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Arbovirus Survey Data

"Arbovirus" is the term used for viruses that are spread by mosquitoes to humans usually from birds. The illness caused by arboviruses in humans can be mild to severe and can even result in death. Therefore early identification of the presence of these viruses is important in order to the control vector species that transmit them.

The Vector-Borne Disease Prevention Program (VBDPP) staff works with the Department of Agriculture Animal Diseases Laboratory (ADL) to conduct tests for the presence of arboviruses throughout the State. The arbovirus testing program includes West Nile virus (WNV), St. Louis encephalitis virus (SLE), and Western equine encephalomyelitis virus (WEE). The objective of this testing is to provide an early warning about the threat of virus transmission to focus mosquito control efforts and to protect the public from becoming infected.

Capturing adult mosquitoes in carbon dioxide traps and pooling them in batches of 50 for arbovirus testing proved to be the most valuable surveillance method for WNV in 2005. This method provides critical information for focusing vector mosquito control and will continue to be used as the main surveillance method in 2006 and beyond. Employees from the VBDP in collaboration with the State Entomologist's staff collect and test mosquito pools from all over the State.

The VBDPP staff also collects blood from five sentinel chicken flocks that reared in various locations throughout Washoe County. The flocks are tested bi-weekly for all three arboviruses. This testing program has been in place for over a decade for SLE and WEE; in 2001, WNV surveillance testing was added. Usually sentinel chicken blood tests show positive results weeks before any human cases of arboviral illness appear.

If any of these tests turn out positive in Washoe County, the VBDP staff quickly responds with abatement methods to isolate and kill infected mosquitoes so that humans and domestic animals are protected.

Testing certain species of dead birds for WNV was an important surveillance method for the early detection of virus early on. During the 2004 season 65.8% of the dead birds picked up tested positive for WNV. In 2005 the percentage of birds that tested positive for WNV dropped to only about 9% showing that the value this method of surveillance for WNV has decreased. In 2006 VBDP will not pick up dead birds for WNV testing.

The presence of WNV continued in 2005 in Nevada and Washoe County in 2005 as shown by the test results in the following table:

Arbovirus Testing - 2005
Type Number of Tests Number Positive Percent Positive
Sentinel Flocks1 732 0 0.0
Dead Birds - Washoe County 72 9 12.5
Mosquito Pools - Washoe County 224 31 13.8
Mosquito Pools Statewide2 1,601 86 5.4

1. 732 samples tested for WEE, SLE (= 1,464 tests) and includes Washoe, Churchill, Humboldt and Lander counties

2. Pools also tested for WEE and SLE for a total of 4,803 tests. Pools included 46,055 mosquitoes.

WNV was first identified in Washoe County and Statewide in Nevada during the 2004 summer season as a result of surveillance testing on sentinel chickens, dead birds and mosquito pools. The following table shows the 2004 testing results for WNV:

WNV Testing - 2004
Type Number of Tests Number Positive Percent Positive
Sentinel Flocks1 1,584 6 0.4
Dead Birds2 79 52 65.8
Mosquito Pools3 744 160 21.5

1. Includes Washoe, Churchill, Humboldt and Lander counties

2. Washoe County only

3. The total number of mosquitoes pooled and tested equals 25,592 collected from all 17 counties in Nevada.

All of above samples were also tested for SLE and WEE in 2004. All were negative except for one mosquito pool in Fernley that tested positive for SLE.

In 2003 all tests were negative for WNV, SLE and WEE as indicated in the table below:

Arbovirus Testing - 2003
Type Number of Tests Number Positive Percent Positive
Sentinel Flocks1 3,000 0 0.0
Dead Birds2 155 0 0.0
Mosquito Pools3 167 0 0.0

1. One thousand samples from Washoe and Churchill Counties were tested for WEE, SLE and WEE totaling 3,000 tests.

2. Washoe County only.

3. The total number of mosquitoes pooled and tested equals 6,143; this figure does not include all the samples collected in Clark County.

Arbovirus testing for mosquito pools is a cooperative effort of Washoe County Health District Vector-Borne Disease Prevention Program and the Nevada Department of Agriculture, Animal Disease Laboratory (ADL). Additional lab results from ADL including testing on horses can be found at http://agri.nv.gov/Animals/Animal_Disease/Animal_Diseases_Food_Safety_Home/. Maps showing surveillance results by county nationwide may be viewed at http://diseasemaps.usgs.gov/wnv_us_human.html.