Properties

Pathogenesis

Clinical findings

Laboratory diagnosis

Treatment & Prevention

Curved, gram-negative rods.

Divided into 2 groups according to nature of its O cell wall antigen.

Epidemiology:

- transmitted by fecal contamination of water & food.

- human carriers are frequently asymptomatic.

- animal reservoirs are marine shellfish, such as shrimp & oysters.

- ingestion of these without adequate cooking can transmit the disease.

Colonization of small intestine:

- large numbers of bacteria must be ingested.

- organism is sensitive to stomach acid.

After adhering, the organism multiplies & produces cholera toxin:

- cholera toxin consists of an active A subunit and a binding B subunit.

- B subunit binds to a ganglioside receptor on the surface of the enterocyte.

- A subunit is inserted into the cytosol, where it catalyzes the addition of ADP-ribose to G_s subunit, causing persistent stimulation of adenylate cyclase.

- the resulting overproduction of cAMP stimulates secretion of CI & water, leading to a massive watery diarrhea without inflammatory cell.

Watery diarrhea in large volumes – ‘rice-water’ stool.

No abdominal pain, & marked dehydration.

Loss of fluid & electrolytes leads to cardiac & renal failure.

Acidosis & hypokalemia occur as a result of loss of bicarbonate & potassium in the stool.

Mortality rate without treatment is 40%.

Vibrio parahaemolyticus:

- causes diarrhea, abdominal pain & vomiting.

- naturally found in salt water environments & causes disease when it contaminates seafood which is eaten raw or undercooked.

- diagnosis: culture stools on TCBS (thiosulphate-citrate-bile salts – sucrose)– large green non-sucrose fermenting colonies.

Special TCBS culture is used: V.cholerae ferments sucrose to produce yellow colonies.

A culture of diarrheal stool containing V cholerae will show colorless colonies on MacConkey’s agar because lactose is fermented slowly.

Organism is oxidase-positive which distinguish it from members of the Enterobacteriaceae.

On TSI agar, an acid slant & an acid butt without gas or H₂S are seen because the organism ferments sucrose.

Treatment consists of prompt, adequate replacement of water & electrolytes, either orally or intravenously.

Tetracycline shorten duration of symptoms & reduce the time of excretion of organisms.

Prevention: public health measures that ensure a clean water & food supply.

Vaccine of killed organisms is only 50% effective in preventing disease for 3-6 months.