ee dl aegis . SNle as hie 5 ead oe te on De at de at mts Table 2. Comparison of number of reported infections with number of viruses handled by laboratories. Numbers in parentheses represent total number of laboratories checked. Viruses handled (No.) 1-5 6-10 11-20 21-30 31-40 41 Unknown Laboratories reporting cases TT Percent No. 29 9 (31) 43 6 (14) I (6) 19 100 23 33 (3) 16 32 42 67 4 (6) 0. 40 64 8 (20) 7 (11) 6 Ne) were probably also acquired by the respiratory route. Although many pathogens decrease in their virulence for the natural host after several passages, a number of overt infections have been acquired in the laboratory by individuals working with “laboratory-adapted” strains of various arboviruses; hence, virus strains that have been passed through animals many times may bestill pathogenic for man (/4, 17). A serious laboratory infection with St. Louis en- 9 1 igo £23] ARBOVIRUSES RECOGNIZED ES|IOVERT 40LABORATORY INFECTIONS 20+ 160+ that had been passed through mice many times over a period of 16 years. Disease occurred in an individual who had worked with a strain of Rift Valley ly acquired infection, but yet they have fever virus that had undergone over to cause disease in man through naturalproduced infections in the laboratory. Acquisition of laboratory infections is not always similar to transmission of the disease when it occurs naturally. For example, none of the cases of encephalitis was believed to be trans- mitted by an arthropod, and only one case of yellow fever was thought to be due to the bite of an infected mosquito. Many persons have become infected while working with dried virus preparations which provide ideal circumstances for aerosol transmission. A case of St. Louis encephalitis and a fatal case of Russian spring-summer encephalitis apparently resulted from aerosol transmission of such materials (14, 15), Contaminated dust from mouse cages was apparently responsi- 150 mouse passages, and Japanese B encephalitis occurred in an individual who had worked with a strain that had been through six passages in a mouse brain and one passage in chickembryo cell cultures. The intensity of activity of the reporting laboratories may be indicated by the number of viruses with which they had been working (Table 2). The percentage of laboratories reporting infections increased directly with the number of viruses being studied in the laboratory. This was the only apparent relation between a characteristic of the laboratory (size of staff, length of operation, and number of viruses handled) listed on the questionnaire and number of infections {including subclini- ble for several infections with the virus cal) reported. litis (76) and for a fatal infection with Machupo virus; thus, these infections directly related to the number of ac- of Venezuelan equine encephalomye- Failure of the size of staff to be quired infections may be due to the increase in the proportion of supportive personnel Table 3. Proved or probable sources of labOratory-acquired arbovirus infections. Cases are distributed according to the one most probable source. Probable source Experimentally infected animals Not indicated Aerosol Agent handled Accidents Preparation of vaccines, antigens, and other Experimentally infected chick embryos Discarded glassware Autopsy (including known accidents) Clinical specimens 8 DECEMBER 1967 Infec- Percent- (No.) total tions age of 93 84 74 70 43 21.7 19.6 17.3 16.4 10.0 35 8.2 9 9 2.1 2.1 8 3 1.9 0.7 in such lJaboratories—indi- viduals who have less exposure to infection. Unless the maintenance of records of acquired infection is an ad- 7 7 4 loot 4 80+ 4 60} 4 40h 20 4 23 22 ge cephalitis virus occurred in an investigator who had worked: with a strain infection (13). Several additional arboviruses (Germiston, Kunjin, Piry, and Nairobi sheep disease) are not known § 13 Fes 5 [ BS. 98 82 23 a oO i oa ‘ SES 7 $8 or Fig. 1. Number of overt laboratory infections and recognition of new arboviruses by decade. In addition, 12 arboviruses have been added to the ACAV catalog; one virus, Canjam, was respon- sible for an overt mild febrile illness. in the number of overt infections acquired in the laboratory correlates with the increase in the recognition of new arboviruses (Fig. 1). Changes in the kinds of agents involved in laboratory infections and in the circumstances resulting in infection to some extent reflect trends in areas of research and interest in certain agents (/8). Prior to 1950, bacterial infections accounted for over one-half of known overt cases of laboratory-acquired infections, while viruses were responsible for about 20 percent of the total. Since 1950, there has been an increasing number of laboratory-acquired infections due to viruses, with more than half due to arbo- viruses, This is probably the result of the marked acceleration in research and diagnosis in connection with arboviruses and a consequent increase in the number of persons and laboratories handling such agents. A large propor- tion of the total number of infections due to arboviruses occurred since 1950 ministrative routine of the laboratory (Fig. 1). Relatively few laboratories worked with these viruses prior to changing, one would not expect a ret- tory infections at that time reflect the and unless surveillance has been un- rospective query to obtain as many early as later cases of infection. The lack of relationship between numberof infections and length of operation of lab- oratories may be ascribed in part to an inadequacy of reporting and in part to changing methodologies. Because of these variables, a man-year exposure index was not presented for any virus. 1941, and the agents causing labora- area of interest prevailing then. The exact source of a laboratoryacquired infection is frequently obscure, Often it is known only that an individual had been working with a particular agent or that he had been in contact with infected animals. In other situations, it is known that the atmosphere of the laboratory had become contaminated. That an aerosol Over 80 percent of the currently may be unwittingly produced by a va- nized since 1950. The steady increase dures has been conyinginghy: dérion- classified arboviruses have been recog- tiety of common laboratory _proce1285