we report an arboviral outbreak that occurred in gabon , central africa , from march through july 2007 , which showed the unexpected extent of the spread of ae . we also describe its association with atypical epidemiologic characteristics such as the co - circulation of chikv and denv-2 and the frequency of human co - infections . the outbreak centered on the capital of gabon ; peaked from april through may 2007 , in the heat of the long wet season ; and subsequently moved north , where the virus sequentially reached several small towns along the route to northern gabon and cameroon ( figure 1 ) . patients with suspected cases exhibited a dengue - like syndrome , including fever , arthralgia , and asthenia . conjunctival hemorrhage , maculopapular rash , headache , and vomiting were also observed in the most severe cases . distribution of the outbreak and location of the 7 towns where suspected cases have been laboratory confirmed by using quantitative reverse transcription chikungunya cases are represented by red circles , dengue cases by blue circles , and cases negative for the viruses by green circles . testing methods are described in the footnote to the table . during the course of the outbreak , 773 early blood samples ( i.e. , obtained during the first week after the onset of the disease ) were collected from febrile patients who visited identified medical health centers in libreville and other towns in gabon ( table ) . samples were tested for the presence of various arboviral rna genomes by using the taqman quantitative reverse transcription  pcr ( qrt - pcr ) technology and specific primers and probes ( protocols available upon request to the corresponding author ) . among these 773 patients , 275 and 54 were positive for chikv and denv , respectively , during may and july 2007 ( table ) , with 8 cases of co - infections . using a dengue serotype - specific qrt - pcr assay , we showed that all denv patients were positive for serotype 2 ( denv-2 ) . in all 7 towns investigated on the route from libreville to cameroon ( 530 km ) , both chikv and denv-2 human cases were reported , except in cocobeach where only laboratory denv-2 confirmed cases were observed ( figure 1 ) . * chikv , chikungunya virus ; denv-2 , dengue-2 virus ; + , positive . rna was extracted from 50 l of plasma by using the abi prism 6100 nucleic acid prepstation according to the manufacturer s recommended procedures ( applied biosystems , foster city , ca , usa ) . fifty - microliter aliquots of extracted rna were then used in 100-l high capacity cdna synthesis reactions according to the manufacturer s instructions ( applied biosystems ) . finally , 10 l of each cdna reaction was then used as template for 50-l quantitative pcrs that contained 200 nmol / l of probe and 900 nmol / l of each primer . the quantitative pcrs were then thermo - cycled in a 7500 real - time pcr system ( applied biosystems ) according to manufacturer s recommended procedures . the probe used for the chikv , denv , and denv-2 assays were fam - labeled with tamra quencher ( applied biosystems ) . to investigate this atypical scenario further , we analyzed 4,807 mosquitoes belonging to various species of aedes ( 2,504 ae . sp . ) , anopheles ( 78 an . gambiae ) and mansonia ( 120 m. africana , 123 m. uniformis ) in 15 different locations in libreville where chikv or denv-2 laboratory confirmed human cases were detected . pools of 20 mosquitoes ( constituted according to species and place of collection ) were homogenized by using genogrinder 2000 ( ops diagnostics , bridgewater , nj , usa ) technology , and then tested for chikv and denv-2 by qrt - pcr . albopictus were positive for chikv and denv-2 , respectively , while no group containing other mosquito species was positive , indicating that ae . these data provide evidence for the presence of chikv and denv in gabon and for their transmission to humans by ae . these epidemiologic results also confirm our previous observation that chikv strains isolated during the gabon outbreak in 2007 belong to the central african lineage and harbor the a226v mutation as a result of adaptation to ae . more surprisingly , our results show that the spread of this mosquito in an area previously occupied predominantly by ae . one denv-2 strain ( designated as libreville 2007 ) , isolated from 1 febrile patient by using e6 vero cells was further characterized by full - length genome sequencing ( 10,695 nt ) . phylogenetic analysis showed that the denv-2 gabon 2007 strain belongs to the cosmopolitan , rather than the sylvatic , genotype ( figure 2 ) . this cosmopolitan genotype includes mainly asian but also related strains isolated in india , australia , mexico , the indian ocean , and africa ( uganda , somalia , and burkina faso ) , presumably the result of travel to these remote locations by viremic patients or the transportation of commercial goods by ship . phylogenetic relationships among dengue-2 virus ( denv-2 ) isolates based on full - length sequences ( 10,695 nt ) . a total of 85 denv-2 genomes were compared with the human isolate obtained during the gabon outbreak . a neighbor - joining tree was constructed by using mega version 3.2 ( www.megasoftware.net ) with the kimura 2-parameter corrections of multiple substitutions . branches are scaled according the number of substitutions per site , and the branch leading to the thailand 94 strain was shortened for convenience . taken together , these findings document chikv and denv-2 co - circulation that resulted in large simultaneous outbreaks in regions where ae . notably , we identified 8 patients with blood samples that tested positive for the presence of both chikv and denv-2 genomes , indicating co - infection of these patients by both viruses . however , while unlikely , genetic exchanges between the 2 viruses , either by recombination or complementation , are not definitively excluded . clinical examination of these patients ( all adults , 5 women and 3 men ) did not identify specific or severe symptoms , although given the limited number of cases and clinical and biologic investigations , this observation should be interpreted with caution . although the denv cases were few , 8 of 48 ( 17% ) denv-2 positive patients from towns affected by the 2 outbreaks tested positive for chikv ( table ) . extrapolation of this result suggests that the total number of denv-2 patients who are superinfected with chikv is likely to be high , which suggests that denv-2 infection is not the antagonist for a secondary chikv infection . in contrast , only 3% of chikv+ patients were also denv-2 + ; however , the starting period of time of infection or the sequence of infection by the 2 viruses can not be assessed . although concurrent infections of dengue and chikungunya have been reported ( 14 ) , such denv-2 and chikv co - infections have never been previously associated with transmission by ae . our study therefore provides a disconcerting example of the unexpected epidemiologic patterns that may be associated with the dispersal of both vectors ( ae . albopictus and ae . albopictus mosquitoes are now present in several temperate countries of the northern hemisphere where , given the opportunity , they could cause future arboviral epidemics . albopictus in northern italy ( 5 ) provides a potential warning of what might occur much more frequently in the future in europe and even in north america . introduction of denv or chikv in these regions are likely to generate indigenous transmission by ae . albopictus .
an outbreak of febrile illness occurred in gabon in 2007 , with 20,000 suspected cases . 
 chikungunya or dengue-2 virus infections were identified in 321 patients ; 8 patients had documented co - infections . 
 aedes albopictus was identified as the principal vector for the transmission of both viruses .