Zika: The Expanding and Deepening Threat

Marc Gozlan, MD


November 14, 2016

Moreover, other routes of nonsexual transmission may also contribute to long-term circulation of the virus, through blood transfusion[110] and even through contact with sweat or tears. A case report published on September 28, 2016, described nonsexual transmission of Zika virus from a patient who had fatal Zika virus infection with a high viral load.[111] In fact, a study[112] published exactly 1 week before had reported that Zika virus infects several different regions of the eye in mice, including the retina. In these experiments, Zika virus RNA was detected in abundance in tears, suggesting that Zika virus might be secreted from lacrimal glands or shed from the cornea.[112]

Research gaps exist with respect to Zika virus transmission. Although accumulating evidence suggests that Zika virus can be transmitted sexually through semen, more data are needed on the risk for sexual transmission and the duration of Zika virus detection in semen. Indeed, on October 16, 2016, French researchers, in addition to confirming the long persistence of Zika virus RNA in semen (in this case for more than 141 days, or more than 4 months), revealed the presence of the Zika virus in the heads of spermatozoa.[113] In this patient, the proportion of infected spermatozoa was estimated at 3.5%.

In two other patients, the presence of Zika virus RNA persisted for 69 and 115 days in the semen.[113] However, how active the Zika virus present in spermatozoa are, and their ability to transmit infection, remain to be determined. Thus, in terms of prevention of sexual transmission, more studies are needed to determine the modes of spermatozoa infection and how long semen contains infectious Zika virus.

In the same issue of Lancet Infectious Diseases,[114] the most prolonged detection (69 days) of infectious Zika virus in semen was also reported. The virus was isolated on cell lines inoculated with Zika virus-positive semen samples. Because virus isolation is the best indicator of transmissibility risk, these findings raise questions about the need to include screening for Zika virus in the testing of sperm donations at fertility centers.[113,115]

At this time, no safe and efficacious vaccine against Zika virus is available. However, two DNA Zika virus vaccine candidates have entered phase 1 human safety testing.[116] Moreover, no effective therapeutic approaches exist yet. In the interim, research is essential for developing new and effective methods for vector control and for better and rapid diagnostic tools.

Almost six decades after its discovery, the long history of Zika virus, until recently considered a virus of apparently minor importance, is just beginning to be understood. As the Zika epidemic expands its range in the Americas, increasing numbers of travelers are transporting the virus to distant regions worldwide.[117] Because competent mosquito vectors could become infected from these travelers in areas where environmental conditions are conducive to the spread of the virus, epidemics could spread within an immunologically susceptible human population.

A very recent study,[52] the first to investigate the potential spread of Zika virus from the Americas into resource-limited areas of Africa and the Asia-Pacific region, found that an estimated 2.6 billion people live in areas of Africa and the Asia-Pacific region where the presence of competent mosquito vectors and suitable climatic conditions could support local transmission of Zika virus. Asia, India, China, the Philippines, Thailand, and Indonesia appear to be high-risk countries because they have large volumes of travelers arriving from Zika virus-affected areas of the Americas and large populations at risk for mosquito-borne Zika virus infection.

In Africa, Angola is at particularly increased risk because it receives the largest number of travelers from Zika virus-affected countries in the Americas, probably because of its historical ties with Brazil.[52] "The epidemiological significance of these ties has already been shown in the context of the exportation of chikungunya from Angola to Feira de Santana, Brazil, which has since affected many countries across the Americas," note the authors.[52]

On September 1, 2016, WHO reported that genetic sequencing has shown that Zika virus isolates from human samples in Guinea-Bissau belong to the African lineage, not the Asian lineage that has been predominant in the global outbreak.[118] It is plausible that Zika virus outbreaks have occurred in recent years in Africa yet went unnoticed, owing to the very low capacity for detection of emerging or reemerging pathogens in most areas of the continent.[109,119] During the ongoing Zika virus epidemic, African countries could be at even greater risk than Asiatic countries because the current Zika virus strain is of Asian, not African, lineage. With the arrival of the Asian strain of the Zika virus in Cape Verde and Guinea-Bissau, African countries might be exceptionally susceptible.[120]

For all of these reasons, and some combinations of climatic conditions and socioeconomic factors,[45,46] the situation is very worrisome. It is more than likely that public health authorities and international medicoscientific communities (epidemiologists, virologists, immunologists, pathologists, and entomologists) will continue to struggle with collaborative efforts against Zika epidemics and its consequences during the years to come, not only in South and Latin America but also in other regions, such as Southeast Asia.

Given the rapid spread of Zika virus and its multiple clinical repercussions, notably its devastating consequences for pregnant women and infants, control of the current Zika epidemic is a more critical global health goal than ever (Figure 9).

Figure 9. Countries, territories with reported autochthonous cases of Zika virus infection in the past three months, as of 30 September 2016. Courtesy of the European Centre for Disease Prevention and Control.


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