Stuart F. Quan

New knowledge on the ecology of sylvatic plague.

The history of plague is an eloqueiit example of changing concepts, of investigation in which numerous hypotheses have been advanced, their limitations described, their positive values extracted, and their remaining skeletons buried in favor of new ideas. Known in classical antiquity, in the illiddle Ages, and in the pandemia of the latter Nineteenth and early Twentieth centuries, plague has been studied primarily as an epidemic disease causing widespread mortality in human populations. Until romparatively recently, plague was thought to be fundamentally a disease of domestic rats in which rat fleas were the major vectors. Ry 1928 it was clear that sylvatic or wild-rodent plague represented an independent epizootiologic entity, and since that time it has been recognized that sylvatic plague may transfer to domestic rodents under certain conditions (Pollitzer, 1954). Detailed historical and ecologic views and reviews of plague in the western United States have been written by Meyer (1942a and 1942b), while the works 01 Pollitzer (1954) and Macchiavello (1954) document the vast world literature. The world distribution of sylvatic plague has been summarized by Garnham (1949). Sylvatic plague is now known to be established in wild rodents and their fleas in the western region of the United States and in western Canada. Recently the presence of wild rodent plague was shown in Mexico with the isolation of the organism from prairie dogs in the northern state of Coahuila (Varela and Vasquez, 1954). FIGURE 1 shows the distribution of plague in animals as a result of extensive surveys, especially from 1936 to 1950, conducted by the TJnited States Public Health Service and by certain states. The predominance of wild-rodent over rat-borne plague is shown in FIGURE 2. Wildrodent plague foci have been found in 131 counties of 15 western states in surveyc from the Pacific Coast to the 100th meridian, an area comprising about 40 per cent of Ihe continental United Stales. The evidence shows that the disease in wild rodents forms a vast enzootic reservoir as a potential threat to humans. Pollitzer (1954) lists 91 plague cases and 57 deaths in 7 wcstern states during the period 1908 to 1951 as having been contracted from wildrodent sources. In June 1956 a fatal human case occurred in Ventura County, Calif.; this was apparently contracted from ground squirrel fleas. FICURE 3 s h o w the distribution of human plague cases resulting from conlaci wiih wild rodeiits, with the exception of the case shown in Michigan, which is a laboratory infection.

Studies on Pasteurella pestis in fleas. IV. Experimental blocking of Xenopsylla vexabilis hawaiiensis and Xenopsylla cheopis with an avirulent strain

Summary An experiment was conducted to determine the ability of an avirulent strain of Pasteurella pestis (A1122) to produce proventricular blockage in two species of fleas, Xenopsylla cheopis and X. vexabilis hawaiiensis . Male X. cheopis and both sexes of X. v. hawaiiensis were infected separately on infective bacterial suspensions in a feeding apparatus and were then maintained separately on young laboratory rats. Bacterial plate counts from the males of both flea species showed an initial decline during the first day after the infectious feeding. Both species showed an increase in the bacterial counts after the first day, but the rise was very gradual in the X. cheopis and much sharper in the X. v. hawaiiensis . The female X. v. hawaiiensis plate counts showed an increase after 3 days. All counts either leveled off or began to decline at 10 days. There appeared to be no major differences between the ability of avirulent plague strain A1122 to cause proventricular blockage in X. cheopis and in X. v. hawaiiensis . Males of both flea species showed similar blocking rate-time curves, but the rise in the X. v. hawaiiensis was more precipitous. The curve for X. v. hawaiiensis females was more gradual and extended over a longer period. It is believed that the rapid blocking rate of the males decimated the populations so rapidly that the percentages of males recovered between 15 and 20 days were less than half that of the percentages of females recovered by the termination of the experiment at 30 days.

Notes on the fate of avirulent Pasteurella pestis in fleas.

Abstract By the use of in vitro feeding techniques, fleas ( Xenopsylla cheopis ) were fed upon six avirulent strains of Pasteurella pestis in suspensions of rat blood. Three of the strains (A1122, “14,” and B741-10-9) showed a capacity to multiply in the fleas and eventually to cause blockage of the proventricular valve. The other strains apparently did not survive in the fleas. Plate counts of the number of bacilli of each strain in fleas, processed at intervals after the infectious blood meal, confirmed the findings with regard to blocking rates. In another experiment, seven different concentrations of avirulent P. pestis strain A1122 were fed to X. cheopis . The results showed a direct relation between the number of bacilli in the infectious blood meal and the blocking rates in the fleas. Thus the vector efficiency of fleas may be limited by a “threshold” of the degree of bacteraemia in wild rodents infected with P. pestis

Efficacy of fluorescent antibody methods for detection of Pasteurella pestis in carcasses of albino laboratory mice stored for various periods.

The methods recommended for diagnosis of Pasteurella pestis infection in animal tissues include microscopic examination of stained impression smears, bacteriological culture followed by biochemical and bacteriophage tests, animal inoculation, and precipitin tests using soluble plague antigens (Baltazard et al. 1956). Bacteriological culture and animal inoculation are sensitive and specific when test materials are well preserved, but these methods seldom give positive results when the specimens are from putrefied or mummified carcasses. In such cases, the modified Ascoli test developed by Larson, Philip, Cicht & Hughes (1951) or the haemagglutination and complement-fixation tests of Chen, Quan & Meyer (1952) and Chen & Meyer (1954) have proved to be of value. Larson reported significant precipitinogen titres in animal tissues stored at 37° C. for periods as long as 14 weeks. Chen reported excellent results with complement fixation tests of plague antigens extracted from decomposed carcasses and from partially desiccated tissues held at 37° C. for 10 days.