Exploration of natural foci of tularemia and plague in Armenia using the serological examination of bird droppings and excrements of predatory mammals]

Forty strains of tularemia and 619 of plague microbes were isolated in 1974 in bacteriological examination of natural plague and tularemia foci from a great number of small mammals and their ectoparasites. At the same time in serological examination (in the antibody neutralization test) of bird pellets, 52 mummified cadavers, and 34 excretion samples of mammalian beasts of prey collected in Armenia (its central and North-Western part) in 1973 the antigen of tularemia microbe was revealed in 73, 8, and 3, and of plagye--in 42, 5, and 1 cases, respectively. In one of the sites examined the number of positive findings failed to exceed 10--17%, this indicating a low intensity of the epizootic in the majority of the places. Judging by the mean titres of the serological test, which varied from 1:12 to 1:1428 in examination for tularemia and from 1:12 to 1428 in examination for plague, it was possible to detect both epizootics coursing during the examination, and those which occurred several monts ago. Tularemia and plague foci were not infrequently present at the same territories, and these diseases could involve the same individual animals of Microtus arvalis (Pall.). The data obtained pointed to the greater effectiveness of examination of bird's pellets and excretions of mammalian beasts of prey for the reconnaisance investigation of the natural foci of plague and tularemia in comparison with the bacteriological methods applied usually.     

Evaluation of the results of animal studies with a scheme of serologic reactions according to a table of standards]

The authors suggest a method of processing of the data of examination of the animals in the system of serological reactions for the purpose of diagnosis of epizootic situations in the natural plague foci. Positive results are recorded in a table in the form of a grid whose cells correspond to all possible variants of the two reaction titres. The table is divided into 6 sections into which the results differing by the correlation and the absolute reaction titres (the standard grid) are placed. The frequency of reactions is determined by the grid sections in the regions in which the character of the epizootic development in nature was studied by bacteriological examination of the animals. The diagnostic scale is drawn by the results of such standard observations. The trial of the suggested method was carried out in the regions of plague epizootics among the Rhombomys optimus Licht. in the Lowland Kara-Kum; it was possible to differentiate the signs of epizootic situations there in greater detail than by the methods suggested earlier.     

Broader investigation into the external environment of the specific antigen of the infectious agent in epizootiological observation and study of the structure of natural foci of plague.

The possibilities of detection of the infectious agent in natural foci of plague can be expanded to some extent by using the method of search for the specific FI capsular antigen of the plague microbe in such objects of the external environment which have not yet been subjected to laboratory analysis and serological analysis in particular. Such objects include soil from the burrow and the substrate of the nest of the rodent as well as the fleas of rodents and regurgitations of birds of prey.The effectivity and suitability of examination of these objects using immuno-adsorption reactions with standard erythrocyte diagnostics (passive haemagglutination reaction, antibody neutralization reaction) were demonstrated experimentally and confirmed under field conditions in various geographical and ecological zones of the Caucasus. Thus in 1971, an earlier unknown natural focus of plague was found in the Mount Elbrus massif by means of serological analysis of the substrate of the nest and of the soil from the burrows of rodents. It can be expected that broadened serological search for the FI antigen of plague will prove useful in the study of the structure of natural foci of plague, in the reconnaissance or retrospective investigation of poorly explored territories and in the solution of some questions concerning the epizootiology of plague, such as the survival of the infectious agent in the inter-epizootic period, telluric plague, etc.     

Results of a trial of a plague monoclonal antibody erythrocyte diagnostic agent

Plague antibody monoclonal erythrocyte diagnosticum was studied in serological tests simultaneously with commercial plague antibody erythrocyte diagnosticum prepared on the basis of hyperimmune horse serum and with commercial plague antigenic erythrocyte diagnosticum. In this investigation the suspensions of numerous strains of Yersinia pestis, other closely related and heterologous organisms, experimentally infected wild and laboratory animals, as well as samples of materials obtained from small rodents caught in several natural foci of plague, were studied. The monoclonal diagnosticum was, practically, not inferior to the similar commercial preparation with respect to the frequency of positive results and the activity of the materials under study in serological tests, but showed greater specificity, as it reacted strictly with Y. pestis capsular antigen.     

Bacteriologic and serologic studies of an outbreak of plague in the State of Paraíba, Brazil

During a plaque outbreak in the Borborema Plateau focus (Paraiba), bacteriological and serological studies were carried out in material from 452 patients (48 positives), 1,938 rodents and other small mammals (75 positives), 4,756 dogs (141 positives) and 2,047 cats (57 positives) obtained from 41 counties (out of which, 21 produced positive samples). Twenty Yersinia pestis strains isolated from material from 3 patients and 17 rodents, displayed biochemical reactions, virulence factors, antibiotic susceptibility and animal experimental pathogenicity similar to those observed in strains previously isolated. According to our findings this recent plague outbreak did not exhibit different factors from those observed during prior outbreaks in other plague foci in the northeast of Brazil.     

A computer model of plague epizootic process in the great gerbil (<Emphasis Type="Italic">Rhombomys opimus</Emphasis>) settlements: Description and test for adequacy

The spread of plague epizootics in areas with natural plague foci, including Rhombomys opimus settlements, is usually studied in the course of annual epizootiological observations. Direct studies on the structure of epizootics and parameters of their spread are labor-intensive and, therefore, rare. Nevertheless, a number of authors have performed such studies using different methods, which made it possible to accumulate a certain factual material. This material has provided a basis for a computer model of the epizootic process in R. opimus settlements with interactively adjustable parameters. Mathematically, the model is based on a probabilistic cellular automation. The model conceptually deals with the same types of settlements as in nature: settlements with acute development of the disease, settlements with a certain proportion of immune individuals among rodents, and those not affected by an epizootic and representing a reserve for its further development. The groups of colonies (microfoci) separated from each other and unevenly distributed in the working space of the model are clearly distinguished. The epizootic process in the working space of the model shows more or less ordered cyclic fluctuations, which resemble the curve of epizootic activity in nature, and advances at a rate of 150 m to 1.5 km per 1.7–2.7 months, which well agrees with parameters recorded in field experiments with isotope-tagged animals and in the course of direct observations on the epizootic process in nature Thus, the main characteristics of the model epizootic process are quantitatively and qualitatively close to its natural analogue, which is evidence that the proposed model is conceptually correct and adequate.     

Serological study of nontriturated fleas following preliminary cultivation of the plague microbe]

Under experimental conditions there was shown a possibility of serological study of nontriturated fleas for the presence of Past. pestis. Hottinger's broth with gentian-violet ((0.0003%) sodium sulfite (0.03%) and a surfactant--O (see article)-7 (3%) served as the growing medium. After the nontriturated fleas were grown for 48 hours at 37 degrees C the capsular antigen was detected with the aid of the passive hemagglutination and antibody neutralization tests. Detection of the capsular antigen in the growing medium pointed to the presence of Past. pestis in the fleas. To test the serological method of study the nontriturated fleas, the insects were collected in the colonies of Rhombomys opimus Licht. at the epizootic territory. The collected fleas were divided into portions (each portion contained not more than 20 fleas). Half of the portions of each collection was studied after trituration by the usual bacteriological method, and another--by serological method without trituration of the insects. In examining the parallel portions (a total of 2400 portions, about 32 000) it appeared that positive results were obtained by both methods with about the same frequency.     

Isolation of pathogens other than Yersinia pestis during plague investigations.

From 1975 to 1978, 37 isolates of Pasteurella multocida, 1 of Salmonella enteriditis, and 5 of Francisella tularensis were recovered from 42 mammalian specimens and 1 flea pool submitted for examination for evidence of infection with Yersinia pestis. Most of the specimens were collected during investigations of either a human plague infection or a reported epizootic among rodent populations. All specimens were of species regularly or occasionally involved in plague or tularemia cycles in nature and most were collected in areas of known plague or tularemia activity.     

Evaluation of three serological tests for the detection of human plague in northeast Brazil.

The passive haemagglutination (PHA) test, enzyme-linked immunosorbent assay (ELISA) and the dot enzyme-immunosorbent assay (DOT-ELISA) were used to detect the levels of IgG antibodies against the Fraction 1 (F1) antigen of Yersinia pestis in sera of plague-infected patients from Northeast Brazil. Twenty three selected PHA-positive sera of subjects with bacteriological confirmation of plague were also positive in the DOT-ELISA but only 19 were detected by the conventional ELISA technique. Another group of 186 serum samples from subjects diagnosed as plague-infected by clinical and epidemiological parameters, but PHA-negative, were screened with DOT-ELISA and 11 gave positive results. The specificity of the assays on the serological detection of plague was confirmed in inhibition tests using purified F1 antigen. These results suggest that DOT-ELISA can be an useful, simple and more sensitive alternative for the serodiagnosis of plague in Northeast Brazil.     

Changes in Flea (Siphonaptera) Vector Activity in the Siberian Natural Plague Foci

Comparative analysis of vector activity of fleas in the Siberian natural plague foci was carried out during two long-term periods of experimental studies: 1967–1980 and 1983–2007. The data on block formation frequency in adult fleas infected with Yersinia pestis were analyzed for 127 experiments with 15 flea species and subspecies. The vector activity of fleas in all the Siberian plague foci (Altai, Tuva, and Transbaikalia) has increased over a rather short time period of 30–40 years. The frequencies of flea blocking were significantly different (P < 0.001) between the analyzed periods in all the three plague foci.

     

New findings concerning the spread of natural foci of infection in the transpolar region of eastern Siberia]

New materials are presented on the presence of the foci of anthropozoonozes in the Extreme North. For the first time there was established the existence in the subarctic tundra of the Taimyr peninsula of the arbovirus foci of the tick-borne encephalitis complex. A virus of the tick-borne encephalitis complex was isolated in 1973 from the gamasida ticks Haemogamasus ambulans Thorel. and Hirstionyssus isabellinus Oudms. and the nests of the Siberian lemming Lemmus lemmus L. This pointed to the existence in the Transpolar region of the foci or arboviruses in the nest-hole biocenoses of the lemmings outside the bird colonies. Cultures of tularemia bacilli (which proved the etiology of the epizootic among the lemmings observed in 1973 and also the presence of the lemming natural foci of tularemia and their combination with the arbovirus foci) were isolated from the lemmings at the same territory. The results of serological examination of the local population and of the animals pointed to the circulation in the Transpolar region of the causative agents of leptospirosis, toxoplasmosis, Q-fever and of the Asian tick-borne rickettsiosis.     

Infectious diseases in natural reservoirs: epidemic situation and morbidity in the Russian Federation and prophylactic measures

The analysis of morbidity in infections with natural foci in Russia for the period of 1999-2000 indicates that a rise in the number of cases of hemorrhagic fever with the renal syndrome, tick-borne encephalitis, Crimean hemorrhagic fever and West Nile fever with a high proportion of urban population among the patients is registered annually. An increase in the epizootic activity of the natural foci of plague is noted in the absence of morbidity among humans. The appearance of the outbreaks of tularemia is linked nor only with the increased activity of the natural foci, but also with the reduced coverage of the population with prophylactic immunization in endemic regions. This analysis infers that morbidity prognosis in infections with natural foci for 2001 remains unfavorable. The growth of morbidity in these infections is mainly due to the reduction of measures for controlling their vectors both is open nature and in settlements, as well as to low coverage with prophylactic immunization. Urgent complex measures which may ensure effective antiepidemic and prophylactic work for controlling infections with natural foci are presented.     

Seasonal dynamics of epizootic process of the plague agent <Emphasis Type="Italic">Yersinia pestis</Emphasis> transmission to the long-tailed suslik <Emphasis Type="Italic">Citellus undulatus</Emphasis> by the flea <Emphasis Type="Italic">Citellophilus tesquorum</Emphasis> in Tuva

Transmission of Yersinia pestis to the long-tailed suslik (Citellus undulatus) by fleas (Citellophilus tesquorum) in the Tuva natural plague focus in different seasons (spring, summer, and autumn) was studied experimentally. Between feeding periods, insects were kept in an artificial nest under temperature and humidity closely corresponding to seasonal ones. The character of the agent transmission was estimated according to the fraction of fleas with the agent in the aggregated state (bacterial lumps, partial blocks of proventriculus), the fraction of blocked individuals, and the fraction of infected susliks and of those with the generalized form of infection. Seasonal dynamics of epizootic process of the Y. pestis transmission corresponded to the results obtained in the epizootic examination of the Tuva natural plague focus and reflected the dynamics of the epizootic process (increase-peak-decline). The activity of the formation of a proventriculus block in C. t. altaicus, the infection ability of the fleas, and the sensitivity of long-tailed Siberian susliks to Y. pestis were the highest in mid-summer (July-first ten days of August), during the period of epizooty activation in the focus. The maximal number of C. t. altaicus with the plague agent at the aggregated state was observed in the cold period, before wintering of insects and after their hibernation.     

Current state of epidemiologic surveillance of natural foci of plague of North Caucasus

Information on the epizootic situation in plague in the natural foci of North Caucasus and on the influence of a number of anthropogenic and natural factors on this situation is presented. The data given in this work indicate that under the conditions of the anthropogenic transformation of landscapes the character of the epizootic manifestations of plague is changed and new factors, capable of aggravating epidemiological situation, appear. In addition, some other factors must be considered, such as the insufficient financing of reliable field surveys at present, the impossibility of making reliable epizootological studies due to causes of the social character (armed conflicts), thus making it impossible to evaluate, with a sufficient degree of reliability, the real epizootic state of a number of territories and, therefore, the risk of human infection. In this connection the necessity to carefully plan prophylactic measures and measures aimed at the localization and liquidation of the probable foci of infection arises.     

Possible vector dissemination by swift foxes following a plague epizootic in black-tailed prairie dogs in northwestern Texas

To determine whether swift foxes (Vulpes velox) could facilitate transmission of Yersinia pestis to uninfected black-tailed prairie dog (Cynomys ludovicianus) colonies by acquiring infected fleas, ectoparasite and serologic samples were collected from swift foxes living adjacent to prairie dog towns during a 2004 plague epizootic in northwestern Texas, USA. A previous study (1999-2001) indicated that these swift foxes were infested almost exclusively with the flea Pulex irritans. Black-tailed prairie dogs examined from the study area harbored only Pulex simulans and Oropsylla hirsuta. Although P. irritans was most common, P. simulans and O. hirsuta were collected from six swift foxes and a single coyote (Canis latrans) following the plague epizootic. Thus, both of these canids could act as transport hosts (at least temporarily) of prairie dog fleas following the loss of their normal hosts during a plague die-off. All six adult swift foxes tested positive for antibodies to Y. pestis. All 107 fleas from swift foxes tested negative for Y. pestis by mouse inoculation. Although swift foxes could potentially carry Y. pestis to un-infected prairie dog colonies, we believe they play only a minor role in plague epidemiology, considering that they harbored just a few uninfected prairie dog fleas (P. simulans and O. hirsuta).     

Natural focal infections in the north of the Far East. I. The detection of natural foci of infections in Chukot]

Results of a 3-year complex (zoo-parasitological, viral, bacteriological, and serological) studies of mammals and birds in Chukotka demonstrated the presence of natural foci of pseudo-tuberculosis, intestinal yersinosis, salmonellosis (heidelberg), and of tick-borne encephalitis. The existence of natural foci of tularemia and endemic rickettsioses is supposed on the basis of serological data.     

Long-term analysis of data on the isolation of Yersinia pestis cultures from rodents during epizootological examinations of natural foci in Muyun-Kum and eastern Kizil-Kum

Results are presented of a long-term examination and evaluation of data on the microbiological procedures of isolating Yersinia pestis cultures from wild mammals and their association animal plague pathogenesis as suggested by investigations in the Muyun-Kum autonomous plague focus and the Eastern Kizil-Kum mesofocus. 1772 Yersinia pestits cultures were isolated largely from Rhombomys opimus as the principle host over 23 years. The authors determined the frequency of pathological alterations in the body organs of infected animals, Rhombomys opimus featuring alterations more often than Meriones sp. Also determined was the rate of Y. pestis isolation from different organs. The authors assessed the validity of examining rodents by the method of direct inoculation of organ microflora on agar plates and by the biological assay. Direct inoculation into agar was shown to yield superior results. The reasons for occasional culture failures are analyzed in group bioassays as well as the advantages of individual bioassays. The significance of collecting and examining dead animals found in the steppe are evaluated as well as focus-specific differences between R. opimus and Meriones sp. with respect to the above parameters. Recommendations are given on assaying animals for plague infection.     

Relationship of the flea Citellophilus tesquorum altaicus (Siphonaptera: Ceratophyllidae) with the plague agent

Some aspects of relationships of the flea Citellophilus tesquorum altaicus and bacterium Yersinia pestis of two strains isolated from different parts of the Tuva natural plague focus were studied. Peculiarities of elimination and blood meal activity of fleas infected with two strains of the plague agent were not revealed. Differences in mortality and alimentary activity are considerably determined by the sex of insects. The ability of examined strains to form a proventriculus block was not identical in the strains examined. This ability was expressed higher in the strain I-3428, which originated from the same part of the natural focus as the insectarium flea culture, than in the strain I-3327. During the spring and first half of summer, the proventriculus block appeared more frequently in females. The increasing of the fraction of blocked individuals was observed in both sexes from spring to summer. As for the ability to transmit the plague agent, similar seasonal increasing was noted in males, but in females, the ability to inoculate the plague microbe was always maintained at the same level.     

A plague epizootic in the white-tailed prairie dogs (Cynomys leucurus) of Meeteetse, Wyoming

Surveillance for sylvatic plague (Yersinia pestis) was conducted near Meeteetse, Wyoming (USA) from 24 May to 14 June 1985. Ten species of fleas were collected from white-tailed prairie dogs (Cynomys leucurus), and from their burrows and associated rodents. Five of these flea species and two adult prairie dogs were positive for plague. The progression of this plague epizootic appeared to be slower and the intensity was less than in previous epizootics in other prairie dog colonies. The plague epizootic occurred within the only known colony of black-footed ferrets (Mustela nigripes) and was a potential threat to the food source of this endangered species.     

Transmission shifts underlie variability in population responses to Yersinia pestis infection

Host populations for the plague bacterium, Yersinia pestis, are highly variable in their response to plague ranging from near deterministic extinction (i.e., epizootic dynamics) to a low probability of extinction despite persistent infection (i.e., enzootic dynamics). Much of the work to understand this variability has focused on specific host characteristics, such as population size and resistance, and their role in determining plague dynamics. Here, however, we advance the idea that the relative importance of alternative transmission routes may vary causing shifts from epizootic to enzootic dynamics. We present a model that incorporates host and flea ecology with multiple transmission hypotheses to study how transmission shifts determine population responses to plague. Our results suggest enzootic persistence relies on infection of an off-host flea reservoir and epizootics rely on transiently maintained flea infection loads through repeated infectious feeds by fleas. In either case, early-phase transmission by fleas (i.e., transmission immediately following an infected blood meal) has been observed in laboratory studies, and we show that it is capable of driving plague dynamics at the population level. Sensitivity analysis of model parameters revealed that host characteristics (e.g., population size and resistance) vary in importance depending on transmission dynamics, suggesting that host ecology may scale differently through different transmission routes enabling prediction of population responses in a more robust way than using either host characteristics or transmission shifts alone.