Pathogenic mechanisms of diarrhoea

what is diarrhoea

• an increase in frequency, fluidity, volume and water content of faeces

what causes diarrhoea as a clinical sign

• single strains of virulent pathogens

• mixed infections

• toxins

why are some animals reservoirs for pathogens?

• because an organism may be pathogenic in one species but not in another

what do we need to consider when implmenting controls, knowing some animals act as reservoirs

• we must consider where we implement these controls; farm, animal, abattoir, retailer or kitchen?

what is the role of diarrhoea?

• enables host to remove infection

• stops pathogen/toxin build up

• may aid disease dissemination

• product of mucosal damage/effect

combination ^

bacterial infections:

1. what do most of them have to colonise the lumen?

2. what can cause the body to react to them?

1. adhesins and host receptors

2. bacteria secrete specific toxin → clinical signs or some interact → response

what are 3 factors that affect whether disease occurs?

1. secretion of exotoxin

2. invasion of target cells → cell death

3. invasion of host cells + lymph → systemic disease

for both viruses and bacteria what does the response of the host depend on (4)

1. cells targeted and their role

2. how cell responds to pathogen/toxins

3. how cell responds to other pathogen regulators

4. other factors (not just pathogens)

state 4 pathogenic mechanisms that lead to diarrhoa

1. hypersecretion

2. villous atrophy

3. infiltrative and proliferative distortion of mucosa

4. mucosal necrosis

how does hypersecretion lead to diarrhoea?

• bacteria → adhesion → toxins (enterotoxins) → hypersecretion

• a functional disturbance is caused to the enterocytes

• increased outflow/decreased re-absorption of water

example bacteria that causes diarrhoea due to hypersecretion?

• enterotoxigenic E.coli

what are the mechanisms by which hypersecretion is caused by bacteria?

• enterotoxin is produced which affects the mucosa → adenylate cyclase/guanylate cyclase activation

• chloride movement is triggered (into lumen)

• sodium follows

• so does water

• net water movement into the lumen → diarrhoea

3 example pathogens that cause villous atrophy

1. attaching + effacing E.coli

2. rotavirus

3. canine parvovirus

which cells do the following target:

1. rotavirus

2. parvovirus

3. coronavirus

1. mature enterocytes near villus tip

2. progenitor cells in the crypts

3. enterocytes

2 example diseases which causes infiltrative and proliferative distortion of the mucosa → diarrhoea

• Johne’s disease

• Lawsonia intracellularis (invades enterocytes)

3 example bacteria that cause mucosal necrosis

1. brachyspira hyodysenteriae

2. clostridium perfringens type c

3. some necrotoxigenic species of E.coli + salmonella

what condition in pigs causes mucosal necrosis

• Brachyspira hyodysenteriae - mucosa of the colon

although there are 4 main pathogenic mechanisms of diarrhoea, what are 4 further factors to consider about how they occur

1. some infections cause a mix of processes

2. some lead to partial effects

3. some infections cause other effects elsewhere as they spread

4. some pathogens vary w/in their species for extent of effect