Potential for rat plague from nonencapsulated variants of the plague bacillus (Yersinia pestis).

Potentials for oral and flea-borne transmission of nonencapsulated Y. pestis were demonstrated when 45% of rats that consumed infected meat died of plague and 22% of the rats that died of plague had bacteremia.

Chronic infections in laboratory rodents from inoculation of nonencapsulated plague bacilli (Yersinia pestis).

Infections with nonencapsulated Y. pestis persisted for 9 weeks in mice and 56 weeks in rats after experimental inoculation. Mice succumbed with bacteremia. Rats developed chronic lesions containing nonencapsulated plague bacilli.

Comparison of passive haemagglutination and enzyme-linked immunosorbent assay for serodiagnosis of plague.

Sera of 42 suspect plague cases from Ovamboland, Namibia, were examined. ELISA proved more sensitive than passive haemagglutination for the detection of F1 antibody and increased positive serodiagnoses of plague by 7-42%.

Correlation of autoagglutination and virulence of yersiniae.

Virulent strains of Yersinia pestis, Y. pseudotuberculosis and Yersinia enterocolitica invariably autoagglutinated in tissue culture media when grown at 36 degrees C. Avirulent strains did not possess this property.

Potency of killed plague vaccines prepared from avirulent Yersinia pestis.

Killed plague vaccines prepared from avirulent strains A1122 and EV76S of Yersinia pestis were more effective in mouse potency tests than samples of Plague Vaccine, USP, prepared from killed Y. pestis of the virulent strain 195/P. Manufacture of vaccine from avirulent Y. pestis would obviate requirements for the large containment facilities that are currently needed for producing Plague Vaccine, USP.

Plague in Central Java, Indonesia.

Plague in man occurred from 1968 to 1970 in mountain villages of the Boyolali Regency in Central Java. Infected fleas, infected rats, and seropositive rats were collected in villages with human plague cases. Subsequent isolations of Yersinia pestis and seropositive rodents, detected during investigations of rodent plague undertaken by the Government of Indonesia and the WHO, attested to the persistence of plague in the region from 1972 to 1974.Since 1968, the incidence of both rodent and human plague has been greatest from December to May at elevations over 1000 m. Isolations of Y. pestis were obtained from the fleas Xenopsylla cheopis and Stivalius cognatus and the rats Rattus rattus diardii and R. exulans ephippium. The major risk to man has been fleas infected with Y. pestis of unique electrophoretic phenotype. Infected fleas were collected most often in houses.Introduced in 1920, rodent plague had persisted in the Boyolali Regency for at least 54 years. The recent data support specific requirements for continued plague surveillance.

Experimental plague in the California ground squirrel.

In experiments to determine the current susceptibility to plague, it was demonstrated that 256 Yersinia pestis (a local strain of high virulence) were required to produce a 50% infectious dose (ID50) in California ground squirrels (Spermophilus beecheyi) that had been collected in southeastern Monterey County, California; 6,070 Y. pestis were required to produce a 50% lethal dose (LD50). (The LD50 was about 24 times the ID50.) The frequency of serologic response to the specific fraction 1 antigen of Y. pestis and mortality were dose-related. Approximately half of the squirrels inoculated with six to 6,070 Y. pestis survived without seroconversion, whereas antibody to specific fraction 1 antigen was always observed in squirrels that survived challenges of greater than or equal to 60,700 Y. pestis. However, titers never exceeded 1:64. The implications of these data for enzootic and epizootic transmission of plague in resistant squirrel populations are examined.

Measuring the efficacy of vaccination in affording protection against plague.

The relationship of F1 antibody titre to protection against plague was investigated by subjecting seropositive laboratory rats to virulent challenge and observing for survival. The passive haemagglutination (PHA) test in microtitre was employed for serology. Rats vaccinated with live vaccine EV76 (51f), killed U.S.P. vaccine, or F1 antigen and challenged by subcutaneous inoculation of 1 x 10(3) to 5 x 10(5)Yersinia pestis survived at similar rates that, overall, equalled 6% at titres less than 1:16, 46% at titres of 1:32-1:64, 90% at titres of 1:128-1:256, and 96% at titres of 1:512-1:1024. Rats vaccinated with F1 antigen and rats that had been infected previously were challenged intranasally with 8.9 x 10(4)Y. pestis and subsequently demonstrated similar rates of survival that was zero at titres less than 1:128, 86% at titres of 1:128-1:256, and 100% at titres of 1:512-1:1024. The significance of titre of F1 antibody as a measure of seroimmunity against acute bubonic or pneumonic plague is discussed for rats, monkeys, and man.

Stylostome formation by Leptotrombidium mites (Acari: Trombiculidae).

The modes of stylostome formation by larvae (chiggers) of Leptotrombidium intermedium, L. fletcheri, L. arenicola, and L. deliense in parasitized mouse skin were studied histologically in relation to their capacity to transmit Rickettsia tsutsugamushi. Three types of stylostome formation were recognized among the different species: the epidermal stylostome formed by the larva of L. intermedium; the mesenchymal stylostome formed by the larva of L. fletcheri; and, the mixed stylostome formed by the larva of both L. arenicola and L. deliense. Dermal inflammations related to the three types of stylostomes were histologically defined. The possible importance of stylostome characteristics to the transmission of rickettsial organisms is discussed.