito-borne disease transmission. PLoS One 2019;14(3):e0212987.
- Kautz TF, Guerbois M, Khanipov K, Patterson EI, Langsjoen RM, Yun R, Warmbrod KL, Fofanov Y, Weaver SC, Forrester NL. Low-fidelity Venezuelan equine encephalitis virus polymerase mutants to improve live-attenuated vaccine safety and efficacy. Virus Evol 2018 Jan;4(1):vey004.
- Forrester NL, Wertheim JO, Dugan VG, Auguste AJ, Lin D, Adams AP, Chen R, Gorchakov R, Leal G, Estrada-Franco JG, Pandya J, Halpin RA, Hari K, Jain R, Stockwell TB, Das SR, Wentworth DE, Smith MD, Kosakovsky Pond SL, Weaver SC. Evolution and spread of Venezuelan equine encephalitis complex alphavirus in the Americas. PLoS Negl Trop Dis 2017 Aug;11(8):e0005693.
- Rey JR, Walton WE, Wolfe RJ, Connelly CR, O'Connell SM, Berg J, Sakolsky-Hoopes GE, Laderman AD. North American wetlands and mosquito control. Int J Environ Res Public Health 2012 Dec 10;9(12):4537-605.
- Adams AP, Navarro-Lopez R, Ramirez-Aguilar FJ, Lopez-Gonzalez I, Leal G, Flores-Mayorga JM, Travassos da Rosa AP, Saxton-Shaw KD, Singh AJ, Borland EM, Powers AM, Tesh RB, Weaver SC, Estrada-Franco JG. Venezuelan equine encephalitis virus activity in the Gulf Coast region of Mexico, 2003-2010. PLoS Negl Trop Dis 2012;6(11):e1875.
- Rülker T, Voß L, Thullier P, O' Brien LM, Pelat T, Perkins SD, Langermann C, Schirrmann T, Dübel S, Marschall HJ, Hust M, Hülseweh B. Isolation and characterisation of a human-like antibody fragment (scFv) that inactivates VEEV in vitro and in vivo. PLoS One 2012;7(5):e37242.
- Deardorff ER, Estrada-Franco JG, Freier JE, Navarro-Lopez R, Travassos Da Rosa A, Tesh RB, Weaver SC. Candidate vectors and rodent hosts of Venezuelan equine encephalitis virus, Chiapas, 2006-2007. Am J Trop Med Hyg 2011 Dec;85(6):1146-53.
- Aguilar PV, Estrada-Franco JG, Navarro-Lopez R, Ferro C, Haddow AD, Weaver SC. Endemic Venezuelan equine encephalitis in the Americas: hidden under the dengue umbrella. Future Virol 2011;6(6):721-740.
- Kirsch MI, Hülseweh B, Nacke C, Rülker T, Schirrmann T, Marschall HJ, Hust M, Dübel S. Development of human antibody fragments using antibody phage display for the detection and diagnosis of Venezuelan equine encephalitis virus (VEEV). BMC Biotechnol 2008 Sep 2;8:66.
- Estrada-Franco JG, Navarro-Lopez R, Freier JE, Cordova D, Clements T, Moncayo A, Kang W, Gomez-Hernandez C, Rodriguez-Dominguez G, Ludwig GV, Weaver SC. Venezuelan equine encephalitis virus, southern Mexico. Emerg Infect Dis 2004 Dec;10(12):2113-21.
- Brault AC, Powers AM, Ortiz D, Estrada-Franco JG, Navarro-Lopez R, Weaver SC. Venezuelan equine encephalitis emergence: enhanced vector infection from a single amino acid substitution in the envelope glycoprotein. Proc Natl Acad Sci U S A 2004 Aug 3;101(31):11344-9.
- Brault AC, Powers AM, Medina G, Wang E, Kang W, Salas RA, De Siger J, Weaver SC. Potential sources of the 1995 Venezuelan equine encephalitis subtype IC epidemic. J Virol 2001 Jul;75(13):5823-32.
- Powers AM, Oberste MS, Brault AC, Rico-Hesse R, Schmura SM, Smith JF, Kang W, Sweeney WP, Weaver SC. Repeated emergence of epidemic/epizootic Venezuelan equine encephalitis from a single genotype of enzootic subtype ID virus. J Virol 1997 Sep;71(9):6697-705.
- Sellers RF, Maarouf AR. Impact of climate on western equine encephalitis in Manitoba, Minnesota and North Dakota, 1980-1983. Epidemiol Infect 1988 Dec;101(3):511-35.
- Calisher CH, Maness KS. Laboratory studies of Venezuelan equine encephalitis virus in equines, Texas, 1971. J Clin Microbiol 1975 Sep;2(3):198-205.
- Bowen GS, Calisher CH. Virological and serological studies of Venezuelan equine encephalomyelitis in humans. J Clin Microbiol 1976 Jul;4(1):22-7.
- Pedgley DE, Tucker MR. Possible spread of African horse sickness on the wind. J Hyg (Lond) 1977 Oct;79(2):279-98.
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