a 29-year - old man ( contact 1 ) transported 8 dromedaries from oman to united arab emirates on may 7 , 2015 ( table 1 ) . the same day , as part of a national policy for controlling mers , samples were collected from the dromedaries at a screening center located at the united arab emirates border .
the samples were tested by reverse transcription pcr ( rt - pcr ) on may 10 and found to be positive for the mers - cov open reading frame ( orf ) 1a and upstream of e genes ( 10 ) . this finding led local public health authorities to conduct active surveillance on humans who had contact with the infected dromedaries .
* contacts 1 and 2 , humans who had direct physical contact with infected dromedaries ; mers - cov , middle east respiratory syndrome ; rt - pcr , reverse transcription pcr . 
a sputum sample collected from contact 1 on may 10 , 2015 , was tested by rt - pcr on may 12 and found to be positive for mers - cov ; the man was admitted to a hospital the same day .
follow - up respiratory samples obtained on may 13 and 14 were still rt - pcr  positive , but a sample obtained on may 18 was negative .
the patient was asymptomatic at hospital admission and throughout his hospital stay ( technical appendix ) .
contact 2 was a 33-year - old man who worked at the screening center mentioned above .
he had direct contact with the same group of infected dromedaries during the sampling procedures .
a nasal aspirate sample was obtained from the man on may 14 , 2015 , and found to be rt - pcr positive for mers - cov .
a follow - up sample obtained on may 18 was rt - pcr negative for mers - cov .
samples from 32 other persons were also tested by rt - pcr ( technical appendix ) .
seven days later ( may 14 ) , follow - up nasal swab samples from 5 dromedaries were still positive by rt - pcr ( table 2 ) ; the animals also had mucopurulent nasal discharge .
the animals were tested for the presence of mers - cov  specific neutralizing antibodies ( 11 ) ; all were seropositive .
two 4-month - old calves ( adfca - hku1 and adfca - hku2 ) had the highest virus loads by real - time rt - pcr and the lowest neutralizing antibody titers ( table 2 ) .
positive by rapid antigen testing ( 12 ) , which suggests the calves were still shedding virus 7 days after the first detection of virus .
the 2 calves were rt - pcr negative for mers - cov , and the whole group of camels was released from quarantine . * ct , cycle threshold ; i d , identification ; mers - cov , middle east respiratory syndrome ; neg , negative ; pos , positive ; rt - pcr , reverse transcription pcr . 
results were determined by open reading frame 1a and upstream of e gene rt - pcr assays ; ct values were from the open reading frame 1a assay .
 results were determined by immunochromatographic tests for mers - cov nucleocapsid protein ( 12 ) . 
respiratory specimens from the 2 infected humans and the 5 dromedaries that were still positive at the second sampling were analyzed by dideoxy sequencing as previously described ( 13 ) ; the nucleocapsid gene sequences of all dromedary samples were found to be identical .
samples from dromedaries adfca - hku13 were selected for further analysis , and a sequence contig encompassing the 3 end of the orf1ab gene through the 3 untranslated region of the mers - cov genome ( 8,900 nt ; sequence coverage 4 ) was obtained from each sample .
contigs from the 3 samples were identical , with the exception of a v221i ( gttatt ) mutation in the orf4b protein of the sample from dromedary adfca - hku2 .
the viral rna content of the 2 human samples available for analysis was too low to provide long pcr amplicons ( cycle threshold 35.5 and 36.9 by upstream of e gene assay ) . however , partial sequences of mers - cov spike ( 466 nt , contacts 1 and 2 ) , orf34a ( 273 nt , contact 1 ) , and nucleocapsid ( 451 nt , contacts 1 and 2 ) gene regions could be detected from the samples .
the 3 sequence contigs obtained from the dromedary samples were phylogenetically closely related to those of viruses detected in humans in the saudi arabia , china , and south korea in 2015 ( figure ) .
all sequences from this cluster , together with the partial orf34a sequence detected in the sample from contact 1 , shared 2 cluster - specific mutations , 79s ( tcatct ) and p86l ( cct ctt ) , in the orf3 protein , suggesting that these viruses may share a common lineage . apart from the unique v221i mutation , the sequences for viruses from the 3 dromedaries shared a unique orf4a - q102e ( gagcag ) mutation that was not found in any published mers - cov genomes .
other than those mutations , all of the orfs ( nonstructural protein 13 , spike , orf3 , orf5 , envelope , membrane , nucleocapsid , and orf8b ) of these virus sequences were unremarkable .
phylogenetic analyses of partial middle east respiratory syndrome coronavirus ( mers - cov ) genomic sequences for viruses detected in dromedaries imported from oman to united arab emirates , may 2015 .
a partial viral rna sequence spanning the 3 end of the open reading frame 1ab gene through the 3 untranslated region of the mers - cov genome ( 8,900 nt ) was used in the analysis .
the phylogenetic tree was constructed with mega6 software ( http://www.megasoftware.net/ ) by using the neighbor - joining method .
symbols indicate mers - covs detected from dromedaries s. scale bar indicates the estimated genetic distance of these viruses .
we report 2 cases of mers - cov infection in men who had direct contact with the same group of infected dromedaries .
neither man had a concurrent medical condition or a history of exposure to human mers cases in the 14 days before their first mers - cov  positive test results .
genomic sequences for the viruses derived from the men and dromedaries and findings from the epidemiologic investigation suggest possible zoonotic transmission of mers - cov from dromedaries to humans .
although it is unlikely , we can not exclude the possibility that the men and dromedaries were independently infected by other sources .
both infected humans were kept in the hospital for 2 incubation periods and were asymptomatic during this period .
clinical observations and positive rt - pcr results suggest that the men were asymptomatically infected with mers - cov .
our findings provide further evidence that asymptomatic human infections can be caused by zoonotic transmission .
nonetheless , our findings highlight the importance of systematic surveillance of persons who have frequent contact with dromedaries .
a recent study demonstrated that persons who have frequent exposure to camels are more likely than the general population to be seropositive for mers - cov ( 4 ) .
the unique border screening program and multisectoral collaborations highlighted in this investigation serve as a model for effective mers - cov surveillance at the animal  human interface .
we did not test serum samples from the human contacts ; such testing would be of interest for follow - up investigation of the patients serologic responses .
we also obtained limited rna samples from these persons , which prevented us from conducting more extensive viral sequence analyses .
mers - cov genomic sequences determined in this study are similar to those of viruses detected in 2015 in patients in saudi arabia and south korea with hospital - acquired infections .
the infected dromedaries in this study were imported from oman , which suggests that viruses from this clade are widely circulating on the arabian peninsula .
sequence analyses of mers - covs found in south korea and china do not suggest that viruses from this clade are necessarily more transmissible variants ( 15 ) .
however , given that a single introduction of mers - cov from this clade caused > 180 human infections in hospital settings ( 2 ) and that viruses of this clade are causing other human infections in saudi arabia , further phenotypic risk assessment of this particular mers - cov clade should be a priority .
additional information regarding 2 persons with asymptomatic mers - cov infection and other persons tested in the study .
 in may 2015 in united arab emirates , asymptomatic middle east respiratory syndrome coronavirus infection was identified through active case finding in 2 men with exposure to infected dromedaries . 
 epidemiologic and virologic findings suggested zoonotic transmission . 
 genetic sequences for viruses from the men and camels were similar to those for viruses recently detected in other countries . 