Epidemiology of Japanese encephalitis in the Philippines: a systematic review

Abstract

Background: Japanese encephalitis virus (JEV) is an important cause of encephalitis in most of Asia, with high case fatality rates and often significant neurologic sequelae among survivors. The epidemiology of JE in the Philippines is not well defined. To support consideration of JE vaccine for introduction into the national schedule in the Philippines, we conducted a systematic literature review and summarized JE surveillance data from 2011 to 2014.

Methods: We conducted searches on Japanese encephalitis and the Philippines in four databases and one library. Data from acute encephalitis syndrome (AES) and JE surveillance and from the national reference laboratory from January 2011 to March 2014 were tabulated and mapped.

Results: We identified 29 published reports and presentations on JE in the Philippines, including 5 serologic surveys, 18 reports of clinical cases, and 8 animal studies (including two with both clinical cases and animal data). The 18 clinical studies reported 257 cases of laboratory-confirmed JE from 1972 to 2013. JE virus (JEV) was the causative agent in 7% to 18% of cases of clinical meningitis and encephalitis combined, and 16% to 40% of clinical encephalitis cases. JE predominantly affected children under 15 years of age and 6% to 7% of cases resulted in death. Surveillance data from January 2011 to March 2014 identified 73 (15%) laboratory-confirmed JE cases out of 497 cases tested.

Summary: This comprehensive review demonstrates the endemicity and extensive geographic range of JE in the Philippines, and supports the use of JE vaccine in the country. Continued and improved surveillance with laboratory confirmation is needed to systematically quantify the burden of JE, to provide information that can guide prioritization of high risk areas in the country and determination of appropriate age and schedule of vaccine introduction, and to measure the impact of preventive measures including immunization against this important public health threat.

Fig 1. Assessment of studies for inclusion in the systematic review.

Fig 2. Geographic distribution of reported JE cases, suspected JE cases and seroprevalence surveys in the Philippines, by province.

Data from published reports and presentations, 1958–2013. Some reports did not specify origin of cases; in these cases, location of the hospital was mapped (black dots). The number of reported JE cases may have been biased by factors other than the incidence of JE. The preference for certain study sites and duration of studies may have unduly increased the number of cases in some provinces. There were additional cases with no detailed residence information: 28 cases in Luzon and 7 cases in Visayas. A seroprevalence survey was also conducted in Manila, but obscured by overlapping black dot for hospital.

Fig 3. Geographic distribution of animal and mosquito studies in the Philippines, 1958–2012.

Fig 4. Monthly distribution of suspected and confirmed JE cases, January 2011 to March 2014.

Suspected JE cases include line-listed cases without laboratory confirmation, sentinel surveillance cases and clinician referral cases. Of 1032 suspected JE cases, 497 (48%) underwent laboratory testing for JE.

Fig 5. Age distribution of suspected (n = 1032) and confirmed (n = 73) JE cases and cumulative percentage of confirmed JE cases, January 2011 to March 2014.

Suspected JE cases include simple line listing of AES cases without laboratory confirmation, case-based sentinel surveillance for JE with laboratory confirmation and clinician referred specimens for testing to RITM.

Fig 6. Geographic distribution of suspected and confirmed JE cases in the Philippines.

Data from surveillance and referral testing, January 2011 to March 2014. There were additionally 21 confirmed JE cases out of 159 cases referred by hospitals in Metro Manila without available data on geographic origin.