host - seeking mosquitoes were collected in and around grand forks , north dakota , by using mosquito magnet traps ( american biophysics corp . , north kingston , ri , usa ) for 4 transmission seasons , from early summer 2002 through fall 2005 .
mosquitoes were sorted by species and tested for wnv by using reverse transcriptase pcr assays .
passerine birds in and around grand forks were surveyed for antibodies to wnv for 3 transmission seasons : june 24october 27 , 2003 , april 4july 7 , 2004 , and may 17august 11 , 2005 . in 2003 and early 2004 ,
birds were captured by using mist nets , blood ( < 0.1 ml ) was obtained by brachial venipuncture , and birds were released . later in 2004 and in 2005 , necropsies were performed on dead birds .
samples were tested for anti - wnv antibodies by using a qualitative epitope - blocking elisa ( 3 ) .
this is the first report of seroprevalence of wnv in passerines in the northern prairies .
a total of 277 birds ( 11 species ) were tested ( table 1 ) . in 2003 , seroprevalence was relatively low ( 17% ) .
the first seropositive bird was captured july 24 , 2003 , 4 days after the first wnv - positive pool of cx .
most seropositive birds ( 11 of 14 ) were collected in september , when migratory species were leaving and vector populations were waning .
thus , a lag occurred between peak abundance of infected vectors in mid- to late august 2003 ( 2 ) and seroconversion of passerines .
seroprevalence rates were significantly higher in 2004 and 2005 than in 2003 ( table 1 , fisher exact tests , p<0.0001 ) and were higher than most seroprevalences reported for passerines in the eastern and southeastern regions of the united states ( 46 ) .
all passerine species sampled in 2004 and 2005 contained seropositive birds , which indicated that all these species were preyed on by vectors regardless of differences in their nesting habitats ( e.g. , cattail marshes , peridomestic ) .
american robins , common grackles , and red - winged blackbirds showed increased seroprevalence from 2003 to 2004 .
high seroprevalence was maintained in passerines in 2005 despite low wnv activity ( i.e. , low natural boosting ) during 2004 ( 2 ) , which suggests that passerine immunity to wnv may last longer than a single season ( 7,8 ) .
previous laboratory and field studies have indicated that most american crows die so quickly from wnv infection that they never have time to seroconvert ( 911 ) . why crows in the red river valley survive wnv infection is not known .
one possibility is that wnv has undergone genetic changes with a concurrent loss in virulence as it spread westward from forest ecosystems with cx .
annual data on seroprevalence in passerines , environmental temperatures , reporting of human cases , and minimum infection rates ( mir ) in vector populations are summarized in table 2 .
time between first and last appearances of host - seeking culex tarsalis mosquitoes in mosquito magnet traps . 
data from north dakota department of public health ( 15 ) .  # no . of wnv - infected cx .
environmental conditions from 2002 to 2005 produced a natural field experiment , which demonstrated the differing magnitudes by which environmental temperature and host immunity affected local wnv activity . despite warm temperatures and high vector abundance , wnv activity was low during its introductory year ( 2002 ) , as indicated by low numbers of human cases , undetectably low seasonal mir , and by low seroprevalence in passerines in 2003 .
because wnv had only recently arrived , presumably there was neither sufficient time nor number of infection nidi to promote extensive amplification cycles . however , 2003 was an epidemic year for wnv , as indicated by the increased number of human cases statewide and the high mir in the local vector population .
the relatively low level of immunity in passerines at the time also likely contributed to the epidemic . in 2004 , unusually cool environmental temperatures prolonged vector larval development , adult emergence , and the arboviral extrinsic incubation period . as a result ,
duration of the 2004 transmission season was nearly half that of the preceding seasons ( table 2 ) .
thus , wnv activity during 2004 was low ( i.e. , reduced vector abundance , number of human cases , and seasonal mir ) , and the virus had insufficient time to undergo extensive amplification cycles , similar to the situation that occurred during the introductory year of 2002 .
however , the epidemic conditions of 2003 had produced a high level of herd immunity in the local bird population in 2004 .
( note : most migratory passerine species live for several years and return each year to the same general locale to breed . ) in 2005 , environmental temperature , length of transmission season , and vector abundance were all nearly identical to those of the epidemic year of 2003 . yet
the high prevalence of immunity during 2005 may have contributed to preventing another epidemic , but it did not totally eliminate wnv activity .
levels of wnv activity during 2005 ( as measured by human cases and mosquito mir ) were intermediate between those of the epidemic year ( 2003 ) and both the introductory year ( 2002 ) and the coldest year ( 2004 ) .
thus , environmental temperature dictated the ultimate success ( or more precisely , the failure ) of wnv amplification within the central red river valley during 2004 , whereas even high levels of herd immunity among the reservoir population exerted only a moderating effect on wnv activity during 2005 . whether the level of wnv activity observed during 2005 will be representative of the arbovirus 's natural equilibrium within the central red river valley remains to be seen .
is wnv reintroduced every spring through infected migratory birds or wind - blown mosquitoes from the south ? or does wnv survive the harsh winters inside mosquitoes undergoing diapause , only to reemerge in the spring ? either or both of these scenarios could be correct , but one conclusion is certain : the incidence of wnv disease in horses from north dakota during june of 2002 and again in may of 2005 ( 15 ) indicates that wnv becomes active in the northern great plains well in advance of the first summer brood of its primary vector , cx .
 west nile virus ( wnv ) epizootiology was monitored from 2002 through 2005 in the area surrounding grand forks , north dakota . 
 mosquitoes were tested for infection , and birds were surveyed for antibodies . in 2003 , wnv was epidemic ; in 2004 , cool temperatures precluded wnv amplification ; and in 2005 , immunity in passerines decreased , but did not preclude , wnv amplification . 
The Study
Conclusions