Entomo-virological study of a suspected Japanese encephalitis outbreak in Muzaffarpur district, Bihar, India

Sir,

Vector-borne diseases are a major cause of human suffering both in terms of morbidity and mortality. Japanese encephalitis (JE) is a mosquito-borne disease, caused by flavivirus that infects vertebrate hosts, primarily birds belonging to the family Ardeidae and swine, naturally maintained in an enzootic cycle12. An outbreak, suspected to be JE occurred in the Muzaffarpur district, Bihar in June, 2011. The symptoms were high fever, followed by bouts of unconsciousness and convulsions akin to a type of JE were noticed. Several children died in Tirhut division alone with the maximum number of deaths concentrated in and around Muzaffarpur district. During the month of June, 125 cases and 43 deaths were reported from both Sri Krishna Medical College and Hospital (SKMCH) and Krishna Daviprasad Kesarival Institution (KDKM) in the district. Although all the afflicted children uniformly suffered from high fever, which gave way to convulsions and seizures, and brought in a semi-consciousness state for treatment, but the possibility of JE was ruled out based on the analysis of human serum samples in Muzaffarpur district during July 2011. As no confirmed reports were available on the aetiology of the present outbreak, the secondary data available confirmed that the mysterious fever was not JE and the cases were also negative for Chandipura and Nipah345. The Centre for Research in Medical Entomology (CRME), Madurai, Tamil Nadu undertook an investigation in July 2011 on entomo-virological aspects to confirm/rule out the JE virus involvement in the vector mosquitoes. Investigation of a mosquito borne epidemic is mainly based on clinical case reporting and abundance of vector mosquitoes. The monitoring of virus infection in vector mosquitoes is an essential component for the confirmation of the same6. The main objective of the investigation was to determine the JEV infection in mosquito vector populations to understand their role in the present outbreak.

During the investigation, the mosquito vector species composition in the affected villages of the district was recorded to know the density and diversity of the prevalent vector. Mosquitoes resting on vegetation and bushes around cattle sheds and pigstys were collected after dusk with the help of oral aspirator and flash torch, and transported to the laboratory for identification to the species level based on standard identification keys available for Culex vishnui subgroup78.

In the Nawada village of Musahari primary health centre (PHC), only Culex quinquefasciatus and Armigeres subalbatus were collected and in Manickpur village of the same PHC Cx. tritaeniorhynchus, Cx. gelidus, Cx. quinquefasciatus, Mansonia uniformis, Anopeles subpictus, An. vagus and Aedes lineatopennis were collected. In Tiwari Tola village of Kanti PHC, the mosquito species namely Cx. tritaeniorhynchus, Cx. gelidus, Cx. quinquefasciatus, Cx. pseudovishnui, An. vagus, Ae. lineatopennis and Ar. subalbatus were collected. In the village Choppara of Bochahan PHC the mosquitoes like Cx. tritaeniorhynchus, Cx. gelidus, Cx. quinquefasciatus, Mn. uniformis, An. vagus, Ae. lineatopennis and Ar. subalbatus were collected. Subsequently collections were carried out in the village Nakta situated in the Motipur PHC where the mosquitoes like Cx. tritaeniorhynchus, Cx. gelidus and Ar. subalbatus were collected. Lastly in the Ganipur Beja village from Sakra PHC, mosquitoes like Cx. pseudovishnui, Cx. fuscocephala, Cx. (Lut.) fuscanus. An. subpictus, Ae. lineatopennis and Ar.subalbatus were collected. Thus, a total of 11 species of mosquitoes (Cx. tritaeniorhynchus, Cx. gelidus, Cx. pseudovishnui, Cx. fuscocephala, Cx. quinquefasciatus, Cx. (Lut.) fuscanus, Ae. lineatopennis, Mn. uniformis, An. subpictus, An. vagus and Ar. subalbatus) were collected. In addition, Cx. infula, An. subpictus, Ae. vittatus and Ae. lineatopennis were also collected from the larval survey conducted in paddy fields, cement cisterns, etc.

Antigen capture ELISA was carried out to detect the JE antigen in the pools prepared with Cx. tritaeniorhynchus (5 pools), Cx. quinquefasciatus (8), Cx. gelidus (4), Cx. pseudovishnui (2), Cx. fuscocephala (1), Cx. fuscanus (1), Cx. infula (1), An. subpictus (5), Ar. subalbatus (8), An. vagus (3), and Mn. uniformis (3) pools. All the 41 pools tested were found negative for JEV. Mosquito pools, Ae. vittatus (2 pools) and Ae. lineatopennis (2 pools) were also tested for dengue and found negative. So far JE virus infection in vector mosquitoes has not been reported from this district of Bihar.

Even though Bihar State is prone to many vector borne diseases, information on vector biodiversity is scanty. The JEV infection in mosquitoes could not be detected from the collected samples of the affected areas. The absence of JEV infection in vector mosquitoes further supports the absence of JE positives in the serological investigations. A detailed in-depth longitudinal study need to be undertaken further to explore the actual aetiological agents for such outbreaks.