we carried out a retrospective study by testing blood and serum samples on 1,740 patients who consulted the emergency department of cayenne hospital seeking treatment for fever compatible with malaria and/or dengue during a 1-year period , july 2004june 2005 ( figure ) .
diagnosis of malaria has always been quick ; dengue diagnosis was initially conducted only in malaria - negative patients . in our study ,
dengue investigations were conducted retrospectively at the pasteur institute of french guiana for 99% of patients ( 1,723/1,740 ) by using serum samples obtained at admission and frozen at 80c .
comparison of confirmed cases of  dengue fever  and of symptomatic malaria in patients  examined at  the emergency department of cayenne  hospital , cayenne , french guiana , january 2004march 2006 .
the black frame corresponds to the period of the retrospective study ( july 2004june 2005 ) .
* cases  confirmed by positive  test results from reverse transcription  pcr or virus isolation ( pasteur institute , cayenne ) .
cases  diagnosed based on recorded fever or history of fever in the previous 24 h associated with microscopic detection of asexual forms of plasmodium spp .  by blood smear .
malaria diagnosis was based on the identification of hematozoa on a thin blood film and/or on a thick blood film stained with giemsa .
the asexual parasite load ( pl ) was quantified in percent parasitized erythrocytes for values > 0.1 % . for lower values
, classes were created using thick smears : class 1 , < 0.00125% but positive ; class 2 , > 0.00125% but < 0.0125% ; and class 3 , > 0.0125% but < 0.125% .
carriage was considered for classes 1 and 2 ( in the absence of prior antimalarial treatment and for case - patients residing > 1 year in an area of malaria transmission ) . virus isolation or reverse transcription 
( 8) was performed on all serum samples obtained during the acute phase of infection , between day 0 and day 4 ( n = 264 ) . for malaria - positive samples ,
virus isolation was conducted on all samples without a date of onset of disease ( n = 163 ) .
serologic immunoglobulin ( ig ) m testing was performed on all serum samples ( n = 1,723 ) .
dengue was detected in 238 case - patients ( 13.8% ) ; among these , 73% ( 174/238 ) were confirmed by positive virologic diagnosis ( isolation or rt - pcr ) , whereas 27% were probable dengue cases ( positive igm serology only ) .
the first group was named early dengue cases ( edc ) and the second group late dengue cases ( ldc ) . of the 1,723 patients , 393 ( 22.8% ) had smear - positive malaria ; of those , 251 ( 63.9% ) were p. vivax , 133 ( 33.8% ) were p. falciparum , 2 were p. malariae , and 7 were mixed p. vivax and p. falciparum .
most malaria - positive case - patients had a parasite count above class 2 ( 371/393 [ 94.4 % ] ) , indicating acute malaria .
concurrent dengue ( edc and ldc ) and malaria were confirmed in 17 of the 1,723 patients ( 1% ) , corresponding to 7.1% ( 17/238 ) of dengue cases and 4.1% ( 16/393 ) of malaria cases , respectively ( table ) .
when considering acute malaria associated with edc , the percentages of confirmed associations were 3.4% for dengue ( 6/174 , 95% confidence interval [ ci ] 0.76.2 ) and 1.6% for malaria ( 6/371 , 95% ci 0.32.9 ) .
all 17 associations were considered clinically as malaria , including the 2 case - patients with low parasite counts .
antimalarial drugs were administered promptly in every case . dengue serology and virology reports were available after the initial episode ; however , these results did not influence patient management . among the 6 acute concurrent infections ,
three patients were hospitalized , all in the igm - seropositive group , i.e. , ldc ; only 1 was severely ill .
this patient , who had p. vivax malaria infection , was hospitalized for interstitial pneumonia with severe anemia .
intubation , blood transfusion , and antimicrobial drugs were required , but he was discharged from the intensive care unit after 11 days .
the second patient was hospitalized for diabetes , the third because treatment with riamet ( arthemether + lumefantrine ) was only available to inpatients . * igm , immunoglobulin m ; rt - pcr , reverse transcription 
pcr ; iso , isolate ; pv , plasmodium vivax ;  , negative ; + positive ; den , dengue ; nd , not done ; pf , p. falciparum ; tp , thrombocytopenia++ ( < 100,000 platelets/l ) ; hb , hemoglobin ( reference range < 10
g / l ) ; bp , arterial blood pressure ( systolic / diastolic ) ; sat , blood oxygen saturation ; hr , heart rate ; bpm , beats per minute . 
 for diabetes requiring insulin .  for treatment with riamet ( arthemether + lumefantrine ) . 
malaria and dengue must be suspected in febrile patients living in or returning from areas endemic for these infections .
although the usual places of contamination by malaria and dengue viruses are quite different in french guiana , considering that the incubation phase is longer for malaria than for dengue and that the population s mobility is high , a simultaneous clinical expression of the 2 diseases is plausible . moreover , in french guiana , dengue viruses have spread to malaria - endemic rural areas ( 9 ) .
the confirmation of malaria is rapid , and after malaria is confirmed , dengue is usually ruled out without screening for it .
two methods can confirm dengue : dengue - specific igm seroconversion or detection of dengue virus particles during the acute phase ( day 0 to day 4 after onset of fever ) by rt - pcr , which is faster and more specific . in published case reports ( 17 ) , the diagnosis of dengue infection is usually made based on positive dengue igm ; however , this can not confirm recent dengue , because igm can persist for months and cross - react with other arboviruses ( 10 ) .
if rt - pcr requires a specific laboratory and can not be performed on site , a new test , the platelia , is now easily included in any laboratory and is indicated particularly for early - acute phase samples ( 11 ) . to investigate the frequency of dengue and malaria co - infection ,
the platelia test should be used in all cases of dengue - like or malaria - like syndrome , even when malaria diagnosis was positive , in regions where both infections may overlap . of the 1,723 patients investigated in this study , 17 had concurrent dengue and malaria . in 10 of these patients , recent acute dengue fever could not be confirmed ( ldc ) .
two patients , 1 of whom was part of the edc group , could have been asymptomatic carriers of plasmodium spp .
( 1 patient with p. falciparum and 1 with p. vivax ) because of low parasitemia ( 12 ) . a true acute concurrent infection ( strictly defined diagnosis )
concurrent acute malaria and recent dengue fever had a lower frequency than predicted by the multiplication of both prevalences , but such reasoning implies the same overlapping contamination areas for malaria and dengue , which it is not the situation in french guiana
. the greater prevalence of ldc than edc associated with acute malaria infection illustrates the prolonged persistence of specific igm or igm cross - reaction , which increases the probability of a malaria case when comparing the short 45 day period corresponding to edc .
virologic investigations using isolation or rt - pcr techniques were not performed on samples taken after the 4th day because of the usual disappearance of viremia .
additional associations where fever was initially caused by malaria and followed by dengue after the 4th day of malaria fever could have been undiagnosed .
edc were diagnosed on average after  4  days  of fever , never 5 .
thus , delayed complications of dengue or malaria may not be detected using this definition .
one of these patients had pneumonia , which has recently been described as a complication of p. vivax ( 13 ) .
although acute concurrent infections were benign in our study , special attention should be given to the possibility of co - infection with malaria and dengue , especially when p. falciparum is implicated .
the distinction between severe dengue and severe malaria must be made in an emergency department or hospital setting because in both situations , early diagnosis is essential for patient care .
 dengue  malaria co - infection reports are scarce . of 1,723 consecutive febrile patients in cayenne hospital , 
 238 had dengue ( 174 early dengue fever cases ) and 393 had malaria ( 371 acute malaria ) ; 17 had both . 
 diagnosis of 1 of these 2 infections should not rule out testing for the other infection . 