[SPECIFIC FEATURES OF RELATIONSHIPS BETWEEN THE FRONTOPSYLLA LUCULENTA LUCULENTA (JORDAN ET ROTHSCHILD, 1923) FLEA AND THE PLAGUE PATHOGEN WITH DIFFERENT PLASMID COMPOSITION].

It was experimentally established that plague pathogen strains with different plasmid composition variously suppressed the viability of Frontopsylla luculenta luculenta fleas. Dead insects were most frequently observed among those infected with a virulent strain having the cryptic plasmid pTP33. The presence of the avirulent and apesticinogenic plasmid I-3480 in the fleas less deteriorated their state. Biofilm formation by different F.l.luculenta strains in the body was characterized by quantitative and qualitative differences. The strains that had the cryptic plasmid and were able to form the biofilm in the F.l.Iuculenta fleas surpassed the three-plasmid strain I-3230 and their formned aggregates achieved very large sizes and frequently persisted until the end of the experiment. Small solitary masses were generally observed in the insects infected with the three-plasmid strain. Thus, the pTP33 plasmid potentiated the pYT plasmid-encoded ability to colonize the F.l.Iuculenta fleas with the plague pathogen; in this case the products of the pYV and pYP plasmids (or one of them) are toxic to ectoparasites.

[Relationships of the plague pathogen and vector from different parasitic systems].

The specific features of interaction of the strains of the plague microbe of the main subspecies, which circulate in the area of natural foci of Mongolia and China, with Citellophilus tesquorum sungaris fleas, the major vector of the plague pathogen in a Transbaikalian natural focus, as well as with Xenopsylla cheopis ones, the classical vector, were revealed. Experiments used virulent Yersinia pestis strains, such as I-3230 isolated from C.tesquorum in Mongolia in 1998 and 2155 isolated from humans in Manchuria (China) in 1947. They established that ectoparasites from other parasitic systems could transmit these strains. At the same time, the Y.pestis strain 1-3230 far exceeded the strain 2155 in its ability to form conglomerates as lumps. It is possible that this fact reflects the adaptive peculiarity of the Y.pestis strain 1-3230 to remain long (during the cold period of a year) in the flea C.tesquorum sungaris that survives winters mainly in the imago stage. The strain 2155 was more active in forming proventricular blocks in the body of X.cheopis, the blocking period in the latter was 3-7 times shorter than that in C.tesquorum sungaris when infected with both strains.

[Reservation forms of plague infectious agent in Tuva natural focus].

Data characterizing the reservation forms of plague infectious agent in Tuva natural focus are presented in the review. Yersinia pestis was shown to persist most of the year in Citellophilus tesquorum altaicus imago --the main carrier, getting into the animal organism only for a short time. An increased ability to aggregate in autumn and accumulate in clumps of C. tesquorum altaicus females that are more adapted to survive the cold season compared with males promote the persistence of the microorganism. The plague infectious agent in the altered form survives in the organism of females not only the winter period but also longer periods of time that is demonstrated by the facts of detection of it after 646 days of staying in the carrier. Moreover Yersinia pestis can persist for more than 400 days in the substrate of the nest of long-tailed ground squirrel infected by excrements and corpses of plague fleas. The substrate of the nest infected in summer-autumn period of the previous year may determine the primary infection of ground squirrels by plague infectious agent in the next epizootic season. On ground squirrels infected during contact with nest substrate, infection of intact fleas may be possible, and so the initiation of a new cycle of transmission of the infectious agent. Adaptation of the plague infectious agent to unfavorable existence conditions in the carrier is expressed in the changes of its morphology and ultrastructure that is evidenced by the facts of isolation of the infectious agent from corpses of fleas situated in the substrate, in the L-form, as well as results of phase-contrast and electron microscopy of the digestive tract of C. tesquorum altaicus.

[Host influence on the frequency of block formation in males and females of Citellophilus tesquorum altaicus Ioff, 1936 (Siphonaptera: Ceratophyllidae) and transmission the plague pathogen by these fleas].

Data obtained during feeding of Citellophilus tesquorum aitaicus Ioff, 1936 infested females and males (Siphonaptera: Ceratophyllidae), the main vectors of plague in Tuva natural plague locus, on the natural host and laboratory animal was analyzed. It was found that sexual differences in fleas depended on the type of the host. Females fed more actively on the longtailed ground Citellus undulatus than on white mouse. Alimentary activity of males on these animals was similar. Higher mortality of fed females and males was noted during feeding on mice. Frequency of formation of the "block" and transmission of the pathogen in males was higher during bloodsucking on the ground squirrel; in females, during feeding on mice. Thus, differences in the transmission of the plague pathogen, revealed in laboratory on white mice, can be quite different in nature. So, extrapolation of experimental data on natural processes of interrelations between plague pathogen and ectoparasites must be performed taking into account revealed peculiarities.

[Epizootological assessment of the possibilities of importing plague from Vietnam to Russia].

To assess whether the plague microbe with vectors or carriers can be imported from Vietnam to Russia, the authors consider the specific features of pathogen circulation in this country's biotopes varying in anthropogenic transformation. The idea that there were natural foci of plague in Vietnam dominated until the late 1990s. The small rat Rattus exulans that inhabits open stations and ensures a parasitic contact with the synanthropic representatives of the fauna was considered to be a major carrier. The recent years have provided conclusive proofs that plague foci are absent in Vietnam wild nature. Anthropurgic foci develop in the network of localities whose conditions are favorable to the existence of synanthropic rodents and the fleas Xenopsylla cheopis. Cases of the plague pathogen, FI antigen and its antibodies being detected in wild mammals are due to their parasitic contacts with synanthropic rats in the agrocultural area around the localities with running epizootias. These contacts are provided by X.cheopis only. Since 2003, there have been no reports on the incidence of human plague or its pathogen isolation from environmental objects in Vietnam. However, all conditions and prerequisites for the formation ofanthropurgic plague foci remain in this country. Further epizootological monitoring is required for appropriate services to rapidly and adequately respond if the epizootological or epidemiological situation of this infection changes.

[Interaction of plague microbe strains varying in plasmid composition with the fleas Xenopsylla cheopis (Roths. 1903)].

The interaction of two Yersinia pestis strains varying in plasmid composition with the fleas Xenopsylla cheopis was studied. The reference virulent strain I-2638 having four plasmids (pCad, pPst, pFra, and pTP 33) and its selected avirulent strain I-3480 that had lost the plasmids pCad and pPst formed a proventricular block in the fleas with equal frequency. There were no differences in the block-forming capacity of these strains among the infected females; however, the stock strain was more active in blocking the proventriculus in females in spring than was the mutant one. in summer. The fleas infected with a defect strain failed to transmit the pathogen. It follows that the presence of a proventricular block is not an indicator of how effectively the fleas transmit the causative agent of plague. While being in the insect, both strains of plague microbe did not alter their biological properties. In experiments, the death rate for insects infected with different strains was similar, but higher in spring than that in summer. The males naturally died more frequently than the females.

[The specific features of relations of the fleas Xenopsylla cheopis L. to the plague microbe of the Altai subspecies, its L-forms and revertant].

The rate and changes in the formation of bacterial lumps and blocks of the proventriculus in Xenopsylla cheopis fleas in their contamination with the plague microbe of the Altai subspecies, its L-form and revertant were studied to clarify the specific features of relations with the vector, including its transmission ability. The plague microbe of the Altai subspecies, which was exposed to L-transformation in the resistant organism of a warm-blooded host (a guinea pig), as well as L-form revertants obtained on nutrient media substantially lost their ability to get acclimatized in the vector and to form a proventricular block. The capacity to form lumps did not greatly differ in L- and original form of the microbe. At the same time, the ratio of the formed blocks and lumps was much lower, which points to the instability of the formed masses. This is supported by the detection of specimens with partial blocks only among the fleas infected with L-forms. The reversed bacterial form was exceeded in the rate of formation of lumps by the original and L-forms. This may be associated with the reversion of L-forms on the nutrient medium rather than in the vector. At the same time, the revertant formed stable blocks in shorter periods than did L-forms and caused a generalization of the infection in plague-infected mice, which may be important for further transmission of the microbe.

[Gender differences in the transmission of plague pathogen by fleas (Siphonaptera)].

The data of a long-term study of the relationships of the plague pathogen with male and female fleas, the vectors of this infection in Siberian natural foci, were analyzed. Gender differences were established in the rate of block formation and vector activity of fleas. In female and male fleas, these indices depend on both the species-specific features of ectoparasites and the pro-feeders used in the experiments, the season of their performance, and keeping conditions. The blocks of the proventriculus more frequently form in males of the majority of flea species, they more actively transmit a plague microbe to animals. The differences in both the rate of proventricular block formation and the pathogen transmission in different seasons were variously shown in males and females. They were clearly marked in one season and insignificant in another. Apparently, flea gender differences may affect the development of an epizootic situation since the sex ratio in these insects may vary in the natural foci of this infection depending on a season.

[Natural foci of plague in India: epizootological characteristics].

In the first half of the 20th century, India was responsible for the incidence of plague in both Asia and the world. The early 21st century was marked by two new epidemic outbreaks of plague (in 2002 and 2004) in this country. The major characteristics of plagues in India, activation of which is a cause of new epidemic outbreaks and necessitates a continuous epizootological monitoring, are analyzed. Historical experience shows that lower focus on surveillance of natural foci of plaque, dissemination of unjustified views on sanitation of endemic areas, and much less their lack, can cause unpredictable epidemiological complications and considerable costs. More scrupulous attention should be given to the importation of goods from North and South India (where there were outbreaks of plague in 1994, 2002, and 2004) to Russia mainly in September to March.

[Role of fleas, the principal and secondary plague vectors, in the circulation of the causative agent in Siberian natural foci].

The paper reviews data on the role of fleas as the principal and secondary vectors of infection in the Siberian natural foci of plague. The role of Citellophilus tesquorum as the principal plague vectors in the Transbaikalian and Tuvinian natural foci is shown to be determined by their ecological characteristics (the degree of specificity to the main host, numbers, and natural infection) to a greater extent and by the rate of block formation to a lesser extent. The significance of principal and secondary vectors is estimated in the circulation of the causative agent in the monovectoral (Transbaikalian and Tuvinian) and multivectoral (Gorno-Altai) foci of plagues. It is suggested that the "microfocal" form (phenotype) of the agent's existence is characteristic of the Siberian natural foci where the rate of block formation does not generally show high indices. This phenomenon might be a basis for the long circulation of the plague microbe in the mountain (Tuva and Gorno-Altai) foci where protracted interepizootic periods have not been registered.

[Plague in China. Threat of transmission to regions of Siberia and Far East].

In China plague has been officially registered from 1754 (638 epidemics with total number of cases 2.5 millions and case-fatality rate 87.5%). Endemic areas started to form on the south of the country and then the disease gradually spread on seaside provinces, where to the end of the 19th century, due to reach of island territories and large international seaports, was characterized by pandemic spread. Epidemic manifestations of plague in China were observed during more than 200 years in 23 out of 36 administrative areas affecting continental and North-Eastern regions of the country, which are immediately adjacent to border of Russia. Pneumonic plague in Manchuria clearly demonstrated the role of transport communications in transmission of this deadly infection and possibility of its spread on border regions of Siberia and Far East. Lengthy country's border, intensive migration flows, large-scale international integration, developing of near-border trade, simplification of policy for transboundary traveling are the reasons for differentiated number of sanitary protective measures on administrative borders of Siberia and Far East.

[Conservation of Yersinia pestis in winter by Citellophilus tesquorum altaicus females and males].

Intersexual differences have been established in Citellophilus tesquorum altaicus Ioff (1936) from a Tuva natural focus in the accumulation of Yersinia pestis in fleas in autumn and its conservation during winter. The ectoparasites were infected and fed on the natural feeder - long-tailed Siberian souslik (Citellus undulates). Small wild animals and insects were infected with the strain Yersinia pestis 1-3226 typical of the focus. In winter fleas survived without the feeder under artificially created conditions of an uninhabited nest for long-tailed souslik. After each feeding, the ectoparasites were microscopically examined, by taking into account the individuals with aggregated agent - bacterial lumps, complete or partial blocks of the proventriculus. The experimental data showed with a high degree of significance that the females had survived the cold season better. Both before and after hibernation of the insects, the plaque agent in the aggregated state was also more frequently recorded in the females than in the males though the proportion of fleas with conglomerates significantly decreased in either sex during that time. If the best female survival and a higher proportion of females with Yersinia pestis are borne in mind, it should be recognized their greater role than that of males in conserving the plague agent during winter.

[The formation of proventriculus block, alimentary activity and mortality of flea Amphipsylla primaris primaris infected with Yersinia pestis].

The results of experiments held in 1982-1983 in Tuva plague natural focus with flea Amphipsylla primaris primaris (Jordan et Rothschild, 1915) from natural populations, whish were inflected and fed on specific host--flat-headed vole (Alticola strelzovi), are analyzed. The initial infectivity of the insects in autumn was higher than in spring: 90 and 50 % respectively. Accumulation of the agent in aggregated form in the organism of A. p. primaris, estimated by the quantity of fleas with <> and partial blocks, was more active in imago of both sexes in autumn than in spring, while sucking flea were observed in spring more often than in autumn. Irrespective of season, the part of males with visible accumulations of Y. pestis was more, and their alimentary activity was higher than that of females. Fleas died much more quickly in spring. Part of the males with proventriculus block exceeded that of females in spring experiment. Females with alimentary canal obstruction prevailed in autumn. Thus, sex of the insect and season of the experiment conducting influenced on all studied indices. Besides that, Y pestis ability for the proventriculus block formation in fleas during different seasons can change by the opposite way depending on sex of the ectoparasites.

[Relationship of the flea Citellophilus tesquorum altaicus (Siphonaptera: Ceratophyllidae) with the plague agent]. / Nekotorye osobennosti vzaimootnoshenii blokhi Citellophilus tesquorum atlaicus (Siphonaptera: ceratophyllidae) s vozbuditelem chumy.

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.

[Seasonal features of the time course of changes in the aggregated state of the causative agent of plague in the organism of Citellophilus tesquorum altaicus (Siphonaptera)]. / Sezonnye osobennosti dinamiki agregirovannogo sostoianiia vozbuditelia chumy v organizme Citellophilus tesquorum altaicus (Siphonaptera).

The data of experiments made in a Tuva natural plague focus with the main vector of the flea C. lesquorum altaicus during different seasons are analyzed. Ectoparasites were infected with the plague microbial strain typical for the focus and fed on natural nourishment, such as long-tailed sousliks. An analysis indicated that estimation of the parameter "the proportion of infected insects containing the causative agent at the aggregated state" is of greater informative value than is the recording of only sporadic gizzard "blocks". In experiments on fleas of a breeding of the current year, the conglomerates of the causative agent of plague were more frequently observed in males. All things being equal, the greatest efficiency of transmission of the microbe to the carriers of infection and the highest block formation in the fleas were observed in summer. The proportion of specimens with microbial conglomerates drastically increased in autumn during the preparation of the body of imagoes for hibernation. At this state, the microbe survived in the organism of a dormant carrier during winter. In spring, in the imagoes infected during the previous season, bacterial conglomerates were observed during the first feeds ten times more frequently than those in the insects of a breeding and infection of the current year.

[Seasonal dynamics of infecting ability of the flea Citellophilus tesquorum altaicus in the Tuva natural focus of the plague]. / Sezonnaia dinamika zarazhaiushchei sposobnosti blokhi Citellophilus tesquorum altaicus v Tuvinskom prirodnom ochage chumy.

The infecting ability of the fleas Citellophilus tesquorum altaicus loff, 1936, the main plague vectors in the Tuva natural focus, was experimentally studied in different periods of the epizootic season. Seasonal dynamics in the efficiency of infecting the long-tailed Siberian souslik with the plague causative agent through flea bites was noticed. Seasonal differences in infectivity of the "blocked" flea bites are revealed. An increase of infected experimental animals with a generalization of infection process in the period of epizooty activation in the natural focus was observed. A resistance of the long-tailed Siberian souslik to the plague agent infection through flea bites in the spring season was registered.

[Differences between two populations of Citellophilus tesquorum altaicus from the Tuva natural focus of plague]. / Razlichiia mezhdu dvumia populiatsiiami Citellophilus tesquorum altaicus iz Tuvinskogo prirodnogo ochaga chumy.

Citellophilus tesquorum altaicus fleas from two natural (Karginsky and Barlyksky) populations have been studied. There are differences between the fleas of these populations in susceptibility to plague microbe. The adaptability and abundance of the causative organism in the body are higher in the fleas of the Barlyksky population. The sizes of the head are significantly higher in the females of the Karginsky population. There are no differences in this index between males. According to all 5 studies signs of chaetotaxia, the chaeta abundance is greater in both females and males of the Karginsky population. The fluctuating asymmetry is higher in the Barlyksky fleas that are more susceptible to the studied type of the causative agent of plague.

[Relationships of parasitic triad co-members in a Tuva natural plague focus: environmental and physiological aspects (experimental studies)]. / Ekologo-fiziologicheskie aspekty vzaimootnoshenii sochlenov parazitarnoi triady v tuvinskom prirodnom ochage chumy (po éksperimental'nym issledovaniiam).

Long-term experimental studies of the environmental and physiological aspects of relationships of parasitic triad co-members, performed in a Tuva natural plague, by using Citellophilus tesquorum altaicus fleas, their natural feeder--a long-tailed Siberian souslik (Citellus undulatus), and the Yersinia pestis strain I-3226 that is typical of the focus were analyzed. A complex of biotic (the environmental features of a causative agent, avector, and a carrier) and abiotic (air temperature and moisture, atmospheric pressure, and seasonal changes) factors that ensure the existence of the enzootic disease plague in this area has been shown to exert a considerable impact on the pattern of relationships of parasitic triad co-members.

[Influence of a temperature factor on the vector capacity of the flea Citellophilus tesquorum altaicus]. / Vliaianie temperaturnogo faktora na vektornuiu sposobnost' blokhi Citellophilus tesquorum altaicus.

The impact of temperatures (2-3 to 23-25 degrees C) of flea keeping on their vector capacity was studied in the experiment. In hibernated fleas, the relationship of their vector capacity to the temperature of their keeping, estimated by the level of plague block in species and by the case rates of animals was insignificant, i.e. there is a trend for these indices to increase with elevated temperatures. In the fleas of current years breeding, the rate of block significantly increased with a rise of temperatures from 5-6 to 23-25 degrees C. There were also increases in the number of transmissions of a plague pathogen to susliks and in the number of animals with generalized infection with a rise in temperature up to 16-18 degrees C. The findings and constructed regression equations may predict the pattern of changes in the vector capacity of the flea C. tesquorum altaicus in the study range of temperatures in the enzootic area of a Tuva natural focus of plague.