California Encephalitis Virus: Transmission And Risks

  • Austeja BakulaiteMSc by Research in Biomedical Sciences (Life Sciences) – the University of Edinburgh

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California encephalitis is a type of encephalitis (inflammation or swelling in the brain) caused by a serogroup (a group of microorganisms that have similar markers on their surface) of viruses that belong to the genus of Bunyavirus within the family of Bunyaviridae.1 Although quite rare, these viruses can cause life-threatening illness, which can even lead to death. Because of this, it is important to understand the routes of transmission as well as the associated risks of this disease. In this article, we will introduce you to what California encephalitis is, how it is spread, the symptoms and risks of this disease as well as some ways on how you can prevent it.

What is encephalitis?

Encephalitis is an inflammation of the brain, which leads to the swelling of the brain that can cause multiple physical and cognitive symptoms such as:

In more severe cases, symptoms might include:

  • Weakness or partial paralysis of arms and legs
  • Double vision
  • Impairment of speech and/or hearing
  • Coma

In the most extreme cases, it can cause brain damage, stroke, or even death.

Even though in around 30-40% of cases the cause of encephalitis cannot be determined, the potential causes of encephalitis include: 

  • viral infections – although quite rare, encephalitis can be caused by common viruses, such as the ones that cause cold sores (herpes simplex), chickenpox, mumps, rubella or measles, spreading to the brain
  • Autoimmune conditions, where sometimes something goes wrong with the immune system (a person’s antibodies or immune cells) and it attacks the brain by mistake, causing it to become inflamed. If antibodies are the ones to attack the brain, then they might target specific receptors or proteins within the brain and this is used to determine the type of autoimmune encephalitis
  • bacterial or fungal infections – however, these are a lot less common causes of encephalitis than viral infections

Additionally, multiple types of encephalitis are spread by ticks, like tick-borne encephalitis; by mammals, for instance, rabies; and by mosquitoes, such as Japanese encephalitis and California encephalitis.

The diagnosis of encephalitis is a complex process, which starts with a usual physical exam and gathering of medical history to investigate recent activities of the affected person, such as recent exposure to insects, ticks or animals, or contacts with sick people, or recent travel, in particular to high-risk areas. Your doctor will also note any pre-existing medical conditions and any medications you use. There are numerous diagnostic tests that your doctor could order to confirm the presence of infection and/or inflammation: 

  • Brain scans, such as CT or MRI - scanning the brain can help with determining if you have encephalitis or another problem such as a stroke, brain tumour or brain aneurysm
  • Lumbar puncture - a lumbar puncture (spinal tap) is a procedure to remove a small amount of fluid from around the spinal cord, so it can be checked for signs of the infection in the brain and the spine or issues with the immune system
  • Other tests – which might include an electroencephalogram (EEG), which looks at the electrical activity in your brain and can show any abnormal brain activity. Also, a blood and urine test could be ordered, which could be used to identify microorganisms or immune system components responsible for the encephalitis

What is California encephalitis and how is it spread?

California encephalitis is one of the types of encephalitis. It is a mosquito-borne disease, which is one of the most common childhood central nervous system (CNS) diseases reported annually.2 The viruses that are known to cause California encephalitis are California encephalitis virus, La Crosse virus, Jamestown Canyon virus, and Tahyna virus.1 La Crosse virus, California encephalitis virus, and Jamestown Canyon virus are causes of California encephalitis predominantly in the United States, meanwhile Tahyna virus is predominantly found in Russia. Most of the human cases have occurred in the western United States and Canada.1  

The first virus within the California encephalitis causing virus group to be isolated and identified was, unsurprisingly, California encephalitis virus.3 It was discovered in the 1940s in Kern County, in the San Joaquin Valley of California, by isolating it from Aedes dorsalis mosquitoes; firstly, and later from Culex tarsalis mosquito species as well.3,4 More recently, it has also been shown to be spread by other mosquito species, such as Aedes aegypti, Aedes communis and Culiseta inornate.5 This virus was shown to be disease-causing in mice, cows, horses, rabbits, squirrels and, importantly, humans.3,4

Because this virus is spread by mosquitoes, it is considered to be an arbovirus (arthropod-borne virus). Initially, mosquitos get infected by sucking blood from an infected animal. The virus, then, can replicate within the mosquito and spread through the mosquito’s saliva when it is sucking the blood from another animal or human. However, while this virus is primarily found in mosquitos, it has also been found in ticks and could be potentially spread through tick bites as well.3

California encephalitis virus group infections are the most reported cause of arboviral encephalitis in the United States. From 1996 to 1998, around three times more human cases of arboviral encephalitis were reportedly caused by California encephalitis viruses as were reported for other arboviruses, such as western equine encephalomyelitis virus, St. Louis encephalitis, and eastern equine encephalomyelitis viruses combined.6 Within the California encephalitis virus group, the La Crosse virus is the most common cause of the disease.1

What are the risks associated with California encephalitis?

La Crosse and California encephalitis viruses most commonly cause the disease in children, meanwhile Jamestown Canyon virus mostly affects elderly people. However, most of the cases are asymptomatic (no symptoms in infected people), with the ratio of asymptomatic to symptomatic infections being 1000 : 1 (i.e. one case per thousand people).1

In the case of symptomatic infections, symptoms of California encephalitis include fever, headaches, vomiting in around 70% of cases, seizures in approximately 46% of cases, as well as altered mental state in around 42% of patients. About 10% of affected people develop intracranial hypertension, a built-up pressure around the brain, which is an acute and life-threatening neurological condition. Rarely, patients develop cerebral herniation, when the pressure is high enough to move the brain tissue within the skull.1 Some of the possible complications associated with intracranial hypertension are:7

Prevention and control measures

Currently, there is no antiviral therapy for the California encephalitis group of viruses, and there is no vaccine that is available for it either.1 Because of this, it is important to focus on prevention by taking action to prevent potential exposure to these viruses. The best way to protect yourself from California encephalitis viruses is by protecting yourself from mosquito (and other insects) bites. According to, these are effective preventive measures to avoid mosquito bites:

  • Use insect repellent - Environmental Protection Agency (EPA)-registered insect repellents with active ingredients, such as DEET, picaridin (also known as KBR 3023 and icaridin outside the United States), IR3535, oil of lemon eucalyptus, para-menthane-diol and 2-undecanone. EPA-registered insect repellents are a proven way to safely and effectively avoid mosquitos
  • Wear loose-fitting, long-sleeved shirts and pants as well as socks and shoes – wearing such clothing is an effective way to prevent mosquito bites. For additional protection, use 0.5% permethrin to treat clothing and gear, such as boots, pants, socks and tents, or you could also buy clothing and gear, which have been pre-treated with permethrin
  • Use screens on windows and doors – to prevent mosquitoes from entering inside your house
  • Use mosquito net - if you will be sleeping outside, you could use a mosquito net to protect yourself. Make sure that your net has no rips, otherwise, the mosquitos will be able to enter
  • Take action to prevent mosquitoes from laying eggs in or near water – at least once a week, empty and clean, cover, or throw out items that hold water, such as tyres, buckets, pools, flowerpots or garbage containers.


Encephalitis is an inflammation of the brain, which can develop into a life-threatening condition and can even be deadly. It can cause symptoms such as fever, headaches and seizures, as well as it can affect your cognitive functions. California encephalitis is one of the types of encephalitis, which is caused by a serogroup of viruses related to each other. It is quite commonly reported to cause CNS disease in children in the United States. However, some types of viruses within this group are also known to affect the elderly.

Most of the infected people will be asymptomatic. However, in the symptomatic cases, the symptoms can be quite severe. This includes the development of intracranial pressure and cerebral herniation, which can be life-threatening and lead to multiple complications, such as strokes, seizures, coma and respiratory arrest.

California encephalitis is a mosquito-borne disease, which means it is mainly spread through bites by infected mosquitoes. The Aedes genus of mosquitoes is the most common culprit. 

The best way to protect yourself from this disease is by protecting yourself from the bites of the infected mosquitos. Some of the measures include: using insect repellent, wearing loose-fitting and long-sleeved clothing, which has been pre-treated with permethrin, using screens and nets to prevent mosquitoes from entering houses and tents, as well as avoiding stagnant water sources.


  1. Beckham JD, Tyler KL. 91 - Encephalitis. In: Bennett JE, Dolin R, Blaser MJ, editors. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases (Eighth Edition) [Internet]. Philadelphia: W.B. Saunders; 2015 [cited 2024 Feb 9]; p. 1144-1163.e3. Available from:
  2. Bente DA. 168 - California Encephalitis, Hantavirus Pulmonary Syndrome, and Bunyavirus Hemorrhagic Fevers. In: Bennett JE, Dolin R, Blaser MJ, editors. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases (Eighth Edition) [Internet]. Philadelphia: W.B. Saunders; 2015 [cited 2024 Feb 9]; p. 2025-2030.e1. Available from:
  3. Newhouse VF, Burgdorfer W, McKIEL JA, Gregson JD. CALIFORNIA ENCEPHALITIS VIRUS. American Journal of Epidemiology [Internet]. 1963 [cited 2024 Feb 9]; 78(1):123–9. Available from:
  4. Hammon WMcD, Reeves WC. CALIFORNIA ENCEPHALITIS VIRUS—A Newly Described Agent. Calif Med [Internet]. 1952 [cited 2024 Feb 9]; 77(5):303–9. Available from:
  5. McLean DM, Grass PN, Judd BD. California encephalitis virus transmission by arctic and domestic mosquitoes. Archives of Virology [Internet]. 1977 [cited 2024 Feb 9]; 55(1):39–45. Available from:
  6. Eldridge BF, Glaser C, Pedrin RE, Chiles RE. The first reported case of California encephalitis in more than 50 years. Emerg Infect Dis [Internet]. 2001 [cited 2024 Feb 9]; 7(3):451–2. Available from:
  7. Sharma S, Hashmi MF, Kumar A. Intracranial Hypertension. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 [cited 2024 Feb 9]. Available from:

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This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Austeja Bakulaite

MSc by Research in Biomedical Sciences (Life Sciences) – The University of Edinburgh

Auste is currently a PhD candidate at the University of Portsmouth working on the development of novel tyrosine kinase inhibitors as cancer drugs. She has several years of experience working on cancer research, biochemistry, molecular biology and drug discovery.

Additionally, Auste is interested in how alternative proteins and plant-based diets can improve public health, and environmental and animal welfare issues.

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