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14 Apr 2017

#Zika virus: an #epidemiological #update (@WHO WER, edited)

 

Title: #Zika virus: an #epidemiological #update.

Subject: Zika Virus Infection and related complication, roundup, WHO.

Source: World Health Organization (WHO), Weekly Epidemiological Record (WER), full PDF document: (LINK).

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Weekly epidemiological record / Relevé épidémiologique hebdomadaire  / 14 APRIL 2017, 92th YEAR / 14 AVRIL 2017, 92e ANNÉE No 15, 2017, 92, 181–192 - http://www.who.int/wer

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Zika virus: an epidemiological update

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E. Garcia{a}, S. Yactayo{a}, V. Millot{a}, K. Nishino{a}, S. Briand{b}

 

The 2015 outbreak of Zika fever in the Region of the Americas has demonstrated how a relatively obscure and mild mosquito-borne disease can become a global health emergency. In late 2015, WHO received reports from the Ministry of Health of Brazil of an unusual cluster of microcephaly and other neurological disorders potentially associated with Zika virus infections, along with retrospective reports from French Polynesia reporting similar findings that had occurred in 2013 and 2014. As a result, on 1 February 2016, WHO declared a Public Health Emergency of International Concern (PHEIC) following the advice of the Emergency Committee under the International Health Regulations (IHR) (Table 1).

Since its emergence in the Americas, Zika virus has been re-introduced to Africa, the region where it was first discovered, and to several Asian and Pacific region countries.

From 1 January 2015 to 1 February 2017, 70 countries have reported Zika virus outbreaks or cases; of these, 59 reported Zika virus infections for the first time and 11 countries with a prior history of Zika virus transmission reported cases or outbreaks indicating possible endemicity.

The actual number of infections by country is unknown due to patients not seeking medical care as symptoms are usually mild and many infections can be asymptomatic.

Several countries, including Brazil and Colombia, reported large outbreaks, most likely due to the low level of pre-existing immunity to the virus and high population densities of competent vectors.

 

Background

Zika virus is an arthropod-borne, single-stranded, positive-sense RNA virus, belonging to the Flaviviridae family, genus Flavivirus.

Other flaviviruses related to Zika virus include dengue virus, Japanese encephalitis virus, West Nile virus and yellow fever virus.

Zika virus is transmitted primarily through the bite of an infected mosquito of the genus Aedes. The virus has been isolated from a variety of Aedes species including A. aegypti, A. polynesiensis, A. albopictus, A. hensilli, A. africans, A. apicoargenteus, A. luteocephalus, A. vitatus, and A. furcifer.

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Table 1 Emergency Committee meetings under the International Health Regulations (IHR) and conclusions on Zika virus disease and its complications

[Date of IHR EC meeting - Conclusions of the Emergency Committee]

{a} Public Health Event of International Concern.

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A. aegypti is the species of mosquito currently most associated with transmission but A. albopictus also may be implicated. Both species are typically found in urban settings of subtropical and tropical regions and are known to transmit dengue and chikungunya viruses.

Zika virus was first isolated in 1947 from a nonhuman primate that was part of a yellow fever sentinel surveillance study in the Zika Forest of Uganda, hence the origin of its name. For half a century, Zika virus was circulating in Africa and Asia and infections in humans were sporadic and caused self-limiting, mild, nonspecific illness.

 

Non-vector borne transmission

In addition to mosquito-borne transmission, there are other routes of transmission, for example person-to-person (mostly via sexual and transfusion transmission), and laboratory contamination.

There are increasing numbers of confirmed cases of Zika virus infection resulting from non-mosquito borne transmission, especially sexual transmission.

Thirteen countries (Argentina, Canada, Chile, Peru and the United States of America; France, Germany, Italy, Netherlands, Portugal, Spain, the United Kingdom of Great Britain and Northern Ireland and New Zealand) have reported person-to-person transmission.

Mostly sexual transmission was reported, with the exception of one case where the mode of transmission was unknown but sexual and vector-borne were excluded.

Given that Zika virus has been detected in semen but not in blood when collected at the same time, viral replication may occur in the genital tract. The evidence of sexual transmission suggests that another mode of transmission may have contributed to the speed in which the virus has spread.

 

Clinical manifestations

The clinical presentation of Zika virus disease is typically self-limiting, nonspecific and can be easily confused with other diseases especially dengue and chikungunya.

However, clinical severity has been reported causing injury to the central nervous system in fetuses and adults. In adults, the clinical severity has been associated with Guillain-Barré syndrome (GBS), neuropathy and myelitis.

In fetuses, Zika virus infection during pregnancy can cause microcephaly and other severe brain anomalies.

 

Geographic dispersion 2007–2016

Concerns were raised only in 2007 about the implications of Zika virus to public health. A large outbreak occurred for the first time in the island of Yap of the Federated State of Micronesia which infected threequarters of the island’s population.

This marked the epidemic potential of the virus and the emergence of Zika infection in the Pacific Islands – beyond its previously known geographic range – and signalled its potential to spread to other Pacific Islands.

Later in 2013 and in 2014, French Polynesia experienced concurrent outbreaks of Zika and dengue infections. In addition, an unexpectedly high number of cases of GBS, temporally associated with these concurrent outbreaks, were reported.

Moreover, during this time, further countries in the region reported outbreaks, including Cook Islands, Easter Island (Chile), New Caledonia and Vanuatu.

The emergence of Zika virus in the Region of the Americas began in the north-eastern region of Brazil in 2015. Several state health authorities reported, retrospectively, an acute exanthematic disease that started in February 2015; by May 2015 the national reference laboratory for arboviruses had confirmed Zika virus.

In October 2015, the Ministry of Health of Colombia reported an outbreak of Zika infection in the state of Bolivar which then spread to other states.

In late 2015 and in 2016, autochthonous Zika virus circulation was reported in a further 46 countries, showing a dramatic geographic spread in Latin American and Caribbean countries.

Until 2015, only sporadic Zika virus infections were reported in the African Region. However, the Ministry  of Health of Cabo Verde reported an outbreak in November 2015.

Of note is that Cabo Verde has a high infestation of A. aegypti and that a large outbreak (>17 000 cases) of dengue fever occurred on the island in 2009.

Due to enhanced surveillance resulting from the emergence of Zika virus in the Americas, several countries in Asia and the Pacific region (American Samoa, Fiji, Indonesia, Malaysia, Maldives Marshall Islands, Micronesia, New Caledonia, Palau, Philippines, Samoa, Singapore, Thailand, Tonga, Viet Nam) reported outbreaks and cases in 2015 and 2016.

It is noteworthy to mention that dengue viruses are endemic in their countries as well.

With regard to Zika virus among travellers returning from endemic or epidemic areas of Zika virus disease, many countries of the Americas, Asia, Europe and the Pacific region have reported imported cases as early as 2013. These importations increase the risk of the virus spreading to areas where competent vectors may be present, thus potentially resulting in local humanmosquito-human transmission.

 

Conclusion

The emergence and re-emergence of dengue, chikungunya, yellow fever, Zika and other arboviruses have become global public health concerns.

Drivers for their emergence or re-emergence include human population growth, urbanization, changes in climatic conditions, globalization and ineffective vector control measures.

New clinical patterns and different modes of transmission for Zika virus continue to be studied, as well as genetic changes that may explain its demonstrated epidemic potential and virulence.

The epidemiological pattern of Zika virus has changed over time, resembling dengue and chikungunya – humans being the amplifying host in urban areas; the same mosquito vectors (A. aegypti and A. albopictus) are implicated which can result in large-scale epidemics.

Because Zika virus infection may have produced serious neurological consequences to newborns and adults, the virus has become a very serious global public health problem.

Although the status of a PHEIC has been lifted in November 2016, WHO recognizes the continued potential public health impact. A global arboviral strategy is needed to address the main challenges for prevention and control through:

  • Strengthening of existing vector borne disease and laboratory surveillance systems according to the country or region specific ecological and epidemiological conditions.
  • Stimulating and enhancing research inlaboratory diagnostics, vector control measures, environmental determinants, vaccines, and immunology;
  • Forming new global alliances to combine efforts and resources efficiently and promptly; and
  • Combining interventions from multiple diseases for effectiveness in disease reduction.

 

Author affiliations

{a} High Threat Pathogens Department, Infectious Hazard Management Cluster, World Health Organization, Geneva, Switzerland; {b} Infectious Hazard Management Cluster, World Health Organization, Geneva, Switzerland (Corresponding author: Erika Garcia, garciaer@who.int)

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Keywords: WHO; Updates; Worldwide; Zika Virus.

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