Most virulent variety of shigellosis is caused by ?

Ans. is 'a' i.e., Shigella dysenteriae . Complications are most often seen in infection with Sh. dysenteriae type 1 and include ahritis, toxic neuritis, conjuctivits, parotitis and in children, intussusception. HUS may occur in severe cases. "Severe dysentery most likely involves infection due to S. dysenteriae type 1, occurs less commonly with S. flexneri and is least likely with S. sonnei or S. boydii" - Harrison 16/h/e p. 904. Remember :? . Incubation period of bacillary dysentry - 1-7 days, usually 48 hours. . Sh. sonnei causes the mildest form of dysentry. . Sh. flexneri is the predominant shigella species (50-85%) in India, Sh. boydii is least common (0-8%). Unique features of shigella dysentriae type-1 : a) Catalase is produced by all except sh. dysentriae type I b) Mannitol is fermented by all except sh. dysentriae type 1 c) It produces a powerful exotoxin, and is an example of an exotoxin produced by Gram (-) ve bacillus. d) Most common cause of HUS in India e) It has invasive propeies. This can be demonstrated by sereny test.