Study shows Zika virus is becoming more effective at infecting humans

Zika virus
Since 2000 the virus has undergone genetic alterations that enable it to confuse the human immune system and replicate in human cells (image: Wikimedia Commons)

On its way from Africa to the Americas via Asia and the Pacific, the Zika virus (ZIKV) has adapted to the human organism by acquiring certain genetic characteristics that have made its replication in the cells of its new host increasingly efficient.

This conclusion was reached by a study posted to the website bioRxiv (pronounced “bio-archive”) by researchers at the University of São Paulo (USP) in Brazil and the Pasteur Institute in Dakar, Senegal. The study calls this adaptation by ZIKV a “humanization” process.

“ZIKV is an African zoonotic agent that mainly infects monkeys and mosquitoes. Previous research suggests there have been sporadic cases of infection in humans in the past. It seems the virus spread out of Africa in or around the mid-twentieth century. The first human outbreak occurred in 2007. There appears to have been a concomitant process of adaptation whereby its genetic code has acquired the capacity to mimic the most expressed human genes so as to produce larger amounts of the proteins that enhance the efficiency of its replication in the new host,” said Paolo Marinho de Andrade Zanotto, a professor at the University of São Paulo’s Biomedical Science Institute (ICB-USP) and one of the article’s co-authors.

Of the genes that ZIKV can most clearly mimic, the most important is the gene encoding NS1, a protein that modulates interaction between the virus and the human immune system.

“NS1 is abundantly expressed and is known to function as a camouflage for flaviviruses like the dengue virus, ZIKV’s closest relative. It confuses the immune system, more or less as warplanes use decoy flares to put heat-seeking missiles off track,” Zanotto said.

The results of the study also show that the adaptive fitness of this species, i.e., its ability to survive and generate progeny that are also capable of surviving and producing, fell drastically around the year 2000, owing to strong selection possibly associated with interspecies traffic. Its fitness rose exponentially from then on. The graphs in the article suggest that the pathogen became as efficient at surviving and reproducing in humans as it already was in monkeys, or perhaps even more so.

The investigation was conducted with FAPESP’s support during Caio César de Melo Freire’s PhD research, which was supervised by Zanotto.

The group analyzed 17 complete viral genome sequences plus additional data, including the years and locations in which the virus was isolated. They deposited the sequences in GenBank, a public database maintained by the National Center for Biotechnology Information (NCBI), part of the United States National Library of Medicine (NLM).

Based on these analyses and on previous research by the same group that was published in 2014 in the journal PLoS Neglected Tropical Diseases, the investigators were able to determine the routes traveled by the African and Asian lineages as well as the genetic alterations that have occurred in the past half-century.

As the authors of the recently posted article explain, the African lineage still mainly infects monkeys and mosquitoes of the Aedes genus, whereas the Asian lineage is spreading via transmission between humans in the Pacific islands and South America.

In addition to mosquito bites, sexual intercourse and perinatal infection may also be routes of transmission,  the authors add.

World alert

The first significant outbreak in humans occurred in Micronesia in 2007 and was caused by the Asian lineage. In 2013 and 2014, it emerged again and caused a large epidemic in French Polynesia, spreading to Oceania and arriving in the Americas via Easter Island, Chile, in 2014. In 2015, it has been reported in at least 14 Brazilian states, mainly in the Northeast region, as well as in other South American countries.

“Analysis based on genetic data suggests the virus is becoming more efficient in producing its proteins in humans, but this hypothesis must now be confirmed through in vitro trials with the African and Asian lineages in human cell cultures to establish comparisons,” Zanotto said.

Zanotto is organizing a partnership with some 25 laboratories in different parts of São Paulo State to monitor the spread of Zika virus through the region. Several units of this network will focus on  congenital brain malformations, in collaboration with neonatal services.

“We’re agreeing on common protocols to identify, characterize and isolate the virus,” Zanotto said. “Given our colleagues’ experience in Africa, isolating the virus from humans may present problems, and we’ll probably have to isolate it from mosquitoes. We also plan to join forces in developing the expression of viral proteins to facilitate detection of the disease, and we’re working out joint actions and exchanging information every day with research groups in other countries. It will be hard work, but we have no alternative because there’s clear evidence that the virus is indeed involved in the many cases of microcephaly that are emerging in Brazil.”

On December 1, the Pan American Health Organization and the World Health Organization (PAHO/WHO) issued a world alert recognizing a link between the Zika virus epidemic and the rising number of cases of microcephaly and Guillain-Barré syndrome in Brazil. The epidemiological alert calls on the organization’s more than 140 member states to reinforce surveillance and raise awareness of the situation, reporting any increase in microcephaly or other neurological disorders in newborns that cannot be explained by known causes. It also recommends that patients be isolated and member states prepare healthcare facilities for a possible increase in demand for specialized care for neurological syndromes, in addition to strengthening antenatal care and following up with pregnant women and newborns in areas where Zika virus is circulating.

“A few days ago, we received from our colleagues at the Pasteur Institute a notice issued by the Health Protection Surveillance Service in Papeete, French Polynesia, saying that a review of the data for pregnancies during the local outbreak of ZIKV infection in 2014 and 2015 had pointed to 12 cases of newborns with severe neurological complications. Four of the mothers concerned were tested and found positive for antibodies against ZIKV, but none had shown symptoms of the disease during pregnancy,” Zanotto said.

More research needs to be conducted to determine whether the dengue virus and ZIKV interact in the development of microcephaly. “The dengue virus is very common in these regions and might just be a red herring,” he added.

In addition to causing similar symptoms, the dengue virus and ZIKV are very closely related phylogenetically. In their latest study, the group showed that both viruses share pieces of NS1 protein considered epitopes, or antigenic determinants. An epitope is a small portion of an antigen recognized by the immune system, specifically by antibodies.

“The findings of this study are highly relevant for health authorities worldwide, so we decided to make it available immediately by posting it to this online public archive. We now intend to submit the article to scientific journals,” Zanotto said.