# Presentation: 003366

## Isolation of West Nile Virus from Mosquitoes and the Importance of Specific Vector Identification

- Harry M. Savage, PhD Research Entomologist
- Centers for Disease Control and Prevention
- Ft. Collins, CO

## Data needed to assess vector  status

**Field infection rate**
**Correct identification and pooling**
**Wide taxonomic coverage of taxa processed**
- Vector competence - Transmission rate
- Blood host preference
- Species abundance and longevity

## Mosquito surveillance, June – October 2000: number of WN virus positive mosquito pools by state with date of first positive,  n = 480

- New York –    358 pools (75%), July 7
- New Jersey –    58 pools, July 18
- Pennsylvania -  46 pools, August 29
- Connecticut –   14 pools, July 11
- Massachusetts – 4 pools, August 8

## Mosquito Species Positive for WN virus, June – October, 2000, 14 nominal species, 17 taxa in 5 genera, n = 480

- 428 isolates (89%) from *Culex* mosquitoes
- 226 from *Cx. pipiens/restuans*
- 131 from *Cx. pipiens*
-  36 from *Cx. salinarius*
-  11 from *Cx. restuans*
-  24 from *Culex* spp.
- 52 isolates (11%) from other genera

## Mosquito Species Positive for WN virus, June - October, 2000 52 isolates (11%) from non-Culex mosquitoes

*Aedes vexans*  - 16 isolates
*Ochlerotatus  japonicus*    - 9 isolates
*Oc. triseriatus*   - 9 isolates
*Oc. trivittatus*    - 8 isolates
*Culiseta melanura* - 3 isolates
*Ae. albopictus, Anopheles punctipennis, Oc. atropalpus, Oc. canadensis, Oc. cantator,  Psorophora ferox* - 1 isolate each

## Mosquito species yielding more than a single WN virus isolate from five NE states, 2000

| Taxa | No. isolates | No. tested | GMIR | 95 %
C.I.  |
| --- | --- | --- | --- | --- |
| Cx. pipiens | 131 | 93,728 | 1.4 | 1.2-1.7 |
| Cx. restuans | 11 | 20,167 | 0.5 | 0.3-1.0 |
| Cx. salinarius | 36 | 26,370 | 1.4 | 1.0-1.9 |
| Ae. vexans | 16 |  56,446 | 0.3 | 0.2-0.5 |
| Oc. japonicus | 9 |   8,137 | 1.1 | 0.6-2.1 |
| Oc. triseriatus | 9 |   4,560 | 2.0 | 1.0-3.8 |
| Oc. trivittatus | 8 |  40,342 | 0.2 | 0.1-0.4 |
| Cs. melanura | 3 |  30,596 | 0.1 | 0.0-0.3 |

| Taxa | GMIR | Transmission rate* | Bloodmeal hosts |
| --- | --- | --- | --- |
| Cx. pipiens | 1.4 | 20% | Avian |
| Cx. restuans | 0.5 | 67% | Avian |
| Cx. salinarius | 1.4 | 54% | Mammal/Avian |
| Ae. vexans | 0.3 | 8% | Mammal |
| Oc. japonicus | 1.1 | 64% | Mammal/Avian? |
| Oc. triseriatus | 2.0 | ND | Mammal |
| Oc. trivittatus | 0.2 | ND | Mammal |
| Cs. melanura | 0.1 | ND | Avian |

- * Experimental data from M. Turell et al. USAMRIID

## Comparison of Molecular and Morphological Identification of 45 blood-fed, field collected Culex specimens from NYC

| Mol. ID | Cx. salinarius | Cx. pipiens | Cx. restuans |
| --- | --- | --- | --- |
| Cx. salinarius | 10 | 0 | 0 |
| Cx. pipiens | 0 | 20 | 0 |
| Cx. restuans | 0 | 3 | 12 |

**Morphological  Identification**

- Overall proportion correct (42/45) – 93 %
- pipiens vs. restuans proportion correct (32/35) – 91%

## Conclusions

- Field infection rates for each species are essential
- Improve quality of mosquito identification
- Increase taxonomic coverage of taxa tested
- Delineation of epizootic and epidemic vectors
- Better control strategies

## West Nile Virus Isolates from Mosquitoes in the U.S., 2000

**West Nile Virus Isolates from Mosquitoes in the U.S., 2000**