Seasonal abundance & role of predominant Japanese encephalitis vectors Culex tritaeniorhynchus & Cx. gelidus Theobald in Cuddalore district, Tamil Nadu.

Background & objectives: Japanese encephalitis (JE) is the leading cause of viral encephalitis in Asia. The first major JE outbreak occurred in 1978 and since 1981 several outbreaks had been reported in the Cuddalore district (erstwhile South Arcot), Tamil Nadu, India. Entomological monitoring was carried out during January 2010 - March 2013, to determine the seasonal abundance and transmission dynamics of the vectors of JE virus, with emphasis on the role of Culex tritaeniorhynchus and Cx. gelidus. Methods: Mosquito collections were carried out fortnightly during dusk hours in three villages viz. Soundara Solapuram, Pennadam, Erappavur of Cuddalore district. Mosquitoes were collected during dusk for a period of one hour in and around the cattle sheds using oral aspirator and torch light. The collected mosquitoes were later identified and pooled to detect JE virus (JEV) infection by enzyme linked immunosorbent assay (ELISA). Results: A total of 46,343 mosquitoes comprising of 25 species and six genera were collected. Species composition included viz, Cx. tritaeniorhynchus (46.26%), Cx. gelidus (43.12%) and other species (10.62%). A total of 17,678 specimens (403 pools) of Cx. gelidus and 14,358 specimens (309 pools) of Cx. tritaeniorhynchus were tested, of which 12 pools of Cx. gelidus and 14 pools of Cx. tritaeniorhynchus were positive for JE virus antigen. The climatic factors were negatively correlated with minimum infection rate (MIR) for both the species, except mean temperature (P<0.05) for Cx. gelidus. Interpretation & conclusions: High abundance of Cx. tritaeniorhynchus and Cx. gelidus was observed compared to other mosquito species in the study area. Detection of JEV antigen in the two species confirmed the maintenance of virus. Appropriate vector control measures need to be taken to reduce the vector abundance.

Entomological investigations into an epidemic of Japanese encephalitis (JE) in northern districts of West Bengal, India (2011-2012).

Background & objectives: Japanese encephalitis (JE) is one of the most important arboviral diseases of human beings with outbreaks in many parts of Southeast Asia including India. We present the entomological findings of an outbreak occurred in northern part of West Bengal during 2011-2012 with special emphasis on the role of JE vectors in different seasons. Methods: Adult mosquito collections were made with the help of mouth aspirators, aided by flash lights during day time resting inside human and animal habitations as indoor, and resting outside field grasses, bushes, underneath of culverts and bridges as outdoor, and in and around the pig enclosures and cattle sheds during dusk period in JE affected villages from Cooch Behar, Dakshin Dinajpur, Darjeeling and Jalpaiguri districts in North West Bengal. In all study villages, a long handled with enamel bowl dipper was used to obtain immature stages of mosquitoes from various breeding habitats. Results: A total of 19 different types of mosquito breeding habitats were examined for vectors of JE. From these habitats, 23.7 per cent were positive for breeding during the study period. Overall, nine different species were recorded through emergence, but none was positive for JE virus when subjected for detection of virus. Adult mosquitoes of more than 50 per cent of the potential JE vector species obtained through dusk and the rest through indoor and outdoor collections in all seasons. Altogether, 27 different species were recorded. Most of these were JE vectors. Interpretation & conclusions: Our results showed that in addition to Cx. vishnui subgroup, detection of JE virus antigen in Cx. quinquefasciatus indicated the possible maintenance of JE virus in nature through poor vector mosquitoes throughout the year. Since, all potential vector species reported elsewhere in India were also found in this region and fluctuated in density in different seasons, a proper integrated vector control programme needs to be implemented to control JE transmission.

Natural vertical transmission of dengue viruses by Aedes aegypti in Chennai, Tamil Nadu, India.

BACKGROUND & OBJECTIVE: Dengue viruses are spread and maintained in an Aedes aegypti-human- Ae. aegypti cycle in urban areas of the tropics. Dengue viruses are also maintained in nature by vertical transmission by Ae. aegypti. A study was undertaken in Chennai, a known endemic city in south India, to comprehend the natural vertical transmission dynamics in Ae. aegypti and to assess its epidemiological importance. METHODS: Ae. aegypti males collected in resting and landing collections were tested for dengue virus infection by antigen-capture enzyme-linked immunosorbent assay (ELISA) and further examined by insect bioassay, Toxorhynchites splendens inoculation-indirect immunofluorescence technique (Toxo-IFA) using serotype-specific monoclonal antibodies (Mabs), if found positive by ELISA. RESULTS: Of the 509 pools of Ae. aegypti males (n=5408) screened, 15 pools, collected in April, June- July, November-December in 2003 and March, May in 2004, were found positive for dengue virus infection and the minimum infection rate (MIR) among adult males was high in June 2003 (28.0/ 1000). Three positive pools could be serotyped as dengue-2 (2 pools) and dengue-3 (1 pool). INTERPRETATION & CONCLUSION: Dengue virus isolations from wild caught males of Ae. aegypti indicate the occurrence of transovarial transmission. Vertical transmission was mainly observed in summer months when dengue infections in humans were low suggesting that dengue viruses adopt a novel strategy of surviving adverse climatic conditions.

Serological and entomological investigations of an outbreak of dengue fever in certain rural areas of Kanyakumari district, Tamil Nadu.

BACKGROUND AND OBJECTIVES: During the first week of July 2003, suspected cases of dengue fever were reported from three villages in Kanyakumari district in Tamil Nadu. Since the fever outbreak occurred for the first time in these villages, serological, virological and entomological investigations were carried out to confirm the aetiology of outbreak. METHODS: A total of 76 plasma samples were collected from suspected cases of dengue fever and screened for the presence of IgM antibodies by Pan Bio ELISA kit. Toxo-IFA system was used for the isolation of dengue virus from the plasma samples. Vector survey employing ovitraps and adult landing collection were carried out in the study villages. Pooled samples of Aedes mosquito were screened for dengue virus antigen by an in-house antigen capture ELISA test employing dengue virus specific monoclonal antibodies. RESULTS: Of the 76 samples tested, 15 (20%) were found positive for dengue virus specific IgM antibodies. Dengue virus serotype-3 was detected from a plasma sample by Toxo-IFA test using virus specific monoclonal antibodies. Entomological survey revealed the abundance of Aedes albopictus (Skuse) mosquitoes in the study area. One pool consisting of 12 Ae. albopictus males were found positive for dengue virus infection. INTERPRETATION AND CONCLUSION: Based on the IgM antibody capture ELISA results, it was evident that the current infection was caused by dengue virus in the affected areas. All the age groups were affected during this outbreak. Detection of dengue virus serotype-3 in plasma samples further confirmed the aetiology of this outbreak. The high prevalence of the mosquito vector Ae. albopictus (Skuse) was observed. Detection of dengue virus antigen in the male mosquitoes confirms that the virus is maintained in wild populations of Ae. albopictus in these areas.

Virus isolation from wild-caught mosquitoes during a Japanese encephalitis outbreak in Kerala in 1996.

Out of 5357 wild-caught mosquitoes in 163 pools tested for virus using antigen capture ELISA and an insect-bioassay (inoculation into Toxorhynchites splendens larvae and identification by IFA using JE virus-specific monoclonal antibody), 16 flavivirus isolations were made of which 12 (75%) were identified as JE virus. Of the 12 JE virus isolations, 7 were from Culex tritaeniorhynchus, 3 from Mansonia uniformis and 1 each from Ma. indiana and Anopheles subpictus. Four isolations from Mansonia species for the first time reported here are noteworthy.

An appraisal of some recent diagnostic assays for Japanese encephalitis.

A study was undertaken in South Arcot district of Tamil Nadu, India to assess relative merits of selected diagnostic techniques for Japanese encephalitis. During the transmission seasons of 1993-1995, a total of 85 patients (mostly pediatric) clinically diagnosed as acute encephalitis or other related central nervous system (CNS) disorders were examined; in 53 (62.4%) a laboratory diagnosis of JE was established. In terms of diagnostic value, immunoglobulin M (IgM) antibody capture ELISA (MAC ELISA) on convalescent serum had the highest sensitivity (89%) and negative predictive value (NPV) (50%). This was followed by MAC ELISA on acute serum and CSF which had similar sensitivity (84%) and NPV (40%). The hemagglutination inhibition test and Toxorhynchites splendens inoculation technique for virus isolation were also similar in sensitivity (68%) and NPV (25%). The virus antigen detection technique by IFA in cells of cerebrospinal fluid (CSF) was the least sensitive (58%). The distinct advantage of the acute serum ELISA is that it can be carried out on a single finger-prick blood specimen. The IFA on CSF cells is the most rapid diagnostic test since it requires only 2-3 hours to complete. Therefore, both these tests also offer potential tools for JE surveillance programs.

Comparative evaluation of bioassay and ELISA for detection of Japanese encephalitis virus in field collected mosquitos.

Comparative evaluation of enzyme-linked immunosorbent assay (ELISA) and bioassay (virus isolation in Toxorhynchites splendens larvae and identification by immunofluorescence test using virus specific monoclonal antibody) was carried out in order to define a suitable strategy for monitoring Japanese encephalitis virus infection in field mosquitos. A total of 8,850 adult female mosquitos in 177 pools (Culex tritaeniorhynchus 91, Cx. vishnui 59 and Cx. fuscocephala 27) collected from an endemic area of Tamil Nadu were examined by both the techniques. In ELISA, 9 pools which had optical densities (OD) equal to the mean of normal infected pools plus > or = 4 standard deviations (SD) mean considered positive and all of them were virus positive by the bioassay also. Sixty-five pools had OD = Mean + 2-3 SD and 103 pools had OD = Mean + < 2 SD of normal pools. From these groups, 12 (18.5%) and 8 (7.8%) pools respectively were found to be virus positive by the bioassay. In total 29 (16%) pools were positive by the bioassay as against 9 (5%) by ELISA. This study demonstrated that the bioassay is sensitive for estimation of true positives and ELISA is a rapid screening system. A protocol has now been developed for surveillance in which field pools are first screened by ELISA and only those with OD = Mean + > or 2 SD are assayed in Toxorhynchites. By excluding a large majority of pools with low OD (Mean + < 2 SD), which are likely to yield to only a small percentage of true positives, the cost, time and labor involved are greatly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)