L3-4. Infectious Causes of Gastroenteritis Flashcards
Why are children more susceptible to the effects of diarrhoea? What are the consequences of this?
They have smaller blood volumes and thus are more susceptible to changes and imbalances of electrolytes and fluids. The major problem is malabsorption and nutrient deficiencies caused by diarrhoea (including dehydration) which leads to higher susceptibility to infection.
What is the normal course of diarrhoea?
It is usually self-limited and most often presents as an acute disease that resolves in less than three weeks. The danger is the dehydration and electrolyte imbalances that ensue in diarrhoea
What is the difference between infection and intoxication in terms of diarrhoea? Give two examples of each.
Bacterial infection refers to ingestion of the pathogen in contaminated food or water leading to replication and colonisation in the gut and subsequent pathology.Eg. Salmonella and E.coliBacterial intoxication is the ingestion of bacterial toxins that contaminate the food or water leading to pathology. Eg. Staphylcoccus and Bacillus cereus
Describe the association between diarrhoea and malnutrition
This occurs in two major cateDescgorical causes:1. Increased loss of Energy: diarrhoea and vomiting and increased metabolic requirements2. Reduced intake of Energy: anorexia and malabsorption
Describe the flow of water through the gastrointestinal system in any one day
About 2 L is ingested in the food per day The body secretes approximately 8L worth of fluid into the gut tube per day through saliva, mucosal secretions, enzymes and acid secretions. At the same time water is being reabsorbed by the gastrointestinal tract. Most of it (5.5L) is absorbed in the small intestine, about 3L in the ileum and about 1.4L in the colon. 10 L into the lumen and 9.9L being reabsorbed.Thus humans defecate only about 100mL of fluid per day in the faeces
What are the four major mechanisms of diarrhoea?
- Secretory (caused by infection)2. Osmotic (caused by decreased production and content of plasma proteins)3. Motility (usually caused by drugs and alcohol)4. Exudative (due to mucosal damages such as Chron’s)
Describe the difference between small bowel and large bowel diarrhoea
Small BowelBecause the majority of water absorption occurs in the small intestine, the diarrhoea involved in small bowel pathology is often more profuse and watery. Fluid entering the bowel»_space; capacity to absorb itThus frequent, high volume, watery stoolsLarge BowelBy the time the stool reaches the large intestine, most water is already reabsorbed. Hence diarrhoea from the large intestinal pathology tends to be less fluid and smaller in volume. The stools tend to contain pus or blood
What is the difference between the causative organisms of diarrhoea between developing and developed countries?
Developing countries: bacterialDeveloped countries: viralthis is only a trend
What is non-specific gastro?What is the cause?Who is most affected?
A general inflammation of the small and large bowelsCaused by variety of bacteria, viruses and protozoaChildren are most affected with this
What is dysentry?Is it only caused by infection?What are the major causative organisms?
Dysentry is the presence of blood, pus and mucous in the diarrhoeaIt is usually caused by gastrointestinal infection but can also be a symptom of oestocolitis, cancer and othersThe major causative organisms are Enteroinvasive E.coli (EIEC) or Shigella and Protozoa including Entamoeba Histolytic and Tryptosporidia spp.
Foodborne diarrhoea is often associated with outbreaks. What are some causes of foodborne diarrhoea and give some features of each cause.
Enterohaemorrhagic E.coli (EHEC) - only a very low dose is required for infectionSalmonella: infects the gut epithelium and colonises leading to inflammation mediated damage and malabsorptionCampylobacter - present in birds (uncooked chicken)Bacilus cereus: toxin producing and found in rice, wheat, potatoesViral: Norvirus (very infectious) and Rotavirus (mainly children)Clostriduium perfringens: ingestion of spores which sporolate in the gut and release toxinsStaph Vibrio cholarea Ciguatoxin is a non-infectious form of intoxication found in fishSSCCC EBVV
Why do many people suffer from Traveller’s Diarrhoea?What are some major causes of it?
Because travelling to different global locations exposes people to microbes that they have never seen before: first exposure response. causes include Enterotoxigenic E.coli (ETEC) and other various viruses and protozoa
What is the major cause of psuedomembranous colitis?What organisms causes this? What is the pathogenesis?In what setting is this most commonly seen?
Pseudomembranous colitis is due to Antibiotic associated Diarrhoea. Antibiotics destroy commensal organisms leaving the resistant Clostridium difficile to colonise (opportunistic) and cause damagePseudomembranous colitis can also sometimes occur due to direct toxicity of the antimicrobial itselfIt is most commonly seen in hospital settings and is difficult to control as they are very resistant bacteria
What is haemorrhagic colitis? How is it different to dysentry?What is the major organism that causes haemorrhagic colitis? And how?
It is diarrhoea that has blood in itIt is different as it doesn’t have pus and mucous like dysentryEnterohaemorrhagic E.coli (EHEC) which produces Shiga like toxins. Shigella dysenterii also causes this (not any other Shigella).
What are the causes of cholera-like diarrhoea?
VibriocholeraEnterotoxigenic E.coli (ETEC)Production of very watery diarrhoea
What are the causes of Enteric fever diarrhoea?
This is only a component of the Enteric Fever (not the main pathology)- Salmonella Typhi and paratyphi ie. Thyphoid fever
What are the different types of E.coli?Give a small description of each and what kind of diarrhoea they produce
- Enterotoxigenic (ETEC): don’t invade but cause toxins that are heat labile and heat stable and produce toxins - traveller’s diarrhoea and cholera like diarrhoea. = WATERY2. Enteropathogenic (EPEC): adhere to the intestinal wall, without invasion and cause brush border damage disease = NON-SPECIFIC GASTRO3. Enteroinvasive (EIEC): adhere to and invade into the intestinal wall = DYSENTRY4. Enterohaemorrhagic (EHEC): adhere to and cause damage to the intestinal walls = BLOODY DIARRHOEA5. Enteroaggregative (EAEC): WATER DIARRHOEA
What two types of E.coli express the adhesion molecule intimin?
EPEC and EHEC
What are the three major types of invasion of the gut lining? Give an example of each type
- Invasion that is limited to the mucosa (move from cell to cell rather than deeper in): Shigella2. Invasion limited to the submucosa: Salmonella and Campylobacter3. Invasion systemically: Salmonella
Describe the two stage model of EPEC adherence:
- Initial sticking in close proximity to the epithelial lining (plasmid mediated)2. Chromosomally encoded release of toxins to adhere more tightly (form effacement for attachment)
Describe the adhesion process of EPEC in terms of its pathogenicity
Type 3 secretion (t3S) is used by the EPEC bacteria to insert toxins directly into the host epithelial cell to cause damage. This damage enables the EPEC to insert (along with the toxins) a receptor TirThis receptor is expressed on the host cell and it binds to Intimin on the EPEC outer membrane. This interaction enables adhesion of the bacteria
How is this adhesion process of Tir and Intimin encoded?
On a pathogenicity islandThis means that there is a collection of genes that play a role in virulence and that are NOT present in the non-pathogenic strains of the same species.
Is it easier for bacteria to colonise the small or large intestine?
Easier to colonise the large intestine because there is much less motility; a lower strength adhesion is required.
What are four major virulence determinants of gastrointestinal bacterium?
Adhesins (by fibriae or non-fibriated means)Invasive abilityExotoxins: cytotonic or cytotoxicAbility to resist killing by the host immune system
What is the difference between cytotoxic and cytotonic exotoxins. Give examples
Cytotonic doesn’t kill the host cell but disrupts its action. Eg. cholera toxin, shiga toxin, ETECCytotoxin: kills the cellEg. Shiga toxin
Is it common practice to determine the causative organisms of diarrhoea? Why or why not?
No Because it is a self limiting disease so is often not requiring of intervention other than supportive therapy (rehydration)
What situations would a causative organism for diarrhoea be determined?
Age: very young or oldPublic health issues and notification (outbreaks)Bloody diarrhoeaPersistence >3 weeksImmunocompromised Recent Travel history
What is the priority in treatment for diarrhoea?
Replace fluid and electrolytes, mainly by oral rehydration (can use IV)
How does the oral rehydration work? What is the magical incredient?
Water and electrolytes are ingested and replaced. They are coupled with glucose as the magical ingredient. Glucose is transported coupled to sodium in a solute:sodium transporter (conserved in diarrhoea). Thus water flows through the osmotic gradient.
What is the premise for using anti-motility agents?
They paralyse the gut to increase the time of the bolus spent in the gut thereby increasing time and amount of reabsorption of fluid. These are generally unfavored for use because the flushing mechanism of diarrhoea is more beneficial to clearing the infection.
Is it common practise to prescribe antibiotics to patients with diarrhoea? For what circumstances would you?
Cholera - has been shown to reduce severity and length of illness and infectiousnessSevere malnutrition and systemic infections (eg. typhoid fever - IV antibiotics)Immunocompromised patientsSevere cases of Shigella: but needs care due to multiresistanceAll protozoal infections are treated: metronidazoleSevere pseudomembranous colitis
For which bacteria is antibiotic treatment strictly contraindicated? Why is this so?
EHEC - phage induced toxin is thought to be induced (stimulated to be released in higher amounts) as a result of antibiotic treatment= makes the illness worse= more likely to suffer HUS
What bacteria is antibiotic treatment generally avoided unless it is severe?
Salmonella infection (among some others) because it is believed that they increase the period of infectiousness of the bacterium.
What is HUS?
Haemolytic Ureaemic Syndrome (HUS)A self-limited illness usually following diarrhoea (primarily associated with infancy)Ingestion of SHIGA TOXIN producing E.COLI (EHEC) leads to the infection and diarrhoea. Absorption of the toxin also ends up in the circulation and transfer to and bind to Gb3 receptors around the bodyGb3 receptors are found in high amounts in the kidney leading to cell death and injury. Also found in capillaries of the CNS. Often leads to kidney failure and injury, heamolytic destruction of RBCs, clotting of platelets and bloody diarrhoea
What is important to do when taking a fecal sample for culture?
Need to take at least 3 samples at different time frames to account for any intermittent shedding of the bacterium (eg. giardia)
What is the trend of incubation periods for toxic infections?
Very rapid onset of symptoms as the toxin is already present and able to act
What is meant by the term ‘infectious dose’
The amount of bacterial organisms required in the host to cause infection. It depends on the virulence of the particular microbe.Some require very little (highly virulent) infectious doses for example EHEC only needs 1-100 organisms while some require high doses like Salmonella 10^6
Are salmonella and shigella part of the gut normal microbiota?
No, never
If a stool sample has a very potent and distinct foul odour, what would you suspect?
An anaerobic organism causing the diarrhoea because the biproducts of anaerobic metabolism classically cause foul smelling gas production.
Describe the typical course of Giardia infection
Typically a slow, chronic course of diarrhoea. Ingestion of hardy cysts which activate in the body into replicating trophozoites. The trophozoites adhere to the intestinal wall and prevent absorption. Diarrhoeal symptoms occur. Then trophozoites and cysts are released in the faeces. Giardia lambia has a very low infectious dose (
What is important to note about an infection with Giardia lamblia?
It is chronic and NOT self-limitingThus it requires treatment:Metronidazole
How does Giardia cause the diarrhoea and malabsorption?
The trophozoits coat the villi and thus form a barrier between the absorptive surface and the nutrients.
