Outbreak of Western Equine Encephalitis Virus in Argentina

The culprit: The primary mode of transmission of the infection to humans is through mosquitoes, which act as vectors for the virus. 
| Photo Credit: Getty/iStockPhoto

On December 20, 2023, the International Health Regulations National Focal Point (IHR NFP) in Argentina alerted the Pan American Health Organization/World Health Organization (PAHO/WHO) of a human case of Western Equine Encephalitis Virus (WEEV) infection. The patient had a sudden onset fever and neurological features on November 19, 2023 and was later admitted to a provincial hospital, and discharged on close follow-up on December 20. The outbreak has grown to 21 confirmed new cases of the infection in Argentina and comes after more than two decades since the last case was reported in the country. The last cases of human infections in Argentina were reported in 1982, 1983, and an isolated event in 1996. The present outbreak also comes in the context of an ongoing outbreak in horses in Argentina and Uruguay. Multiple outbreaks and human cases of Westen Equine Encephalitis have been reported in the past from the U.S. and Canada and over 3,000 cases of infection have been reported over these years.

Western Equine Encephalitis is a mosquito-borne infection caused by the Western Equine Encephalitis Virus (WEEV), which belongs to the Togaviridae family of viruses. The virus has an approximately 11.5 kilobases long single-stranded RNA genome and is a recombinant of the eastern equine encephalitis virus (EEEV) and a Sindbis-like virus. Passerine birds are thought to be the reservoir and equine species as intermediate hosts. The primary mode of transmission of the infection to humans is through mosquitoes which act as vectors for the virus. While most of the infections are asymptomatic, the infection may lead to severe consequences in rare cases. An estimated 4-5% of cases may manifest as infection/inflammation of the brain, resulting in neurological symptoms and sequelae of disease. With no specific antiviral treatment, symptomatic care is crucial, especially for neurologic symptoms.

Since December 2023, there have been 374 laboratory-confirmed cases of WEEV infections in animals in Argentina and Uruguay, and an additional 21 human cases in Argentina. In Argentina, there have been 1,258 cases of infections in animals in 15 provinces, with the province of Buenos Aires having the highest number of cases. In Uruguay, there have been 56 cases of Western Equine Encephalitis Virus in equines, with no reported human infections. After the detection of the initial case of the ongoing outbreak in Argentina reported in December 2023, a total of 91 additional suspected cases have since been tested across 11 provinces in the country out of which 21 have been confirmed positive for the virus. Of these cases, eight individuals needed intensive care unit care, 13 required general hospitalization and one case resulted in a fatality. In the cases for which exposure history is available, three individuals were veterinarians, two were construction workers, and two reported engaging in outdoor recreational activities.

While PAHO/WHO has issued an alert on the risk of the spread of WEE in the Americas, enhanced surveillance and detection of infections will be crucial for preventing the spread of the virus. Following a One Health approach is essential for the control of the virus. Environmental modifications, vector control, and vaccination for equines in affected and high-risk regions to curb the potential spread of the virus.

(Bani Jolly is a senior scientist at Karkinos Healthcare and Vinod Scaria is a senior consultant at Vishwanath Cancer Care Foundation)

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