Yellow fever in the Americas: the growing concern about new epidemics

Introduction

Yellow fever (YF) is a haemorrhagic viral, vector-borne disease with a high case fatality rate (CFR), spread by infected mosquitoes. It has reappeared as a threat to global public health, evidenced by new epidemics in several countries in Africa and South America through autochthonous transmission, and in Asia with imported cases¹. In Asia, but also Europe and North America, Nevertheless, potential spreads beyond the borders of the endemic countries is a matter of global concern. Currently, there are around 1 billion people, from 49 endemic countries, that are considered at risk^1,2.

Recent outbreaks

Although relatively wide scale YF vaccination has been applied, a growing number of outbreaks have been documented in several African countries in the last decade. The most recent outbreak occurred in Angola, resulting in 7,344 suspected cases, 962 laboratory-confirmed cases and 137 deaths (with a CFR of 14.2%), and lasting from December 2015 to October 2016². In addition to spread of YF by autochthonous transmission, confirmed imported cases of YF were identified in China and Kenya^1–3. Other countries, such as Chad, Ghana and Guinea have also reported outbreaks or sporadic cases not linked to the outbreak in Angola^1–3.

The concern raised from Brazil

Even though no new cases have been confirmed since the last year in Angola, the global threat continues, now with its epicentre in South America. An ongoing outbreak of YF has started in Brazil since December 1, 2016. Up to February 22, 2017, a total of 1,336 cases of YF infection have been reported (292 laboratory confirmed, 920 suspected and 124 ruled out), resulting in 215 deaths (101 confirmed, 109 suspected, 5 ruled out) across six states of the country (Bahia, Espírito Santo, Minas Gerais, Rio Grande do Norte, São Paulo and Tocantins). The current CFR is 35% (from confirmed cases) and 12% (from suspected cases)³.

The geographical spread of the cases in Brazil has led to major concern, because cases are no longer being reported just in the jungle, but also in the most densely populated cities and states such as Minas Gerais and São Paulo. Fortunately, these regions have a long history of high YF vaccination coverage in young people, in contrast with the low vaccination rates in other major urban centres of Brazil⁴.

Although the epidemiology and clinical manifestations of YF should be familiar to healthcare workers in endemic countries, where clinical manifestations can overlap with other acute viral haemorrhagic fevers and other etiologies of the febrile syndrome, a rapid spread of misinformation about this harmful disease in social media and a lack of online training for healthcare workers has been reported in the recent outbreak of 2016–2017 in the Americas^5,6. In addition to limited health resources, this highlights that early identification could be a challenge in Latin America, as has been observed in the past with Zika and chikungunya virus outbreaks in this region, particularly in countries such as Brazil and Colombia^7,8.

Conclusions

There seems to be an almost imminent risk of YF outbreaks turning into a large epidemic⁹. Unvaccinated travelers heading to the affected states in Brazil are at risk of spreading the virus in to areas where YF risk factors (human susceptibility, prevalence of competent vector, and animal reservoirs) are present. Ecological factors and enzootics would promote the necessary spillover that would lead to an epidemic^10–12. Moreover, the vast border of Brazil, with 10 neighboring countries/territories (Uruguay, Argentina, Paraguay, Bolivia, Peru, Colombia, Venezuela, Guyana, Suriname and French Guiana), the lack of efficient health policies and surveillance systems, and the distribution of Aedes vectors (as well the uncontrollable sylvatic vector species in the genus Haemagogus and Sabethes), raise the possibility of the widespread YF throughout the Americas, including the USA. The USA has suitable conditions for autochthonous cases in areas such as South Florida, where Aedes albopictus is present and has been linked to transmission of dengue virus (another flavivirus), chikungunya and possibly Zika.

Mass vaccination of the at-risk population¹³, and public health awareness and preparedness is urgently needed to control the current 2016–2017 outbreak in Brazil and prevent a possible epidemic related to this deadly disease. More studies, as well as new innovative strategies for vector control (e.g. involving community participation), early prevention (e.g. sampling in risk areas to look for asymptomatic subjects), warning and enhanced surveillance (using smart phones), are necessary in order to improve the scenario of this reemerging arboviral threat^14,15.