Risk Factors for Yellow Fever Transmission in Masaka district, March-April 2016
Authors: Leocadia Kwagonza1, Ben Masiira1,Daniel Kadobera1, Alex R. Ario1, Bernard Lubwama2; Affiliations: 1Public Health Fellowship Program- Ministry of Health, 2 Ministry of Health-ESD
On 28th March, 2016, the IDSR Focal Person of Masaka district alerted Ministry of Health of a cluster of three deaths from a hemorrhagic strange disease all from the same family. We investigated the extent and risk factors for the disease. We found that the population was not vaccinated against Yellow Fever. Cultivating in forested or swampy areas were associated with Yellow Fever transmission (OR; 5.7, 95% CI: 1.5-22.0) and (OR; 5.2, 95% CI: 1.5-17.5) respectively. The entomological assessment revealed evidence of the vectors (Ae. africanus), the reservoir (Sylvatic monkeys) and the breeding sites for the vectors seen around the cultivation areas. This was suggestive of sylvatic Yellow Fever transmission. We recommended Mass Yellow Fever vaccination and enhanced surveillance to prevent future outbreaks
Introduction: Yellow fever is an acute viral hemorrhagic dis- ease transmitted by the Aedes mosquito. The Yellow fever virus is an RNA virus that belongs to the flavivirus genus and is common- ly found in tropical regions of Africa and South America. Yellow fever is considered to be a re-emerging disease due to its increased outbreaks in the recent years. This may be attributed to declining population immunity, increased human activities such as deforestation, population movements and climate change. On 28th March, 2016, the IDSR Focal Person of Masaka district alerted Ministry of Health of a cluster of three deaths from a hemorrhagic strange disease all from the same family. We investigated the epi- demiologic characteristics, risk factors for the transmission and developed recommendations to control future outbreaks.
Methods: We defined a suspect case as any person who lived in Masaka District from January 2016 onwards presenting with acute onset of fever (T>37.5°C) AND had failed to respond to ma- laria treatment with at least two of the following: Abdominal pain/diarrhea, Headache, Jaundice, Unexplained bleeding; A confirmed case was a suspect with a positive PCR or IgM laboratory test. We actively identified cases through visiting affected com- munities, responding to alerts and reviewing health facility records.
We assessed for history of Yellow Fever vaccination, cultiva- tion in forested and swampy areas.We conducted descriptive analysis on the 19 cases to generate hypotheses which were tested using a case control study. The ana- lytical study was composed of 19 cases and 73 controls. We matched one case-patient to four village controls of same sex and similar age-group (±5 years). We defined a control as any person who had no life history of symptoms that match with suspected case definition above.
Results: A total of 19 cases were identified. Their mean age was 29 (SD=13), ranging from 9 to 64 years. Males were more affected, attack rate = 0.3/1,000 compared to females, attack rate = 0.1/1,000. 30- 39 year olds were more affected, attack rate =0.4/1000 compare to the other age groups (Figure 1). Their main occupation was subsistence farming. Buwunga Sub-county was the most affected with an attack rate of 0.4 per 1000 as shown in figure….
The first case had a date of onset in the week of 14th to 20th February 2016 and the highest number of cases had date onset of symptoms during the week of 20th to 26th March, 2016 (Figure 1).
Environmental findings: Entomological investigations revealed presence of both the vector (Aedes egypti) and the sylvatic monkeys (the host) with abundant breeding sites.
Case control findings: Yellow Fever was strongly associated with; cultivating in swampy areas (OR 6.6; 95% CI: 2.0-21.0), cultivated in forests (OR 5.7; 95 % CI: 1.5-25.0), visiting more than one forests (OR 5.2; 95% CI: 1.5-17.5) and visiting more than one swamp (OR 8.1; 95% CI: 1.8 -45.2). All participants were not vaccinated against Yellow Fever
Discussion: In this area of the outbreak, it is majorly the young male adults that undertake activities around forests and swamps including clearing of forests and swamps for agricultural activities. This may explain the higher risk observed among young
males in this study. This is similar to what was observed during the 2010 large outbreak of yellow fever in the Northern part of Uganda [1].
Entomological investigations and observations made by the investigation team showed that both the vector (Aedes mosquitoes) and Sylvatic monkeys (a host for yellow fever) are present in forests, swamps and homes around the outbreak area. This case control study was able to epidemiologically link these forests and swamps to increased risk of development of yellow fever among people who culti- vate or visit these ecosystems. To our knowledge, there are no studies that have assessed human activity around forests and swamps as a risk factor for development of yellow fever in Uganda and elsewhere. The findings of this study indicate that this outbreak was possibly as a result of Jungle (Sylvatic) mode of yellow fever transmission.
In the population where this outbreak occurred, no one was vaccinated against yellow fever. This implies that there is little or no immunity against yellow fever and therefore this area remains prone to future outbreaks if no intervention is put in place. WHO recommends Yellow fever mass vaccination as the most effective means of controlling yellow fever outbreaks [2]. To prevent transmission in a region with an outbreak, mass vaccination should target vaccinating ≥80% of the population. However, immediate interventions should include sensitizing the community to reduce the risk of mosquito bites while engaging in activities in and around forests or swamps and seeking early treatment. On 19th May 2016, Ministry of Health and its partners implemented the Mass Yellow fever vaccination too the affected/ susceptible communities and reached a coverage of over 90%.
Conclusions and recommendations: This was a yellow fever outbreak that occurred in a population without immunity against yellow fever virus. The out- break was linked to working or travelling to forests and swamps which is suggestive of Jungle Sylvatic transmission. We recommended mass yellow fever vaccination and community sensitization focusing on reducing the risk of mosquito bites. Yellow fever sur- veillance system should be strengthened so that cases if any are detected early enough and further spread prevented.
References.
1. Joseph F. Wamala a, et al., Epidemiological and laboratory characterization of a yellow fever
outbreak in Northern Uganda, October 2010–January 2011
2.WHO, Yellow fever Fact sheet Updated March 2016. 2016.
