Mosquitoes are known to be vectors - carriers of disease-causing pathogens like parasites, bacteria, and viruses. Some of these viruses can cause debilitating infections of various organs including the brain. In California in the 1950s, one such group of viruses was found to cause acute encephalitis - a sudden inflammation of the brain causing mild to severe neurological and physical symptoms.

The disorder was found to be carried by mosquitoes. In later decades, several variations of this virus were found to cause encephalitis, with California Encephalitis now being identified as one of the most common viral-borne forms of encephalitis in the United States. 

This article discusses the California Encephalitis Virus, its history, and how it infects and affects children. We will also see how the disease is identified and how symptoms develop and progress. Finally, we will see how the symptoms are treated and what to expect if your child gets infected.

Introduction

California Encephalitis was the name given to a group of mosquito-borne viruses in California, causing encephalitis. The first of these diseases was identified in residents of Kern County, California where an alarming 11% of residents tested positive for the virus.¹

The virus was antigenically related to similar viral cases later seen in other parts of California and the United States such as the La Crosse Encephalitis virus and the Jamestown Canyon Encephalitis. Similar but unrelated viruses also caused encephalitis in both horses and humans in warm regions of the USA, specifically the Eastern Equine Encephalitis (EEE) virus, and the St. Louis Encephalitis (SLE) virus. 

What is California Encephalitis?

California Encephalitis is a form of encephalitis caused by arboviruses - viruses carried by airborne mosquito vectors, first discovered in Kern County, California.¹ These viruses were found to cause encephalitis in affected individuals.

Encephalitis is a disorder characterised by the inflammation of the brain. This can cause symptoms ranging from headaches and fever to even permanent brain damage and death.²

Transmission

Arboviruses enter the gut of a mosquito when it sucks infected blood (of certain animals). In doing so, the virus finds a new home within the gut lining where it develops and spreads to other parts of the mosquito including the saliva. As such, when this mosquito bites a human victim, the virus, vis-a-vis the saliva enters the bloodstream.³

California Encephalitis is carried by specific species of mosquito, notably - Aedes dorsalis¹ Aedes triseriatus,⁴ and Aedes melanimon⁵, These mosquitoes are endemic to the warm and marshy areas of parts of California, such as in Marin County, and the Central Valley. 

Notably, the virus does not spread from human to human, through cough, touch, fluids, or even mosquito intermediates. In other words, humans are considered “dead-end hosts”.⁴

History of California Encephalitis

A study conducted in the 1950s, following the first identification of the California Encephalitis Virus, concluded that approximately 11% of the Kern County population had been, at some point, infected with the virus. However, inapparent infections (infection in which symptoms do not occur) were seen in the vast majority who tested positive for the virus without the development of encephalitis.¹

Cases of California Encephalitis are mostly rare, in addition to which, only 1 in 1000 positive cases develops symptoms of encephalitis.⁶ Since the identification of the original Kern County variety of California Encephalitis, several related viruses causing acute encephalitis have been identified including the La Crosse Encephalitis virus and the Jamestown Canyon Encephalitis virus. 

Since the 1960s, spikes in the number of reported cases have resulted in the identification of the California Serogroup of encephalitis-causing arboviruses, as the leading arboviral cause of encephalitis in the United States⁵, averaging 50-150 cases annually in the United States.^7,8

California Encephalitis in children

While the original serotype of California Encephalitis virus infects adults at a higher rate than children, the La Crosse variant has been identified as the leading arboviral cause of acute encephalitis in children.⁹

The La Crosse Encephalitis virus is the most commonly reported and most infective member of the California Serogroup. First identified in a four-year-old in La Crosse, Wisconsin, the virus is found to cause the most severe symptoms in children under the age of 16.⁷

Prognosis

Early identification, diagnosis, and intervention are necessary to minimise the risk of complications. In general, cases of encephalitis have a mortality rate of around 10%. With early identification, combined with advancements in treatment, the risk of complications, such as permanent damage to the brain tissue can be minimised.¹⁰ 

While the mortality rate for La Crosse Encephalitis in children is low (less than 1% of cases), serious symptoms such as seizure and brain damage can cause permanent effects on the patient.⁹

Symptoms of California Encephalitis

Symptoms seen in children suffering from California Encephalitis are wide-ranging, reminiscent of other forms of encephalitis. The incubation period of the virus ranges from 5-15 days⁷, meaning the onset of symptoms may begin within a week following the mosquito bite.

Low severity symptoms

Low severity symptoms include:

• Headache
• Lethargy
• Nausea
• Muscle soreness^9,11

Moderate severity symptoms

These may include the following:

• Fever
• Vomiting
• Disorientation, cognitive/behavioural changes
• Tremors⁹

Severe symptoms

Severe symptoms often develop in children under 16. These can include:

• Cerebral herniation
• Aseptic meningitis
• Seizures
• Vision loss
• Paralysis
• Coma^7,9

Diagnosis

Diagnosis is important to identify the cause of the encephalitis, as the infection may be caused by bacteria, fungus, virus etc. Early identification and treatment of encephalitis can minimise the risk of long-lasting complications such as brain damage, impaired concentration, and death.¹⁰ 

Diagnosis for California Encephalitis is initially done by identifying the symptoms experienced by the patient. Electroencephalograms (EEG) and CT scans are taken to detect inflammation of the brain. Next, a history of residence or travel to areas associated with the virus, especially during high-risk seasons like summer, increases the chance of exposure to mosquitoes carrying the responsible virus. 

Finally, the presence of the virus, or antibodies produced by the body against the virus is tested for.⁷ This is usually done by testing the spinal fluid by spinal tapping or serological testing for:

• California Encephalitis virus-specific antibodies
• Abnormal WBC count
• Low levels of sodium in serum

PCR testing for antibodies against La Crosse Encephalitis virus is currently under study.⁹ This will improve the sensitivity and efficiency of testing for the virus.

Treatment and management

No vaccine or treatment exists for the prevention of the California Encephalitis group of diseases. The CDC recommends the following for management of mild to moderate symptoms:

• Adequate bedrest
• Sufficient fluid intake
• Over-the-counter pain medication

However, severe symptoms may necessitate hospitalisation. Several phenomena may occur in severe cases of California Encephalitis, including coma, seizures, or severe brain inflammation in which case hospital treatment is required. Mechanical ventilation or intravenous fluid administration may be needed if and when such complications arise.⁷

More often than not, the disease passes without any such complications. Fevers and headaches managed at home usually subside within 1-2 weeks.⁷ Adequate follow-up must be conducted for children who have suffered moderate to severe symptoms, to evaluate the effect of the encephalitis on the cognitive and behavioural health of the child. Rehabilitation could assist in their recovery.

Preventative measures extend to mosquito control measures and general hygiene practices in children. The CDC recommends approved insect repellents and minimising exposure of the skin when outside. Responsible species of mosquito lay eggs in tree holes containing water, so adequate measures must be taken to reduce mosquito survival, by draining such holes and reducing stagnation of water.⁷

As of yet no antiviral or other therapy has been approved for the treatment of California Encephalitis. In some emergency cases, however, approval has been provided for the treatment of severely affected children with ribavirin, a drug seen to show some results in tick-borne arboviral encephalitis.^9,12

It is evident that, with the increasing incidence of California Encephalitis subgroups since the 1960s-1990s, an increased effort towards developing treatments or at the very least vaccines is necessary to limit the impact the disease can have on the general population.

FAQs

How is California Encephalitis transmitted?

California Encephalitis is transmitted by the bite of infected mosquitoes, specifically those found in specific warm-climate regions of the USA like the Central Valley of California. It is not transmitted between infected people.

What are the early signs of California Encephalitis in children?

Early signs of California Encephalitis include high fever, severe headache, and muscle soreness coupled with fatigue for the first 2-3 days of the disease. Confusion and disorientation come in, and in severe cases, seizures may occur.

How is California Encephalitis diagnosed in children?

California Encephalitis is diagnosed in children via CT scans or EEGs which map the brain and indicate inflammation or swelling. It is confirmed by spinal tapping/lumbar puncture where the spinal fluid is drawn out and analysed, as well as blood serum testing to identify the presence of the virus or antibodies produced against it.

Are there vaccines to prevent California Encephalitis in children?

There have not yet been any vaccines developed to prevent California Encephalitis in adults or children.

What are the potential long-term effects of California Encephalitis?

Following severe cases there is some risk of reduced cognitive ability, behavioural problems, and even some impaired physical functioning.

Summary

In conclusion, The California (CAL) serogroup of arboviral, encephalitis-causing viruses is a group of closely related viruses yielding similar symptoms in affected individuals. While the actual development of encephalitis occurs only in a small population of patients testing positive for the viruses, certain varieties, especially the La Crosse virus, have a severe effect on children.

Symptoms can be crippling ranging from fever and fatigue to debilitating encephalitis, seizures, or even comas. Regardless, the mortality is relatively low with less than 1% of patients with encephalitis succumbing completely to the disease.

With the lack of vaccines or therapeutics preventing or treating California Encephalitis, the measures taken to reduce risk are reduced to mosquito control and hygienic practices. More research and reporting are necessary for the treatment of this disease, which has shown increasing prevalence in recent decades.