Epidemiological and laboratory characterization of a yellow fever outbreak in northern Uganda, October 2010-January 2011.

Background: In November 2010, following reports of an outbreak of a fatal, febrile, hemorrhagic illness in northern Uganda, the Uganda Ministry of Health established multisector teams to respond to the outbreak.

Methods: This was a case-series investigation in which the response teams conducted epidemiological and laboratory investigations on suspect cases. The cases identified were line-listed and a data analysis was undertaken regularly to guide the outbreak response.

Results: Overall, 181 cases met the yellow fever (YF) suspected case definition; there were 45 deaths (case fatality rate 24.9%). Only 13 (7.5%) of the suspected YF cases were laboratory confirmed, and molecular sequencing revealed 92% homology to the YF virus strain Couma (Ethiopia), East African genotype. Suspected YF cases had fever (100%) and unexplained bleeding (97.8%), but jaundice was rare (11.6%). The overall attack rate was 13 cases/100000 population, and the attack rate was higher for males than females and increased with age. The index clusters were linked to economic activities undertaken by males around forests.

Conclusions: This was the largest YF outbreak ever reported in Uganda. The wide geographical case dispersion as well as the male and older age preponderance suggests transmission during the outbreak was largely sylvatic and related to occupational activities around forests.