Anthrax is an acute disease that primarily affects domesticated and wild herbivores and is caused by the spore-forming bacterium Bacillus anthracis. Human anthrax results from cutaneous infection or, more rarely, from ingestion or inhalation of the pathogen from contaminated animal products (1). Anthrax has also caused concern as a possible agent of biological warfare (2).

Early in 1980, reports appeared in the Western press of an anthrax epidemic in Sverdlovsk, a city of 1.2 million people 1400 km east of Moscow (3, 4). Later that year, articles in Soviet medical, veterinary, and legal journals reported an anthrax outbreak among livestock south of the city in the spring of 1979 and stated that people developed gastrointestinal anthrax after eating contaminated meat and cutaneous anthrax after contact with diseased animals (5, 6, 7). The epidemic has occasioned intense international debate and speculation as to whether it was natural or accidental and, if accidental, whether it resulted from activities prohibited by the Biological Weapons Convention of 1972 (8).

In 1986, one of the present authors (M.M.) renewed previously unsuccessful requests to Soviet officials to bring independent scientists to Sverdlovsk to investigate. This resulted in an invitation to come to Moscow for discussions with four physicians who had gone to Sverdlovsk to deal with the outbreak (including another of the present authors, O.Y., who was a clinician in the intensive care unit set aside to treat the victims.)  In 1988, two of these Soviet physicians visited the United States, where they gave formal presentations and participated in discussions with private and government specialists. According to their account, contaminated animals and meat from an epizootic south of the city starting in late  March 1979 caused 96 cases of human anthrax with onsets from 4 April to 18 May. Of these cases, 79 were said to be gastrointestinal and 17 cutaneous, with 64 deaths among the former and none among the latter (9).

The impression left on those of the present authors who took part in the U.S. meetings (J.G., A.L., M.M., and A.S.) was that a plausible case had been made but additional epidemiological and pathoanatomical evidence was needed. Further requests by M.M. for an invitation led to an on-site study in Sverdlovsk, initiated there in June 1992, and a return visit in August 1993. 

Starting in 1990, several articles about the epidemic appeared in the Russian press (10). These included interviews with Sverdlovsk physicians who questioned the foodborne explanation of the epidemic and with officials at the military microbiology facility. These officials said that in 1979 they had been developing an improved vaccine against anthrax but knew of no escape of anthrax pathogen. Late in 1991, Russian President Boris Yeltsin, who in 1979 was the chief Communist Party official of the Sverdlovsk region, directed his Counsellor for Ecology and Health to determine the origin of the epidemic (11). In May 1992, Yeltsin was quoted as saying that "the KGB admitted that our military developments were the cause" (12). No further information was provided. Subsequently, the chairman of the committee created by Yeltsin to oversee biological and chemical disarmament expressed doubt that the infection originated at the military facility and stated that his committee would conduct its own investigation (13). The results of that investigation have not yet appeared.

Pathoanatomical evidence that the fatal cases were inhalatory, recently published by Russian pathologists who performed autopsies during the epidemic (14,15,16), is summarized in an earlier report from the present study (17). Here we report epidemiological findings that confirm that the pathogen was airborne, and we identify the location and date of its escape into the atmosphere.