California encephalitis: history, transmission, and public health implications

California Encephalitis

Although relatively rare, California encephalitis (CE) can be a highly lethal disease that is caused by the Balamuthia mandrillaris ameba. In fact, of the 10 cases of CE reported to the California Encephalitis Project during the period from 1999 through 2007, all but one patient died. Today, though, the majority of victims of CE survive the condition, but a significant percentage (about 20%) experience long-term complications as a result. To determine the facts about this potentially deadly human pathogen, this paper reviews the literature to provide the history of CE including its first outbreak, how the disease is transmitted, and the epidemiology of CE. In addition, a discussion concerning the search for a vaccine for CE is followed by description of the treatments and public health considerations of CE. Finally, an examination of the concern CE has for public health is followed by a summary of the research and important findings concerning this disease in the conclusion.

History of California encephalitis

First isolated in 1943 from mosquitoes in Kern County in the Central Valley of California, the arbovirus California encephalitis virus was caused the first recorded deaths of three humans in 1945, and all three deaths were located in Kern County (Eldridge, Glaser, Pedrin & Chiles, 2001). California encephalitis belongs to the California serogroup of viruses but a number of the other viruses that belong to this serogroup are not found in California (California serogroup - pathogen safety data sheet, 2015) but are found throughout other parts of North America, including Wisconsin and Illinois (Vanderhoof-Forschner, 2003). In fact, there were 13 confirmed cases of CE in Illinois in 1996 alone (Racki, 1997).

Many victims of CE, especially adults, frequently fail to demonstrate any symptoms until the condition is well progressed (California serogroup viruses, 2015). For instance, according to the Wisconsin Department of Health Services, "People infected with California serogroup viruses may have no apparent symptoms. Some people have symptoms of illness that may range from mild fever to encephalitis or mengioencephelitis" (California serogroup viruses, 2015, para. 2). Although precise numbers are unavailable and the potential for misdiagnosis is high in CE cases, clinicians and researchers have reported about 150 cases of balamuthiasis worldwide since 1990 (Balamuthia Amebic Encephalitis -- California, 1999 -- 2007, 2015).

How CE is transmitted

The ameba that is responsible for CE in humans is ubiquitous in the soil and like yellow and dengue fever, can be directly transmitted through breaks in the skin, through direct contact with skin lesions (Mahy, 1998) or through airborne inhalation (Balamuthia Amebic Encephalitis -- California, 1999 -- 2007, 2015). According to Massey (2015), the ameba responsible for CE is carried by intermediate vectors such as mosquitoes and ticks. In this regard, Massey advises that, "Mosquito-borne illnesses include viruses that can cause life-threatening encephalopathy and meningitis [including] the Lacrosse virus resulting in California encephalitis" (p. 3). Symptom severity in CE cases varies widely, and adults may be completely asymptomatic; however, infants and children typically experience more severe symptoms (California encephalitis, 2015). Human-to-human transmission of CE is not possible since humans are the dead end hosts for the pathogen (California serogroup - pathogen safety data sheet, 2015).

Epidemiology of CE

Despite ongoing and extensive research, the epidemiology of all types of encephalitis in the United States remains characterized by a majority of cases that have unknown origins (Trevejo, 2004). The research to date, though, implicates exposure to mosquitoes as being the most probable route of infection for many victims in the Central Valley of California (Eldridge et al., 2001). In this regard, Eldridge and his associates report that, "Further studies are needed to assess the risk for human infection by California encephalitis in coastal California and the role of various mosquito species in transmission" (2001, p. 452). In addition, ticks are also known disease transmission vectors (Vanderhoof-Forschner, 2003) and chipmunks, squirrels and other small mammals such as are known intermediary hosts (California serogroup - pathogen safety data sheet, 2015; Romanucci-Ross & Moerman, 1997).

Although many of the viruses that belong to the California serogroup are not found in California, they occur worldwide in a broad range of climates, including: tropical, coastal temperate marshland, lowland river valleys, alpine valleys, high lands, deserts, and arctic regions (California serogroup - pathogen safety data sheet, 2015). The group of viruses that belong to the California serogroup that are currently found in North America include CE, La Crosse, Snowshoe hare, Jamestown Canyon, and Trivittatus viruses (California serogroup - pathogen safety data sheet, 2015).

Moreover, the pathogen that causes CE can survive the winter months in an intermediary host, and the potential for infection increases as the weather warms (Anderson & Schneider, 2005). In this regard Vanderhoof-Forschner advises that, "The seasonality of arboviral transmission is variable and depends on the geographic location of exposure, the specific cycles of viral transmission, and local climatic conditions" (2003, p. 221). It is noteworthy that i the case of mosquitoes, the transmission range can extend widely, also depending on seasonal conditions. For instance, Massey reports that, "The mosquitoes that carry these diseases do not only stay with the local area. Some species, especially after heavy rains, will travel up to 10 miles to find a blood meal" (2015, p. 3).

Vaccine for CE

Although there is no vaccine currently available for the treatment of CE (Zoonotic diseases, 2015), the California serogroup viruses are susceptible to in vitro treatments with ribamydil (ribavirin) (California serogroup - pathogen safety data sheet, 2015).

Public health concerns

The California Encephalitis Project (CEP) reports that of the preliminary diagnoses for the 10 cases of CE reviewed during the period from 1999 through 2007 included viral meningoencephalitis, neurocysticercosis, neurotuberculosis, as well as acute disseminated encephalomyelitis, and only one of these patients survived the disease (Balamuthia Amebic Encephalitis -- California, 1999 -- 2007, 2015). According to the CDC, "These findings underscore the importance of increasing awareness among clinicians, epidemiologists, and public health officials for timely recognition and potential treatment of Balamuthia encephalitis" (Balamuthia Amebic Encephalitis -- California, 1999 -- 2007, 2015, para. 1).

Conclusion

The research showed that California encephalitis (CE) is a disease caused by the free-living Balamuthia mandrillaris ameba that is transmitted to humans through intermediary hosts such as mosquitoes and ticks, then to small mammals such as chipmunks and squirrels. The disease can be directly transmitted to humans through breaks in the skin or skin lesions and by airborne inhalation. Although the mortality rate due to CE has been reduced since the 1990s, the condition remains difficult to diagnose, especially in adults who may remain asymptomatic. The disease was first isolated in the mid-20th century but despite ongoing research, a vaccine for the disease has still not been developed. There are some steps that public health officials and individuals living in CE-prone regions of the country can take to reduce the risk of this disease, though, including mosquito-eradication programs, enhanced surveillance and laboratory safety protocols.