Insects carrying deadly disease that causes brain to swell found in new US cities

Mosquitoes in California have tested positive for a deadly virus that causes brain swelling.

Health officials found that a batch of the blood-sucking insects sampled in the city of Brawley - located 130 miles east of San Diego - were carrying St. Louis Encephalitis (SLE). 

The mosquito pool was identified in the vicinity of Palm Avenue, which is a residential area east of the city center. Brawley is home to more than 28,000 people, while the Greater San Diego area has a population of over 3.3 million. 

SLE circulates between mosquitoes of the genus Culex and wild birds, and in about 30 percent of human cases, can cause brain swelling, vomiting, seizures and death.

Most people infected with the virus will show no symptoms, but those who do become ill may experience fever, headache, nausea, and tiredness.

The Imperial County Public Health Department confirmed two human cases in Imperial County in 2019, and the virus has been detected in mosquito pools in Imperial and other California counties, such as Fresno, Kings, and Madera, in recent years. 

This summer, there have been numerous cases detected in mosquitoes around California, Arizona, Utah, Nebraska and Louisiana, with these emerging mainly over the summer months.

While no human cases of SLE have been recorded in California this year, health officials are warning residents to be wary. 

The CDC has recorded an average of 14 SLE cases annually in the US between 2003 and 2023 in humans. 

During this decade-long period there have been 20 deaths. While the virus has a wider geographic range from Canada to Argentina, the majority of human cases are in the US, making travel within these areas a primary factor for exposure, not usually travel outside the US.

SLE is traditionally most common in the eastern and central US, particularly in the Mississippi Valley and Gulf Coast regions, as these regions are prime breeding grounds for Culex mosquitoes, which transmit the virus, with warmer temperatures and the presence of stagnant water.

Dr Stephen Munday, who serves as the Imperial County Health Officer, said: 'While it's not unusual to detect mosquito activity during the summer months, the identification of multiple positive pools in different areas is a reminder for all of us to stay alert.

'We want residents to take precautions, like eliminating standing water and using insect repellent, to help reduce the risk of mosquito-borne illness and protect their families and communities.' 

There are 52 mosquito traps deployed throughout Imperial County - which has a population of more than 180,000 - primarily within city limits. 

The traps are checked several times a week, and mosquito pools are tested weekly for various viruses. 

SLE was first recognized in 1933 when an epidemic in St. Louis, Missouri, resulted in over 1,000 cases of encephalitis, or inflammation of the brain. 

Several epidemics have occurred sporadically throughout the US since then, with the majority of cases occurring in eastern and central states such as Florida, Texas and California. 

The virus circulates in the environment between mosquitoes and birds typically found in freshwater swamps. 

Some people may develop neuroinvasive disease, such as encephalitis or meningitis (inflammation of the membranes that surround the brain and spinal cord). In rare cases, long-term disability or death can occur. 

In the two human cases of SLE previously reported in Imperial County, the individuals were admitted to a local hospital with severe headache, fever and nausea, and were diagnosed with viral meningitis.

There are no vaccines or specific antiviral medications to treat SLE.

The CDC states that the best way to avoid contracting the virus is to protect yourself from mosquito bites.

Bite prevention tips include using insect repellent, wearing long-sleeved shirts and pants, and using mosquito repellent devices outside and around the home. 

Following the detection of SLE in Imperial County, residents are being encouraged to limit their time outdoors during dawn and early evening and remove standing water from their properties. 

Larvicides - a type of insecticide used to control mosquitoes indoors and outdoors - can also be used to treat any areas where mosquitoes could lay eggs. 

As of September 3, there have been no known human cases of SLE in the US in 2025. In 2024, there were only two cases reported, down from 21 the previous year.  

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