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Abstract & Introduction

Background

Materials & Methods

Results & Discussion

References

Appendices

RESULTS AND DISCUSSION

Positive West Nile virus results were found at the following locations (Table 1).

Based upon the 2002 collections made by IUS in southern Indiana, we expected to find positive mosquito pools in our area. Mosquito species testing positive in Floyd, Clark and Harrison Counties for West Nile virus included three genera—Culex, Aedes, and Anopheles. All species testing positive are on the CDC list of mosquitoes that have previously tested positive for WNV (24).

The presence of Culex pipiens/restuans at 1905 Grantline Rd. and Jenny Lane behind the Scottsville Road Fire Station in Floyd County and on 1900 Winburn Drive in Clark County was not surprising. Each site offers ideal habitats for Cx. pipens/restuans. Silverly (1972) notes these mosquitoes are prevalent in old tires, ditches and stagnant roadside pools (23). The Grantline Road location is adjacent to an urban stream, Falling Run and a failed sewer line was noted in the vicinity (Craig Lalor, personal communication). The high levels of precipitation in the summer of 2003 and the presence of urban pollution provide an ideal habitat for this species.

Cx. pipiens/restuans are known to thrive in areas that provide high levels of organic material ideal for breeding. The Jenny Lane location in Floyds Knobs, IN is an area in which failing septic systems have been documented by the Floyd County Health Department. Figure 13 illustrates the total numbers of mosquitoes collected by IUS in 2003. The Pauls Lane and Jenny Lane numbers in Floyd County are indicated individually as they are from the only locations in all three counties where traps were set directly by failing septic systems. These data clearly indicate that faulty septic systems are ideal breeding sites for Cx. pipiens/restuans mosquitoes.

A literature review provided some insight into mosquito population dynamics. In 1976, IUS students completed a study of mosquito larvae in our area to discover arbovectors of St. Louis encephalitis. The study is illuminating because these researchers discovered that early mosquito larvae pools in Floyd County are dominated by Culex restuans. Culex pipiens were taken much later in the season beginning in late June (25).

In studies in Connecticut in 2000, scientists discovered a similar pattern with very early amplification of the WNV virus associated with Culex restuans populations (26). Culex pipiens is highly ornithophilic, and it is probable that this group is responsible for the dramatic further amplification of the virus in July and August. We began our studies in June, but we observed that in late August and early September large numbers of dead birds were being reported. Crows with very high viremia are thought to remain sedentary for several days prior to dying providing ideal hosts for the spread of the virus to other mosquito species. It is probable, also, that other urban birds such as house sparrows, starlings and pigeons act as reservoirs for the virus. These birds, although they may carry the disease, rarely become victims (27). Connecticut scientists also did not discount the presence of vertical transmission of the virus in their state. Overwintering Culex with the virus have been found in NYC, and male Culex sp. harboring the virus have been found in other countries (26).

The model of Culex restuans followed by Cx. pipiens and other species appears to mesh with our particular geographic region. For example, starlings banded in Arkansas were recovered in New York State suggesting that the birds may have flown through the Ohio River Valley (28). It is interesting that Arkansas was one of the first states west of the Mississippi to report the presence of dead infected birds in 2001.

The species testing positive in Harrison County for West Nile Virus was the Aedes vexans mosquito. Ae. vexans is known as an “inland floodwater” mosquito for good reason. They overwinter as eggs singly on moist, but not waterlogged ground in areas that are likely to be subject to flood or inundation by rain, seepage, or flood water at some future point (29). There is also research to suggest that Ae. vexans prefers the presence of a dense cover of detritus on the soil in selection of oviposition (29). After a period of drying, eggs hatch when the oviposition site is flooded (29). Not all eggs hatch at once, however, and as long as eggs are kept moist, some of the eggs will not hatch until after subsequent flooding, sometimes a year or more after initial flooding occurs (29). Hatching is stimulated by a low level of dissolved oxygen in the water, created by a high number of bacteria and yeasts. The Blue River location with flooding and rainwater pools is ideal for the growth of this species. It did not come as a surprise to find these mosquitoes in abundance in the Blue River flood plain along Harrison Springs Road.

West Nile virus appears established in Clark, Floyd and Harrison Counties. It is likely that many individuals in our counties exposed to the virus now have immunity. Similarly, living birds exposed to the virus now have immunity. As more individuals are infected and become immune to the virus, the number of acute cases will decline in proportion to those infected. In parts of Africa, the number of immune individuals in the population is approaching 100%. Thus in a few years, the rate of West Nile infections, although epidemic at this time, is expected to decline.

Unlike St. Louis encephalitis, WNV has had a novel devastating impact on some native bird populations. Crows, blue jays, and some species of hawks are especially susceptible to infection by WNV, and have been killed by the virus. The long-term recovery of these species will be an interesting aspect of the overall WNV outbreak.