A PDF version of this shigellosis laboratory case definition (PDF file 26Kb) is available.
Consensus Date: 29 May 2000
Shigella causes bloody diarrhoea (dysentery) and non-bloody diarrhoea. Shigellosis often begins with watery diarrhoea accompanied by fever and abdominal cramps but may progress to classical dysentery with scant stools containing blood mucus and pus. All four species of Shigella are capable of causing dysentery, but Sh. dysenteriae 1 has been associated with a particularly severe form of the illness thought to be related to its production of Shiga toxin.
In 1998, 527cases of shigellosis were notified to the National Enteric Pathogens Surveillance Scheme
(Anonymous. 1999). The dominant types were Sh. sonnei and Sh. flexneri 2a.
Anonymous, 1999 NEPSS Human Annual Report 1998, 3/99
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2. Laboratory diagnosis/tests
2.1 Culture for Shigella species
While shigellae are classed as non-fastidious organisms, because of the nature of the specimens for culture, Shigella detection is still highly dependent on employing appropriate culture media.
For optimal isolation of Shigella, two different selective media should be used:
a general - purpose plating medium of low selectivity ( e.g., MacConkey-MAC) and
a more selective agar medium (e.g. xylose lysine desoxycholate agar-XLD).
Salmonella -shigella agar (SS) should be used with caution because it inhibits some strains of Sh.dysenteriae There is no reliable effective enrichment medium for all types of Shigella, but gram-negative (GN) broth and Selenite broth are frequently used (Murray et.al. 1999).
2.1.2 Suitable specimens
Faeces collected in an appropriate sterile container, transported to the laboratory ASAP (if longer than 2h keep at 4° C), may be stored at 4° C for 24 hours before culture.
Rectal swabs put into a tube transport medium containing modified Stuart's medium, transported to the laboratory ASAP. May be stored at RT for 24 hours before culture.
2.1.3 Test sensitivity
No mathematical data available.
Depends on the quality of the specimen, the type of media chosen, the number of organisms and the time of collection after the onset of diarrhoea. Antibiotic treatment may inhibit growth of shigellae
2.1.4 Test specificity
No mathematical data available.
The isolation of a Shigella like organism, confirmed by specific biochemical and preliminary serological techniques. At this point it is a "presumptive Shigella"
The isolation of a Shigella species is significant.
2.1.5 Predictive values
A negative culture does not exclude the diagnosis of shigellosis.
2.1.6 Suitable acceptance criteria.
On MAC, XLD or SS a non-lactose fermenting, non-spreading colonies which biochemically fit a Shigella species.
2.1.7 Suitable internal controls
Properly documented, relevant, quality control program for each type and batch of medium used.
Use of a control Shigella with each batch of cultures is not recommended because of the risk of cross contamination of the test organisms.
2.1.8 Suitable test validation criteria
Isolation of a Shigella species, confirmed by both biochemical and serological parameters, is the gold standard.
2.1.9 Suitable external QC programme
Royal College of Pathologists of Australia Quality Assurance Programs Pty. Ltd. (RCPA)
2.1.10 Special considerations
Based on DNA-DNA hybridization studies the genera Escherichia and Shigella are very closely related are essentially the same genus. For practical and historical reasons both groups are still treated separately. It is essential therefore, before a final designation of Shigella is made the organism must conform to both recognised biochemical and serological parameters. Preferably, the isolate identity should be confirmed by a designated enteric reference laboratory.
Ewing, W.H. 1986 Edwards and Ewing's Identification of Enterobacteriaceae. 4Th Edition, Elsevier Publishing Company, New York.
Murray, P.R., Baron, E.J., Pfaller, M.A., Tenover, F.C. and Yolken, R.H. 1999. Manual of Clinical Microbiology, ASM Press, Washington, D.C.
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2.2 Identification of Shigella species
There are two levels of Shigella identification
- Diagnostic laboratories - identify the Shigella using biochemical parameters and may perform preliminary serology.
- Reference laboratories - both the state and Australian reference laboratories.
2.2.1 Conventional biochemical tests
220.127.116.11 Suitable specimen
A pure culture on solid medium.
Various specific biochemical substrates
18.104.22.168 Test sensitivity
Depends on the biochemical tests performed. "Any strain that blackens triple sugar iron, is urease positive, or is able to grow on Simmons' citrate or decarboxylates lysine, produces phenylpyruvic acid from phenylalanine, or is motile does not belong to the genus Shigella" (Ewing, 1986). Christensen's citrate sodium acetate and sodium mucate media are of considerable value in the differentiation of Shigella from Escherichia (Ewing, 1986). (Particularly from anaerogenic E. coli)
22.214.171.124 Test specificity
Of 139 isolates submitted to an enteric laboratory as "probable Shigella species" eleven (7.9%) were not confirmed as a Shigella species (MDU unpublished data). Not all shigellae exhibit the same biochemical properties. The notable exception is Sh.sonnei which is the only Shigella serotype to ferment lactose. A few particular types produce gas in mannitol.
126.96.36.199 Predictive values
A negative culture does not preclude a diagnosis of shigellosis.
188.8.131.52 Suitable test criteria
An isolate which exhibits biochemical characteristics consistent with documented reactions for Shigella species.
184.108.40.206 Suitable internal controls
Each batch of biochemical substrate tested with positive and negative control strains.
Results of all testing recorded and the records maintained.
220.127.116.11 Suitable validation criteria
Correct biochemical reactions exhibited by a standard Shigella strain.
18.104.22.168 Suitable external QC program
RCPA Quality Assurance Programs Pty. Ltd.
22.214.171.124 Special consideration
As indicated in 126.96.36.199 biochemical variants of shigellae occur. Even if an isolate is not biochemically typical of Shigella, serology may be performed on suspicious isolates. The major biochemical problem is the differentiation of Shigella from inactive or atypical E. coli. An isolate which biochemically resembles a Shigella must be confirmed by serotyping.
Ewing, W.H. 1986. Edwards and Ewing's Identification of Enterobacteriaceae. 4th Edition, Elsevier Publishing Company, New York.
Various kits and automated machines exist for the identification of Enterobacteriaceae
Of the numerous kits available in Australia Microbact (Medvet Science Pty. Ltd., Adelaide, Australia, Micro - Id (Organon Teknika N.V. Belgium) and API would be the most commonly used.
MicroScan (Baxter, Baxter Healthcare Corporation, West Sacramento, USA), Vitek (bioMerieux, Marcy- l'Etoile, France), MicroStation (Biolog, Heyward, USA) and Cobas (Becton Dickinson Instrument Systems, Sparks, USA) are the major systems.
The information stated in sections 188.8.131.52 - 184.108.40.206 also applies to both kits and automated identification systems.
220.127.116.11 Special considerations
For accurate results it is imperative to follow the manufactures procedure exactly.
2.2.3 Latex agglutination kit
A latex agglutination kit has been described for screening for shigellae from selenite enrichment broth (Wellcolex Color Shigella; Murex Diagnostics, Inc., Norcross, GA.). This can also be used to screen individual colonies from primary plates. Suspect colonies may be inoculated into media to biochemically identify Shigella. This technique only identifies Shigella serological groups; it does not give a complete serological identification.
NB Isolates must be sent to a designated reference centre for complete serological identification.
"A preliminary report of suspect Shigella infection may be issued if biochemical or serologic screening tests are positive "(Murray et. al., 1999)
2.2.4 Molecular identification
Probes for Shigella have been developed for the detection and identification of shigellae, however, to date they are not used routinely in clinical laboratories.
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2.3 Serological identification of Shigella species
2.3.1 Suitable specimen
Pure bacterial culture.
Verified polyvalent and monovalent somatic Shigella antisera. Many enteric (salmonella) reference laboratories raise their own antisera in rabbits. Prior to use each serum is titred for the specific antigen and for cross reactions to any other Shigella antigens. It is imperative to do this on all sera used in the reference section. All testing/titres must be fully documented. The production of Shigella antisera is more complex compared to the production of Salmonella antisera. Commercial antisera are available, but it is recognised that not all brands of antisera are reliable. For reference laboratories, which test large numbers of isolates, commercial antisera are expensive and not as useful because of cross-reactions and lack of information about the antigen titres.
2.3.3 Test sensitivity
Unless an isolate is rough, most Shigella isolates will serotype.
2.3.4 Test specificity
No data available.
2.3.5 Predictive values
Failure to completely serotype a strain does not mean it cannot be serotyped. Using a different batch of antisera, using a younger subculture or another isolate from the patient may achieve serotyping of the Shigella infecting the patient. Occasionally a culture will not give a satisfactory serotype. This may indicate that the culture is mixed.
2.3.6 Suitable test acceptance criteria
Agglutination, to the specified titre, with specific Shigella antisera.
2.3.7 Suitable internal controls
Regular serotyping of a Shigella type strain.
2.3.8 Suitable validation criteria
Unequivocal demonstration of a recognised serotype.
2.3.9 Suitable External QC program
2.3.10 Special considerations
Because members of the genus Enterobacteriaceae are closely related there is antigenic cross over. Therefore it is imperative that been confirmed biochemically as a Shigella and only properly quality controlled antisera is used for serotyping. Very occassionally an isolate will be biochemically indistinguishable from Shigella species but no agglutination with the standard antisera is observed. This isolate may represent a new serotype. The isolate must be sent to the WHO designated Shigella Reference Centre at CDC, Atlanta, USA. for verification.
Ewing, W.H. 1986 Edwards and Ewing's Identification of Enterobacteriaceae. 4th Edition Elsevier Publishing Company, New York.
2.4 Further subtyping
Biotyping and colicin typing (particularly for Sh.sonnei), plasmid profiling and various other molecular typing techniques may be used for the subtyping of shigellae, however to date they are not in use routinely.
2.5 Rapid methods
Commercial rapid diagnostic tests have been developed, but currently these techniques are not being used in the clinical diagnostic laboratories.
"Several groups have developed serodiagnostic assays based on several antigens possessed by Shigella. These assays are practical only in research settings for seroepidemiology surveys and are not currently used for the diagnosis of infection in individual patients who are acutely ill" (Murray et .al., 1999).
3. PHLN laboratory definition
3.1 Condition Shigellosis
Definitive Criteria: Isolation of a Shigella species.