we began an ongoing longitudinal serosurvey in 2004 , sampling macaques from each of gibraltar s 6 groups for 6 known enzootic nonhuman primate borne viruses : siv , cehv-1 , rhesus cytomegalovirus ( rhcmv ) , stlv , sfv , and simian retrovirus ( srv ) . trapping , sampling , and assay protocols none of the animals showed evidence of having been infected with siv , srv , an alphaherpesvirus antigenically related to cehv-1 , stlv , or with a cmv antigenically related to rhcmv . the complete absence of seroreactivity to cehv-1 and rhcmv by elisa was particularly striking because both viruses are considered enzootic in populations of wild - caught and captive macaques ( 9,13 ) . in contrast , more than half of the population showed evidence of infection with sfv ( table ) . prevalence of sfv infection increased with age , from 20% among young juveniles to 90% among adults . no differences in sfv antibody prevalence were detected between male and female macaques or among the 6 groups . none of the 5 initially sfv - seronegative macaques had seroconverted by the second sampling . similarly , all 3 of the initially sfv - seropositive macaques remained seropositive at second sampling . * simian foamy virus ( sfv ) seroprevalence for 2004 samples determined using enzyme immunoassay . subsequent samples screened as part of the whole - virus multiplex flow cytometric assay ( 11 ) . simian immunodeficiency virus ( siv ) , simian retrovirus ( srv ) , cercopithecine herpesvirus 1 ( cehv-1 ) , simian t - cell lymphotropic virus ( stlv ) seroprevalence for 2004 samples determined by using multiplex microbead assay ( 10 ) ; seroprevalence for subsequent samples determined by using whole - virus multiplex flow cytometric assay ( 11 ) . rhesus cytomegalovirus ( rhcmv ) seroprevalence for the 2004 samples ( n = 40 ) determined by elisa according to previously published protocols ( 12 ) . subsequent samples not screened .   age class determinations made on the basis of dental eruption sequence and morphometrics : young juvenile , < 1 y of age ; older juvenile , 13 y ; subadult , 35 y ; adult , > 5 y. the population shows no evidence of infection with either cehv-1 or rhcmv , in contrast to other macaque populations studied in which animals seroconvert early in life ( 14 ) . both these viruses are old viruses that are thought to have coevolved with nonhuman primate species over millions of years . one possible explanation for the lack of cehv-1 and rhcmv in this population is that the m. sylvanus populations in north africa from which they were derived are not infected with these viruses . at present if north african m sylvanus are infected with cehv-1 and rhcmv , our results could be explained by a founder effect or the loss of genetic variation when a new colony is established by a very small number of individuals from a larger population . because seroconversion begins early in life , this hypothesis would imply that the progenitors of the present population were translocated from north africa when they were young , likely < 1 year of age , and therefore less likely to be infected with the viruses . sfv seroprevalence in gibraltar macaques mirrors that of other studied macaque populations in south and southeast asia with the exception that they seroconvert to sfv at a later age . for example , 81% of free - ranging macaques in thailand are sfv positive by the age of 3 years , compared with only 38.7% of gibraltar macaques ( 15 ) . this observation could be explained by decreased rates of aggression because bites are thought to be a principal mode of virus transmission . properties of the virus or physiologic or immunologic characteristics of the macaques could also influence virus transmission . the relatively low rates of interspecies aggression observed between macaques and humans in gibraltar and the low seroprevalence of enzootic nonhuman primate viruses detected in this serosurvey suggest that the risk for virus transmission from macaques to humans in gibraltar is lower than this risk in other locations where humans come into contact with free - ranging macaques . nevertheless , the abundance of contact between humans and macaques in gibraltar implies that the possibility of bidirectional cross - species transmission of infectious agents , including sfv , remains and has implications for humans and macaques . this prospect justifies continued vigilance by local park authorities to monitor and regulate contact between humans and macaques at the upper rock reserve . we recommend strengthened efforts to discourage tourists and locals from feeding macaques and strict enforcement of rules enjoining visitors from seeking contact with them . macaques should be fed exclusively in designated areas by trained park personnel ; such feeding should compensate for decreased feeding by the general public . these changes will preserve the public s ability to observe the macaques while simultaneously reducing the risk for cross - species transmission of infectious agents .
because gibraltar 's macaques ( macaca sylvanus ) have frequent contact with humans , we assayed 79 macaques for antibodies to enzootic primate viruses . 
 all macaques were seronegative for herpesvirus b , simian t - cell lymphotropic virus , simian retrovirus , simian immunodeficiency virus , and rhesus cytomegalovirus . 
 seroprevalence of simian foamy virus reached 88% among adult animals .