Evaluation of surveillance methods for detection of West Nile virus activity in East Baton Rouge Parish, Louisiana, 2004-2006.

A 3-year study was conducted to determine if testing mosquitoes collected in modified sentinel chicken boxes for West Nile virus (WNV) or testing sentinel chickens for WNV antibody would detect WNV activity before onset of human cases in East Baton Rouge Parish, LA. In each year mosquitoes tested positive for WNV before the onset of human cases were detected, but seroconversions of sentinel chickens were detected after the human cases occurred. In 1 year we also compared the effectiveness of Centers for Disease Control and Prevention (CDC) light traps, gravid traps, and sentinel chicken box traps for collecting WNV-positive mosquitoes. Gravid traps collected more WNV-positive mosquitoes than CDC light traps or sentinel chicken box traps. However, WNV was detected earlier in mosquitoes collected from sentinel chicken box traps than in mosquitoes collected with gravid traps or CDC light traps. In total, 1,222 pools containing 19,353 mosquito specimens representing 18 species were tested for WNV. West Nile virus was detected in 59 mosquito pools from 4 species; 87% of the positive pools were detected from Culex quinquefasciatus, which was the most abundant species collected in all 3 years.

West Nile virus detection in mosquitoes in East Baton Rouge Parish, Louisiana, from November 2002 to October 2004.

The prevalence of West Nile virus (WNV) was determined in mosquitoes between November 2002 and October 2004 in East Baton Rouge Parish, LA. A total of 244,374 female mosquitoes were collected and tested by viral isolation. Additionally, 131,896 female mosquitoes were collected in 2003 and tested by VecTest and 167,175 female mosquitoes were collected in 2004 and tested by reverse transcriptase-polymerase chain reaction (RT-PCR). West Nile virus was isolated by cell culture from 17 (47.2%) out of 36 mosquito species collected over the study period. In 2003, WNV was detected in 9 (33.3%) out of 27 species tested by VecTest. In 2004, 14 (50%) out of the 28 mosquito species tested by RT-PCR were positive for WNV. The species with the greatest number of WNV-positive pools detected by all 3 testing methods was Culex quinquefasciatus. A significantly greater proportion of Cx. salinarius pools collected in light traps placed at a 3-m height were positive for WNV by viral isolation than in pools collected in light traps placed at a 1.5-m height.

Evidence of vertical transmission of West Nile virus in field-collected mosquitoes.

Male and nulliparous female mosquitoes were surveyed for evidence of vertical WNV infection in East Baton Rouge Parish, Louisiana. Adult male mosquitoes collected by trapping and aspiration, and adult male and nulliparous female mosquitoes reared from field-collected larvae were tested. Adult male Culex spp., female Aedes albopictus (Skuse), and female Culex quinquifasciatus Say mosquitoes that were collected as larvae were test-positive for WNV RNA. Infectious WNV was detected using virus isolation in field-collected male Aedes triseriatus Say and Culex salinarius Coquillett; these data represent the first field evidence of vertical transmission of WNV in Ae. triseriatus and Cx. salinarius.