Scientifically Speaking | Tracing the Zika virus — from Uganda to UP
There is currently a Zika outbreak in India. It was first detected in Kerala, and then in Maharashtra, and is now spreading across Uttar Pradesh (UP).
At the time of writing, over 120 cases of Zika infection had been reported from Kanpur, Lucknow, Kannauj, and Unnao in UP. Overall, many people more are infected compared to those who have tested positive, because there may be some immunity from prior infection in the population. Up to 80% of cases of Zika are asymptomatic.
Zika virus is an RNA virus that belongs to the same family of viruses that includes the viruses that cause dengue, yellow fever, and West Nile fever. It was first isolated in the Zika forest in Uganda from a feverish monkey in 1947. There is currently no approved vaccine for Zika, though a number of vaccines are in various stages of development.
Zika can spread from mother to foetus, from sexual activity, from organ transplantation, and from blood transfusion. However, the current outbreak seems to be fuelled mainly by spread from the primary mosquito vector, Aedes aegypti (and possibly also from the competent vector, Aedes albopictus) to humans.
In 1954, the first reported case of infection in humans was reported from Nigeria. For decades, the virus sporadically caused mild or asymptomatic infections in humans in parts of Africa and Asia. Scientists traced the genetic history of the virus and split it into two different branches — the African and the Asian lineage. Each lineage has different mutations that distinguish it from the other.
In 2007, Zika was identified in Yap Island in the Pacific Ocean. By 2013, the virus had spread to other Pacific Islands such as Tahiti, Easter Island, and Cook Island. The virus spread from French Polynesia in the Pacific Ocean to Brazil probably in 2013, when it caused a devastating epidemic. Few people had heard of Zika before it became associated with severe microcephaly, a condition in which infants born to infected mothers have abnormally small heads. Advisories were issued for those who were pregnant or wishing to conceive warning against travel to countries where the epidemic was ongoing.
Initially, Zika was thought to have been brought over to Brazil during the 2014 FIFA World Cup. But Pacific Island nations did not participate in the FIFA World Cup. Then, it was thought to have been brought over by athletes from French Polynesia who took part in a boat race in Brazil in August 2014. Subsequent studies estimated that it arrived earlier.
A number of research studies have found evidence that Zika may have arrived in Brazil from French Polynesia during the Confederations Cup in 2013. The virus found in Brazil was closely related to the Asian variant found in Polynesia. Subsequently, this damaging virus became known as the American subtype.
It is plausible that transmission could’ve occurred in conjunction with one particular match played between Tahiti (where there was an outbreak ongoing) and Uruguay in Recife, Brazil. Tahiti lost the match 8-0 and Uruguay secured a spot in the Cup semi-finals. The match was also noteworthy because Luis Suárez came off the bench to score two goals to become Uruguay’s all-time leading goal-scorer (with 35 goals). This theory is unproven, but it fits with the timeline of events.
However, it is also possible that the virus hitched a ride at a different time or entered another country before being first detected in Brazil. Reconstructing the early history of an outbreak is difficult (as we also know from the multiple theories on the origin of the virus causing the Covid-19 pandemic).
In South America, Zika spread silently in an unsuspecting population for many months. The first verified case of Zika infection was identified much later in 2015. Brazil reported over 200,000 cases of Zika infections and over 2,300 cases of severe microcephaly and birth defects by the end of 2016. It is likely that over a million people were infected in total. Some cases of Guillain-Barré syndrome, a neurological disease, were also associated with viral infection. From Brazil, the virus spread across the Americas to over 45 countries.
This is a common theme among many viral outbreaks — there are many cryptic infections in a population that has some built immunity. Then the infectious agent spreads to a new location where there is a susceptible population that has not faced the pathogen before. Often the pathogen mutates to become more transmissible or virulent.
This year, on July 8, a pregnant woman in Thiruvananthapuram with no recent history of travel became the first-ever reported person testing positive for Zika in Kerala. She delivered a child, and fortunately, her newborn was reported to be in good health. But it is almost certain that she is not the first person to be infected. In retrospective testing in Kerala — which has top-notch public health infrastructure compared to many other states — nearly 12% of blood samples collected in July (out of nearly 600) tested positive for Zika. Most of these cases were from the capital. On July 31, a case was reported from a village in the Pune district of Maharashtra.
It is possible that the Asian strain of Zika has a reservoir in the mosquito population in India. Periodic mosquito surveillance and blood tests of humans have indicated that there may be some low-level presence in populations in India. Why the cluster of cases is emerging now is still unknown.
Here is the good news. So far, not a single case of microcephaly or Guillain-Barré syndrome has been tied to this outbreak. To my knowledge, no case of Zika-associated microcephaly has ever been reported in India.
But that should not be a cause for complacency. The Zika epidemic in South America is recent history, and the Covid-19 pandemic is ongoing. As a virus spreads, it has a greater opportunity to mutate into more concerning variants. Proper surveillance, control of mosquito vectors, and prevention of mosquito bites must be implemented to keep the spread of Zika in check. Should cases of severe microcephaly in association with Zika infection be detected, advising birth control in affected areas may be necessary.
Anirban Mahapatra, a microbiologist by training, is the author of COVID-19: Separating Fact From Fiction.
The views expressed are personal