Genus Shigella contains 4 species of importance to man as the main causes of Shigellosis also known as bacillary dysentry (as opposed to amoebic dysentry).
In descending order of symptom severity:
- Shigella dysenteriae
- Shigella boydii
- Shigella flexnri
- Shigella sonnei
Shigella are Gram-negative, non-motile rods, capable of both aerobic and anaerobic respiration. They are bile-tolerant, non-fastidious, lactose non-fermenters.
To completely identify the strain requires biochemical and serological tests for O antigens. Commercial kits for this exist.
The main route of infection is faecal-oral, and infection requires only a very small dose (as low as 10-100 organisms). Like Vibrio Cholerae shigellae are spread through human contact, and do not have an animal reservoir. However, unlike the vibrios, they are not found in the environment and are not commonly spread through contaminated water or food.
Shigella attach to and invade the mucosal epithelium of the distal ileum and colon, causing inflammation and ulceration. Enterotoxin is produced, however, its role is uncertain as toxin-negative mutants still produce disease.
The symptoms of infection with Shigellae are:
- Diarrhoea (watery at first, but containing mucus and blood later on)
- Stomach cramps
- Dehydration
Treatment should be supportive only, pain management, rehydration and nutrition. Antibiotics should not be given as many strains of Shigellae are resistant to multiple drugs. Persons infected will continue to excrete shigellae for a number of weeks after symptoms subside.
Shigellosis is a major problem in the third world, especially among children. The organisms attack the lining of the gut, making the uptake of water and nutrients more difficult. This can lead to malnutrition, dehydration and possibly conditions like Kwashiorkor, the protein deficiency syndrome. The disease is entirely preventable through health and hygiene education and the provision of safe food and water supplies.
Medical Microbiology.Mims et al. 1994 Mosby.