A letter in The Lancet: Preparing for emerging infections means expecting new syndemics.
When several sources of morbidity come together to produce interlinked and worse health outcomes, they are sometimes called a syndemic. Research on syndemics explores how two or more diseases cluster together; how social, political, economic, and ecological factors drive those clusters; and how clustered conditions interact via biological, psychological, or social pathways. Syndemic interventions target underlying patterns of multidirectional causality to achieve better overall health in susceptible and often dispossessed populations.
So far, syndemic theory has found limited traction in fields such as ecoepidemiology that dominate emerging infectious disease research, despite a natural fit for three major challenges: proliferation of closely-related emerging viruses; co-occurence with endemic infections; and broader interactions with other aspects of health, including non-communicable diseases, mental health, and stigma. We show the relevance of syndemics for ecoepidemiology by showing how each of these complicates our understanding of the Zika virus epidemic in Latin America and the Caribbean.
First, the idea of a syndemic formalises existing knowledge about co-occurrence of Aedes mosquito-borne viruses. Dengue and chikungunya viruses have had substantial comorbidity and co-infection with the Zika virus, including at least one case of triple coinfection. Many studies implicitly examine the three viruses as syndemic but do not refer to this linkage using the term. All three viruses are linked through Aedes mosquitoes to urbanisation, driving a broad spatial correlation between poverty, chikungunya, and Zika congenital syndrome (ZCS) in Brazil. Thinking syndemically could lead to mosquito control and public health education that targets several Aedes-borne viruses at once.
Second, syndemic thinking has the potential to broaden conversations about co-occurring morbidities without adding confusion about causality. After scientific consensus resolved that ZCS was responsible for Brazil's microcephaly epidemic, conversations about other possible drivers were mostly sidelined. However, Brazil is a global hotspot of congenital toxoplasmosis, and microcephaly cases attributable to toxoplasmosis were observed during the Zika virus outbreak.
Zika, toxoplasmosis, and microcephaly could create a syndemic, even without links between toxoplasmosis and the 2015–16 ZCS epidemic; poverty and environmental degradation connect them, as could fear and stigma. Syndromic surveillance for emerging pathogens like Zika the virus might be weakened by syndemic clustering between endemic and emerging infections of similar pathology or etiology, especially in poor and undersurveilled populations.
Third, sexual transmission of the virus and fetal effects of ZCS introduced two new topics—safe sex and abortion access—that had never before been extensively discussed in an arboviral outbreak. Already, work focused on the syndemic of Zika virus, HIV, and unintended pregnancy, has identified marital and cultural risk factors that can help target interventions such as condom use and social policy, emphasising the promise of syndemic thinking, and the need for social scientists' involvement in outbreak prevention and management.
The long-term effects of Zika virus might also be syndemic, as surviving babies become adults with a range of disabilities and sequelae. Although virologists work tirelessly to understand ZCS, this basic medical research remains disconnected from broader anthropological inquiries about the ongoing social effects of Zika. Bridging this gap is imperative to understand and improve quality of life in affected communities.
The Zika virus outbreak in Latin America and the Caribbean will not be the last major outbreak; several new outbreaks have been reported across India. Other diseases will also raise complicated issues regarding safe sex or access to safe and legal abortions, and virologists have begun to anticipate sexual transmission and teratogenic effects of other emerging viruses. Increased knowledge of these select pathogens can help emerging disease researchers plan for syndemics of the future.
