Epidemiological Profile of Japanese Encephalitis in Nepal 1996 - 1997

Abstract

In Nepal, during the past two decades, Japanese Encephalitis (JE) is known to occur mainly in southern terai districts of Nepal, but recently during 1996 and 1997, the Kathmandu valley particularly, the southern part of Lalitpur district first reported outbreak of JE, which was confirmed both serologically and virologically. Epidemiological surveillance study was carried out in all 18 JE endemic terai districts of Nepal during the months of November & December of 1996 and 1997 respectively. Results of epidemiological, entomological and serological investigation are presented, tabulated, analyzed and discussed. There was a total of 1687 cases with 353 deaths (21% Case fatality rate) in 1996 and a total of 1740 cases with 126 deaths (7% CFR) in 1997. All age & sex groups are infected with the disease, but below 15 years children were more prevalent with 21% & 7% CFR in male and 16% and 6% CFR in female children during 1996 and 1997 respectively. Whereas in adults, both male and female showed 23% and 25% CFR respectively. There is a seasonal difference in occuring diseases; the maximum number of cases appeared from June to October, and August was observed as a peak period during both years.

It has long been recognised that the feeding behaviour of Culicin mosquitoes is of paramount importance in the epidemiology of mosquito-borne JE pathogens. Host feeding pattern of vector mosquitoes is crucial for the maintenance of the complex natural cycle of JE virus, which includes pigs and birds and sometimes large animals like buffaloes and cattle. The virus is transmitted from pig to pig and bird to pig by Culex species of mosquito. The human beings are only incidental hosts of JE. The role of pigs and ducks as amplifier hosts is well established in Nepal as well as in other southeast Asian countries. Except dog Cx. vishnui was found to feed on man, cattle, buffalo, pig, bird and goat, showing its wide range of feeding. Among them bovids were found to be preferred hosts after pigs and ducks. Pigs and ducks possess a high body temprerature and attract a large number of mosquitoes, thus maintaining the pig-mosquito-pig cycle, duck-mosquito-duck-mosquito-pig cycle. In Nepal, in recent outbreaks of 1996 and 1997, the feeding behaviour of mosquito on bovid in the endemic areas, where there are no pigs and ducks population, has been observed. This means the epidemiological pattern of the disease transmission could be buffalo-mosquito-buffalo and cattle-mosquito-cattle cycle with more contact with humans infection through vectors. JE virus isulation studies in the endemic areas from mosquitoes, animals and diseases human beings are needed to confirm this hypothesis.

The prevention and control of JE has been a challenging problem in view of non-availability of effective chaemoprophylaxis against the virus and limitations of immuno-prophylaxis. There have been attempts for effective prevention and control of JE by malathion spraying, directing the control measures against the vectors-Culicine mosquitoes. However, this approach too has its own limitation because the known vectors of JE in Nepal namely Culex species mainly breed in large paddy fields and ponds limiting the effectiveness and feasibility of the larval control. JE virus transmission is from domestic animals and birds as reservoirs because the JE virus in nature is found in pigs, birds and large animals like water buffaloes and cattle. In Nepal pigs and ducks have been found as the main reservoirs of JE virus. In the present senario, the epidemiology of JE has undergone considerable changes over the last two decades. There are no reports of antibodies in 'sentinel' pigs from terai districts of the country. Therefore, seroconversion profile of sentinel pigs and birds with its relation to endemicity pattern of JE in Nepal must be carried out urgently. The human cases of JE have been known to occur between mid June to mid October with peaks in August and September. Therefore, the JE transmission period epidemiologically can be considered as July to October with a peak on September.

The present study has revealed that the JE situation in Nepal from 1978 onward showed increasing trend in the incidence of JE from the human population point of view, coverage of geographical area, seasonal occurance, vector bionomics, animal reservoir hosts and environmental and climatic changing effects. Since it is a vector-borne viral zoonotic disease, so far Culicine mosquitoes species viz. Culex tritaeniorhynchus, Cx. vishnui, Cx. pseudovishnui, Cx. gelidus and Cx. fuscocephalus have been incriminated and identified as vectors of JE. The female mosquitoes are known to feed on a wide range of vertebrates including man. Whereas some Culicine species have a restricted host range and distinct feeding preferences. JE is a viral zoonotic disease transmitted by mosquito vector bite and caused by arbovirus (flavivirus) which affects the central nervous system. The virus is maintained in nature in animals reservoirs like pigs, cattle and birds. This disease was first recognised in Japan in 1924 and in the Indian adjoining border of southern terai districts of Nepal, Gorkhpur district of Uttar Pradesh in 1978. Since then the JE has become a major public health problem in all Terai districts of Nepal since 1978. The current epidemiological survey study has revealed that both the ricefields and the ponds situated in every endemic villages are chiefly contributing towards the population density of Culicine mosquitoes. In view of recurrent outbreaks in Terai districts and widespread breeding places of mosquitoes, there is an urgent need to asses the feasibility of suitable vector control measures including antilarval operations and ultra-low volume application of insecticides in all the affected endemic districts.