THE ZIKA VIRUS I - GEOGRAPHICAL DISTRIBUTION

This is the first of a three part series of briefs dealing with the Zika virus. It will discuss its identification and spread, correlating the spread with the incidence of Aedes mosquitoes.  The second will describe the new health implications for humans, and the third will discuss what the World Health Organization and the United States Centre for Disease Control are currently doing to keep the infection rate under control.

History

The Zika virus was first identified in a rhesus monkey in the Zika forest of Uganda in 1947.  The next year, the virus was recovered from an Aedes Africanus mosquito caught in the forest.  The first human infections were detected in Uganda and Tanzania in 1952 through serological studies, though it was not established until 1964 that the virus causes human disease.  From the 1960s, human infection was found across equatorial Africa and the virus was isolated from more than 20 mosquito species, mainly in the genus Aedes.  .   From 1969 the virus was also found in equatorial Asia, including India, Pakistan, Malaysia and Indonesia.   The two most prominent species are Aedes aegypti (the yellow fever mosquito) and Aedes albopictus (the Asian tiger mosquito) which transmit the viruses that cause dengue fever, yellow fever, West Nile fever, chikungunya, eastern equine encephalitis, and Zika virus.  In both regions, seroprevalence studies indicated widespread exposure in humans, but up to 2007 only 14 cases of illness had been documented worldwide.  The incidence of illness must have been much higher but, since symptoms are usually mild and the disease easily confused with dengue and chikungunya, little attention was paid to it. 

In 2007, the virus reached the island of Yap in the Federated States of Micronesia, causing 185 cases of suspected Zika disease.  An estimated 73% of the population over three years of age were found to be infected.  No evidence of viral mutation was found, and it was suspected that the higher reported incidence of the disease in Yap may have been caused by lack of population immunity in Micronesia and the under-reporting of cases before 2007.

In 2008, it was discovered that the virus could be sexually transmitted as well as through infected mosquitoes, and there is a risk of transmission via transfusions.  In 2012, two distinct lineages of the virus were distinguished: African and Asian.  There is further differentiation within each lineage.  It appears that Zika in Latin America belongs to the Asian lineage[1].  In 2013 and 2014, the virus spread to other Pacific Islands and it has now been found in 72 countries, as indicated in Figure 1, published by the WHO in early September 2016.  This map represents new Zika notifications since 2013. The darker the blue, the more recent the notification.  Circulation of Zika virus in Thailand, Cambodia and Lao People’s Democratic Republic (marked in dark grey) started before 2013. Countries where sexual transmission occurred are not represented in this map. Available information does not permit measurement of the risk of infection in any country; the variation in transmission intensity among countries is therefore not represented on this map. Zika virus is not necessarily present throughout the countries/territories shaded in this map.

Figure 1. Global spread of Zika virus, 2013-2016  

Figure 2 maps the probability of finding the Aedes aegypti  and Aedes albopictus mosquitos.

Aede aegypti

Aede albopictus

Source:   Kraemer, M. U. G., et al., 2015. The global distribution of the arbovirus vectors Aedes aegypti and Aedes albopictus
Note:      The probabilities vary from zero (blue) to one (red)

Figure 2 indicates the possibility of Zika infection in a number of countries which have not yet reported it.  Recent research in Brazil has shown that the Culex mosquito may also transmit Zika. 

On 6 July 2016, the Department of Health advised travellers that there is no threat of contracting the Zika virus in South Africa, noting that it has not been found south of Uganda[2].   The National Health Laboratory Service points out that the typical African subspecies tends not to bite humans[3], and that local Aedes aegypti mosquitoes have very limited flight ranges (measured in a few metres) and tend not to enter buildings.  The Aedes aegypti mosquito has been present in South Africa for many years, without any case of Zika infection being reported.  Professor Leo Braack of the University of Pretoria reports that it is common in certain areas of South Africa, especially in the warmer, northern areas, such as along the KwaZulu-Natal coastline, but it is also found elsewhere, including some urban areas. This species typically lays its eggs in small volumes of water, such as you would find in discarded tin cans, blocked roof gutters, pots of stored water in rural areas and bird baths that are not flushed regularly, or on the leaf axils of Strelitzia nicolae (“banana trees”).   He points out that viruses transmitted by insects usually have an animal that functions as an amplifier host, in which the virus multiplies to sufficient levels to provide an important source of infection. Such an animal is either absent or the virus has never been introduced to a susceptible amplifier host in this country.

However:

1. The Zika virus was found in a visitor from Colombia in February 2016.
2. There have been no serological studies in South Africa with the Zika disease having very little reported cases in Africa, and the virus has spread fast internationally.

The probability of a Zika outbreak in South Africa may well be low, but it cannot be discounted completely.

Conclusion

The Zika virus is found mainly in tropical areas, but it is found in sub-tropical areas as well.  The list of countries reporting its presence is growing, and the possibility of its spread through human to human contact needs to be kept in mind.  What is more, the list of conditions which may present themselves in infected people is growing.  This will be the subject of the next brief.

Arvitha Doodnath                             Charles Simkins
Legal Researcher                            Head of Research
Helen Suzman Foundation              Helen Suzman Foundation                          
arvitha@hsf.org.za                          charles@hsf.org.za