This is the first study in Bangladesh to estimate the population-based incidence of JE. The estimated incidence varied in different parts of the country, with the highest in the Rajshahi area, in northwestern Bangladesh (2.7/100,000 population). This rate is similar to the JE incidence of some other JE endemic countries before the introduction of a JE vaccine into the national immunization programs; in Taiwan the incidence was 2.1 and in Thailand the incidence was 3–5/100,000 population.27,28
Nepal has recently focused on a mass immunization program of JE in their 20 endemic and four hyperendemic districts where the incidence of JE was 1.6/100,000 population in JE-endemic districts.29–31
Identifying the highest JE incidence in the Rajshahi Medical College Hospital catchment area is consistent with the hospital-based JE study conducted in 2003–2005, where over half of the cases (11 of 20) were identified from this hospital among four study hospitals.9
A higher number of JE cases in Rajshahi may result from the local ecology. The mosquito vector of JE breeds prolifically in wet fields, especially in paddy fields.32
The Rajshahi area is famous for rice and other agricultural products and the proportion of people directly involved in agriculture is higher than any other area of the country.33
Although pig raising is not common in Bangladesh's predominately Muslim communities, indigenous and tribal people commonly raise pigs, including the Santal,
the most populous indigenous community living in northwestern Bangladesh.34
The actual pig density is not known, but a recent pig census conducted in three catchment districts of Rajshahi Medical College Hospital indentified over 11,000 pigs, which were well distributed throughout the districts (Khan SU, personal communication).
The vast majority of JEV infections are asymptomatic and the clinical features of JE have a broad spectrum, ranging from a mild flu-type illness to a severe meningoencephalitis.6
The case definition of suspect meningoencephalitis in the catchment survey included similar clinical signs to those used for the case definition in surveillance hospitals to enroll the patients for JE testing. However, the presenting clinical signs, in terms of duration of altered mental status and unconsciousness, were more severe among admitted cases in surveillance hospitals than those of cases who were not admitted. The severity of illness of these admitted patients could be higher because surveillance at the tertiary level hospital captures patients with more severe illnesses.35,36
In Bangladesh, people initially seek care from local health care providers, either qualified or unqualified. Of the identified suspected meningoencephalitis cases, only 1% of cases visited a surveillance medical college hospital as their first visit to any health care provider even though many of these people lived quite close to that surveillance hospital.
Although only 45% of hospital-confirmed JE cases were < 15 years of age, in the catchment area of all study hospitals the estimated incidence of JE was higher among children compared with adults, which is similar to other studies conducted in JE-endemic countries.30,37,38
In the catchment area more than two-thirds of the suspected meningoencephalitis cases were found in children < 5 years of age, but these cases were less likely to be admitted to the surveillance hospitals. For tertiary level health care, people preferred pediatric hospitals for the treatment of their children instead of medical college hospitals (). However, as no pediatric hospital is available in the Rajshahi area, all children admitted to pediatric hospitals were identified from the Chittagong and Khulna area.
The approach used in this study, to estimate the population-based incidence of JE by adjusting crude incidence based on healthcare facility usage, builds on approaches used by other investigators in other settings to estimate the incidence of other diseases of public health importance.35,39,40
Incidence estimation of a disease with low frequency requires a large sample. The average population of our survey area (20 unions) in each hospital catchment area was more than 500,000. Rather than traditional door-to-door visits, which would be relatively expensive, we deployed a novel low-cost practical approach leveraging the strong social networks of rural Bangladeshi communities to identify persons with symptoms compatible with meningoencephalitis in the rural study areas. A typical village in Bangladesh is made up of a number of neighborhoods (paras
) that are composed of a group of homesteads (baris
usually consists of 6–7 households around a single courtyard and generally it's members are patrilineally related; in a para
people are generally involved in the same occupation and are closely related to each other.41,42
Hence, because of deep kinship prevailing in rural Bangladesh, residents are quite knowledgeable of major events that occurred in the other's households. As the field team visited the village doctors, educational institutions, and various groups in the village market, in addition to courtyard gathering, they usually received information of a suspected meningoencephalitis case from multiple sources. Although this community networking approach did not work in the three urban clusters because of weaker social networks among the urban dwellers, the approach may be efficient in identifying persons with severe diseases in other areas where strong social networking exist.
We note certain study limitations. First, the adjusted incidence assumes that meningoencephalitis patients who were admitted to the study hospital had a similar likelihood of having JEV infection as patients who received care elsewhere. However, the cases who were admitted to the study hospital were older, sicker, and lived closer to the hospital than the other meningoencephalitis cases. However, whether these differences would result in an increased or decreased proportion of the hospitalized cases being caused by JE is unknown and, therefore, could not be accounted for in the analysis. Second, in the surveillance hospitals only 45% of admitted patients had a CSF or serum specimen collected ≥ 7 days of onset of illness. This limited availability of convalescent specimens would underestimate the incidence of JE, so we made an additional estimate of incidence that accounted for this underestimation. Our estimate would be more precise if we were actually able to test the convalescent sera of all patients. Third, we collected information on persons who had symptoms compatible with meningoencephalitis using a community-based survey, usually collected months after the acute illness. However, as our trained field team explained the symptoms of meningoencephalitis to the villagers in local terms, we believe that the reports of seizures and altered consciousness, especially coma, were reliably reported by lay persons and therefore provide a reasonable estimate of the proportion of persons with serious central nervous system infections living in hospital catchment areas.
The data obtained from this study provides a credible estimate of the JE incidence in three wide geographic areas in Bangladesh that can be used as a rationale for policymakers to take effective control measures. Vaccination is the most effective means to control the incidence of JE.43
Current JE vaccines are safe and inexpensive3
; the neighboring countries of Bangladesh, such as India, Nepal, and Sri Lanka, have recently introduced JE vaccine into their JE endemic regions. Bangladesh should consider a vaccine demonstration project in areas where the incidence is high, especially in Rajshahi, and should assess the cost-effectiveness of vaccine introduction in the routine childhood immunization program.