a passive surveillance system of encephalitis in equine and avian species was set up to detect any occurrence of clinical signs of wnv infection .
at the same time , a serologic investigation for wnv was conducted in guadeloupe archipelago .
a cross - sectional study was performed on the most susceptible animal species ( birds and horses ) .
maarten island ( 635-185 ) on domestic ducks ( family : anatidae , anas species ) , domestic geese ( family : anatidae , anser sp . ) , and on laughing gulls ( larus atricilla ) , a resident wild species .
the french part of the island belongs to guadeloupe s archipelago and is located 270 km north of the main island .
it is a major resting place for migratory birds , which spend some days or weeks on the ponds before migrating south in the fall ( or north in the spring ) .
therefore , st . martin was chosen to increase the probability of detecting the earliest serologic conversions on resident birds and to prove the circulation of wnv among resident birds and domestic avian species .
a total of 50 ducks and geese from four backyards as well as ducks from the st .
three gulls were caught and released after blood collection . on guadeloupe island ( 6130-1615 ) , blood samples were drawn from 20 chickens from two different farms neighboring a horse - riding center in december 2002 . the survey on horses was planned to be as exhaustive as possible in guadeloupe and the nearby island of marie galante ( 6115-1555 ) .
serum samples from 360 of 400 horses thought to live in guadeloupe were collected in july 2002 ( figure ) . in locations where positive horses were detected during the first survey ,
another sampling was drawn from horses from december 2002 to january 2003 to measure the rate of serologic conversion and the incidence of the infection .
insert : locations of horse and chicken sampling places in guadeloupe and marie galante islands .
red triangle , farm with seropositive chicken ; blue circle , equine center with seropositive horses identified from july 2002 ; gold circle , equine center with seropositive horses identified from december 2002 to january 2003 ; black circle , equine center with no seropositive horses .
enzyme - linked immunosorbent assays ( elisa ) were performed to detect specific immunoglobulin ( ig ) g antibodies to wnv in horses , ducks , geese , and chickens .
additional immunocapture igm elisa was performed on horses positive for wnv by igg elisa ( 8) .
most positive serum samples were tested by plaque reduction neutralization test ( prnt 80 ) for both wnv and st .
all the birds ( 36 ducks , 14 geese , and 3 gulls ) sampled from five farms and one pond in st .
maarten at the end of july 2002 tested negative for wnv by igg elisa ( the elisa test for the gulls has not been validated ) . in july 2002 ,
10 of 360 horses tested positive for wnv by igg elisa , and 2 of them were also positive by igm elisa .
seropositive horses were located in four different places , two in guadeloupe and two in marie galante .
the results of the survey undertaken in december 2002 to january 2003 in equine centers where positive animals were detected in july 2002 indicated a high rate of wnv seroconversion in horses in these locations ( table 1 ) . in july 2002 ,
the overall wnv prevalence rate ( igg elisa ) was 2.8% , reaching 10.4% in places where infected horses were found ( locations a , b , c , d ) . in january 2003 , in these and related places ( where some horses were moved from the former areas in july 2002 ) , the prevalence rate increased to 50% .
on paired samples , 54 of 114 horses that tested negative in july 2002 were positive in january 2003 .
the incidence rate calculated for the places where outbreaks were noticed ( a to i ) is 7.9% per month . in december 2002 and january 2003
chickens were collected from two backyards ( 10 chickens in each place ) neighboring one horse - riding club where positive animals were detected .
eleven of these chickens tested positive by igg elisa in december 2002 . to confirm the specificity of the results ,
positive horses samples were tested by prnt against wnv and slev ; all the animals showed a higher titer for wnv than for slev ( table 2 ) .
the serologic survey conducted on horses indicated an active focus of wnv infection in guadeloupe , probably linked to the first infestation of the archipelago by the virus .
the absence of igm antibodies in horses at the end of 2002 indicates that the seroconversions did not occur during the last weeks of the year but earlier .
these results ( i.e. , the presence of igm antibodies in 2 of 10 positive animals in july 2002 ) suggest that the first wmv infections in horses probably occurred during the first 6 months of 2002 and spread in the equine population in the middle of the year . when birds migrate , they cross the lesser antilles ( 6 )
. a migratory bird , infected before leaving north america or the caribbean islands , may develop viremia when reaching st .
martin , marie galante , or guadeloupe islands and transmit the virus to mosquito vectors during the resting period .
migratory birds from the north usually arrive in guadeloupe later than july ; thus , the infection observed in horses in july 2002 was probably not derived from migrating birds that year .
after one or more introductions , the virus may have gradually spread in the local vector populations and amplified in resident birds even over the winter , when vectors are still active in the caribbean .
then , 6 months later , the virus spread to susceptible species ( horses ) , in which it was first detected . in guadeloupe
, both animal and human surveillance systems have been set up and are interacting to detect virus circulation . in that respect ,
the surveillance of susceptible animal species can provide important indicators for the possible appearance of the disease in humans . as shown in the united states ,
the death of wild birds is a pertinent indicator for human risk ( 9 ) . in avian species , mortality and sentinel surveillance
this could be related to the absence in guadeloupe of species known to be extremely susceptible to the infection ( corvidae ) , vector competence , or the virus strain .
a random survey is being implemented on domestic birds to assess the geographic distribution of the infection , in june through july 2003 ( beginning of the rainy season ) , when populations of vectors increase markedly in guadeloupe .
a new serologic prevalence survey in horses is also in process , and clinical surveillance is ongoing . despite a high rate of wnv - seropositive animals , no clinical disease has been observed .
this situation could be related to the virus titer , the rate of infected vectors ( which could be too low during the first year after wnv is introduced ) , or the virus strain . in 2003 , mosquito surveillance was implemented in places where deaths in birds or encephalitis cases in horses were observed .
virus detection using reverse transcription  polymerase chain reaction will be used in our laboratory to test dead birds and pools of mosquitoes .
these surveys are intended to provide the public health services with distribution and prevalence maps .
 to determine whether west nile virus ( wnv ) had reached the archipelago of guadeloupe , a serologic study in horses and birds was conducted in 2002 . 
 immunoglobulin ( ig ) g , igm , enzyme - linked immunosorbent assay , and seroneutralization tests identified wnv infection in horses and chickens . 
 six months later , a high rate of seroconversion was observed in horses . 
The Study
Conclusions