Statistical analysis of shigellosis data

In 1997, a large outbreak of shigellosis infections occurred in a west Texas city between the months of June and December. The local city health department is interested in determining where transmission of the bacteria was most likely to occur (home or school), in order to better educate on the prevention of contamination.

Shigellosis (bacillary dysentery) as defined by Benenson(1995) is an intestinal disease caused by shigella bacteria. Infection is characterized by diarrhea, high fever, nausea, vomiting, cramps, and occasionally toxemia. In some cases, the stools of infected individuals contain blood and mucus. Convulsions may be an important complication in young children. The incubation period for shigellosis is between 1 and 3 days. Illness usually lasts 4 to 7 days, but the bacteria are communicable up to 4 weeks after infection. Mild and asymptomatic cases can occur, usually in adults, and are capable of transmitting infection (Lukefahr, 1994).

There are 4 serotypes of Shigella: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. Most infections involve a short clinical course and an almost negligible case-fatality rate, except in compromised hosts. Two-thirds of all shigellosis cases world-wide are in children under 10 years of age. Outbreaks are commonly associated with crowded living conditions, or where personal hygiene is poor. The only significant reservoir is people. The disease is transmitted by direct or indirect fecal-oral transmission from a carrier of shigella bacteria. Infection may occur after the ingestion of very few (10-100) of the bacteria. Individuals responsible for transmission are those who fail to wash hands and clean under fingernails after defecation. Transmission also occurs through contaminated food and water as seen in Frost (1995). The most effective methods in controlling the spread of infection are those which improve the personal hygiene standards of any at-risk individuals. An organized effort to promote careful hand washing with soap and water is the most important control measure to decrease transmission rates in most settings.