Salmonella Infections in Humans

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Salmonella Infections in Humans

• Mark Pallen

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Salmonella infections in humans

• Enteric fever
• typhoid and paratyphoid fevers
• S. typhi, paratyphi A, B, C
• systemic infection
• infects only humans
• GI symptoms may not be evident
• Salmonella gastroenteritis
• non-typhi serovars
• zoonosis predominantly food-borne
• can be complicated by septicaemia
• more common with some serovars, e.g. S. dublin
  (15 mortality rate when septicemic in the
  elderly)
• Metastatic disease, e.g. osteomyelitis

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Overview

• Bacteriology
• Epidemiology
• Clinical features
• Pathogenesis
• Diagnosis
• Treatment
• Prevention

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Bacteriology

• Salmonella enterica
• one species, 2000 serovars
• Non standard nomenclature common
• S. enterica serovar Typhimurium
• or S. typhimurium
• rod-shaped, non-spore-forming Gram-negative
  bacterium
• belongs to the family Enterobacteriaceae
• close relative of E. coli
• Motile by peritrichous flagella (H antigen).
• nonmotile exceptions S. gallinarum and S.
  pullorum

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Antigenic Structure

• Kauffmann-White antigenic scheme
• agglutination reactions with specific antisera
  against Salmonella antigens
• O antigens
• characteristic sequence of repeating
  polysaccharide units in LPS.
• H antigens
• flagellar antigens (protein) and may occur in one
  of two phase variations.
• Vi antigen
• a capsular polysaccharide homopolymer of N-acetyl
  galactosamineuronic acid

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EpidemiologyEnteric fever

• person-to-person spread
• no animal reservoir
• contamination with human faeces
• usual vehicle contaminated water.
• occasionally, contaminated food (usually handled
  by an individual who harbours S. typhi)

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EpidemiologyNon-typhoidal serovars

• zoonosis with enormous animal reservoir
• common animal reservoirs are chickens, turkeys,
  pigs, and cows
• contaminated food is major vehicle, usually
• red and white meats, raw eggs, milk dairy
  products
• many other possibilities, from spices or
  chocolate to cannabis
• can follow direct contact with infected animals
  (e.g. farm trip, reptiles as pets)

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EpidemiologyNon-typhoidal serovars

• outbreaks common
• In Catering establishments
• In Hospitals
• Stanley Royd Hospital outbreak
• now careful attention to hospital kitchen hygiene

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EpidemiologyNon-typhoidal serovars

• Food-borne transmission by
• contamination of cooked food by raw food
• failing to achieve adequate cooking temperatures.
  
• secondary cases by person to person spread are
  common in outbreaks
• food handlers who practice good hygiene very
  rarely responsible for outbreaks

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Salmonella in eggs

• various Salmonella serovars isolated from the
  outside of egg shells
• S. enteritidis PT4 present inside the egg, in the
  yolk
• vertical transmission
• deposition of the organism in the yolk by an
  infected layer hen prior to shell deposition.

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Infectious dose

• typically about 1,000,000 bacteria
• much lower if the stomach pH is raised
• much lower if the vehicle for infection is
  chocolate
• protects the bacteria in their passage through
  the stomach
• an infectious dose of about 100 bacteria

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Epidemiologycarrier states

• carrier state may last from many weeks to years
  with faecal shedding
• convalescent carrier
• chronic carrier
• 3 of persons infected with S. typhi
• 0.1 of those infected with non-typhoidal
  salmonellae
• potential for cross-contamination of foods by the
  infected handler
• Typhoid Mary Mallone
• but more common in textbooks than in real life

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Clinical FeaturesEnteric Fever

• incubation period 10 to 14 days
• septicaemic illness
• myalgia and headache
• fever
• splenomegaly
• leukopenia
• abdominal pain
• Rose spots (macular rash on abdomen)
• 10 fatal
• positive blood, urine, and stool cultures
• Sequelae intestinal haemorrhage and perforation

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Clinical featuresGastroenteritis

• incubation period depends on dose
• symptoms usually begin within 6 to 48 hours
• Nausea and Vomiting
• Diarrhoea
• Abdominal pain
• Myalgia and headache
• Fever
• duration varies, usually 2 to 7 days
• seldom fatal, except in elderly or
  immunocompromised

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Pathogenesis Gastroenteritis

• Pathogenic salmonellae ingested in food survive
  passage through the gastric acid barrier
• invade intestinal mucosa
• invasion of epithelial cells stimulates the
  release of proinflammatory cytokines
• induces an inflammatory reaction
• causes diarrhoea and may lead to ulceration and
  destruction of the mucosa

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PathogenesisEnteric Fever

• Bacteria invade mucosa or Peyer's patches of
  small intestine (?M cells), pass into mesenteric
  lymph nodes where they multiply and then enter
  the blood stream via the thoracic duct
• Primary bacteraemia cleared by RES, bacteria
  multiply in RES cells and destroy them
• Facultative intracellular parasites

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PathogenesisEnteric fever

• Secondary bacteraemia occurs and results in
  spread to other organs.
• Infection of the biliary tract.
• Multiplication in biliary tract leads to seeding
  the intestine with large numbers of bacteria.
• Involvement of intestinal lymphoid tissue may
  lead to necrosis and ulceration.
• In untreated nonfatal cases, temperature drops in
  3 to 4 weeks (onset on immunity?)

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S. typhimurium in the mouse

• S. typhimurium
• causes gastroenteritis in humans
• causes typhoid-like disease in mice
• infection can be established orally or
  systemically
• used as model of typhoid
• primary mechanisms of pathogenesis
• invasion of the intestine
• survival and growth in macrophages

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Invasion

• membrane ruffling
• depends on Spi1 Type III secretion system
• effectors of invasion
• SopE affects actin cytoskeleton
• SipA binds to actin, inhibits depolymerization
• SopB inositol phosphate phosphatase.
• SptP PTPase, disrupts the actin cytoskeleton
• Invasome
• Bacterial surface appendage expressed when in
  contact with host cells

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Survival in cells

• Spi2 Type III secretion system
• expressed in cells
• activated by acidic pH in phagosome
• mutants severely attenuated in mice
• currently under intense investigation
• PhoP/PhoQ
• Pags
• Prgs
• Induction of apoptosis
• Spi1-dependent
• SipB-mediated

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Laboratory Diagnosis

• Isolated from stool, blood and urine in enteric
  fever (blood cultures need to be taken!)
• Isolated from stool in gastroenteritis
• Appears as a non-lactose fermenter
• on MacConkey agar or similar selective agar

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Laboratory Diagnosis

• Biochemical tests and serological tests must be
  done in parallel
• Some other bacteria, e.g. Citrobacter, may have
  similar serological profiles
• Commercial kits commonly used, e.g. API20
• Phage typing done for epidemiological purposes
• E.g. to find source of outbreak
• Certain phage types predominate nationally
• S. typhimurium PT4
• S. enteritidis DT109

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Treatment

• Gastroenteritis
• replace fluid loss by oral and intravenous routes
• antibiotics are not recommended for uncomplicated
  gastroenteritis
• do not shorten illness
• prolong excretion.
• antibiotic therapy reserved for the septicaemic
  and metastatic disease
• Typhoid fever and enteric fevers should be
  treated with antibiotics
• usually ciprofloxacin
• rise of resistance

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Prevention

• Remove source
• Salmonella free life-stock
• Vaccinate chicks
• Interrupt transmission
• Good food hygiene
• Cook food properly
• Keep raw and cooked foods apart
• Public Health clean water
• Strengthen host
• vaccination

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Salmonella vaccines

• Vaccination of travellers against typhoid
  recommended, but does not remove need for good
  hygiene
• Three licensed vaccines
• Traditional heat-killed
• very reactogenic
• Vi subunit vaccine
• live oral vaccine, S. typhi Ty21A
• Salmonellas can act as live attenuated carriers
  for other antigens
• So far only experimental
• No vaccines for gastroenteritis

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Salmonella Genome Sequencing Projects

• S. typhiumurium LT2 at WashU
• Multi-resistant S. typhi from Vietnam at Sanger
  Centre
• Will allow delineation of how Salmonella became a
  pathogen

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Online bibliography

• http//www.sanger.ac.uk/Projects/S_typhi/
• http//genome.wustl.edu/gsc/bacterial/salmonella.s
  html
• http//www.salmonella.org/
• http//www.who.int/inf-fs/en/fact149.html
• http//www.bmj.com/cgi/content/full/313/7065/1094
• http//vm.cfsan.fda.gov/mow/chap1.html
• http//129.109.112.248/microbook/ch021.htm