By Tyler M. Sharp Ph.D. (LCDR,USPHS)
Most travelers to Africa know to protect themselves from malaria. But malaria is far from the only mosquito-borne disease in Africa. Recent studies have revealed that dengue, a disease that is well recognized in Asia and the Americas, may be commonly misdiagnosed as malaria in Africa. So if you’re traveling to Africa, in addition to taking anti-malarial medications you should also take steps to avoid dengue.
Dengue is a mosquito-transmitted illness that is recognized as a common illness throughout Southeast Asia and much of the Americas. In fact, a study published in 2013 estimated that 390 million dengue virus infections occurred throughout the tropics in 2010. Although 70% of infections were predicted to have occurred in Southeast or Southcentral Asia, the next most affected region (16% of infections) was Africa, followed by the Americas (14% of infections). The large estimated burden of dengue in Africa came as a surprise to some, since dengue is not often recognized to be a risk in Africa.
Dengue is Hard to Diagnose in Africa
There are several reasons why dengue has limited recognition in Africa. First, the lack of laboratory-based diagnostic testing leads to many patients not being diagnosed with dengue. This can be perilous because without early diagnosis and appropriate clinical management, dengue patients are at increased risk for poor outcome. However, in order for a clinician to request dengue testing, they must first be aware of the risk for dengue. This awareness usually comes in the form of a positive diagnostic test result. Hence, without testing there is limited clinical awareness, and without clinical awareness there is limited testing.
Finding Dengue in Africa
Brown indicates countries in which dengue has been reported in residents or returned travelers and where Aedes aegypti mosquitoes are present. Light brown indicates countries where only Ae. aegypti mosquitoes have been detected.
How do we know that there actually is dengue in Africa? First, since 1960 at least 15 countries in Africa had reported locally-acquired dengue cases. In addition, travelers returning home with dengue had been detected after visiting more than 30 African countries. Still more African countries are known to have the Aedes mosquitos that transmit the 4 dengue viruses. These findings together provide strong evidence that dengue is a risk in much of Africa.
Thus, it was not a surprise in the summer of 2013 when dengue outbreaks were detected in several sub-Saharan African countries. In many cases, detection of dengue was facilitated by the availability of rapid dengue diagnostic tests that enabled on-site testing.
Dengue Field Investigations in Angola and Kenya
In a past blog I described the initial findings of a dengue outbreak in Luanda, Angola, in west-central Africa outbreak: dengue cases were initially identified with a rapid diagnostic test and confirmatory diagnostic testing and molecular epidemiologic analysis performed as CDC demonstrated that the virus had actually been circulating in the region for at least 45 years. This provided strong evidence that dengue was endemic in the area. During the outbreak investigation, CDC and the Angola Ministry of Health conducted house-to-house surveys wherein blood specimens and questionnaires were collected. Of more than 400 participants, 10% had been recently infected.
Teams from the Angola Ministry of Health conduct a dengue serosurvey in Mombasa, Luanda. Image courtesy of the Angola Field Epidemiology Training Program.
Though nearly one-third reported recently dengue-like illness, and half had sought medical care, none of the patients with laboratory evidence of infection with dengue virus had been diagnosed with dengue, including one person who had symptoms consistent with severe dengue. Although this investigation yielded more questions than answers, it was clear that there was much more dengue in Luanda than was being recognized clinically. By improving clinical awareness through training of clinicians and strengthening disease surveillance, the ability for diagnosis of individuals ill with dengue or other emerging infectious diseases was improved.
On the opposite coast of Africa in Mombasa, Kenya, although dengue outbreaks had been reported for decades, the first outbreak to be confirmed with laboratory diagnostics occurred in the early 1980s. When an outbreak of non-malarial illness was reported in 2013, blood specimens were sent to a laboratory at Kenya Medical Research Institute (or KEMRI) to determine the cause of the outbreak. Three out of the four dengue viruses were detected during this outbreak, which alone suggested that dengue was endemic in the area. To get a better idea for how much dengue there was in Mombasa, CDC and the Kenya Ministry of Health conducted a representative survey in a populous neighborhood of Kenya. Over 9 days, 1,500 people were enrolled in the serosurvey and testing revealed that 13% of participants were currently or recently infected with a dengue virus. Nearly half of infected individuals reported a recent dengue-like illness, most of which had sought medical care.
Field workers from CDC and the Kenya Ministry of Health conduct a dengue serosurvey in Mombasa, Kenya. Image courtesy of Dr. Esther Ellis.
However, nearly all patients had been diagnosed with malaria. Because Mombasa is a port city that is also popular tourist destination, not only was the apparent magnitude of the outbreak a concern for patient diagnosis and care in Mombasa, it also meant that visitors to Mombasa may not be aware of the risk of dengue and therefore could be getting sick and/or bringing the virus home with them.
There is not yet a vaccine to prevent infection or medication to treat dengue. Unlike the night-time biting mosquitoes that transmit malaria, the Aedes mosquitoes that spread dengue are day-time biters. Consequently, both residents of and travelers to Africa should protect themselves from mosquito bites to avoid dengue by using mosquito repellent. Other strategies, like staying in places with air conditioning and screens on windows and doors and wearing long sleeve shirts and pants, can also help whether you’re traveling to Africa or other regions of the tropics. For clinicians, if travelers recently returned from Africa with acute febrile illness, consider dengue as a potential cause of the patient’s illness.
We still have much to learn about dengue in Africa, but learning where there is risk of dengue is the first step to avoiding it.