L~ttm to the Editor

that not only first but also recurrent events of disease during the study period are taken into account. Thus a rate difference, ER, - ERr, between the index (exposed) and reference populations indicates the excess number of episodes per persontime in the exposed population. A twosided confidence interval can be calculated based on the normal distribution assumption RD(1 ± ZJX), where Z is the critical ratio (1.96 with 95% confidence interval [Cl]) and X is test statistic. The chisquare test statistic is calculated using the observed (0) and expected number of episodes (E) in polluted and reference populations during the study period: X2 =

(O-E)/E

In our studywe used information collected during a MMR vaccination program, where the interviewer asked the mothers about the number of upper respiratory infection episodes experienced by the child during the previous 12 months (Table 1). Unfortunately, the information was coded in categories (0, 1-4, 5-9, 10-), which lead us to use the odds for experiencing one or more episodes during the past year. However, a crude estimate of ER in different study populations can be estimated by assiging for each category an average number of episodes: 0, 2.5, 7.0 and 10. The average for the last category is the most arbitrary. The episode rate of URI among the younger children of the polluted city was 3.1 episodes per person-year and 2.6 in the reference cities, giving a RD of 0.5 episodes per person-year (95% CI: 0.4-0.6) (Table 2). The corresponding figures for the older children were 2.0, 1.7, and 0.3 (0.1-0.5). Accordingly, ER for the younger children in the more polluted area (3.3) compared to those of less polluted area (2.8) gave a RD of 0.5 (0.1 - 0.9) and the corresponding figures for the older children were 2.2, 1.9, and 0.3 (-0.1 0.7). These crude RDs are not likely to be seriously confounded, while the corresponding crude odds ratios did not differ essentially from those adjusted for the confounders in the logistic regression analysis.3 The different analyses do not change qualitatively our findings, which suggest that children's respiratory health can be jeopardized from air pollution in concentrations lower than reported in most earlier studies from Europe. We would also like to point out that the estimated magnitude of the effect, 0.3 to 0.5 extra epiJune 1992, Vol. 82, No. 6

sodes a year, has a considerable public health importance. 0 Jouu J.L Jaakkok MD 0( P. Heinen, MD, DSc Requests for reprints should be sent to Jouni J.K. Jaakkola, MD, Helsinki University of Technology, Kl/Ko 302, Otakaari 4 A, SF02150 Espoo, Finland.

References 1. Miettinen OS. Theoretical epidemiology. P,incpls of occurence research in medicine. New York, NY: John Wiley & Sons; 1985:246-249. 2. Rothman KJ. Modem ep iogy. Boston, Mass: Little, Brown and Company; 1986:24. 3. Jaakkola JJK, Paunio M, Virtanen M, Heinonen OP. Low-level airpollution and upper respiratory infections in children.AmJPublic Health. 1991;81:1060-1063.

Plague Surveillance in Los Angeles County In 1986 Los Angeles County began a plague surveillance program because the number of human plague cases had increased. (uring the past 50 years, all human cases have occurred since 1978, including 3 in 1984.) Reagents and quality control were supplied by the CDC Plague Laboratory. Groups participating in the study included the countyveterinarian, animal shelters, county vector control, the agriculture commissioner, and the California State Polytechnic University, Pomona. Between 1986 and 1990,4,484 animal sera were tested for passive hemagglutination antibodies to Yerismia pestis, with 113 animals having antibody titers of 1:16 or higher. The titers ranged from 1:16 to 1:512, with the most common (modal) titer being 1:32. The largest number of seropositive animals were seen in 1987 (43) and the smallest in 1990 (2). Ground squirrels, dogs, coyotes, and black bears accounted for 109 (96%) of the seropositive animals. Additional seropositive animals included two cats, one fox, one raccoon, and one wild boar. Over half (60) of the seropositive animals were ground squirrels. Of the over 2000 shelter dogs tested, less than 1% were seropositive. Owned dogs were useful in areas of suspected plague epizootics. Two camp dogs were seropositive (1:32, 1:64) in 1987 at Camp Barley Flats, a correctional facility. Yersinia pestis was isolated from a dead ground squirrel found on the premises.

The animals most often seropositive were black bears (75%), wild boar (9%o), coyotes (6%), and ground squirrels (5%).

The seropositive rate for ground squirrels and coyotes was comparable to what was detected 10 years earlier.' Autopsies were performed on most of the coyotes. There were no histopathologic lesions suggestive of plague in any coyote examined. Most coyotes examined were under 1 year of age. Urban coyotes are useftl in plague surveillance of large areas. Wild carnivore provide a practical plague monitoring system. Although black bears and wild boars make good sentinels, they are uncommon in the semi-urban areas of Los Angeles County where most human exposure occurs. Ground squirrels are useful in pinpointing a location where a plague epizootic is occurring or has recently occurred. When ground squirrels are not available, domestic and wild carnivores are useful monitors.2-4 [] C. Pabick RPwn, DVM, MPH P. Eric Hughes, DVM GeorW Ma lsmoto, BA, MT Requests for reprints should be sent to C. Patrick Ryan, County of Los Angeles, Department of Health Services, Comparative Medical and Veterinary Services, 12824 Erickson Avenue, Downey, CA 90242.

References 1. Nelson BC, Madon MB, Tilzer A. The complexities at the interface among domestic/ wild rodents, fleas, pets, and man in urban plague ecology in Los Angeles County, California. In: Proceedings of the Twelfth Vertebrate Pest Conference; 1986:88-96; University of California, Davis. 2. Rust JH, Miller BM, Bahmanyar M, Marshall JD, et al. The role of domestic animals in the epidemiologyof plague. II. Antibodyto

Yeristiair in sera of dogs and cats.Journal of Infectious Diseases 1971;124(5): 527-31. 3. Rust JH, Cavanaugh DC, O'Shita R, Marshall JD. The role of domestic animals in the epidemiology of plague. I. &perunental infection of dogs and cats. Joumal ofInfectious Diseases 1971;124(5):522-26. 4. Nelson JH, Decker MD, Barnes AM, et al. Plague surveillance usingwild boar and wild carivore sentinels.JownalofEnviron,nen-

tal Health 1985;47(6):306-309.

Epidemiologic Studies on Electromagnetic Fields and Cancer Articles in the popular press alleging hazards of electromagnetic fields (EMFs) have contained misconceptions. In particular, in articles by Paul Brodeur in The New Yot*r magazine (and a book dealing with essentially the same information) that state a strong association between exposure to EMFs and cancer, the scientific

American Journal of Public Health 897

## Plague surveillance in Los Angeles County.

L~ttm to the Editor that not only first but also recurrent events of disease during the study period are taken into account. Thus a rate difference,...