during july and august 2012 , investigators from cdc s viral special pathogens branch reviewed shipping records from facility b and subsequent distributors and notified health departments in states that had received potentially infected mice during january 1may 7 , 2012 ; frozen mice were considered a low public health risk and were not traced . health departments were provided with a list of facilities that had purchased these mice , educational resources about lcmv , and an algorithm to determine whether potentially infected mice remained at these purchasing facilities resulting from the presence , comingling , or breeding of these mice , which would maintain lcmv among the mouse population ( figure 1 ) . as a result of varying state statutes concerning regulation and licensing of pet stores and animal breeders or distributors , the government agencies that had jurisdiction to perform these investigations included local and state departments of public health , environmental health , food safety , and agriculture . algorithm used to determine whether mice were potentially infected with lymphocytic choriomeningitis virus ( lcmv ) during a multistate investigation , united states , 2012 . this algorithm was used to determine whether 1 ) potentially infected mice remained at the facilities being assessed , 2 ) mice from the original shipment remained , 3 ) offspring from these mice remained , or 4 ) shipments of mice had been comingled or had shared equipment with mice from the original shipment . lcmv is easily maintained in a mouse colony , and a clear break among the population ( i.e. , a time when no remaining mice are maintained and equipment is disinfected ) is necessary to ensure that no ongoing infection continues . state investigators interviewed purchasing facility managers by telephone , mail , email , or in person to determine whether potentially infected mice remained on the premises and to encourage euthanization of these mice . interviews also assessed whether pregnant , ill , or immunocompromised employees might have been exposed to lcmv by directly handling potentially infected mice or bedding or equipment used for the mice . because of risk for severe disease , facility managers were asked to offer serologic testing to these employees for lcmv igm and igg , which was performed by cdc by using elisa as described ( 10 ) . no additional case - finding activities were conducted . because of resource limitations , diagnostic testing of live mice at purchasing facilities was not conducted . reviews of shipping records indicated that 304,000 live mice distributed by facility b were shipped to 561 purchasing facilities : 543 pet stores , 11 breeders or distributors , and 7 zoos or aquariums in 21 states , potentially exposing thousands of employees and pet store mouse purchasers to lcmv . facility b had shipped mice to 4 subsequent distributors ; the largest was located in georgia , and it had shipped > 183,000 mice to 420 purchasing facilities in 16 states ( figure 2 ) . interviews of facility managers at purchasing facilities revealed that 48% still had potentially infected mice ; > 10,000 mice were subsequently euthanized . the most common reason for still having potentially infected mice was comingling of rodent shipments , followed by breeding or still having mice from the original shipments . the distribution of mice potentially infected with lymphocytic choriomeningitis virus originating from facility a to 500 pet stores and other animal facilities in 21 states , united states , 2012 . serologic testing was performed on blood samples from 34 pet store or zoo employees from 6 states who self - identified as pregnant or ill , were potentially exposed to lcmv , and agreed to serologic testing . fourteen were pregnant ; 1 had aseptic meningitis ; and 23 reported nonspecific symptoms including fever , headache , body aches , cough , and vomiting . these captive feeder mice had a wide and complex distribution chain , potentially exposing thousands of persons to lcmv . no additional human cases were identified after distribution of these mice ; none of the pet store or zoo employees tested had serologic evidence of infection . although no additional human cases were identified , euthanasia of all potentially infected rodents was recommended to mitigate potential risk . wild mice that access captive breeding populations are often the source of infection of captive rodent populations ( 8,11 ) . after being introduced , lcmv transmission is easily maintained among mouse colonies and is difficult to recognize because mice do not appear ill . because persistently infected mice pass infection to their offspring , the number of infected mice in a breeding colony can quickly multiply . mice can be persistently infected without having serologic evidence of infection ( 12 ) ; thus , lcmv can be missed by serologic screening alone . therefore , preventing introduction of the virus into breeding colonies , depopulation of infected rodents , and correct use of personal protective equipment are the most efficient ways to mitigate human exposure . we recommend preventive measures at each point in the distribution process , both domestically and abroad ( table ) ( 1315 ) . more research is needed to develop methods for detecting lcmv in rodents at distributors and pet stores . employees tested were a fraction of those who had had contact with potentially infected mice . also , pet store mouse purchasers and purchasing facility employees were difficult to contact , and no pet store customers were tested . frozen feeder mice are considered pet food and can be regulated by the food and drug administration ( fda ) , but neither fda nor the us department of agriculture has the authority to regulate live mice and rats because they are not regulated under the animal welfare act ( 7 cfr 2132 , may 13 , 2002 , www.aphis.usda.gov/animal_welfare/downloads/awa/awa.pdf ) and the food , drug and cosmetics act ( 21 cfr 500 , april 1 , 2012 , www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfrsearch.cfm?cfrpart=500&showfr=1 ) . therefore , regulatory authority falls to the states , which have an array of regulations governing the handling , breeding , and distribution of rodents , including the licensing of pet breeders and distributors ( 15 ; thomas edling , pers . because of the lack of consistent regulation , we recommend that state and federal partners and rodent industry advisory groups work with breeders , distributors , and pet stores to increase awareness of lcmv infection and implement recommended best practices ( table ) to prevent introduction of lcmv into captive rodent populations , prevent subsequent dissemination of potentially infected rodents , and reduce the potential for human exposure and disease among employees and consumers of pet stores and rodent breeding facilities . multistate lcmv outbreak , united states , july 2012 : recommendations for trace - back and safe disposal of potentially lcmv - infected mice .
during follow - up of a 2012 us outbreak of lymphocytic choriomeningitis virus ( lcmv ) , we conducted a trace - forward investigation . 
 lcmv - infected feeder mice originating from a us rodent breeding facility had been distributed to > 500 locations in 21 states . 
 all mice from the facility were euthanized , and no additional persons tested positive for lcmv infection .