May 24, 2017—Few researchers studied the Zika virus (ZIKV) before an outbreak that started in Brazil two years ago spread to 40 countries and territories throughout Latin America and infected more than one million people. A new study led by researchers from Harvard T.H. Chan School of Public Health fills in a gap in ZIKV’s history by showing that it’s been silently circulating in West Africa for the past two decades.
The study was published April 8, 2017 in the Journal of Infectious Diseases.
ZIKV is primarily transmitted to humans via infected mosquitoes, and recent research has shown that it can also be transmitted from mother to fetus and between sexual partners. Symptoms vary, from mild fever and rash to severe neurological disorders, including microcephaly, a birth defect in which a baby is born with an abnormally small head. To date, there is no effective vaccine against ZIKV and no drugs to control infection.
According to the World Health Organization (WHO), ZIKV was first identified in a rhesus monkey in the Zika forest in Uganda in 1947 and detected in humans in the same country in 1952. Over the next 50 years, just 14 confirmed cases of ZIKV were reported in Africa and Asia.
“There was a long gap when nobody did any research on it, nobody cared about it,” said Phyllis Kanki, professor of immunology and infectious diseases and senior author of the study. “Then it popped up in Southeast Asia, and then it became a big epidemic in Brazil. The real question was, ‘What happened to it in Africa after it was first identified?’ This paper goes back multiple decades and shows that it was always there.”
Kanki and colleagues have been conducting HIV/AIDS research in Africa for decades, so were well-positioned to investigate. Their new study examined blood samples collected from 387 patients presenting with a fever in Senegal and Nigeria between 1992 and 2016 to find out if ZIKV was present: 119 samples collected from an HIV study of Senegalese female sex workers; 80 malaria-confirmed samples collected from two regions in Senegal; and 188 samples collected from a Nigerian HIV treatment cohort.
The researchers found that 24 samples tested positive for ZIKV IgM antibodies, resulting in an overall ZIKV seroprevalence of 6.2% (seroprevlance is the overall occurrence of a disease in a population based on blood samples).
To ensure that patients were accurately diagnosed, ZIKV IgM positive samples were also tested for dengue, another mosquito-borne virus in the flavivirus family. The results were negative.
The researchers concluded that ZIKV, though largely unreported, has been circulating in West Africa for the past two decades, demonstrating a previously unrecognized persistence in human populations.
First author Bobby Brooke Herrera, a PhD candidate in Kanki’s lab, and colleagues stressed the need for improved detection methods for ZIKV and other flaviviruses in order to correctly diagnose the cause of fever among patients and to determine how ZIKV may be affecting patient outcomes.