Enteric Infections Flashcards
1
Q
Gastrointestinal Natural Resistance to Infection
A
- We routinely cope with low-level bacterial infection in food due to natural defenses in GI tract
- Gastric acid - fat in diet may protect pathogens in transit
- Bile salts - in duodenum disrupt some cell surfaces
- Normal commensal flora
- Modify environment with metabolites
- Compete for nutrients
- Produce natural antibiotics
- Mucosal immunity
- Cell-mediated immunity
- Secretory IgA (also in breast milk)
- Motility - diarrhoea probably important in clearing pathogens
2
Q
Hydration/Rehydration
A
- Our survival depends upon the fluid absorption capacity of the gut
- Adult gut handles 6,500 ml of fluid/day
- Food
- Saliva
- Gastric/pancreatic/biliary fluids
- Volume reduced to 1,500ml by distal ileum
- Reduced further in colon –> stool egestion of <250ml a day
- Adult gut handles 6,500 ml of fluid/day
- Diarrhoea due to:
- Secretory (chloride or calcium mediated)
- +/- Osmotic (damage to villous brush border)
- Oral rehydration therapy - oral rehydration salts (ORS)
- Rely on coupled transport of sodium and glucose into enterocytes so that water follows osmotic gradient
- Now a standard of care
3
Q
Types of Enteric Infection
A
- Type 1: Non-inflammatory (watery diarrhoea) e.g.
- Toxin-mediated
- C.perfringens
- B.cereus
- S.aureus
- Giardia
- Cryptosporidium
- Rotavirus
- Norovirus
- ETEC
- EPEC
- Toxin-mediated
-
Type 2: Inflammatory (dysentery, faecal leukocytes, lactoferrin) e.g.
- Shigella
- VTEC
- C.difficile
- C.jejuni
- S.enteritidus
- Entamoeba
-
Type 3: Penetrating (enteric fever) e.g.
- S.typhi
- Yersinia
4
Q
Standard Management
A
- Often uncomplicated and self-limiting
- Mainstay of treatment is supportive
- Rehydration therapy
- Little role for anti-diarrhoheal agents
- Some bacteria or parasites may require specific antimicrobial therapy
- Assessing dehydraion correctly and early important
- Infants more susceptible as:
- Higher body surface to volume ration
- Smaller fluid reservoir
- Dependent on other for fluid intake
- Signs of severe dehydration
- Apathy
- Tachycardia (bradycardia if extreme)
- Weak pulse
- Deep breathing
- Deep dunken eyes
- No tears
- Parched mouth
- Skin recoil >2 seconds
- Minimal urine output
- Infants more susceptible as:
5
Q
Basic Diagnosis (Bacteria/Parasites)
A
- Rarely possible on clinical features alone
- Epidemiology - as part of an outbreak
- Microbial investigation
- Rarely necessary unless:
- Dehydrated
- Febrile
- Blood or pus in stool
- Part of outbreak
- Stool +/- blood culture
- Sekective indicator growth mediator
- Diagnostic yield of stool culture approximately 5%
- Microsopy of stool for:
- Ova
- Cysts
- Parasites
- Specific typing
- Rarely necessary unless:
6
Q
E.Coli
A
- Major part of normal gut flora
- Until recently role uncertain as difficult to distinguish pathogens from commensals
- Enterotoxic E.coli (ETEC)
- Produces two main types of polypeptide toxins (similar to cholera) –> hypersecretion of fluid into lumen
- Verycytotoxic (VTEC) or Enterohaemorrhagic (EHEC)
- Produces cell killing cytotoxin (similar to shigella)
- Can ==>
- Haemmorhagic colitis
- Haemolytic uraemic syndrome (HUS)
- Enteroinvasive (EIEC)
- Enteropathogenic (EPEC)
- Enterotoxic E.coli (ETEC)
7
Q
E.Coli - Epidemiology
A
- Transmission by:
- Faecal-oral
- Direct
- Food or water
- ETEC
- Commonest cause of bacterial diarrhoea in children in areas of poor hygeine
- Uncommon in Western Europe
- Important cause of travellers’ diarrhoea
- Reservoir = human GI tract
- VTEC
- Several types - commonest is 0157
- Now a common cause of acute renal failure in Western countries
- Reservoir = GI tract of healthy cattle
- Transmission by:
- Contaminated food/animal carcases (e.g. hamburgers)
- Unpasteurised milk
- Farms
- Paddling pools
- Rarely person to person e.g. nurseries
- Several types - commonest is 0157
8
Q
E.Coli - Clinical Features
A
- Incubation period usueally 1-5 days (can be up to 14)
- Abrupt onset vomiting and diarrhoea
- ==> later profuse watery diarrhoea only
- Mild fever
- Little pain
- Similar presentation to viral gastroenteritis/salmonellosis
9
Q
E.Coli - Haemorrhagic Colitis
A
- Possible complication of 0157 infection in children and adults
- Typically diarrhoea escalates to bloody with abdominal pain
- Fever usually low
- May be mistaken for acute inflammatory bowel disease
10
Q
E.Coli - Haemolytic Uraemic Syndrome
A
- May accompany colitis as a complication of infection
- Affects 10% of children in outbreaks
- Characterised by:
- Rising urea and creatinine (AKI)
- Haemolytic anaemia
- Thrombocytopenia
- Raised BP
- Prognosis
- >50% need haemodialysis
- Only fatal in <5% - mostly elderly
- Preceeding GI illness may go unrecognised
- Shiga-toxin binds to glomerular epithelium ==> apoptosis + bind leukocytes + become thrombogenic ==> activated platelet complexes lodge in capillaries and arterioles ==> destruction of RBCs (microangiopathic haemolysis)
11
Q
E.Coli - Laboratory diagnosis
A
- Difficult as pathogen and normal flora are the same species
- 0157:
- Phage typing
- Doesn’t ferment sorbitol
- Immunological cytotoxin detection
- PCR detection of cytotoxin gene
12
Q
E.Coli - Management
A
- Mostly supportive
- Many E.Coli are resistant to broad spectrum antibiotics e.e.g penicillins, cephalosporins, trimethoprim
- Rx -* ciprofloxacin* 500mg BD for 3-5 days
- **! **- Avoid antibiotics in HUS as bactericidal ==> toxin release
- Antimotility drugs probably increase chance of HUS through delayed clearance of toxin
13
Q
Salmonella
A
- Common cause of food poisoning
- Infects humans and other animals
- >2000 serotypes
- Commonest are S.enteritidis, S.typhimurium and S.virchow
-
Cause of typhoid and paratyphoid fevers
- These are exclusive human pathogens
14
Q
Salmonella - Food Poisoning
A
- Contaminated poultry/dairy products common source
- Not usually from food handlers or person to person spread
- Reservoir - GI tract of birds, reptiles, amphibians
- Commoner in summer/hot weather
- Microbiology:
- Identified on specific media by biochemical features:
- Non-lactose fermenters
- Produce H2S
- LPS is O antigen, flagellae H antigen - defines serotypes
- Identified on specific media by biochemical features:
15
Q
Salmonella - Food Poisoning - Pathogenesis
A
- Infection of gut epithelium
- Does not extend beyond basement membrane
- ==> Excess fluid secretion from ileum/jejenum
- If transported through cells ==> systemic infection
- Survives in macrophages
16
Q
Salmonella - Food Poisoning - Clincal Features
A
- Incubation period 12-72 hours
- Symptoms:
- Malaise
- Nausea
- Vomiting
- Fever
- ==> Watery-brown diarrhoea follows rapidly
- Abdominal pain common but not severe
- Often resolves in several days, some cases last several weeks
- Children and elderly at risk of hypovolaemia
17
Q
Salmonella - Food Poisoning - Complications
A
- Salmonella colitis
- Affects up to 10% ==> colic and bloody stools
- Bacteraemia
- ==> Seeding to bones/joints in sickle cell disease patients
- ==> Aneurysms
- Post-infectious reactive arthritis
- Prolonged excretion in:
- Diverticulosis
- IBD
- HIV
20
Q
Shigellosis (Bacillary Dysentery) - Epidemiology
A
- Worldwide problem
- In Western countries endemic Shigellae usually cause mild illness
- Few thousand cases a year in the UK
- Tropical strains tend to be more severe and persistent
- Transmission via
- Person to person spread
- Contaminated food and water
- Reservoir = human GI tract
25
Q
Campylobacter
A
- Commonest cause of food poisoning
- >50,000 cases per year in the UK
- Mostly sporadic - undercooked poultry, bird pecked milk
- Large food/waterborne outbreaks can occur
- Incidence higher in summer
- Person to person spread uncommon
- Animal pathogen - several species can infect humans:
- C.jejuni
- C.coli
- C.fetus
- C.lari
30
Q
Clostridium Difficile
A
- Commonest cause of hospital-acquired diarrhoea
- Gut commensal in 3% of healthy adults and 66% of children
- Some antibiotics disturb normal balance of microbial flora ==> rapid multiplication of C.difficile ==> toxin production ==> mucosal injury and inflammation ==> diarrhoea
34
Q
Viral Gastroenteritis
A
- Common pathogens:
- Rotavirus
- Adenovirus
- Norovirus
- Astrovirus
- Commonest cause of symptomatic intestinal infection in Western world
- Rarely severe or fatal in UK
- Significant cause of infant mortality in resource poor countries
37
Q
Rotavirus
A
- Commonest cause of viral gastroenteritis in young children
- 1 million deaths/yr worldwide
- >10,000 cases/yr UK - under-reported
- Peak incidence 6-24 months, uncommon >5yrs but adult infection occurs and can be symptomatic – may cause outbreaks in elderly care homes
- Seasonal, late winter
41
Q
Calciviruses
A
- Types:
- Norovirus
- Sapovirus (SRSV)
- ssRNA, non-enveloped, does not grow in routine cell culture
- Reservoir human GI tract
- May be concentrated in bivalve molluscs
43
Q
Enteric Adenoviruses
A
- Second most common cause of infantile diarrhoea in temperate climates
- Adenoviruses cause many diseases
- Non-enveloped, dsDNA
- Virology
- Subgroups A-F - gastroenteritis agents are group F: types 40/41
- Poor growth in cell culture
- Standard clinical picture:
- Incubation period up to 10 days ==>
- Watery diarrhoea
- Mild fever
- Illness may last longer but in general (3-11 days)
44
Q
Astroviruses
A
- Infants and elderly exhibit significant illness
- Severity lower than other agents
- Often co-infection with rotavirus/norovirus
- <5% hospitalised cases viral gastroenteritis
- Commoner in winter time
- Virology
- +ssRNA
- Non-enveloped
- 5-6 pointed star
- Several serotypes
