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SHIGELLA
Important Properties:
@ Gram-negative, Lactose negative rods.
@ Distinguished from Salmonella by:
# No gas from glucose fermentation
# Do not produce H2S.
# Non-motile.
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SPECIES:
Shigellae are divided into the following: @ Group (A): S. dysenteriae, serotypes 1-10
@ Group (B): S. flexneri, serotypes 1-6.
@ Group (C): S. boydii, serotypes 1-15.
@ Group (D): S. sonnei, only one serotype.
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Antigens:
@ All shigellae have polysaccharide O antigens in their cell walls.
@These antigens are used to divide the Shigella genus into 4 groups: A, B, C, & D
Disease:
Shigella causes enterocolitis (dysentery).
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Pathogenesis:
@ Shigella has a very low ID50
@ ID50 (the 50 % infectious dose) is the
number needed to cause infection in half the hosts.
@ LD50 (the 50 % lethal dose) is the number
needed to kill half the hosts.
@ Ingestion of as few as 100 organisms causes disease
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@ Shigellosis is a disease of humans only.
@ It leads to bloody diarrhoea (dysentery) by invading the mucosa of the distal ileum and colon.
@ This leads to inflammation and ulceration
@ Shigella will not penetrate the intestinal wall or enter the bloodstream (unlike Salmonella)
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@ Some Shigella strains produce an enterotoxin, but its role in pathogenesis is uncertain.
@ The evidence for this: @ Shigella strains not producing enterotoxin can still cause dysentery.
@ Non-invasive Shigella strains are non- pathogenic.
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Clinical Findings:
@ First fever, abdominal cramps, then watery diarrhea, which later contains blood and mucus.
@ The severity of the disease depends on:
# The species (S. dysenteriae is the most severe and S. sonnei is the mildest) # The age of the patient (children and elderly most affected).
@ Antibodies are not protective
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Laboratory Diagnosis:
@ Shigellae form non-lactose-fermenting colonies on Mac Conkey & EMB agar.
@ On TSI agar, they give an alkaline slant and an acid butt, without gas or H2S.
@ Confirmation by slide agglutination
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@ Methylene blue stain of feces detects polymorpho-nuclear cells (PMNs).
# If present, an invasive organism is involved, e.g. Shigella, Salmonella, Campylobacter # If absent, a toxin-producing organism is involved, e.g. V cholerae, E coli, Clostridium perfringens.
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Epidemiology:
@ Shigella is transmitted from person to person by asymptomatic carriers.
@ The four F's are the main sources of infection (Fingers, Food, Faeces, Flies)
@ Portal of entry is the faecal-oral route.
@ Food-borne outbreaks outnumber water- borne outbreaks by 2 to 1.
@ Children under 10 yrs account for half of Shigella positive stool cultures.
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Treatment:
@ Main therapy is fluid & electrolyte replacement
@ In mild cases, no antibiotics. In severe cases, ciprofloxacin is the best.
@ Incidence of drug resistance is high, hence sensitivity tests are performed.
@ Antiperistaltic drugs (Lomotil) are not used so as not to prolong the symptoms
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Prevention:
@ Interrupt the fecal-oral transmission by:
# Proper sewage disposal.
# Chlorination of water.
# Hand washing by food handlers.
# No vaccine, or antibiotic prophylaxis.
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FeatureShigellaS. typhiOther SalmonellaeReservoirHumansHumansAnimals, Eggs,
PoultryInfectious doseLowHighHighDiarrhoea as a prominent feature
YesNOYes
Invasion of blood stream
NOYesYes
Chronic carrier stateNOYesInfrequentLactose fermentationNONOH2 S productionNOYesYesVaccine availableNOYesNO
Important Features of Salmonella and Shigella
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