Via The Lancet, an important article: Zika in Africa—the invisible epidemic? Excerpt:
Scientists now know that Zika is asymptomatic in up to 80% of those infected, and that only a small fraction of infections during pregnancy result in microcephaly. A wide spectrum of more insidious neurological sequelae is just starting to be defined. But before May, 2015, health officials had little reason to be concerned; what weak health systems could not detect, epidemiologists and scientists could not see, much less study.
“It's one of those things where sometimes we don’t realise there's a problem, so we don’t know to look for it”, said Ann Powers, acting chief of the arboviral diseases branch at the US Centers for Disease Control and Prevention (CDC). “Until we know there's a problem, there usually isn’t funding for it.”
Zika has probably been circulating in nature in a sylvatic cycle for many decades, spilling undetected into human populations across Africa with unknown regularity, according to Powers. Indeed, studies have claimed to show widespread human exposure to Zika in at least 25 countries across the continent, and scientists recently reported definitive evidence that Zika has been continuously circulating across west Africa for decades. On analysing 387 frozen blood samples taken from febrile patients in Senegal and Nigeria between 1992 and 2016, 6·2% were positive for IgM antibodies to Zika virus. “We were a bit surprised by how much we found”, said Phyllis Kanki, a virologist from Harvard University, MA, USA, and the study's senior author.
Four patients in the study also showed infection with an African Zika virus strain by real-time PCR. “It certainly is possible that there is a level of ongoing Zika virus infection in sub-Saharan Africa that is associated with microcephaly in fetuses of women infected during pregnancy”, said Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases. However, he said, the region could be “well past the outbreak stage and now has a low level of endemic infection that may result in some cases of microcephaly”.
In February, 2017, two studies suggested another potential explanation for the lack of observed birth defects associated with Zika in Africa. A research group at the University of Missouri, USA, reported that human trophoblasts making up the early placenta are killed much more rapidly by a Ugandan Zika isolate than by a Cambodian one. “This virus is so destructive of placental cells that pregnancy loss may be occurring very early on”, said coauthor and obstetrician Danny Schust. “If so, women may not even realise they were pregnant.”
A team of immunologists and obstetricians at Johns Hopkins have extended these findings to mice. Alongside Zika isolates from across Latin America and southeast Asia, they infected immunocompetent pregnant mice with a strain collected in Nigeria in 1968 and found “no significant differences in relative viral load, transmission rate, or fetal outcome”. Each strain tested was able to cross the placenta, and to result in either spontaneous abortion or neuroinflammation and cortical thinning of neonatal mouse brains.
“This all could have been going on for 100 years for all we know, and we just didn’t notice it”, said Michael Wells, a Harvard neurobiologist, who with his colleague Max Sallick showed that the Ugandan strain can infect primitive brain structures with effects that are nearly indistinguishable from those caused by the Asian lineage strain. There is a critical need for sensitive and specific diagnostics to do proper surveillance.