Zika virus was first discovered in monkeys and mosquitoes in the Zika Forest of Uganda in the late 1940s and later found to be present in many other African and South Asian countries in the decades that followed.  It was not thought to have much clinical significance in humans until 2015, when a large number of congenital birth defects were noticed in association with a large, de novo outbreak of zika infection in NE Brazil.

The strain of the zika virus that had struck the Americas, and Western Pacific two years earlier, was determined to be of the Asian lineage of Zika virus.  This strain, first seen in Malaysia, has since been found to have undergone two key mutations, rendering it: a) more transmissible from humans on to mosquitoes, and b) more pathogenic, targeting neural tissue in the developing foetus.  A range of congenital birth defects have since been reported in babies born to mothers who were pregnant when infected, from microcephaly through to deafness, difficulty swallowing, brain damage and eye defects.  

The risk of congenital birth defects if infected with Zika virus during pregnancy is approximately 30x higher than the background risk associated with a 'normal' pregnancy*.

By contrast, there has been no similar association so far detected with pregnancies infected with the African lineage of zika virus, despite large numbers of people infected (e.g. 6% seroprevalence in Western and NW Provinces of Zambia in 2015).  One plausible theory is that this may be due to the African strain more commonly leading to early miscarriage, rather than to full term births.

The pandemic of zika in the Americas and Caribbean subsided for a few years, but is now on the rise again, particularly in Brazil in early 2024, following El Niño.  Thailand now has the second highest reported incidence (873 cases reported in 2023, of whom 33 were pregnant and 13 (39%) had babies with congenital zika syndrome).  Asian lineage Zika virus has also recently been confirmed in 5 febrile adults in Lamu, Kenya (2024), where a landmark study is now underway to determine the true risk in Africa.

Clinical symptoms

• 80% of people infected with zika experience no symptoms whatsoever
• The remaining 20% will typically get mild symptoms of fever, rash, headache, joint pain and eye symptoms such as conjunctivitis or pain, lasting only a few days.
• A very small proportion may develop Guillain-Barré syndrome, which is a non-specific immune reaction to a number of infections and vaccines.
• The main significance is therefore for women who are already pregnant, or couples who are planning a baby after their travels.