Shigella species

Essentials of Diagnosis

• Enteritis caused by Shigella species may be watery (Shigella sonnei, Shigella boydii) or dysenteric (Shigella dysenteriae, Shigella flexneri).
• Risks include ingestion of faecally contaminated food or water and contact with infected individuals.
• A definitive diagnosis requires microbiological isolation and identification of Shigella species or molecular evidence of infection.

General Considerations

Epidemiology

Dysentery is an ancient disease and has been described throughout the ages. It was not until the 19th century that dysentery was recognised as being caused by either parasitic amoebae or certain bacteria. In 1898, Shiga recognised and isolated bacteria from patients with dysentery that would agglutinate when exposed to the patient's serum. Today, the most commonly recognised agents of bacterial dysentery are Shigella species and the EIEC (see above).

Shigella species are unique among bacterial enteric pathogens in that < 200 and possibly = 10 organisms may traverse the gastric acid barrier and cause disease. For this reason, person-to-person transmission is common. Person-to-person transmission results in increased frequencies of shigellosis in day care centres, schools, and custodial-care facilities. Disease is most common in infants and young children and frequently occurs in family members of patients. Peak incidence occurs in summer, and common houseflies are thought to contribute to disease spread. Outbreaks have also occurred from faecally contaminated food. Transmission through contaminated water is most common in developing countries that lack adequate sewage and water treatment facilities.

In the United States, S sonnei is the most commonly encountered Shigella species, whereas S boydii has a worldwide distribution. The prevalence of shigellae appears to be cyclic, with replacement of the predominant strain approximately every 20 years. This cycling of prevalence is presumably secondary to slowly acquired herd immunity in a given host population. Epidemic shigellosis, caused by S dysenteriae and S flexneri, is prevalent in underdeveloped countries, but may develop anywhere that poverty, overcrowding, or conditions of war exist.

Microbiology

Shigella are non-motile, facultative anaerobic, gram-negative bacilli that are closely related to the genus Escherichia. At least 40 serotypes comprise four groups or species. These are S dysenteriae (serogroup A), S flexneri (serogroup B), S boydii (serogroup C), and S sonnei (serogroup D).

The numbers of shigellae present in the stool vary with the course of disease. Early in the watery diarrhoea phase, shigellae are abundant and number 103-109 shigellae/g of faeces. During this phase, shigellae are easily recovered on MacConkey or eosin methylene blue (EMB) agar, where they appear as lactose non-fermenting colonies. Later in the course of disease, in the dysentery and post-convalescent phases, bacterial stool counts decline to 102-103 shigellae/g of faeces. In addition, the recovery of shigellae is inversely proportional to specimen transport time, especially in stool specimens with low numbers of shigellae. During the latter phase of disease, culture is best accomplished by rapid specimen transport or bedside medium inoculation, combined with the use of enrichment broth and moderately to highly selective media, such as xylose-lysine-desoxycholate medium and Shigella-Salmonella medium.

In many laboratories, suspect colonies (lactose non-fermenters) are screened using a three-tube set: (i) one tube containing triple sugar iron (TSI) or Kligler iron agar (KIA), (ii) a second containing lysine iron agar (LIA), and (iii) a third containing Christensen's urea agar (CU) (also see the Salmonella Microbiology section below). On TSI and KIA, shigellae characteristically produce an alkaline slant and acid butt without gas production. Rare isolates may produce gas. Negative reactions are produced on LIA and CU, because shigellae do not decarboxylate lysine or hydrolyse urea. In addition, shigellae do not produce hydrogen sulfide, which is detected by the TSI, KIA, and LIA systems. Group antisera may be used to attempt to agglutinate organisms suspected to represent Shigella species. Isolates with a suggestive screening profile are further characterised by additional biochemical reactions in either traditional or automated systems.

Useful clues in the identification of shigellae include the following: the majority of shigellae cannot ferment mucate, cannot use acetate, and are negative for indole and ortho-nitrophenyl-ß-galactopyranoside.

Pathogenesis

Shigellosis may produce either predominantly watery diarrhoea or watery diarrhoea that progresses to dysentery. The severity of disease is largely determined by the invading organism. S dysenteriae and S flexneri are the agents most commonly associated with bacillary dysentery, whereas the other Shigella species more often produce watery diarrhoea.

The watery diarrhoea phase of bacillary dysentery is caused by a combination of luminal bacterial replication and superficial mucosal invasion in the small intestine. During this phase of disease, large numbers of shigellae are present in the lumen of the small intestine. This phase of disease correlates with the onset of cramping abdominal pain, fever, and toxaemia.

Within days, shigellae are no longer present in the luminal contents of the small intestine, and the colon becomes the site of infection. Shigellae invade the colonic mucosa and occasionally invade to the level of the submucosa. Factors important for invasion are present on the bacterial chromosome, as well as on a 140-MDa plasmid. Eventually, epithelial cell death occurs and the mucosa sloughs, possibly secondary to shigatoxin production. Mucosal loss evokes an intense inflammatory response and allows the introduction of coliform bacteria. Microabscesses, epithelial ulcerations, and pseudomembranes that consist of sloughed epithelial cells, bacteria, fibrin, and inflammatory cells may be seen. This phase of disease correlates with tenesmus and fractionated stools that contain blood, mucus, and inflammatory debris.

Clinical Findings

Signs and Symptoms

Early in the course of disease, when bacteria are present in the small intestine, patients develop acute watery diarrhoea, fever, and abdominal pain. Patients may become toxemic, and fever may reach as high as 104 °F (40 °C). Later in the course of disease, the colon becomes the primary site of infection. In this phase, fever continues but is usually less pronounced. Pain is usually present in the lower abdominal quadrants. Stools become dysenteric, consisting of a mixture of neutrophils, blood, mucus, and debris. Frequent, small-volume or fractionated stools may occur, and tenesmus is often present. Patients experience pain on rectal examination. Colonoscopy discloses hyperaemic and friable-to-ulcerated colonic mucosa.

Differential Diagnosis

When watery diarrhoea predominates, other bacterial, parasitic, and viral enteric pathogens must be considered. Entamoeba histolytica and EIEC must also be considered in patients with dysentery. Infection with E histolytica is most commonly associated with travel to, or living in, endemic locations. These organisms are readily recognised on microscopic examination of the stool for ova and parasites. Dysentery caused by EIEC, however, may pose a diagnostic challenge for the clinical microbiologist. EIEC, unlike other E coli, are often non-motile, may not ferment lactose or may ferment it slowly, are lysine decarboxylase negative, and may cross-react with Shigella antisera. Laboratory personnel must recognise this potential pitfall and exclude this organism through additional testing (see above).

Non-infectious causes of diarrhoea must also be considered. The differential diagnosis of non-infectious colitis is extensive and includes inflammatory bowel disease, lymphocytic/collagenous colitis, neoplasia, and numerous other disorders. Patients with inflammatory bowel disease may also have faecal leukocytes, limiting the usefulness of this test. An accurate diagnosis may be achieved through a thorough history and physical examination, exclusion of enteric pathogens through appropriate microbiological studies, and obtaining and reviewing gastrointestinal biopsies via endoscopy and histopathological studies.

Complications

The most common complications of bacillary dysentery include dehydration and a protein-losing enteropathy. In rare instances, toxic megacolon may occur and may result in perforation, intra-abdominal haemorrhage, peritonitis, and possibly death. Some patients, particularly those with HLA-B27 phenotypes, may develop post-infectious arthritis or Reiter's syndrome.

Diagnosis

Patients with acute diarrhoea, which may be watery or dysenteric, fever, abdominal pain, and systemic symptoms/toxaemia may have shigellosis. A history of exposure to individuals with shigellosis, travel to endemic areas, and exposure to a high-risk population, such as people in a custodial-care facility, should raise the index of suspicion. The presence of leukocytes in the stool, although supportive, is not definitive for shigellosis. Faecal leukocytes may be present in the stools of patients with other bacterial enteritides, amoebic dysentery, pseudomembranous colitis, and non-infectious disease, such as inflammatory bowel disease. A definitive diagnosis requires microbiological identification of a Shigella species.

Shigellae are particularly susceptible to environmental changes and die rapidly during transport. Therefore, it is imperative to transport stool specimens rapidly to the laboratory when shigellosis is suspected. This is especially important for patients in the latter stages of disease, in whom the number of shigellae in the stool is relatively low.

Where available, molecular tests can complement stool culture and may increase detection in patients who present late in the course of illness or who have already received antibiotics.

Treatment

Fluid and electrolyte replacement is necessary for patients with dehydration. In most instances, this is readily achieved with oral rehydration. Unlike in many other bacterial enteritides, antibiotic therapy is important in the treatment of shigellosis. Antibiotic therapy limits the clinical course of disease, may decrease the likelihood of intestinal complications, and reduces faecal excretion of viable pathogenic organisms, which in turn diminishes transmission. Fluoroquinolones are the treatment of choice for adults. TMP/SMX is the treatment of choice for children. Alternatives include ampicillin, chloramphenicol, and nalidixic acid. In areas with known resistance to TMP/SMX, such as parts of Southeast Asia, Africa, and South America, quinolones should be used for adults, and one of the above-mentioned alternatives should be used for children with shigellosis. When available, the antimicrobial susceptibility profile should guide therapy.

Antimotility agents, such as diphenoxylate, should not be used. Inhibition of diarrhoea increases contact between the intestinal mucosa and pathogenic organisms and their toxins, and may cause more fulminant disease.

In all cases, antimicrobial choice should take into account local resistance patterns and individual factors, including age, comorbid conditions, pregnancy status, and history of drug allergy.

Prognosis

The prognosis is generally good for patients with endemic or sporadic shigellosis. Infants and older people, especially if malnourished, suffer the highest mortality. Epidemic shigellosis caused by S dysenteriae, however, is a severe and often life-threatening disease with mortality rates from 5% to 20%. This disease must be treated aggressively with antimicrobial and rehydration therapies.

Prevention & Control

The development and refinement of sewage disposal and drinking water treatment systems are important in developing countries. In both developed and developing countries, personal hygiene, good hand washing practices, and clean living conditions are important preventive measures, particularly in custodial-care facilities. Fly control and hygienic food preparation practices should also reduce the incidence of disease.

Because the infectious dose is low and person-to-person spread is efficient, even modest improvements in hygiene and environmental sanitation can substantially reduce transmission in high-risk settings.