Causes of Shigellosis
The causative agents are gram-negative immobilized bacteria of the genus Shigella of the Enterobacteriaceae family. According to the current classification, shigella are divided into 4 groups (A, B, C, D) and, accordingly, into 4 species – S. dysenteriae, S. flexneri, S. boydii, S. sonnei. Each of the species, except for Shigella Sonne, includes several serovars. Among S. dysenteriae, 12 independent serovars are distinguished (1 – 12), including Grigoriev-Shigi (S. dysenteriae 1), Stutzer-Schmitz (S. dysenteriae 2) and Large-Sachs (S. dysenteriae 3-7). S.flexneri includes 8 serovars (1-6, X and Y), including Newcastle (S.flexneri 6). S. boydii include 18 serovars (1 – 18). S. sonnei do not serologically differentiate. In total, there are about 50 Shigella serovars. The etiological role of different Shigella is not the same. Of the greatest importance in almost all countries are Shigella Sonne and Shigella Flexner – pathogens of the so-called large nosological forms. The etiological significance of the individual Shigella serovars is not the same. Among S. flexneri, subserovars 2a, lb and serovar 6 dominate, among S. boydii – serovars 4 and 2, among S. dysenteriae – serovars 2 and 3. Biochemical variants of He, Ilg and 1a prevail among S. sonnei.
The causative agents of bacterial dysentery are distinguished by enzymatic activity, pathogenicity and virulence. All shigella grow well on differential diagnostic environments; temperature optimum 37 ° C, Sonne bacteria can multiply at 10-15 ° C.
Shigella are not very stable outside the human body. Virulence of bacteria is quite variable. The virulence of Shigella Flexner, especially subserovar 2a, is quite high. Shigella Sonne is the least virulent. They are distinguished by a large enzymatic activity, unpretentiousness to the composition of nutrient media. They multiply intensively in milk and dairy products. At the same time, their storage time exceeds the time frame for the sale of products. The pronounced deficiency of virulence in Shigella Sonne is fully compensated by their high biochemical activity and the rate of reproduction in the infected substrate. To accumulate a dose of S. sonnei, which infects adults, in milk at room temperature, it takes from 8 to 24 hours. In the hot season, these periods are minimal: to accumulate a dose of bacteria sufficient to infect children, it takes only 1-3 hours. In the process Shonella Sonne breeding in contaminated products accumulates thermostable endotoxin, which can cause severe damage in case of negative results of bacteriological studies of infected foods. S. sonnei also has a high antagonistic activity against saprophytic and lactic acid microflora.
An important feature of Shigella Sonne is their resistance to antibacterial drugs. Outside the body, the resistance of Shigella of different species is not the same. Shigella Sonne and Flexner can be stored in water for a long time. When heated, shigella quickly die: at 60 ° C – for 10 minutes, when boiled – instantly. Least resistant S.flexneri. In recent years, thermoresistant (capable of surviving at 59 ° C) strains of Shigella Sonne and Flexner are often isolated. Disinfectants in usual concentrations act shigella destructively.
The reservoir and source of infection is a person (a patient with an acute or chronic form of dysentery, a carrier, convalescent centilitransitory carrier). The greatest danger is presented by patients with mild and erased forms of dysentery, especially persons of certain professions (working in the food industry and persons equated to them). Shigella begin to stand out from the human body at the first symptoms of the disease; the duration of the allocation is 7-10 days plus the period of convalescence (on average 2-3 weeks). Sometimes the selection of bacteria drags on for several weeks or months. The tendency to chronicity of the infectious process is most characteristic of Flexner’s dysentery, the least – Sonne dysentery.
The transmission mechanism is fecal-oral, transmission routes are water, food, and contact-household. In Grigoriev-Shiga dysentery, the main transmission route is contact-household, which ensures the transmission of highly virulent pathogens. In Flexner’s dysentery, the main transmission route is water; in Sonne dysentery, it is food. Sonne bacteria have biological advantages over other shigella species. Yielding to them in virulence, they are more stable in the external environment, under favorable conditions they can even multiply in milk and dairy products, which increases their danger. The predominant effect of certain factors and transmission methods determines the etiological structure of the disease with dysentery. In turn, the presence or predominance of different transmission routes depends on the social environment and living conditions of the population. The area of Flexner dysentery mainly corresponds to the territories where the population still uses epidemiologically unsafe water.
The natural susceptibility of people is high. Post-infectious immunity is unstable, species-specific and type-specific, repeated diseases are possible, especially with Sonne dysentery. The immunity of the population does not serve as a factor regulating the development of the epidemic process. At the same time, it was shown that after Flexner’s dysentery, a post-infectious immunity is formed, which can protect against repeated illness for several years.
The main epidemiological signs. Bacterial dysentery (shigellosis) is a ubiquitous disease. Making up the bulk of the so-called acute intestinal infections (or diarrheal diseases, according to WHO terminology), shigellosis is a serious public health problem, especially in developing countries. The widespread spread of intestinal infections in developing countries leads to a beggarly level of people living in unsanitary housing, customs and prejudices that contradict basic sanitary standards, poor water supply, malnutrition against the background of an extremely low level of general and sanitary culture and medical services. The spread of intestinal infections is also facilitated by various kinds of conflict situations, migration processes and natural disasters.
The development of the epidemic process of dysentery is determined by the activity of the mechanism of transmission of infection pathogens, the intensity of which directly depends on social (level of sanitary and communal improvement of settlements and sanitary culture of the population) and natural and climatic conditions. Within the framework of a single fecal-oral transmission mechanism, the activity of individual pathways (water, domestic and food) is different for different types of shigellosis. According to the developed V.I. Pokrovsky and Yu.P. Solodovnikov’s (1980) theory of etiological selectivity of the main (main) ways of transmitting shigellosis, the spread of Grigoriev-Shiga dysentery is carried out mainly by the contact-household way, Flexner dysentery – by water, Sonne dysentery – by food. From the standpoint of the theory of correspondence, transmission paths that ensure not only wide distribution, but also the preservation of the corresponding pathogen in nature as a species, become the main ones. The cessation of activity of the main transmission path ensures the attenuation of the epidemic process, which is unable to be constantly supported only by the activity of additional paths.
Describing the epidemic process in shigellosis, it should be emphasized that these infections include a large group of epidemiologically independent diseases, including the so-called large (Shigellosis Sonne, Flexner, Newcastle, Grigoryev-Shigi) and small (Shigellosis Boyd, Stutzer-Schmitz, Large -Sachsa et al.) Nosological forms. Large nosological forms are constantly widespread, the epidemiological significance of small forms is small. However, it should be mentioned that over the past century the importance of individual shigellosis in human pathology has changed. So, at the beginning of the 20th century, during the years of the civil war and intervention, famine and poor sanitary conditions, high morbidity, severe forms and mortality were associated with the spread of Grigoryev-Shiga dysentery. In the 40-50s, up to 90% of diseases were caused by Flexner Shigella, while the second half of the century was marked by the prevalence of Sonne dysentery. The indicated regularity was determined by the biological properties of the pathogen and the socio-economic changes in human society at different stages of its development. Thus, changes in the social environment and living conditions of the population turned out to be the main regulator of the etiology of dysentery. In recent years, attention has again been drawn to the Grigoriev-Shigi dysentery. Three large foci of this infection (Central America, Southeast Asia and Central Africa) have formed in the world and cases of its importation to other countries have become more frequent. However, for its rooting, certain conditions are needed that are available on the territory of the states of Central Asia. World experience indicates the possibility of the spread of shigellosis and secondary ways. So, large water outbreaks of Grigoriev-Shiga dysentery are known that arose in many developing countries during the late 60-80s against the background of its global spread. However, this does not change the essence of the epidemiological patterns of individual shigellosis. As the situation normalized, Grigoriev-Shigi dysentery again gained predominant distribution through the household.
The dependence of the incidence on sanitary and communal amenities has made Sonne dysentery more widespread among the urban population, especially in kindergartens and collectives united by a single source of nutrition. Nevertheless, Sonne shigellosis still remains predominantly a childhood infection: the proportion of children in the morbidity structure is more than 50%. This is because children, more than adults, consume milk and dairy products. In this case, children under the age of 3 are more likely to fall ill. There is an opinion that the high incidence of children, detected much more fully, is a direct consequence of the wide spread of undetected infection among the adult population. Children who are more susceptible to infection than adults, require a much lower dose of the pathogen to develop the disease. Undetected patients and bacteria carriers form a massive and fairly constant reservoir of the pathogen among the population, which determines the spread of shigellosis both in sporadic cases and in the form of epidemic morbidity. Most outbreaks of Sonne dysentery associated with the infection of milk and dairy products (sour cream, cottage cheese, kefir, etc.) occur as a result of their contamination by undetected patients at various stages of collection, transportation, processing and sale of these products.
Citizens are sick 2-3 times more often than rural residents. Dysentery is characterized by summer-autumn seasonality of the disease. The natural (temperature) factor mediates its effect through the social, contributing to the creation in the warm season of the most favorable (thermostatic) conditions for the accumulation of Sonnet shigella in contaminated dairy products. Similarly, heat provides an increase in the intensity of the epidemic process in Flexner’s dysentery, mediating its effect through the main route of transmission of this nosological form – water. In the hot season, water consumption increases dramatically, which leads to the activation of the water factor, which is mainly realized in the form of chronic epidemics, against the background of poor water supply to the population. There is evidence that a decrease in the incidence of Sonne dysentery occurs amid a sharp decline in the production and consumption of milk and dairy products. The intensification of the epidemic process in the course of Flexner’s dysentery is obviously associated with the socio-economic conditions of life that have changed in recent years. Flexner shigellosis spreads mainly via the secondary food route through a wide variety of food products (a chronic decentralized food transmission route is implemented without prior accumulation of pathogens that are highly virulent and have an extremely low infectious dose). A high incidence and mortality rate is mainly recorded among adults from the group of socially disadvantaged and dysfunctional populations.
It must be pointed out that in recent years, with Sonne dysentery, as with other intestinal anthroponoses, an increase in the proportion of adults is noted. This is due to the fact that in the new socio-economic conditions of life, a significant part of the population is forced to purchase the cheapest products, especially dairy, far from guaranteed quality – flask milk, weighted cottage cheese and sour cream, still sold in the city under unauthorized street trading. In addition, unfavorable social factors of recent years, including the emergence of large contingents of asocial groups of the population (people without a fixed place of residence, vagrants, etc.) have a pronounced effect on the epidemic process. As a result, the proportion of older age groups of the population, including pensioners, significantly increased among patients, and against this background, the specific significance of the child population significantly decreased. This unequivocally proves that among the adult population of this contingent, a kind of independent epidemic process develops that does not actually affect children, as a result of the most pronounced adverse social impact on the spread of dysentery among this adult population.
Symptoms of Shigellosis
In accordance with the features of clinical manifestations and the duration of the disease, the following forms and variants of dysentery are currently distinguished.
Acute dysentery of varying severity with options:
- typical colitic;
- atypical (gastroenterocolitic and gastroenteric).
Chronic dysentery of varying severity with options:
Shigellosis bacterial excretion:
A variety of forms and variants of dysentery is associated with many reasons: the initial state of the macroorganism, the timing of the onset and nature of treatment, etc. The type of pathogen that caused the disease is also of certain importance. So, dysentery caused by Shigella Sonne is distinguished by a tendency to develop lighter and even erased atypical forms without destructive changes in the intestinal mucosa, short-term course and clinical manifestations in the form of gastroenteric and gastroenterocololytic variants. For dysentery caused by Shigella Flexner, a typical colitic variant with intense damage to the colon mucosa, severe clinical manifestations, and an increase in recent years in the frequency of severe forms and complications is more characteristic. Grigoriev-Shiga dysentery is usually very severe, prone to the development of severe dehydration, sepsis, and toxic toxic shock.
The incubation period in the acute form of dysentery ranges from 1 to 7 days, averaging 2-3 days. The colic variant of acute dysentery most often occurs in moderate form. A sharp onset is characteristic with an increase in body temperature to 38-39 ° C, accompanied by chills, headache, a feeling of weakness, apathy and lasting for the first few days of the disease. Appetite is quickly reduced up to complete anorexia. Often there is nausea, sometimes repeated vomiting. The patient is concerned about cutting, cramping abdominal pain. At first they are diffuse in nature, later they are localized in the lower abdomen, mainly in the left iliac region. Almost simultaneously, a frequent loose stool appears, first of fecal character, without pathological impurities. The fecal nature of the bowel movements is quickly lost with subsequent bowel movements, the stool becomes sparse, with a lot of mucus; later streaks of blood, and sometimes impurities of pus, often appear in feces. Such bowel movements are referred to as rectal spitting. The frequency of bowel movements increases to 10 times a day or more. The act of defecation is accompanied by tenesmus – painful pulling pain in the rectum. False desires are frequent. The frequency of stool depends on the severity of the disease, but with a typical colic variant of dysentery, the total amount of excreted feces is small, which does not lead to serious water-electrolyte disorders.
When examining a patient, dryness and lining of the tongue are noted. Palpation of the abdomen reveals soreness and spasm of the colon, especially in its distal section (“left colitis”). However, in some cases, the greatest intensity of pain is noted in the cecum (“right colitis”). Changes in the cardiovascular system are manifested by tachycardia and a tendency to hypotension. With colonoscopy or sigmoidoscopy, recently rarely used in the typical colitic variant of acute dysentery, a catarrhal process or destructive changes in the mucous membrane in the form of erosions and ulcers are detected in the distal colon. The expressed clinical manifestations of the disease usually fade away by the end of the first – the beginning of the 2nd week of the disease, but complete recovery, including repair of the intestinal mucosa, requires 3-4 weeks.
The mild course of the colitic variant of acute dysentery is distinguished by short-term subfebrile fever (or body temperature does not increase at all), moderate abdominal pain, defecation frequency only several times a day, catarrhal, less often catarrhal-hemorrhagic changes in the colon mucosa.
In severe cases of the disease, hyperthermia is observed with pronounced signs of intoxication (fainting, delirium), dry skin and mucous membranes, stools in the form of “rectal spitting” or “meat slops” up to tens of times a day, sharp abdominal pains and painful tenesmus, marked changes hemodynamics (persistent tachycardia and arterial hypotension, deafness of heart sounds). Possible intestinal paresis, collapse, toxic toxic shock.
The gastroenterocolitic variant of acute dysentery is distinguished by a short (6-8 h) incubation period, an acute and rapid onset of the disease with an increase in body temperature, early onset of nausea and vomiting, and abdominal pain of a diffuse cramping nature. Almost simultaneously, multiple, quite plentiful loose stools without pathological impurities join. Tachycardia and arterial hypotension are noted.
This initial period of gastroenteric manifestations and symptoms of general intoxication is short and very reminiscent of the clinical picture of foodborne toxic infection. However, in the future, often already on the 2-3rd day of the disease, the disease acquires the character of enterocolitis: the amount of excreted feces becomes scarce, mucus appears in them, sometimes with streaks of blood. Abdominal pains are predominantly localized in the left iliac region, as in the colitic variant of dysentery. During the examination, spasm and pain of the colon are determined.
The more pronounced gastroenteric syndrome, the more demonstrative the signs of dehydration, which can reach the II-III degree. The degree of dehydration must be taken into account when assessing the severity of the course of the disease.
The gastroenteric variant begins acutely. The rapidly developing clinical symptoms are very similar to those for salmonellosis and foodborne toxic infection, which makes clinical differential diagnosis extremely difficult. Repeated vomiting and frequent loose stools can lead to dehydration. In the future, symptoms of colon damage do not develop (a hallmark of this variant of dysentery). The course of the disease is stormy, but short-lived.
The current course of dysentery is now quite common; this condition is difficult to diagnose clinically. Patients complain of a feeling of discomfort or pain in the abdomen of a different nature, which can be localized in the lower abdomen (usually on the left). Manifestations of diarrhea are insignificant: stool 1-2 times a day, mushy, often without pathological impurities. Soreness and spasm of the sigmoid colon in most cases is clearly determined by palpation. Body temperature remains normal or rises only to subfebrile digits. Confirmation of the diagnosis is possible with repeated bacteriological examination, as well as with a colonoscopy, in most cases revealing catarrhal changes in the mucous membrane of the sigmoid and rectum.
The duration of the course of acute dysentery is subject to significant fluctuations: from several days to 1 month. In a small percentage of cases (1-5%), a prolonged course of the disease is observed. At the same time, intestinal dysfunction in the form of alternating diarrhea and constipation, abdominal pain of a spilled character or localized in the lower abdomen is constantly preserved for 1-3 months. In patients, appetite worsens, general weakness develops, weight loss is observed.
The chronic form of dysentery is a disease with a duration of more than 3 months. Currently, it is rarely observed. Clinically, it can occur in the form of relapsing and continuous options.
The recurrent variant of chronic dysentery during periods of relapse in its clinical picture is basically similar to the manifestations of the acute form of the disease: periodically expressed intestinal dysfunction with abdominal pain, spasm and sigmoid colon pain during palpation, low-grade body temperature. Changes in the mucous membrane of the sigmoid and rectum are mainly similar to those in the acute form, however, alternation of the affected sections of the mucous membrane with little changed or atrophied ones is possible; vascular pattern is enhanced. The timing of the onset, the duration of relapses and the “bright gaps” between them, which differ in the quite satisfactory state of health of patients, are subject to significant fluctuations.
A continuous version of the chronic course of dysentery is much less common. It is characterized by the development of profound changes in the digestive tract. Symptoms of intoxication are weak or absent, patients are concerned about abdominal pain, daily diarrhea from one to several times a day. The chair is mushy, often with a greenish color. Remissions are not observed. Symptoms of the disease are constantly progressing, body weight is reduced in patients, irritability appears, dysbiosis and hypovitaminosis develop.
The pathogenesis of protracted and chronic dysentery is still poorly understood. The role of autoimmune processes in the development of these conditions is currently being discussed. They are facilitated by a variety of factors: previous and concomitant diseases (primarily gastrointestinal diseases), impaired immunological response in the acute period of the disease, dysbiosis, dietary disorders, alcohol use, inadequate treatment, etc.
Shigellosis bacteria can be subclinical and convalescence. Short-term subclinical carriage of bacteria is observed in individuals in the absence of clinical signs of the disease at the time of the examination and 3 months before it. However, in some cases, it is possible to detect antibodies to Shigella antigens in RNGA, as well as pathological changes in the colon mucosa during endoscopic examination.
After clinical recovery, the formation of a longer convalescence of bacterial carriers is possible.
Complications are currently rare, but with severe dysentery of Grigoriev-Shiga and Flexner, infectious toxic shock, severe dysbiosis, intestinal perforation, serous and perforated purulent peritonitis, paresis and intussusception, fissures and anus erosion, hemorrhoids can develop rectal mucosa. In some cases, after the disease, intestinal dysfunctions develop (post-dysenteric colitis).