Japanese Encephalitis in South Arcot District, Tamil Nadu, India: a Three-Year Longitudinal Study of Vector Abundance and Infection Frequency

Abstract

In the South Arcot district, an area endemic for Japanese encephalitis in Tamil Nadu, Culex tritaeniorhynchus Giles, Culex vishnui Theobald, Culex gelidus Theobald and Culex fuscocephala Theobald constituted 93.6% of 422,621 adult females representing 27 culicine species collected between August 1991 and July 1994. Vector abundance was lowest in the hot and dry season (April-June) and highest in the cool and wet season (October-December). Overall, 285,531 adult female mosquitoes (5,710 pools) were tested for virus using an enzyme-linked immunosorbent assay or by inoculation into larvae of Toxorhynchites splendens Wiedemann and identification by immunofluorescent test using JE virus specific monoclonal antibody or by both. In total, 91 isolations were made, of which 80 (88%) were identified as JE virus; 58 isolations were from Cx. tritaeniorhynchus, 22 from Cx. vishnui, 6 from Cx. fuscocephala and 5 from Cx. gelidus, giving similar minimum infection rates (MIR) of 0.28, 0.41, 0.39, and 0.52, respectively. Vector abundance and MIR increased from July concurrently with the initiation of rice cultivation. MIR peaked in September followed by a decrease in October, but mosquitoes remained abundant until March. The decrease in MIR from October onward coincided with rising herd immunity in pigs. Although MIRs in October (0.47) and November (0.42) were lower than in September (0.92), a comparable high risk of infection for humans continued because of high vector abundance and human biting rates. In the South Arcot district, the probability of a child receiving an infective bite was 0.53 per JE transmission season.

The Abundance and Biting Patterns of Culex quinquefasciatus Say (Culicidae) in the Coastal Region of Nigeria

“…All the mosquito species were more abundant during the rainy season than during the dry season. This agrees with the patterns observed by Lindsay et al [23] and by Gajanana et al [24]. This is understandable as the rains make more breeding sites available.…”

Nature, Nurture and Evolution of Intra-Species Variation in Mosquito Arbovirus Transmission Competence

“…Although genes will be selected because they increase fitness after exposure to the virus in the small portion of the population encountering the arbovirus, they may decrease in the great majority of mosquitoes not encountering the virus and more so in populations where the arbovirus is not present at all. For naturally occurring infections in mosquitoes, the minimum infection rate (MIR) generally recorded as the number of infected mosquitoes per 1,000, is normally <1 to 10 with rare instances being on the order of 50 for brief periods [182,183,184,185,186,187,188]. The overwhelming proportion of the mosquito population never encounter a virus during an epidemic and more often 100% of the mosquitoes in a population will not encounter an arbovirus during the long inter-epidemic periods when the arbovirus is not present or at low levels in the population.…”

Modelling and assessment of combining gilt vaccination, vector control and pig herd management to control Japanese Encephalitis virus transmission in Southeast Asia

“…A time interval of 14 days (or more) with less than one viremic pig ( I state) in the epidemic dynamic was considered a proxy for epidemic die-out, Indeed, the lifespan of a Culex mosquito is between 21 and 30 days (62), and the intrinsic incubation period varies between 7 and 15 days (63, 64). Assuming a newly emerged female would bite a host and get infected the same day, it would become infectious 7 to 15 days later.…”