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Plague is transmitted by fleas or contaminated aerosols. To successfully produce disease, the causal agent (Yersinia pestis) must rapidly sense and respond to rapid variations in its environment. Here, we investigated the role of 2-component regulatory systems (2CSs) in plague because the latter are known to be key players in bacterial adaptation to environmental change. Along with the previously studied PhoP-PhoQ system, OmpR-EnvZ was the only one of Y. pestis' 23 other 2CSs required for production of bubonic, septicemic, and pneumonic plague. In vitro, OmpR-EnvZ was needed to counter serum complement and leukocytes but was not required for the secretion of antiphagocyte exotoxins. In vivo, Y. pestis lacking OmpR-EnvZ did not induce an early immune response in the skin and was fully virulent in neutropenic mice. We conclude that, throughout the course of Y. pestis infection, OmpR-EnvZ is required to counter toxic effectors secreted by polymorphonuclear leukocytes in the tissues.

Protection against pneumonic plague by the oral administration of a live, attenuated Yersinia pestis vaccine, EV76 (Paris) F, was evaluated in the vervet (Cercopithecus aethips). Six animals were vaccinated with a dose of 1.175 X 10(9) colony-forming units (cfu); all tolerated the dose and developed antibodies to Fraction I of Y. pestis. Three immunized animals were challenged with an inhaled dose of 3.2 X 10(6) cfu (160 LD50 [50% lethal dose]) of virulent Y. pestis strain 195/P and two survived; the other three received a challenge dose of 4.9 X 10(6) cfu (245 LD50) and one survived. The three surviving monkeys had high titers of antibody to Fraction I. At necropsy of the animals that succumbed to the challenge infection, almost complete consolidation of the lungs was apparent, and sizable effusions were present in the thoracic cavities. Indeed, the striking pathology indicated the severest form of pneumonic plague. Since it can be assumed that the animal surviving challenge with the larger inoculum would have survived the smaller dose, we conclude that the immunization procedure protected half of the vervets that were exposed to challenge.

Studies with sera collected during a 20-year period from 117 individuals who received multiple inoculations of plague vaccine, USP, revealed three patterns of serologic response. Eighty-four individuals responded with high titers of indirect hemagglutinating antibody, 23 produced intermediate antibody titers, and 10 did not produce significant antibody titers. After approximately five booster inoculations, each individual attained a plateau level of antibody that remained constant regardless of subsequent administration of booster inoculations. Long-term intervals between booster inoculations did not result in a marked decline in antibody titers. Following suspected episodes of exposure to virulent Yersinia pestis, antibody titers of 29 individuals increased abruptly but returned to previously established plateau levels within 18 months.

Plague is an acute bacterial infection of man caused by Yersinia pestis. The natural reservoirs of the organism are predominantly urban and sylvatic rodents, and it is transmitted among animals and occasionally to man by bites of infected fleas. Plague has a cosmopolitan distribution with significant foci of infection in the Americas, Africa, and Asia. Y. pestis has caused devastating pandemics throughout history with high mortality rates, in which pneumonic man-to-man transmission has occurred in addition to the usual flea-to-man spread. Today, in the United States, urban plague carried by rats has been eliminated, but sylvatic plague persists in rodents and other mammals of rural areas in the southwestern states with occasional transmission to man. The most common clinical form is acute regional lymphadenitis, called bubonic plague. Less common forms include septicemic, pneumonic, and meningeal plague. Mortality is high in untreated patients, but antibiotic treatment with streptomycin administered early in the course of disease markedly reduces fatalities.

Early concepts of fever as a major feature of illness range from those of Hippocrates to those present in the Bible and have influenced cultural attitudes during several major European pandemics of both black plague and tuberculosis, the "white plague" of the early 19th century. Evolving ideas of thermoregulation and fever in the 19th century followed the first extensive use of the clinical thermometer by Wunderlich. Experimental studies on the pathogenesis of fever during the last 30 years suggest that fever and certain aspects of both immunoregulation and inflammation are produced by a single hormone, the monokine interleukin-1, which has presumably been selected by evolution to protect the host against infection.

cemic. We reviewed these cases to better describe the clinical presentation of this disorder and to identify risk factors for developing septicemic plague. The symptoms (fever, chills, malaise, headache, and gastrointestinal symptoms) and signs (tachycardia, tachypnea, and hypotension) of septicemic plague are similar to those of other forms of gram-negative septicemia. Abdominal pain was reported in nearly half of the cases, and differential white blood cell counts revealed a marked shift to the left. The risk of developing septicemic plague was higher for persons >40 years of age. Because of empirical antibiotic treatment of older persons, deaths from septicemic plague occurred primarily among persons

Due to the erroneous concept relating reactogenicity to immunogenicity, killed plague vaccines were initially produced and standardized to insure that the majority of persons receiving them would experience severe local and systemic reactions. Subsequent studies have resulted in the production of relatively nontoxic, killed plague vaccines. Throughout the world the use of highly immunogenic, living, attenuated plague vaccines has resulted in frequent adverse local and systemic reactions. Modification of the route of administration of these vaccines has not substantially altered either the frequency or the severity of adverse reactions. The evidence presented in the literature indicates that programs of mass immunization with highly immunogenic, living, attenuated strains of Yersinia pestis are accompanied by harmful side effects.

In 22 Vietnamese patients with fever and bubo, Yersinia pestis infection was diagnosed by positive culture or serologic response. All strains of Y. pestis except one isolated from these patients were susceptible to most antimicrobial drugs tested, including trimethoprim. Common clinical features were tachycardia, hypotension, and leukocytosis. One patient died; the others survived while receiving streptomycin, chloramphenicol, trimethoprim-sulfamethoxazole, or combinations of these drugs. Intravascular coagulopathy was demonstrated in 19 patients who had elevated titers of fibrinogen-fibrin degradation products in serum; these titers decreased during convalescence. At the time of discharge, peripheral blood smears of most patients showed eosinophilia. The pathogenesis and treatment of plague are considered. From a peak incidence of 5,568 cases per year reported to the World Health Organization in 1967, the incidence of bubonic plague in Vietnam has been steadily declining [1]. Nevertheless, Vietnam continues to report more cases than other countries. During the first six months of 1973, there were approximately 30 confirmed cases of plague in the Saigon area. These patients, who afforded us the opportunity to observe hospital courses and to perform studies relating to the pathogenesis of plague, form the basis of this report.

During a plagpe survey* in late summer of 1942, 11 pools of fleas from Rattus norvegicus, Microtus californicus and Peromyscus maniculatus and 2 pools of lice from R. norvegicus were proven to be infected with plague. Since this area in southern Marin County was adjacent to the San Francisco Bay, an intensive survey** was made to find livers and lymph nodes of from 2 to 12 mammals of the same species from a single area. Each pool of tissue was thoroughly pulped in a mortar, and suspended in a normal saline solution. The suspensions were inoculated subcutaneously into guinea pigs. Ectoparasites removed from the rodents were refrigerated in salt solution until they

The value of the bacteriophage as a therapeutic agent has been investigated extensively both experimentally and clinically. In the attempt to utilize the highly potent homologous bacteriophage to control experimental mouse typhoid some years ago, one of us found that it did not influence the course of the experimental infection produced by feeding of B. pestis caviae to mice. These experiments indicated that the ineffectiveness of bacteriophage therapy was due to the failure on the part of the bacteriophage to destroy the susceptible bacteria and not to the production of phage resistant strains.1 Other investigators2 and particularly Topley, Wilson and Lewis3 also used bacteriophage in mouse typhoid caused by B. aertrycke and came to a similar conclusion. They found that the infected mice eliminated the organisms in the excreta even though bacteriophage was fed to them over a period of three months. Aside from the reports of successful application of bacteriophage therapy by d'Herelle4 in the hemorrhagic septicemia of buffalo (Barbone), and in typhoid of fowls, the reports of results of bacteriophage therapy in experimental infections have been uniformly disappointing.5 Although clinical success of bacteriophage therapy has been reported in such diseases as bacillary dysentery, typhoid fever, cystitis and pyelitis, plague and cholera, the pyogenic infections have received by far the most attention. A large number of clinicians used local applications of staphylococcus bacteriophage in the treatment of boils and related infections, Brynoghe and Maisin6 being apparently the first to have reported favorable

Swine were shown to be clinically refractory to experimental plague. Yersinia pestis, formerly known as Pasteurella pestis, was not isolated from blood or from buccal-cavity cultures 4 hr or more after oral challenge. In spite of the lack of clinical evidence of infection, all of the swine developed high titers of indirecthemagglutination (IH) antibody specific for Fraction I antigen of Y. pestis within nine days of exposure. Peak titers of IH antibody occurred by day 28 and remained essentially unchanged for a period of 200 days. Swine did not produce complement-fixing antibody against the Fraction I antigen of Y. pestis.

Experiments undertaken for the purpose of ascertaining the influence upon the virulence of B. pestis by passage from rat to rat without intermediate culture on artificial media showed that a certain number of the wild rats (Mus norvegicus) in San Francisco were immune to plague infection. This result led me to undertake a series of experiments to determine the extent of this immunity. The Indian Plague Commission1 found that 59 per cent of the wild rats of Bombay were immune when inoculated by the cutaneous method with splenic material containing a large number of B. pestis. They also found a certain percentage of the rats immune to subcutaneous inoculation of agar culture of B. pestis. The smallest dose of plague culture used by the Indian Commission in their work was

The subject of this report was a boy (J. M.) born in California of Portuguese parents. His age was 13 years and he was rather more than ordinarily well developed physically for his age. There was nothing in the previous history, medical or otherwise, having any bearing on the present illness, with the exception of the fact that, according to his own statement, he had been shooting ground squirrels in Alameda County, California, about a week before he was taken sick. According to the father's statement the boy had been out squirrel hunting just five days prior to his becoming ill. While on this hunting trip he shot four or five squirrels which he brought home with him. He stated that he had seen sick squirrels in the same vicinity, but did not shoot nor handle any of them.1 This boy had not been more than a few miles from his home in his life and indeed had never seen a trolley car until he was brought to the hospital in Oakland. There is no evidence that he was in any place where he might have been infected with plague except on this hunting trip. There had been no known previous case of plague in man in California for nearly a year, and none in rodents, except ground squirrels, for about seven months. The boy became sick on Tuesday, July 27, with headache and violent vomiting. The vomiting was almost continuous for about 24 hours. The next day he was seen by a physician and was found to have a temperature of 1040 F. On Thursday, 48 hours after the beginning of the illness, the glands in the left axilla were found to be enlarged and tender. The patient was taken to a hospital in Oakland

In a previous paper1 one of us described a disease found in nature in California ground squirrels {Citellus beecheyi Richardson) which is experimentally transmissible to other rodents, and which in practically all susceptible animals gives rise to plague-like lesions. The following is the summary of the observations reported in the paper mentioned: A disease which presents lesions very similar to those of plague has been found among ground squirrels. The disease is readily transmitted to guinea-pigs, mice, rabbits, monkeys, and gophers, and plague-like lesions are produced in at least some of these animals. Rats are but moderately susceptible to the infection. Cats, dogs, and pigeons appear to be immune.

In a previous study it had been found that domestic fowl from areas in California endemic for the virus of western equine encephalomyelitis were more resistant to inoculation of this strain than were birds from supposedly nonendemic regions.' A few resistant fowl were among the latter group, however, even among those without neutralizing antibodies in the blood stream. These substances were found in a small percentage of chicken serums before inoculation, usually in animals from endemic localities. Likewise in man a certain correlation had been observed between

While it is generally held that vaccination is of value for controlling insectborne bubonic plague, most workers in this field doubt or even deny its efficacy in combatting pneumonic plague. This skeptical attitude, taken by several authorities in regard to vaccines made with killed organisms,1 has been shared by the advocates of vaccination with live avirulent plague bacilli. Otten2 was so convinced that his Tjiwidej strain conferred no protection against pneumonic plague that he excluded cases of this nature from the statistics he com

The incidence of bubonic plague in humans in the United States has increased markedly since 1965; there were nineteen cases in 1969-1970. All but four were acquired in New Mexico from sylvatic plague endemic in wild rodents. An epizootic in these animals has not been proved. Increasing outdoor recreation and establishment of rural communes may bring more people into contact with infected rodents, while suburban pets may transport fleas from rodents to man. The current patients were younger and included more females than previously reported. Initial clinical manifestations were often milder than expected, although there was one death. All patients had fever and a bubo, while malaise or myalgia, headache or confusion, and abdominal complaints were found in one-half of the cases. When this disease is kept in mind, early recognition and specific therapy are usually possible. However, delayed diagnosis may result from atypical manifestations, or illness that develops after travel to places distant from endemic areas such as the Southwestern United States. In the 40 years from 1925 to 1964, 42 cases of bubonic plague were identified in the United States; one or two cases were reported each year [1-3]. In the past six years (1965-1970), 38 cases have been recognized, including six in 1969 [4] and 13 in 1970. Among these thirty-eight there have been five deaths, or approximately one per year; the last death was reported in November, 1970. The outbreak of 1970 was the largest in any year in the United States since 1924, when an urban epidemic hit Los Angeles. Most of the recent cases have occurred in the Southwest, espe

During the study of the prevalence of Leptospira hemorraghiae in the wild rats of the city of Sao Paulo, a group of interesting organisms were encountered which were very puzzling at first in that they were so similar to the plague bacillus. It was necessary to catch normal appearing wild rats in various parts of the city, choloroform them, macerate the kidneys in a sterile mortar, with careful aseptic, precautions, and inject the resulting emulsion into the peritoneal cavity of guinea-pigs. The work, went well up to a certain period, when we began to examine rats that had been caught in the chicken yard of Santa Casa, the large general hospital. One of every three or four of the guinea-pigs that were injected with kidneys of rats caught in this, location died within 12 to 24 hours after they were inoculated, though the rats used in the experiments had all appeared normal. The dead guinea-pigs presented all the typical characteristics of animals inoculated with the bacillus of bubonic plague. This finding was rather alarming, for though there were no known cases of plague in the city, the rats came from the grounds of a hospital at which it was strongly suspected that a recent death had resulted from plague, and the port of Santos, only 40 miles away, was suffering at this time with an outbreak of the disease. Our alarm was not quieted by subsequent study. Smears from the peritoneal cavity of the dead guinea-pigs showed myriads of gram-negative, bipolar rods, with rounded ends, their morphology being characteristic of the plague bacillus. Pure cultures of the organism were readily obtained from the heart blood of the guinea-pigs. It was obvious that we were working with an organism which, if it was not the plague bacillus, was certainly closely related to it, and we therefore made an immediate differential study of the organism.

The brief note here given represents a few experiments which were made incidental to a study aiming at immunity to septicemic and pneumonic plague. Friedberger1 discovered that powerful poisons could be produced if complement, as present in normal guinea-pig serum, was allowed to act on suspensions of bacteria. These poisons, or "anaphylatoxins," as he named them, give acute shock on injection and cause the death of the animal. Recently Zinsser2 showed that animals may acquire distinct tolerance of such poisons and survive large doses of the "proteotoxins" without evidencing any noticeable shock after the preliminary dose, which is so measured as to give a slight shock to the animal. It was also demonstrated that these products apparently possess aggressinlike properties, which, if injected in combination with sublethal doses of bacilli, such as typhoid bacilli, cause death through a resulting bacteriemia.

THIS report is based upon 70 plague-infected ground squirrels (Citellus beecheyi) which have come under observation in California during the months of June, July, and August, 90o9. During this period approximately 200 squirrels have been reported as plague infected to Surgeon Rupert Blue, Public Health and Marine Hospital Service, commanding the plague suppressive measures. In the case of the majority of these, the diagnosis was based upon the gross lesions alone or upon the gross lesions in addition to the result of the microscopical examination of the smear preparations from the buboes or from other tissues. In the case of 68 of the squirrels, however, tissues were inoculated into laboratory animals and when these animals died, cultures were made from their tissues and the specific bacillus isolated in about 30 cases. In the other cases the postmortem lesions presented by the laboratory animals were so characteristic of plague that it did not seem necessary to make cultures from them. In a few cases the cultures were hopelessly contaminated and the effort to isolate the plague bacillus was abandoned. In two cases the bacillus was isolated directly from the tissues of a naturally infected ground squirrel. Indeed the attempt to recover the plague bacillus in this manner has been made on only about a half-dozen occasions. The reason that the effort was not made to

Although numerous investigators have examined the blood of patients with plague for B. pestis, no one has yet advanced any convincing explanation of the relation of bacilli in the blood to the development of plague. From the results of examination of test animals, Tsurumi and his coworkers 1 and others state that in plague infection the circulating blood acts only as a carrier of bacilli from the infected location to other tissues, but that the appearance of bacilli in blood is not a specific condition for the development of the disease. According to Sata,2 who studied this subject in detail both bacteriologically and anatomically, at the beginning of the infection, bacilli are held chiefly at the point in which inoculation took place, or in neighboring lymph nodes. The bacilli, however, have a great tendency in time to enter the circulating blood and grow in it. As a result, there occurs an intensive production of toxin with the characteristic severe symptoms and plague septicemia develops. As to the presence of bacilli in the bile, no detailed examination appears to have been made. In Tsurumi's experiments in 13 fatal human cases of plague and 9 animal cases, no positive result was obtained, and he ascribed this to the unfavorable conditions in bile for the growth of pest bacilli. In a previous paper 3 on B. pestis in blood of patients with plague, it is pointed out that the examination for B. pestis in the circulating blood is best made by cultivating on bile medium or 1% sodium citrate broth. By this method, 72.2% of patients with pest gave positive results. Consequently, pest septicemia is not rare as reported heretofore, and its prognosis not absolutely bad. The presence of B. pestis in the blood should not be explained as due only to the invasion of the blood after it has been growing at the point of invasion; it may grow at the same

In 1976, in a small, remote Libyan village, one apparently sick camel was slaughtered and skinned, and the camel meat was distributed for human comsumption. A few days later, 15 villagers suffered a severe febrile illness. Of the five individuals who had participated in the killing and dispensation of the camel, all were dead within four days. When samples of serum from nine of the remaining patients were examined, seven were found to be positive for plague as determined by the passive hemagglutination test. Another six persons became ill after killing two goats, and the serum of one goat contained antibodies to Yersinia pestis. Because all of the remaining patients except one were treated early enough, they recovered. These incidents confirm previous reports that the camel and the goat are susceptible to naturally occurring plague infection and have a significant role in the dissemination of human plague.

The animals (other than controls) used in this experiment were trapped upon the U.S. Marine Hospital Reservation at Fort Stanton, New Mexico. After they had been kept in captivity for several weeks an effort was made to determine the susceptibility of the several species to infection with the plague bacillus. The culture of B. pestis used was one that had been isolated from the spleen of a naturally infected ground squirrel about two months prior to the present experiments. The third generation on agar was used, the culture having been grown for 72 hours at 370 C. In the case of the animals in which the culture was inoculated