a group of 30 us citizens spent june and july 2005 in kenya and uganda as part of an educational program . their return journey took them from nairobi to london , where they spent a week before their flight to the united states . by the time they arrived at the airport departure lounge , they were referred by airport staff to the infectious diseases unit at northwick park hospital , london . all arrived at the unit within 3 hours of referral and were proven to have falciparum malaria on peripheral blood smears . of these 6 persons , all were 1922 years of age , and 5 exhibited initial clinical features of severe malaria , e.g. , impaired consciousness ( disorientation as to time , place , or person ; or prostration [ inability to sit up and drink ] ) . these 5 persons required intravenous quinine and intensive monitoring ; the other person had less severe disease and was treated with a combination of oral atovaquone and proguanil . temp , body temperature ; bp , blood pressure ; hgb , hemoglobin ; alt , alanine aminotransferase ; nd , not done . reference range < 100 m / l . to convert to mg / dl , divide by 88.4 . < 19 m / l . to convert to mg / dl , divide by 17.1 . # reference range 1950 u / l .  * * in mmol / l . to convert to meq / l , divide by 1.0 . in light of these cases , all other members of the group who had returned to the united states were alerted to contact their healthcare providers if they became symptomatic . another person became febrile while in the united states and was initially treated with conventional antimicrobial drugs ; this person was later recalled to a hospital and treated appropriately for falciparum malaria after the diagnosis of his colleagues became known . unconfirmed reports were received that 2 more patients had received treatment for malaria ( species unidentified ) within 6 months of return . the high attack rate in this group of students and the close temporal clustering of cases led us to examine the possibility that the group had experienced a mini - outbreak as a result of a point - source exposure . a study - specific , self - administered questionnaire gathered information about demographics , onset and nature of symptoms , medical history , malaria protective measures , and chemoprophylaxis and adherence . the questionnaire was given to patients 16 in the uk hospital and to patient 7 by email . clinical data for patients 16 were obtained during their uk hospital stay , and clinical data for patient 7 was obtained from the diagnosing and treating physician in the united states . genetic diversity of parasite isolates was investigated by pcr amplification of polymorphic regions of the p. falciparum merozoite surface protein 1 ( pfmsp1 ) and pfmsp2 loci as previously described ( 5 ) . all had slept in shared accommodations under unimpregnated malaria nets . despite weekly malaria education sessions , . only 1 of 7 patients had taken malaria prophylaxis , but this person did not continue using chemoprophylaxis for the full period at risk . none of the 7 patients recalled being bitten by mosquitoes . however , when returning to nairobi on july 2 , 2005 , the group was delayed for several hours by the roadside in the rice - growing area of mwea ( northern kenya ) . many had little protective clothing with them and while waiting until dusk that evening , noticed a large number of mosquitoes of unknown species around them . after visiting uganda , the group returned to nairobi on july 11 , arrived in london on july 13 , and went to london s heathrow airport on july 20 for the flight back to the united states . paired peripheral blood samples from pretreatment and day 1 posttreatment were provided by 6 of the 7 patients ; for the other patient , only a pretreatment sample was available for dna analysis . no 2 patients had identical alleles at both pfmsp1 and pfmsp2 loci ( figure ) . pretreatment and posttreatment samples were similar except for those of patient 2 , for whom postquinine allelic patterns differed for pfmsp1 and pfmsp2 on day 1 and day 0 . molecular typing of malarial parasites from 6 us travelers with falciparum malaria returning from east africa in 2005 . plasmodium falciparum merozoite surface protein 1 ( pfmsp1 ) ( upper gel ) and pfmsp2 ( lower gel ) allelic variation among isolates was determined by nested pcr and agarose gel electrophoresis . no parasites of the ro33 allelic family of msp1 were found ( data not shown ) . malaria - endemic areas , such as east africa , are increasingly popular destinations for us travelers . therefore , suboptimal adherence to malaria chemoprophylaxis and effective preventative measures against mosquito bites ( such as use of bed nets ) is cause for concern ( 3 ) . antimalarial drug use was poor among the case - patients , for whom emphasis was placed on personal protective measures rather than chemoprophylaxis . the high attack rate for this group is consistent with rates reported by other studies demonstrating the injudicious risk of traveling to a malaria - endemic area without taking effective antimalarial drugs ( 6,7 ) . the course of disease for patients 16 displayed remarkable synchrony ; onset of symptoms occurred within 24 hours of each other and illness proceeded rapidly . these 6 patients received prompt diagnosis and treatment at a specialist unit and recovered fully . had it had not been for alert airport staff , these 6 patients ( 5 with severe malaria ) would have been on a 9-hour flight to the united states ; consequences for the patients could have been fatal . these cases highlight the benefits of vigilant staff at airports , even for passengers boarding from non  disease - endemic areas . prompt contact tracing enabled a missed diagnosis of malaria to be rectified at a us facility and highlights the advantage of actively seeking all those who might have shared the same exposure . the clustering of the onset of symptoms for patients 16 suggested a common timing of infection , although other unproven factors , such as the effect of atmospheric pressure changes during air travel , may have led to the synchronous onset of the cases . molecular evidence suggests that despite synchrony of symptoms , the genetically heterogeneous parasite variants were unlikely to have come from 1 infectious mosquito . we estimate that at least 12 distinct haploid parasite genotypes were circulating among the 6 patients . our data also demonstrate , for patient 2 , that circulating parasite genotypes can change profoundly immediately after treatment , as previously observed for quinine - treated persons ( 8) . the main rice - growing season starts in june , which is also the time of anopheles spp . however , the mosquitoes seen at mwea were not positively identified as anopheles , and although the students did not recall being bitten by mosquitoes on other occasions , we can not rule out infection later in the journey , which would be more probable given the date of onset of symptoms in nonimmune persons . this mini - outbreak demonstrates the need to encourage travelers to take malaria prophylaxis and to improve their knowledge about the risk of malaria in their area of proposed travel . it also underscores the value of alerting fellow travelers in the same party to the possibility of contracting malaria .
a cluster of 6 cases of plasmodium falciparum malaria occurred in a uk airport among 30 travelers returning to the united states from east africa . 
 molecular genotyping analysis indicated that all had been exposed to different parasites . 
 the travelers use of chemoprophylaxis was poor ; their perception of risk was limited .