Dengue – Sao Tome and Principe

On 13 May 2022, the Ministry of Health (MoH) of São Tomé and Príncipe notified WHO of a dengue outbreak in São Tomé and Príncipe. From 15 April to 17 May, 103 cases of dengue fever and no deaths have been reported. This is the first reported dengue outbreak in the country.

Description of the cases

From 15 April to 17 May 2022, 103 cases of dengue fever, confirmed by rapid diagnostic test (RDT), and no deaths have been reported from five health districts in São Tomé and Príncipe (figure 1). The majority of cases (90, 87%) were reported from the Água Grande health district followed by Mézochi (7, 7%), Lobata (4, 4%); Cantagalo (1, 1%); and Autonomous Region of Principe (1, 1%) (figure 2). The most commonly affected age groups were: 10-19 years (5.9 cases per 10 000), 30-39 years (7.3 cases per 10 000), 40-49 years (5.1 cases per 10 000) and 50-59 years (6.1 cases per 10 000). The most frequent clinical signs were fever (97, 94%), headache (78, 76%) and myalgia (64, 62%).

Figure 1. Confirmed cases of dengue in São Tomé and Príncipe by notification date, 15 April to 17 May 2022

Figure 2. Distribution of confirmed cases of dengue in São Tomé and Príncipe by district, 15 April to 17 May 2022

A subset of 30 samples confirmed by RDT were sent to an international reference laboratory in Lisbon, Portugal, which were received on 29 April. Further laboratory testing confirmed that the samples were positive for early acute dengue infection, and that the predominant serotype was dengue virus serotype 3 (DENV-3). Preliminary results suggest the possibility of other serotypes present within the batch of samples.

A dengue outbreak alert was initially triggered when a suspected dengue case was reported at a hospital in São Tomé and Príncipe on 11 April. This case, who presented with symptoms suggestive of dengue infection, had travel history and was later diagnosed as having a past dengue infection.

Epidemiology of the disease

Dengue is a viral infection transmitted to humans through the bite of infected mosquitoes. Dengue is found in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas. The primary vectors that transmit the disease are Aedes aegypti mosquitoes and, to a lesser extent, Ae. albopictus. The virus responsible for causing dengue, is called dengue virus (DENV). There are four DENV serotypes and it is possible to be infected four times. Many DENV infections produce only mild illness, and over 80% of cases do not exhibit symptoms (asymptomatic). DENV can cause an acute flu-like illness. Occasionally this develops into a potentially lethal complication, called severe dengue.

There is no specific treatment for dengue; however, a timely detection of cases, identifying any warning signs of severe dengue, and appropriate case management are key elements of care to prevent patient deaths due to dengue. A delay in seeking medical care is often related to deaths from dengue virus disease.

Occasional cases acquired overseas have been reported among individuals returning from an area with active dengue transmission, however, there is no conclusive evidence that there have been imported cases of dengue to São Tomé and Príncipe. Before this outbreak, São Tomé and Príncipe has never reported a dengue outbreak, despite being in the global dengue belt and having favourable conditions to transmit dengue.

The risk at national level is currently assessed as high due to the (i) presence of the mosquito vector Aedes aegypti and Aedes albopictus; (ii) favourable environment for mosquito breeding grounds following heavy rains and floods since December 2021; (iii) concurrent outbreaks of diarrheal disease, malaria, COVID-19 among other health challenges; and (iv) decreased functionality of sanitation and water management systems in health facilities due to structural damage after heavy flooding. The reported numbers are likely an underestimate because a high proportion of dengue cases are asymptomatic, and there are limitations to the capacity to conduct surveillance and diagnose cases. Clinical management of severe dengue cases is also a challenge. Community awareness in the country is low, and risk communication activities are insufficient.

The overall risk at the regional and global levels are assessed as low. The likelihood of further spread from São Tomé and Príncipe to other countries is unlikely because the country is an island that does not share land borders and it would require the presence of susceptible vectors.