Executive summary
On March 28, 2016, a cluster of deaths resulting from jaundice and hemorrhagic symptoms occurred in Masaka, Central Uganda. We investigated this outbreak to deter- mine the nature of disease, identify risk factors, and recommend control and prevention measures. We conducted medical record reviews and active community case-finding. We also conducted entomological studies and environmental assessments. At least 32 cases (7 confirmed) were identified. The overall attack rate (AR, per 100,000) was 0.48 overall (6.4 Masaka, 3.1 Rukungiri and 1.9 Kalangala); the case-fatality rate was 22% (7/32). Men (AR=1.2/10,000) and persons aged 30-39 years (AR=4.0/10,000) were most affected. The commonest symptoms were fever, abdominal pain/ diarrhea, headache, jaundice, and unexplained bleeding. No one interviewed during this investigation was vaccinated against yellow fever yet sylvatic monkeys and Aedes mosquitoes the known reservoirs were identified on the farmlands in four villages. This investigation revealed that the Ugandan population is not vaccinated against Yellow fever infection yet Uganda lies within the yellow fever belt. In addition to the hot humid conditions that enable both the sylvatic monkeys and the Aedes mosquitoes to thrive. We recommend yellow fever vaccine to be included in the Ugandan routine immunization schedule.
Introduction
Yellow fever, a disease of international importance, is an acute viral hemorrhagic disease transmitted from human to human or animal to human by the Aedes mosquito. Yellow fever is considered to be a re-emerging disease due to increasing reports of its occurrence in different parts of the world in the recent years (1). World- wide, the number of yellow fever cases has in- creased over the past twenty years. This may be attributed to declining population immunity to infection (2), increased human activities such as deforestation, urbanization(3), population movements(4) and climate change (5).
In 2013, it was estimated that yellow fever affected 130,000 people in Africa and that about 78,000 people died from the disease.(6). There is no specific treatment for Yellow fever, thus only supportive treatment to manage symptoms. Up to 50% of severely affected persons without treatment die from yellow fever (7).
WHO recommends Yellow fever mass vaccination as the most effective means of con- trolling yellow fever outbreaks (8). Uganda is among the so called 32 African countries considered at risk of Yellow Fever transmission (lies in the yellow fever belt) (9). The first documented outbreak of yellow fever in Uganda was reported in Bwamba County, Western Uganda in 1941 with subsequent out- breaks in 1952, 1959, 1964, 1971 and 2010 (10, 11).
The largest yellow fever outbreak in Uganda occurred in northern Uganda in 2010 and affected 181 people of which 45 died (CFR=24.8%) (10). On March- April 2016, there was a Yellow fever out- break in the Masaka, Rukungiri and Kalangala Districts.
The aim of this investigation was to identify risk factors and to develop evidence based recommendation for prevention and control of future out- breaks.
Approaches and Results
To be able to identify as many cases as possible, we used two strategies i.e. health facility and community case finding strategies. The team visited health facilities and reviewed out-patient, in-patient and laboratory records to identify current or previous patients that fulfilled the case definition. Patients who had been seen at a health facility and discharged were followed up and assessed.
With the help of Village Health Teams (VHTs), the team also visited affected villages to identify more cases. We conducted a matched case-control study using a ratio of 1 case: 4 controls. Controls were persons from the same village who never had any symptoms resembling yellow fever from January 2016 onwards, matched by sex and age (±5 years).
We selected controls using systematic random sampling method from the village household list from the same village as the case-persons. We assessed potential risk factors such as, presence of monkeys on farm land and homes, yellow fever vaccination, cultivation in forested and/or swampy areas and travel history. Entomological assessments were conducted to identify breeding sites, presence of the vectors (using light traps) and reservoirs.
We identified 32 case-persons (7 confirmed and 7 probable). Males were more affected than females (Men (AR=1.2/10,000)). The most affected age group was 30- 39 years with an attach rate of 40 cases per 10,000 populations. The overall attack rate (AR, per 100,000) was 0.48 overall (6.4 Masaka, 3.1 Rukungiri and 1.9 Kalangala); the case-fatality rate was 22% (7/32).
The commonest symptoms were fever, abdominal pain/diarrhea, headache, jaundice, and unexplained bleeding. At least 62% of case-persons cultivated in forest areas while 69% cultivate in swampy areas. Yet sylvatic monkeys- the known reservoirs of yellow fever virus were part of the ecosystems due to massive deforestation. It was common to find Monkeys around homes and gardens. Entomological assessments found the presence of the Aedes spp mosquitoes and numerous breeding sites around farm- lands and homesteads. None of the persons interviewed were vaccinated against yellow fever infection.
Conclusion
This was a yellow fever outbreak that occurred in a population without immunity against yellow fever virus. The outbreak was associated with per- sons who cultivated in forests and swampy areas which is suggestive of Jungle (Sylvatic) transmission.
Implications
If the population is not vaccinated against Yellow fever virus, similar outbreaks are likely to appear in other parts of the country. This will result in disruption of economic activities such as tourism and travel as well as loss of lives since the disease has a potential to cause high morbidity and mortality. Vaccination of the population is the cheapest control measure than dealing with costs associated with repeated outbreaks.
Recommendations
Ministry of Health should look at the available evidence and plan to include the yellow fever vaccination into the routine immunization schedule.
The Integrated Disease Surveillance and Response (IDSR) should strengthen yellow fever surveillance in the entire country.
References
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