Laboratory Diagnosis, Treatment and Prevention of Shigella dysenteriae
The most common specimen for the laboratory diagnosis of Shigella dysenteriae is stool. Stool samples should be collected as soon as possible after the onset of symptoms, preferably within 48 hours. The samples should be transported to the laboratory under refrigeration or in a transport medium such as Cary-Blair or buffered glycerol saline. Stool samples should be processed within 2 hours of collection or stored at 4°C until processing.
Other specimens that can be used for the diagnosis of Shigella dysenteriae include rectal swabs, sigmoidoscopy or colonoscopy specimens, and blood cultures. Rectal swabs are less sensitive than stool samples and should be avoided if possible. Sigmoidoscopy or colonoscopy specimens can provide direct visualization of the colonic mucosa and allow for the collection of biopsy material or pus. Blood cultures can be positive in severe cases of shigellosis, especially in children and immunocompromised patients.
The quality and quantity of the specimens are important for the accurate identification of Shigella dysenteriae. The specimens should be representative of the infection and contain sufficient material for microscopic examination and culture. The specimens should also be free of contaminants such as urine, water, soil, or food particles. The specimens should be labeled with the patient`s name, date and time of collection, and type of specimen. The specimens should be accompanied by a request form that includes relevant clinical information such as age, sex, travel history, symptoms, duration of illness, and antibiotic treatment.
One of the methods to diagnose Shigella dysenteriae infection is to examine the stool smears under a microscope. This can reveal the presence of inflammatory cells and bacteria in the feces.
To prepare the stool smears, a small amount of fresh stool sample is mixed with a drop of saline or water on a glass slide. The smear is then stained with methylene blue or Wright stain and observed under a light microscope.
The microscopic examination can show the following features:
- Higher number of polymorphonuclear leukocytes (PMN cells) in the stool, indicating an inflammatory response in the colon .
- Gram-negative, small, rod-shaped, non-motile, non-capsulated bacteria that are consistent with Shigella species .
- Absence of motile bacteria or parasites that could cause other types of diarrhea.
The microscopic examination of stool smears is a quick and simple test that can provide presumptive evidence of Shigella dysenteriae infection. However, it is not specific or sensitive enough to confirm the diagnosis. Therefore, it should be followed by culture and biochemical analysis of the stool sample to identify the exact species and serotype of Shigella .
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Microscopic examination of stool smears can also help to monitor the response to treatment and clearance of the infection. A decrease in the number of PMN cells and bacteria in the stool indicates improvement, while persistence or increase suggests treatment failure or relapse. However, this test cannot differentiate between viable and non-viable bacteria, so it should not be used as the sole criterion for stopping antibiotic therapy.
To identify Shigella dysenteriae from stool samples, the specimens are streaked on differential media and selective media that can distinguish them from other enteric bacteria. Differential media, such as MacConkey agar, EMB agar, BCP agar or SS agar, allow the detection of lactose fermentation by the colonies. Selective media, such as Hektoen enteric agar or xylose-lysine-deoxycholate agar, inhibit the growth of other Enterobacteriaceae and gram-positive organisms .
After overnight incubation, pale non-lactose-fermenting colonies are tested by standard biochemical and sugar utilization tests to differentiate them from other enterobacteria. Some of the tests that are commonly used are:
- TSI agar: This medium contains glucose, lactose and sucrose as fermentable sugars, as well as sodium thiosulfate and ferrous sulfate as indicators of hydrogen sulfide (H2S) production. Shigella dysenteriae produces acid but not gas in the butt and an alkaline slant in TSI agar medium, indicating glucose fermentation only. It also fails to produce H2S .
- Motility test: This test determines the ability of bacteria to move by flagella or other mechanisms. Shigella dysenteriae is nonmotile .
- Mannitol fermentation test: This test detects the ability of bacteria to ferment mannitol, a sugar alcohol, by producing acid. Shigella dysenteriae is characteristically negative for mannitol fermentation .
- Catalase test: This test detects the presence of catalase enzyme, which breaks down hydrogen peroxide into water and oxygen. Shigella dysenteriae produces catalase.
- ONPG test: This test detects the presence of beta-galactosidase enzyme, which hydrolyzes o-nitrophenyl-beta-D-galactopyranoside (ONPG) into galactose and o-nitrophenol. The latter is a yellow compound that can be measured spectrophotometrically. Shigella dysenteriae is occasionally ONPG positive .
If the results of these tests are consistent with Shigella dysenteriae, the colonies are further subjected to slide agglutination by specific Shigella antisera to confirm the serotype .
Differential media and selective media are two types of culture media that are used to isolate and identify different types of bacteria. Differential media allow several types of bacteria to grow, but also contain indicators that reveal differences among the bacteria, such as colony color, shape, size, or hemolysis. Selective media inhibit the growth of some bacteria while allowing the growth of others, usually by adding chemicals that target specific metabolic pathways or cell wall components.
For the isolation and identification of Shigella dysenteriae, several differential and selective media can be used, such as:
Xylose lysine deoxycholate (XLD) agar: This is a selective and differential medium that contains xylose, lactose, sucrose, lysine, sodium deoxycholate, sodium thiosulfate, ferric ammonium citrate, and phenol red. XLD agar inhibits the growth of gram-positive bacteria and some gram-negative bacteria by sodium deoxycholate. It also differentiates bacteria based on their ability to ferment xylose, lactose, and sucrose, and to produce hydrogen sulfide. Shigella dysenteriae does not ferment any of these sugars and does not produce hydrogen sulfide, so it forms colorless colonies with a black center on XLD agar .
Salmonella Shigella (SS) agar: This is a moderately selective and differential medium that contains lactose, bile salts, sodium citrate, sodium thiosulfate, ferric citrate, brilliant green dye, and neutral red. SS agar inhibits the growth of most gram-positive bacteria and some gram-negative bacteria by bile salts and brilliant green dye. It also differentiates bacteria based on their ability to ferment lactose and to produce hydrogen sulfide. Shigella dysenteriae does not ferment lactose and does not produce hydrogen sulfide, so it forms colorless colonies with a black center on SS agar.
Hektoen enteric (HE) agar: This is a highly selective and differential medium that contains lactose, sucrose, salicin, bile salts, sodium thiosulfate, ferric ammonium citrate, bromothymol blue, and acid fuchsin. HE agar inhibits the growth of most gram-positive bacteria and some gram-negative bacteria by bile salts. It also differentiates bacteria based on their ability to ferment lactose, sucrose, and salicin, and to produce hydrogen sulfide. Shigella dysenteriae does not ferment any of these sugars and does not produce hydrogen sulfide, so it forms green colonies with a black center on HE agar.
These media are commonly used in clinical and food microbiology laboratories to isolate Shigella dysenteriae from stool samples or other specimens. They can be used in conjunction with other biochemical tests or molecular methods to confirm the identification of Shigella dysenteriae.
After isolating non-lactose-fermenting colonies from differential and selective media, they should be further tested by standard biochemical and sugar utilization tests to differentiate them from other enterobacteria. Some of the common tests are:
- Triple sugar iron (TSI) agar: Shigella species produce acid but not gas in the butt and an alkaline slant in TSI agar medium. They also do not produce hydrogen sulfide (H2S), unlike Salmonella species .
- Urea broth: Shigella species are urease negative, meaning they do not hydrolyze urea to ammonia and carbon dioxide. This can be detected by a color change of the broth from yellow to pink .
- Motility test medium: Shigella species are nonmotile, meaning they do not spread from the stab line in the semisolid medium. This can be contrasted with motile bacteria such as Proteus or E. coli .
- Potassium cyanide (KCN) broth: Shigella species are inhibited by KCN, which blocks the cytochrome oxidase system of aerobic respiration. This can be observed by a lack of growth or turbidity in the broth .
- Malonate broth: Shigella species do not use malonate as the sole carbon source for growth, unlike some strains of E. coli or Citrobacter. This can be detected by a color change of the broth from green to blue .
- Tryptone (tryptophane) broth: Shigella species do not produce indole from tryptophane, unlike E. coli or Salmonella typhi. This can be detected by adding Kovacs` reagent to the broth and observing a red ring at the surface .
- MR-VP broth: Shigella species are methyl red positive and Voges-Proskauer negative, meaning they produce mixed acids but not acetoin from glucose fermentation. This can be detected by adding methyl red indicator or Voges-Proskauer test reagents to the broth and observing a color change .
- Christensen citrate agar: Shigella species do not use citrate as the sole carbon source for growth, unlike Klebsiella or Enterobacter. This can be detected by a lack of color change or growth on the slant .
In addition to these tests, Shigella species can be further differentiated by their ability to ferment various carbohydrates, such as adonitol, salicin, rhamnose, glucose, inositol, lactose, mannitol, raffinose, sucrose, xylose, dulcitol, and glycerol. These tests can be performed using bromcresol purple broth supplemented with each carbohydrate at a level of 0.5%. A positive result is indicated by a color change from purple to yellow due to acid production . Some examples of carbohydrate fermentation patterns are:
- Shigella dysenteriae: Ferments glucose only; mannitol negative; ONPG negative; catalase negative .
- Shigella flexneri: Ferments glucose and mannitol; mannitol positive; ONPG negative; catalase positive .
- Shigella boydii: Ferments glucose and some other sugars depending on the serotype; mannitol variable; ONPG negative; catalase positive .
- Shigella sonnei: Ferments glucose and lactose after prolonged incubation; mannitol positive; ONPG positive; catalase positive .
Other biochemical tests that can be used to identify Shigella species are acetate agar, mucate broth, decarboxylase basal medium (lysine and ornithine), and serology. However, these tests are not routinely performed in most laboratories.
Biochemical and sugar utilization tests are useful for confirming the presence of Shigella species and distinguishing them from other enteric bacteria. However, they may not be sufficient for identifying the specific serotype or subgroup of Shigella. For this purpose, molecular methods such as PCR or NAATs may be more reliable and accurate.
MacConkey agar is a differential and selective medium that inhibits the growth of gram-positive bacteria and allows the growth of gram-negative bacteria. It also distinguishes between lactose-fermenting and non-lactose-fermenting bacteria based on the color of the colonies. Lactose-fermenting bacteria produce pink or red colonies, while non-lactose-fermenting bacteria produce colorless or transparent colonies. Shigella dysenteriae is a non-lactose-fermenting bacterium that produces colorless colonies on MacConkey agar .
Hektoen enteric agar is a differential and selective medium that inhibits the growth of most gram-positive bacteria and some gram-negative bacteria, and allows the growth of Salmonella and Shigella species. It also differentiates between these two genera based on their ability to produce hydrogen sulfide (H2S) and ferment lactose, sucrose, and salicin. H2S-producing bacteria produce black or greenish-black colonies with a metallic sheen, while non-H2S-producing bacteria produce blue-green colonies. Lactose-, sucrose-, or salicin-fermenting bacteria produce yellow to salmon-colored colonies, while non-fermenting bacteria produce blue-green colonies. Shigella dysenteriae is a non-H2S-producing and non-fermenting bacterium that produces blue-green colonies on Hektoen enteric agar .
BCP agar (bromocresol purple agar) is a differential medium that distinguishes between lactose-fermenting and non-lactose-fermenting bacteria based on the pH indicator bromocresol purple. Lactose-fermenting bacteria produce acid from lactose, which lowers the pH of the medium and turns it yellow. Non-lactose-fermenting bacteria do not change the pH of the medium and keep it purple. Shigella dysenteriae is a non-lactose-fermenting bacterium that produces purple colonies on BCP agar.
SS agar (salmonella-shigella agar) is a differential and selective medium that inhibits the growth of most gram-positive bacteria and some gram-negative bacteria, and allows the growth of Salmonella and Shigella species. It also differentiates between these two genera based on their ability to produce H2S and ferment lactose. H2S-producing bacteria produce black centers in their colonies, while non-H2S-producing bacteria do not. Lactose-fermenting bacteria produce red or pink colonies, while non-lactose-fermenting bacteria produce colorless or transparent colonies. Shigella dysenteriae is a non-H2S-producing and non-lactose-fermenting bacterium that produces colorless colonies on SS agar .
These four media are commonly used to isolate and identify Shigella dysenteriae from stool samples or other specimens suspected of containing this pathogen. They can help to differentiate Shigella dysenteriae from other enteric bacteria based on their biochemical characteristics.
TSI agar is a triple sugar iron agar that contains glucose, lactose and sucrose as well as ferrous sulfate and phenol red indicator. It is used to differentiate enteric bacteria based on their carbohydrate fermentation and hydrogen sulfide production.
Shigella species are non-lactose fermenters and do not produce gas or hydrogen sulfide. Therefore, they produce an alkaline slant and an acid butt in TSI agar medium. The slant is red and the butt is yellow. This can be contrasted with other enteric bacteria such as Salmonella, which produce an alkaline slant and an acid butt with blackening due to hydrogen sulfide production.
To perform the TSI agar test, a pure culture of the organism is inoculated by stabbing the butt and streaking the slant. The inoculated tube is incubated at 35°C for 18-24 hours and observed for color changes and gas production.
The TSI agar test is useful for screening Shigella from other enteric bacteria, but it is not sufficient for identification. Further biochemical and serological tests are required to confirm the species and serotype of Shigella.
Hektoen agar is a selective and differential medium that is used to isolate and differentiate Salmonella and Shigella from other enteric bacteria. It contains bile salts and dyes (bromthymol blue and acid fuchsin) that inhibit the growth of most gram-positive organisms and some gram-negative organisms, allowing only Salmonella and Shigella to grow on the medium. It also contains various carbohydrates (lactose, sucrose, and salicin) and indicators (ferric ammonium citrate and sodium thiosulfate) that allow the detection of lactose fermentation and hydrogen sulfide production.
Salmonella and Shigella are both non-lactose fermenters, which means they do not produce acid from the carbohydrates in the medium. Therefore, they form blue-green colonies on Hektoen agar. However, Salmonella can produce hydrogen sulfide gas from the sulfur source (sodium thiosulfate) in the medium, which reacts with the iron salt (ferric ammonium citrate) to form a black precipitate. This gives Salmonella colonies a black center or a completely black appearance on Hektoen agar. Shigella does not produce hydrogen sulfide gas, so it forms blue-green colonies without any blackening on Hektoen agar.
Other enteric bacteria that can grow on Hektoen agar are usually lactose fermenters, which means they produce acid from the carbohydrates in the medium. This lowers the pH of the medium and changes the color of the dyes from blue-green to yellow or orange. Therefore, lactose fermenters form yellow or orange colonies on Hektoen agar. Some lactose fermenters can also produce hydrogen sulfide gas, which gives them a black center or a completely black appearance on Hektoen agar. However, these colonies can be distinguished from Salmonella by their yellow or orange color.
Hektoen agar is a useful medium for isolating and differentiating Salmonella and Shigella from clinical specimens such as stool samples. It can be used as a primary plating medium or as a subculture medium from enrichment broths. However, it should be noted that some rare strains of Salmonella can ferment lactose and some strains of Shigella can produce hydrogen sulfide gas, which may cause confusion in interpreting the results. Therefore, additional biochemical and serological tests are recommended to confirm the identification of Salmonella and Shigella isolates from Hektoen agar.
Serology is the study of serum antibodies that are produced by the immune system in response to infection. Serological tests can be used to detect and identify Shigella dysenteriae infection by measuring the specific antibodies against the lipopolysaccharide (LPS) antigens of the bacteria . LPS antigens are part of the outer membrane of Shigella dysenteriae and are responsible for its serological classification into 15 serotypes .
Serological tests for Shigella dysenteriae include slide agglutination, enzyme-linked immunosorbent assay (ELISA), and latex agglutination. These tests use purified LPS antigens or monoclonal antibodies as reagents to detect the presence or absence of serum antibodies in a patient`s blood sample. Serological tests can also be used to determine the serotype of Shigella dysenteriae by using specific antisera for each serotype.
Serological tests have some advantages and limitations for the diagnosis of Shigella dysenteriae infection. Some advantages are:
- They are simple, rapid, and inexpensive to perform
- They can be used when culture methods are not available or reliable
- They can provide epidemiological information on the prevalence and distribution of different serotypes
Some limitations are:
- They may not be sensitive or specific enough to detect low levels of antibodies or cross-react with other bacteria
- They may not be able to distinguish between current and past infections or between different strains within a serotype
- They may not be available routinely in some laboratories or regions
Therefore, serological tests should be used as a complementary tool to culture methods for the diagnosis of Shigella dysenteriae infection, and should be interpreted with caution and in conjunction with clinical and epidemiological data .
Molecular methods are useful for the rapid and accurate detection of Shigella dysenteriae in clinical and environmental samples. They can also provide information on the genetic diversity, virulence factors and antimicrobial resistance of the isolates. Some of the molecular methods that can be used for Shigella dysenteriae are:
Polymerase chain reaction (PCR): This is a technique that amplifies specific DNA sequences from a sample using primers and a DNA polymerase enzyme. PCR can be used to target the genes encoding the invasion plasmid antigen H (ipaH), which is present in all Shigella species and is involved in the invasion of epithelial cells . PCR can also be used to detect the genes for Shigella enterotoxin 2 (ShET-2), which is produced by some strains of S. dysenteriae and causes fluid secretion in the intestine. Additionally, PCR can be used to identify strains with the genes encoding an aerobactin-mediated iron uptake system, which enhances their survival and virulence in iron-limited environments. PCR can be performed as a single or multiplex assay, depending on the number of targets and primers used.
Whole genome sequencing (WGS): This is a technique that determines the complete DNA sequence of an organism`s genome. WGS can provide detailed information on the phylogenetic relationships, genetic diversity, virulence factors and antimicrobial resistance of Shigella dysenteriae isolates. WGS can also help to identify novel genes and variants that may be associated with pathogenicity or drug resistance. WGS can be performed using different platforms and methods, such as Illumina, PacBio, Nanopore or hybrid approaches.
Commercial nucleic acid amplification tests (NAATs): These are tests that use different methods to amplify and detect nucleic acids from a sample, such as loop-mediated isothermal amplification (LAMP), transcription-mediated amplification (TMA) or helicase-dependent amplification (HDA). Commercial NAATs are available that can directly detect Shigella dysenteriae in fecal samples along with some of the other major enteric pathogens, such as Salmonella, Campylobacter, Escherichia coli and Vibrio . Commercial NAATs are convenient, fast and sensitive, but they may have limitations in terms of specificity, cost and availability.
Molecular methods are valuable tools for the laboratory diagnosis of Shigella dysenteriae infections. They can complement or replace conventional methods, such as culture and serology, depending on the resources and objectives of the laboratory. Molecular methods can also provide insights into the epidemiology, transmission and evolution of Shigella dysenteriae strains.
One of the molecular methods that can be used to detect Shigella is the polymerase chain reaction (PCR) test targeting the genes encoding the invasion plasmid antigen H (ipaH). The ipaH genes are found in multiple copies on both the invasion plasmid and on the chromosome of Shigella and enteroinvasive E. coli (EIEC), and they encode a family of T3SS effectors that modulate host cell ubiquitination and degradation . The ipaH genes are highly conserved among Shigella and EIEC strains, and they are not present in other enterobacteria . Therefore, they are an appealing target for diagnostic tools, as they remain detectable despite the loss of the plasmid.
The PCR test for ipaH genes can be performed on stool samples or culture isolates, and it can provide a rapid and specific identification of Shigella and EIEC infections. The test can also be combined with other PCR tests for Shiga toxin genes or serotype-specific genes to further characterize the strains . The PCR test for ipaH genes has been validated in several studies, and it has shown high sensitivity and specificity compared to conventional culture methods . However, some limitations of this test include the need for specialized equipment and trained personnel, the possibility of false-negative results due to PCR inhibitors or low bacterial load, and the inability to distinguish between Shigella and EIEC .
Shigella enterotoxin 2 (ShET-2) is one of the virulence factors associated with Shigella spp., especially Shigella sonnei and Shigella dysenteriae. ShET-2 is encoded by the sen gene, which is located on a large plasmid. ShET-2 can cause watery diarrhea and intestinal inflammation by activating the adenylate cyclase pathway in the host cells.
ShET-2 PCR is a molecular method that can detect the presence of the sen gene in Shigella isolates or fecal samples. It is based on the amplification of a specific DNA fragment using polymerase chain reaction (PCR) with primers designed for the sen gene . The PCR product can be visualized by gel electrophoresis or hybridized with a probe for further confirmation.
ShET-2 PCR has some advantages over other methods for detecting Shigella enterotoxins, such as cell culture assays or immunoassays. It is more sensitive, specific, rapid and cost-effective. It can also differentiate ShET-2-producing strains from non-producing strains, which may have implications for the clinical management and epidemiology of shigellosis .
ShET-2 PCR can be performed as a single test or as part of a multiplex PCR that can simultaneously detect other Shigella virulence determinants, such as the invasion plasmid antigen H (ipaH) or Shigella enterotoxin 1 (ShET-1) . Multiplex PCR can provide more comprehensive information about the pathogenicity and diversity of Shigella strains.
ShET-2 PCR is a useful tool for the laboratory diagnosis of shigellosis caused by ShET-2-producing strains. It can also help to monitor the prevalence and distribution of ShET-2 among Shigella spp. in different geographic regions and populations .
NAATs (nucleic acid amplification tests) are molecular methods that can directly detect the genetic material of pathogens in clinical samples. They are highly sensitive and specific, and can provide rapid results compared to culture-based methods.
There are several commercial NAATs that directly detect shigellae in fecal samples along with some of the other major enteric pathogens. These include:
- The Luminex xTAG Gastrointestinal Pathogen Panel (GPP), which is FDA cleared and can simultaneously detect 14 enteric pathogens, including Shigella/EIEC.
- The BioFire FilmArray Gastrointestinal (GI) Panel, which is also FDA cleared and can simultaneously detect 22 enteric pathogens, including Shigella/EIEC.
- The Seegene Allplex Gastrointestinal (GI) Panel, which is CE marked and can simultaneously detect 18 enteric pathogens, including Shigella/EIEC.
Commercial NAATs offer several advantages for the diagnosis of shigellosis, such as:
- They can detect asymptomatic or low-level infections that may be missed by culture or microscopy.
- They can reduce the need for multiple stool specimens and selective media.
- They can provide results within a few hours, which can facilitate timely treatment and infection control measures.
- They can differentiate Shigella from EIEC (enteroinvasive E. coli), which share similar virulence genes and clinical features.
However, commercial NAATs also have some limitations, such as:
- They are expensive and require specialized equipment and trained personnel.
- They may not be widely available or accessible in resource-limited settings where shigellosis is endemic.
- They may not detect all strains or serotypes of Shigella, especially those that are rare or emerging.
- They may not provide information on antimicrobial susceptibility, which is important for guiding therapy and monitoring resistance.
Therefore, commercial NAATs should be used as a complement to conventional methods for the diagnosis of shigellosis, and not as a replacement. Culture and serology should still be performed whenever possible to confirm the results of NAATs and to provide epidemiological data.
Shigella dysenteriae is a bacterial infection that causes severe diarrhea and dysentery. It can lead to serious complications such as dehydration, sepsis, hemolytic uremic syndrome, and toxic megacolon if left untreated. Therefore, it is important to seek medical attention if you or someone you know has symptoms of shigellosis, such as bloody or watery stools, abdominal pain, fever, and tenesmus.
Treatment of shigellosis depends on the severity of the infection, the age and health status of the patient, and the susceptibility of the bacteria to antibiotics. In most cases, shigellosis is self-limiting and resolves within a week without any specific treatment. However, in some cases, especially in young children, elderly people, or those with weakened immune systems, antibiotic therapy may be necessary to shorten the duration of symptoms and prevent complications .
The World Health Organization (WHO) recommends the use of ciprofloxacin, ceftriaxone, or pivmecillinam as the first-line antibiotics for the treatment of dysentery in children. These antibiotics are effective against most strains of Shigella dysenteriae and can reduce the risk of transmission and resistance. However, some strains may be resistant to one or more of these antibiotics, so it is important to perform laboratory tests to confirm the diagnosis and identify the appropriate antibiotic for each case .
Other antibiotics that may be used to treat shigellosis include ampicillin, azithromycin, doxycycline, and trimethoprim-sulfamethoxazole. However, these antibiotics may have lower efficacy or higher side effects than the first-line antibiotics. Therefore, they should be used only when the first-line antibiotics are not available or contraindicated .
In addition to antibiotic therapy, patients with shigellosis should drink plenty of fluids or oral rehydration solutions to prevent dehydration and electrolyte imbalance. They should also avoid anti-diarrheal drugs such as loperamide or diphenoxylate with atropine, as these drugs may worsen the condition by slowing down the intestinal motility and increasing the toxin absorption . They should also get adequate rest and eat a bland diet that is easy to digest until their symptoms improve.
Shigella dysenteriae is a serious infection that can cause life-threatening complications if not treated properly. Therefore, it is essential to seek medical attention as soon as possible if you suspect you have shigellosis. With prompt diagnosis and appropriate treatment, most cases of shigellosis can be cured within a few days without any long-term consequences.
The best way to prevent Shigella dysenteriae infection is to follow good hygiene and sanitation practices, such as:
- Washing hands frequently with soap and water, especially before eating or preparing food, after using the toilet or changing diapers, and before any sexual activity.
- Avoiding swallowing water from ponds, lakes, or swimming pools that may be contaminated with fecal matter.
- Drinking only safe water and eating only cooked or peeled foods when traveling to areas that lack sanitation.
- Cleaning and disinfecting surfaces and utensils that may have come into contact with raw or undercooked meat, poultry, eggs, or seafood.
- Disposing of human and animal waste properly and keeping flies away from food sources.
In addition to these measures, people who are infected with Shigella dysenteriae should take steps to avoid spreading the infection to others, such as:
- Staying home from school, work, or childcare until at least 48 hours after the diarrhea has stopped.
- Not preparing food for others or sharing food with anyone while sick.
- Not swimming or having sex for at least two weeks after the diarrhea has ended.
- Washing hands thoroughly after handling soiled diapers, clothing, or bedding and disposing of them in a sealed plastic bag.
Furthermore, public health authorities should implement surveillance and outbreak response strategies to identify and control Shigella dysenteriae infections in the community, such as:
- Testing stool samples from suspected cases and contacts for Shigella dysenteriae and performing antibiotic susceptibility testing.
- Reporting confirmed cases to local and national health agencies and notifying exposed individuals.
- Providing health education and counseling to affected populations on how to prevent and treat Shigella dysenteriae infection.
- Tracing the source of infection and removing or treating contaminated food or water supplies.
Finally, research and development of effective vaccines against Shigella dysenteriae is ongoing and may offer a promising way to prevent this infection in the future. Some candidate vaccines that are in advanced stages of development include:
- A conjugate vaccine composed of O-antigen polysaccharides from Shigella dysenteriae type 1 linked to a carrier protein.
- A live attenuated vaccine derived from a mutant strain of Shigella dysenteriae type 1 that lacks the genes for Shiga toxin production.
- A subunit vaccine based on recombinant proteins that mimic the invasion plasmid antigens (Ipa) of Shigella dysenteriae.
These vaccines have shown promising results in animal models and human trials, but further studies are needed to evaluate their safety, efficacy, and immunogenicity in different populations.
Shigella dysenteriae is a bacterial infection that causes severe diarrhea and dysentery. Treatment with a suitable antibiotic is necessary in some cases, especially for young children, elderly people, or those with weakened immune systems. Antibiotics can shorten the duration of symptoms and reduce the risk of complications and transmission.
However, not all antibiotics are effective against Shigella dysenteriae, as many strains have developed resistance to multiple drugs. Therefore, it is important to choose an antibiotic based on the local susceptibility patterns and the severity of the infection.
According to the World Health Organization (WHO), the recommended antibiotics for the treatment of Shigella dysenteriae are:
- Ciprofloxacin, a fluoroquinolone that inhibits bacterial DNA synthesis. It is given orally as a single dose of 500 mg for adults or 15 mg/kg for children.
- Ceftriaxone, a third-generation cephalosporin that interferes with bacterial cell wall synthesis. It is given intramuscularly as a single dose of 1 g for adults or 50 mg/kg for children.
- Pivmecillinam, a penicillin derivative that also inhibits bacterial cell wall synthesis. It is given orally four times a day for 3 to 5 days at a dose of 400 mg for adults or 20 mg/kg for children.
These antibiotics are usually effective against most strains of Shigella dysenteriae, including those that produce Shiga toxin and those that are resistant to other drugs. However, resistance to ciprofloxacin and ceftriaxone has been reported in some regions, so it is advisable to perform culture and sensitivity testing whenever possible.
Other antibiotics that may be used in some cases include:
- Azithromycin, a macrolide that inhibits bacterial protein synthesis. It is given orally once a day for 3 days at a dose of 500 mg for adults or 10 mg/kg for children. It is often used to treat children with shigellosis, as it has a good safety profile and oral availability.
- Ampicillin, another penicillin derivative that inhibits bacterial cell wall synthesis. It is given orally four times a day for 5 days at a dose of 2 g for adults or 50 mg/kg for children. It may be effective against some strains of Shigella dysenteriae that are susceptible to penicillins.
- Doxycycline, a tetracycline that inhibits bacterial protein synthesis. It is given orally twice a day for 5 days at a dose of 100 mg for adults or 2 mg/kg for children. It may be effective against some strains of Shigella dysenteriae that are susceptible to tetracyclines.
- Trimethoprim-sulfamethoxazole, a combination of two drugs that inhibit bacterial folate synthesis. It is given orally twice a day for 5 days at a dose of 160/800 mg for adults or 8/40 mg/kg for children. It may be effective against some strains of Shigella dysenteriae that are susceptible to sulfonamides.
However, these antibiotics are not recommended as first-line options, as they have higher rates of resistance and adverse effects than the ones mentioned above. They should only be used if the preferred antibiotics are not available or contraindicated.
Antibiotics should not be used in asymptomatic carriers of Shigella dysenteriae, as they do not reduce the duration of bacterial excretion and may increase the risk of resistance development. They should also not be used prophylactically in contacts of infected persons, as they do not prevent infection and may cause adverse effects.
In addition to antibiotic therapy, patients with Shigella dysenteriae should receive supportive care, such as oral rehydration therapy, zinc supplementation, and nutritional support. They should also practice good hygiene measures, such as washing hands with soap and water, disposing of feces safely, and avoiding contact with contaminated food and water.
One of the major challenges in the treatment of Shigella dysenteriae infections is the emergence and spread of multiple drug resistance (MDR) among the strains. MDR Shigella strains are resistant to several commonly used antibiotics, such as ampicillin, tetracycline, cotrimoxazole, nalidixic acid, and fluoroquinolones . This limits the therapeutic options and increases the risk of treatment failure and complications.
MDR can be transmitted by plasmids, which are small circular pieces of DNA that can move between bacteria. Plasmids can carry genes that encode resistance to different antibiotics, as well as genes that enhance the virulence and survival of Shigella . MDR plasmids can also spread to other enteric bacteria, such as Escherichia coli and Salmonella, creating a potential public health threat.
The prevalence and patterns of MDR vary by geographic region, serotype, and population group. For example, in the United States, about 5% of Shigella infections reported to CDC in 2022 were caused by extensively drug-resistant (XDR) strains, which are resistant to all commonly recommended empiric and alternative antibiotics. XDR Shigella infections have been associated with international travel, men who have sex with men, people experiencing homelessness, and people living with HIV. In contrast, in Bangladesh, a re-emergence of MDR Shigella dysenteriae type 1 was observed in 2004 after a decline in the previous decade. The MDR strains were resistant to ampicillin, tetracycline, cotrimoxazole, chloramphenicol, and nalidixic acid.
The detection and surveillance of MDR Shigella strains are important for guiding the appropriate use of antibiotics and preventing the further spread of resistance. Laboratory testing of stool samples can identify the serotype and antimicrobial susceptibility profile of Shigella isolates . Molecular methods, such as polymerase chain reaction (PCR) and whole genome sequencing (WGS), can also be used to detect specific resistance genes and plasmids, as well as to track the genetic relatedness and evolution of Shigella strains .
The prevention and control of MDR Shigella infections require a multifaceted approach that involves health care providers, public health authorities, patients, and communities. Some of the key strategies include:
- Prescribing antibiotics only when indicated and following the local or national guidelines for empiric and definitive therapy
- Ordering laboratory testing for Shigella infection and reporting the results to the local or state health department
- Educating patients about the proper use of antibiotics and the importance of adherence to treatment
- Advising patients to wash their hands carefully with soap and water after using the toilet and before preparing or eating food
- Instructing patients to stay home while they are ill and avoid preparing food for others or having sex until their diarrhea resolves
- Implementing infection prevention and control measures in health care facilities, child care centers, food service establishments, and other settings where Shigella transmission may occur
- Enhancing surveillance and monitoring of antimicrobial resistance trends and outbreaks among Shigella strains
- Promoting research and development of new antibiotics and vaccines against Shigella
MDR Shigella infections pose a serious challenge for the management and control of shigellosis. By following these strategies, we can help reduce the burden of MDR Shigella infections and preserve the effectiveness of existing antibiotics.
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