Introduction Zika, a flavivirus transmitted mainly by mosquitos in the genus Aedes, was discovered in 1947 in Uganda. (1) From the 1960s to 1980s, human infections were found across Africa and Asia, typically accompanied by mild illness. The first large outbreak of disease caused by Zika infection was reported from the island of Yap (Federated States of Micronesia) in 2007, as the virus moved from south-east Asia across the Pacific. During an outbreak in French Polynesia in 2013-2014, Guillain-Barre syndrome was linked to Zika infection and cases of microcephaly in newborn children were also retrospectively linked to this outbreak. The World Health Organization (WHO) received the first reports of locally-transmitted infection from Brazil in May 2015. In July 2015, health ministry officials from Brazil reported an association between Zika virus infection and Guillain-Barre syndrome in adults. In October 2015, WHO received reports from Brazil of microcephaly in babies whose mothers had been exposed to Zika during pregnancy. At this time, there was no proof of a causal link between Zika infection and these neurological complications. In February 2016, as infection moved rapidly through the range occupied by Aedes mosquitos in the Americas, WHO declared that Zika infection associated with microcephaly and other neurological disorders constituted a Public Health Emergency of International Concern (PHEIC). By the start of February 2016, local transmission of Zika infection had been reported from more than 20 countries and territories in the Americas and an outbreak numbering thousands of cases was under way in Cabo Verde in western Africa. Beyond the range of its mosquito vectors, Zika virus infections are expected to be carried worldwide by people as they travel and be transmitted by travellers to sexual partners who have not been to places where the virus is endemic. Methods To illustrate the spread of Zika virus and associated neurological complications, we did a literature search in PubMed using "Zika" and "ZIKV" as the search terms and cross-checked our findings for completeness against other published reviews. (2, 3) In addition, we drew on formal notifications to WHO under the International Health Regulations (IHR), (4) which are archived in the WHO Event Information Site (EIS). EIS contains information about public health events of potential international concern notified to WHO as required by the IHR. EIS notifications sometimes contain confidential patient information and therefore are not publicly available. Other details of specific events can be provided by the authors on request. Results The first reported case of Zika virus dates to 1947 when the virus was isolated in samples taken from a captive sentinel rhesus monkey by scientists conducting routine surveillance for yellow fever in the Zika forest of Uganda. (1) The virus was recovered from Aedes (Stegomyia) africanus, caught on a tree platform in the forest. (1) Laboratory infection experiments showed the virus to be neurotropic in mice. (5) The timeline presented in this paper includes numerous serological surveys that purportedly detected antibodies to Zika virus in the 1950s and 1960s in Africa and Asia. Because serological (antibody detection) tests for Zika cross-react with antibodies stimulated by other viral infections, the presence of Zika virus is ideally confirmed by the detection of viral nucleic acids by polymerase chain reaction (PCR) testing or by virus isolation. A chronological map of the presence of Zika in those countries for which there is evidence of autochthonous transmission by mosquitos is presented in Fig. 1. The map excludes the many countries from which imported Zika infections have been reported. The country-by-country spread of Zika virus infections, from the earliest published report in 1947 to a World Health Organization, avenue Appia 20, 1211 Geneva 27, Switzerland. January 2014 is summarized in Table 1. …

Zika: the origin and spread of a mosquito-borne virus.

First discovered in 1947, Zika virus (ZIKV) infection remained a little-known tropical disease until 2015, when its apparent association with a considerable increase in the incidence of microcephaly in Brazil raised alarms worldwide. There is limited information on the key factors that determine the extent of the global threat from ZIKV infection and resulting complications. Here, we review what is known about the epidemiology, natural history, and public health effects of ZIKV infection, the empirical basis for this knowledge, and the critical knowledge gaps that need to be filled.

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Assessing the global threat from Zika virus

The number of mosquito-borne viruses (‘moboviruses’) occurring in Europe since the twentieth century now stands at ten; they belong to three families—Togaviridae (Sindbis, Chikungunya), Flaviviridae (West Nile, Usutu, Dengue), and Bunyaviridae (Batai, Ťahyňa, Snowshoe hare, Inkoo, Lednice). Several of them play a definite role in human or animal pathology (Sindbis, Chikungunya, Dengue, West Nile, Ťahyňa). Mobovirus outbreaks are strictly determined by the presence and/or import of particular competent vectors of the disease. Ecological variables affect moboviruses considerably; the main factors are population density of mosquito vectors and their vertebrate hosts, intense summer precipitations or floods, summer temperatures and drought, and presence of appropriate habitats, e.g., wetlands, small water pools, or intravillan sewage systems. A surveillance for moboviruses and the diseases they cause in Europe is recommendable, because the cases may often pass unnoticed or misdiagnosed not only in free-living vertebrates but also in domestic animals and even in humans.

Mosquito-borne viruses in Europe

The serum dilution neutralization test was evaluated for serological diagnosis of California group arbovirus infections and identification of virus isolates. The technical advantages and the degree of subtype specificity of the serum dilution neutralization test over the hemagglutination inhibition test and the complement fixation test were demonstrated with paired specimens from human cases, single human survey sera, and sentinel rabbit sera. Twenty-one virus isolates from various geographical areas of the United States were also used to evaluate the efficacy of the serum dilution neutralization test for specific virus identification.

Serum dilution neutralization test for California group virus identification and serology.

Key aspects of 36 mosquito-borne arboviruses indigenous to Africa are summarized, including lesser or poorly-known viruses which, like Zika, may have the potential to escape current sylvatic cycling to achieve greater geographical distribution and medical importance. Major vectors are indicated as well as reservoir hosts, where known. A series of current and future risk factors is addressed. It is apparent that Africa has been the source of most of the major mosquito-borne viruses of medical importance that currently constitute serious global public health threats, but that there are several other viruses with potential for international challenge. The conclusion reached is that increased human population growth in decades ahead coupled with increased international travel and trade is likely to sustain and increase the threat of further geographical spread of current and new arboviral disease.

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Mosquito-borne arboviruses of African origin: review of key viruses and vectors

Sera collected in 1960 from 492 indigenous residents of 14 widely scattered localities in Angola were tested for HI antibodies against 18 viruses: 5 from Group A, 9 from Group B, 1 from Group C, 2 from the Bunyamwera Group, and Rift Valley fever virus. Results of these tests indicated that the prevalence of antibodies was greatest in the 6 localities sampled in the north-western coastal and plateau region. Sera positive in the HI test were further tested with the respective viruses by mouse neutralization test. Analysis of these results according to sera positive for only one virus of Group A, B or Bunyamwera indicated that Sindbis, chikungunya, Semliki Forest, Wesselsbron, Zika, H 336, West Nile, Bunyamwera and Germiston viruses have been active, predominantly in the north-western region, and that Spondweni virus has been present. There was no evidence that Mayaro or Middelburg virus has infected the population sampled, or that Ntaya, dengue 1 or dengue 2 virus has been present. In 3 localities in the north-west interpretation of results with Group B viruses was complicated by the fact that some of the donors had been vaccinated against yellow fever. HI test results with Oriboca virus suggest that this or a closely related member of Group C exists in Angola. In additional neutralization tests, one or more sera from children and adults in each of the 6 north-western localities protected against Lumbo (California complex), Pongola or Bwamba fever virus.

Survey for antibodies against arthropod-borne viruses in the sera of indigenous residents of Angola.

Summary
Twelve strains of a hitherto unknown virus have been isolated from Aedes (Skusea) pembaensis collected in 1959 and 1960 at Lumbo in northeastern Mozambique. The name proposed for the agent is Lumbo.
Lumbo virus is filterable and withstands lyophilization but is sensitive to sodium desoxycholate. It is pathogenic for adult mice and hamsters by the intracerebral but not the intraperitoneal inoculation route. Vervet monkey, yellow baboon, bushbaby and guinea pig develop an immune response to the virus without showing signs of illness. Viremia was detected on the 3rd and 4th postinoculation days in a vervet monkey. Of 128 human sera collected at Lumbo and Ndumu, northern Natal, in the tropical corridor in southeastern Africa, 16 contained neutralizing antibodies; all of 175 human sera collected in Angola and the Caprivi Strip were negative.

Isolation of viruses from mosquitoes collected at Lumbo, Mozambique. I. Lumbo virus, a new virus isolated from Aedes (Skusea) pembaensis Theobald.

Isolation of Viruses from Mosquitoes collected at Lumbo, Mozambique. III. Isolation of Spondweni Virus from Aedes (Ochlerotatus) fryeri (Theobald) and/or Aedes (Aedimorphus) fowleri (D'Emmerez de Charmoy).

M. P. Weinbren

Papers

Survey for antibodies against arthropod-borne viruses in the sera of indigenous residents of Angola.

Isolation of viruses from mosquitoes collected at Lumbo, Mozambique. I. Lumbo virus, a new virus isolated from Aedes (Skusea) pembaensis Theobald.

Isolation of Viruses from Mosquitoes collected at Lumbo, Mozambique. III. Isolation of Spondweni Virus from Aedes (Ochlerotatus) fryeri (Theobald) and/or Aedes (Aedimorphus) fowleri (D'Emmerez de Charmoy).