Marburgvirus and Ebolavirus, the two genera of the Filoviridae family of virus, cause human diseases known as Marburg virus disease (Marburg) and Ebola virus disease (Ebola), respectively. They are highly lethal diseases, with case fatality ratios typically between 20% and 90%, depending on the virus species.1–3 Evidence suggests that cave- and forest-dwelling fruit bats of the Pteropodidae family (suborder Megachiroptera) are the natural reservoir of both viruses.2–7

The frequency of filoviral disease outbreaks in Uganda has increased in recent years (Figure 1). Before 2007, there had only been one reported outbreak (of Ebola) in Uganda.8,9 However, since 2007, nine filoviral disease outbreaks have occurred: six of Ebola and three of Marburg.10–16 One of these outbreaks in 2007 and 2008 involved Bundibugyo ebolavirus, which was the first new species of filovirus to be described in several years, an occurrence that has been ascribed to improved surveillance capacity.11,12

Similar patterns have been observed elsewhere: there were no reported outbreaks of Ebola globally between 1979 and 1994, but this period was followed by near simultaneous reemergence of the disease in Gabon, Cote D'Ivoire, and the Democratic Republic of the Congo (Figure 2).17,18 Similarly, there were no known cases of Marburg globally between 1987 and 1998,17 whereas three outbreaks have occurred in Uganda since 2007.

The reasons for the apparent increasing frequency of filoviral disease outbreaks in Uganda are unknown. We present the most likely explanations and suggest steps to further elucidate filoviral transmission dynamics and guide the healthcare response to the changing context.

Three scenarios are suggested by these explanations. (1)

The increase in outbreak frequency is perceived and not real. This scenario would result from successful strengthening of the national surveillance system. We would expect to see similar increases in other filoviral-endemic countries in which capacity building has been, or will be, implemented.

The most likely of these scenarios is the first, because strengthening of the Ugandan national surveillance system has demonstrably taken place in recent years. However, scenarios 2 and 3 are both plausible given the possible explanations described above, and therefore they necessitate additional investigation as to their roles in the observed increased frequency of outbreaks of filoviral disease.

These scenarios imply different interpretations and would require different responses in those countries that experience more frequent filoviral disease outbreaks. However, the available evidence in support of any of these possible scenarios is mostly circumstantial; therefore, the roles that any, all, or none of these scenarios play in the increased frequency of filoviral disease outbreaks in Uganda must be investigated. Activities should focus on the following topics. (1)

Reinforcing targeted surveillance activities, particularly around areas that have experienced multiple outbreaks and areas inhabited by bat colonies proven to be seropositive for filoviruses. Establishing in-country capacity for ongoing, sustained surveillance (with concommitant reduced reliance on external intervention) is essential for a variety of diseases, including filoviral disease, in the (rural) African context.

These activities could help healthcare providers to adapt and respond appropriately to the new paradigm. This information is relevant not only for Uganda, but also for other African countries that have experienced outbreaks of filovirus disease during the past 45 years, particularly areas that have experienced multiple outbreaks, including Western Equatoria state in South Sudan, Durba and Kampungu in the Democratic Republic of the Congo, and the Ogooué-Ivindo region of northeastern Gabon (Figure 2) (where Marburg virus has been found to be enzootic).18,25 Lessons from the Ugandan experience, whatever they may be, must be investigated, learned, and shared.