# Presentation: 756671

- Michael J. Turell, Michael R. Sardelis, David J. Dohm, and Monica L. O’Guinn
- U.S. Army Medical Research Institute of Infectious Diseases, Fort Detrick, Maryland

- Potential for North American Mosquitoes to transmit
- West Nile Virus

## Joseph Dudley, Ph.D. U.S. Army Environmental Programs Directorate

**Joseph Dudley, Ph.D. ****U.S. Army Environmental Programs Directorate**

**Versar, Inc.**

## West Nile Viral Activity in the Continental US

- Year		Human	     Horses      States
- 1999		62		25	4
- 2000		31		63	12
- 2001		66		733	27
- 2002		2,741		14,901	44

## Overview of West Nile virus

**West Nile virus**

**Mosquito vector**

**Bird reservoir hosts**

**Overview of West Nile virus**

**Incidental  infections**

**Incidental  infections**

- WHY SHOULD WE
- STUDY THE DISEASE
-  TRANSMISSION CYCLE?

## A BETTER UNDERSTAND OF THE NATURAL TRANSMISSION CYCLE ALLOWS US TO:

**  ****learn which vectors and vertebrates are involved in the transmission cycle**
**  ****predict when disease outbreaks may occur**
**  ****prevent these outbreaks by:**
**     ****- controlling the vector**
**     ****- controlling the reservoir host**
**     ****- vaccinating the susceptible population**

- WHAT DO WE KNOW
- ABOUT POTENTIAL
-  VECTORS IN
- NORTH AMERICA

- DETECTION OF
- WEST NILE VIRUS IN
-  FIELD-COLLECTED
-  MOSQUITOES

## Field isolates of WNV from NA mosquitoes

**	****	****	****	****	****	****	****	****	****	****	****	****	**
***Culex pipiens***
***Culex restuans***
***Culex nigripalpus***
***Culex quinquefasciatus***
***Culex salinarius****** ***
***Culex tarsalis***
***Culex territans***
**	*****	***
***	******	******	******	***

## Field isolates of WNV from NA mosquitoes

**	****	****	****	****	****	****	**
***Oc. canadensis ******	******	****** ******	******Ae. aegypti***
***Oc. cantator ******	****** ******	****** ******	******Ae. albopictus ******	***
***Oc. japonicus******	******	****** ******	******Ae. ******cinereus ***
***Oc. sollicitans******	******	****** ******	******Ae. vexans****** ***
***Oc. triseriatus***
***Oc. trivittatus****** *****	**
**	*****	***
***	******	******	******	***

## Field isolates of WNV from NA mosquitoes

**	****	****	****	****	****	****	****	****	****	****	****	****	**
***An. punctipennis******	****** ******	******	******Cq. perturbans****** ***
***An. quadrimaculatus******	****** ******	******	***
***	******	******	******	******Cx. (Dei.) cancer***
***Ps. columbiae****** ***
***Ps. ciliata******	******	******	******Cs. inornata******	******	***
***Ps. ferox******	******	*****	*****Cs. melanura***** **
**	**
***Or. signifera******	******	******	******Ur. sapphirina***** *****	***
***	******	******	******	***

## Vector Incrimination

## Criteria for Vector Incrimination

**Repeated isolations of virus from field-collected individuals of that species**** **
**Susceptibility of the arthropod to infection in the laboratory**** **
**Ability of the arthropod to transmit the virus in the laboratory**** **
**  ****Association in nature between the arthropod and naturally infected vertebrate hosts**** **
**  ****A temporal association between the arthropod’s activity and virus transmission   ****	****	**

## Vector Competence

- Virus in the blood meal, but mosquito not infected

- Mosquito infected, but limited to midgut

- Virus disseminated to hemocoel,
- but salivary glands not infected

- Salivary glands infected, ready to transmit by bite

## STUDY PROCEDURE

**ALLOW MOSQUITOES TO FEED**
**ON INFECTED CHICKEN**
**	****	****	****ENGORGED                      UNENGORGED**
**         ****	****	****(Hold 12-14 d)           ****	****    (Inoculate for**
**	****	****	****	****	****                  ****transmission study)**
**	****	****            ****REFEED****	****	**
**	****	****	****	****	****	**
**	****	****        ****TRITURATE**
**	****	****     ****(Legs + Bodies)**
**	****	****	****	****	****      ****ASSAY FOR VIRUS**

**Vector competence NA mosquitoes for WNV**

## Potential vectors of West Nile virus

*	**Ae. vexans*
*Cx. salinarius* *	**	**Cq. perturbans*
*Cx. tarsalis* 	*Cx. nigripalpus** **	*
*	**Cx. pipiens** **	**Oc. canadensis *
*Oc. atropalpus *	*Cx. quinquefasciatus**	**Oc. cantator*
*Oc. j.  japonicus* *	**Cx. restuans** **	**Oc. sollicitans *
- 	 *	**Oc. taeniorhynchus*
*	**Oc. triseriatus **	*
*	*

**  ****Efficient****	****Moderate****	****Inefficient**

**Potential vectors of West Nile virus**
**based on laboratory vector competence studies**

## Infection and dissemination rates for mosquitoes that ingested 107.0 +0.5 PFU/ml of West Nile virus 

- 	**      ****	****	**** ****Number****	**** Infection ****	**** Dissem.**
**      ****Species****	****  tested****	**** rate (%) ****	**** rate (%)**
***Ae. vexans*****	****	****75****	****	****44****	****	****17**
***Cx. nigripalpus*****	****	****127****	****	****84****	****	****12**
***Cx. pipiens*****	****	****95****	****	****81****	****	****23**** **
***Cx. tarsalis*****	****	****71****	****	****96****	****	****86**
***Oc. triseriatus*****	****	****28****	****	****32****	****	****25**
***Ps. ferox*****	****	****24****	****	****33****	****	****0**
- Department of Vector Assessment, Virology Division, USAMRIID

- Virus disseminated to hemocoel,
- but salivary glands not infected

## Transmission rates for mosquitoes with a disseminated infection with West Nile virus 

- 					**	****	****	**
**      ****	****	****	**** ****Number****	****  Number****	**** Trans. ****	**
**      ****Species****	****    fed****	****	****    trans.****	****   rate ****	****     **      _                                                                                                                                                 _
***Ae. vexans*****	****	****   16****	****	****15****	****	****94****	****	**
***Cx. nigripalpus*****	****	****15****	****	****13****	****	****87****	****	**
***Cx. tarsalis*****	****	****6****	****	****6****	****	****100****	****	**
***Cq. perturbans*****	****	****17****	****	****4****	****	****24****	****	**
***Oc. triseriatus*****	****	****3****	****	****2****	****	****67****	****	**
***Ps. ferox*****	****	****4****	****	****0****	****	****0**
**	**
- Department of Vector Assessment, Virology Division, USAMRIID

## Transmission rates for mosquitoes with a disseminated infection with of West Nile virus 

- 					**	****	****	**
**      ****	****                               ****Number****	****Number****	**** Trans. ****	**
**      ****Species****	****  fed****	****	****  trans.****	****   rate ****	****     **      _                                                                                                                                                 _
***Ae./Oc. spp.*****	****	****88 ****	****	****80****	****	****91****	****	**
***Cx. (Cul.) spp.*****	****	****75****	****	****63****	****	****84****	****	**
***Cq. perturbans*****	****	****17****	****	****4****	****	****24****	****	**
***Ps. ferox*****	****	****4****	****	****0****	****	****0**
**	**
- Department of Vector Assessment, Virology Division, USAMRIID

## ENVIRONMENT

**ENVIRONMENT**

**VIRUS**

**HOST**

**VECTOR**

## Effect of Environmental temperature

**ALLOW MOSQUITOES TO FEED**
**ON INFECTED CHICKEN**
**	****	****	****     ****ENGORGED          UNENGORGED (discarded)**
**         ****	****	****	****	****	****	**
**            ****17 C for 7 days****	****	**
**then transfer to 10 C for 1 month          26 C for 14 days **
**	****	**
**	****	****10 C****	****	****	****26 C****	****	****	****	**
**	****	****       **
**	****	****	**** ****TRITURATE****	****	****      ASSAY FOR VIRUS**

**Effect of over-wintering temperature**

## Effect of over-wintering temperatures

**           ****Days at        Days at****	**** No.                      ****	**** % **
**             ****10****o****C****	****              26****o****C          Tested                 Infec.**
**   **
**	****	****>1 mo****	****0****	****50****	****0**
**  **
**	****	****1 mo****	****7 ****	****37****	****81 **
** **
**	****	****0****	****14****	****23****	****96 **
**	****	****1 mo****	****14 ****	****12****	****100 **
** **

## Effect of over-wintering temperatures

**              ****Days at            Days at****	**** No.                        ****	****%   ****	****    **
**	****             ****10****o****C                26****o****C                Tested                  Infected**
**   **
**	****	****> ****1 mo****	****0****	****50****	****0**
** **
**	****	****41****	****1 ****	****16 ****	****31**
** **
**	****	****39****	****3****	****13****	****69**
** **
**	****	****37****	****5 ****	****13****	****69   **
** **
**	****	****35****	****7 ****	****13****	****54**
** **** **
**	**

## Effect of Environmental temperature on Vector Competence

**ALLOW MOSQUITOES TO FEED**
**ON AN INFECTED CHICKEN**
**	****	****	****     ****ENGORGED          UNENGORGED ****	****	****	****	****	****	****	****(discarded)**
**         ****	****	****	****	****	****	****	****	**
**         ****Place in cardboard cages maintained at **
**	****	**
**	****	****18 C****	****	****20 C   ****	****26 C****	****	****30 C**
**	****  ****ASSAY FOR VIRUS AT SELECTED INTERVALS**

**Effect of environmental temperature**

## Days of Extrinsic Incubation

**Days of Extrinsic Incubation**

**Mean Body Titer**

**EFFECT OF TEMPERATURE ON VIRAL REPLICATION**

## Days of Extrinsic Incubation

**Days of Extrinsic Incubation**

**Percent Disseminated**

**EFFECT OF TEMPERATURE ON VIRAL DISSEMINATION**

## Slide 34

## Vertical transmission

**Species                     Male       ****	****Female****	****Totals****	****MFIR**
** **
***Cx. pipiens******	*****1,633 (5)****	****1,657 (1)****	****3,290 (6)****	****1.8**
** **
***Ae. albopictus*****	****6,704 (0)****	****6,739 (0) ****	****13,443 (0)****	****<0.1**

## Bionomics of potential vectors

**Host preference**
**Population density **
**Biting behavior **
**Longevity**
**Feeding time**
**Seasonallity**

## Potential vectors

**Enzootic/maintenance:  **
**Epizootic/epidemic**
**Minor/incidental**

## Enzootic/maintenance

**Principally avian feeders**
**Competent vectors**
**Do not need to be involved in transmission to humans or horses**

## Epizootic/epidemic(Bridge vectors)

**General feeders**
**Competent vectors**
**May not be able to maintain infection in nature without enzootic vectors**

##              SUMMARY

**Field isolates (PCR-positive pools)**
**Isolates from more than 30 distinct mosquito species**
**Vast majority from *****Culex *****(*****Culex*****)***** spp*****. **

##              SUMMARY

**Vector competence**
- Most *Culex* (*Culex*) spp. were competent, though only moderately efficient, laboratory vectors of WNV.
- Ae. *albopictus*, *Oc. japonicus*, and *Cx. tarsalis* were the most efficient laboratory vectors tested.
- With very few exceptions, the transmission rate for individuals with a disseminated infection was high (>75%).

##              SUMMARY

**Bionomics**
**Characteristics of *****Culex***** spp. support the role of these mosquitoes in maintaining WNV in nature**
**Selected *****Aedes***** and *****Ochlerotatus***** species probably serve as bridge vectors transmitting WNV from the *****Culex*****/avian cycle to humans and equines**** **