Zika virus

January 2016

Key Facts

  • Zika virus is transmitted to humans by infected mosquitoes. It causes mild fever and rash. Other symptoms include muscle pain, joint pain, headache, pain behind the eyes and conjunctivitis.
  • Zika virus disease is usually mild, with symptoms lasting only a few days.
  • The disease has similar clinical signs to dengue, and may be misdiagnosed in areas where dengue is common.
  • There is no cure for Zika virus disease. Treatment is focused on relieving the symptoms.
  • Prevention and control relies on reducing the breeding of Aedes mosquitoes and minimizing contact between mosquito vectors and people by using barriers (such as repellents, insect screens), reducing water-filled habitats supporting mosquito larvae in and close to dwellings, and reducing the adult mosquito populations around at-risk communities.

Background

Zika virus is a mosquito-borne flavivirus closely related to dengue virus. It was first isolated from a rhesus monkey in Zika forest, Uganda in 1947, in mosquitoes (Aedes africanus) in the same forest in 1948 and in humans in Nigeria in 1954. Zika virus is endemic in parts of Africa and Asia and was first identified in the South Pacific after an outbreak on Yap Island in the Federated States of Micronesia in 2007. (1)

Transmission

Zika virus is primarily transmitted to humans through bites from Aedes mosquitos, which often live around buildings in urban areas and are usually active during daylight hours (peak biting activity occurs in early mornings and late afternoons).

Some evidence suggests Zika virus can also be transmitted to humans through blood transfusion, perinatal transmission and sexual transmission. However, these modes are very rare.

The incubation period is typically between 2 and 7 days.

Signs and symptoms

Zika virus infection is characterized by low grade fever (less than 38.5°C) frequently accompanied by a maculopapular rash. Other common symptoms include muscle pain, joint pain with possible swelling (notably of the small joints of the hands and feet), headache, pain behind the eyes and conjunctivitis. As symptoms are often mild, infection may go unrecognized or be misdiagnosed as dengue.

A high rate of asymptomatic infection with Zika virus is expected, similar to other flaviviruses, such as dengue virus and West Nile virus. Most people fully recover without severe complications, and hospitalization rates are low. To date, there have been no reported deaths associated with Zika virus.

Diagnosis

Several methods can be used for diagnosis, such as viral nucleic acid detection, virus isolation and serological testing. Nucleic acid detection by reverse transcriptase-polymerase chain reaction targeting the non-structural protein 5 genomic region is the primary means of diagnosis, while virus isolation is largely for research purposes. Saliva or urine samples collected during the first 3 to 5 days after symptom onset, or serum collected in the first 1 to 3 days, are suitable for detection of Zika virus by these methods. Serological tests, including immunofluorescence assays and enzyme-linked immunosorbent assays may indicate the presence of anti-Zika virus IgM and IgG antibodies. Caution should be taken with serological results as IgM cross reactivity with other flaviviruses has been reported in both primary infected patients and those with a probable history of prior flavivirus infection.

Treatment

There is no commercial vaccine or specific antiviral drug treatment for Zika virus infection. Treatment is directed primarily at relieving symptoms using anti-pyretics and analgesics.

Prevention and control

The proximity of mosquito vector breeding sites to human habitation is a significant risk factor for Zika virus infection. Prevention and control relies on reducing the breeding of mosquitoes through source reduction (removal and modification of breeding sites) and reducing contact between mosquitoes and people. This can be achieved by reducing the number of natural and artificial water-filled habitats that support mosquito larvae, reducing the adult mosquito populations around at-risk communities and by using barriers such as repellants, insect screens, closed doors and windows, and long clothing. Since the Aedes mosquitoes are day-biting mosquitoes, it is recommended that those who sleep during the daytime, particularly young children, the sick or elderly, should use insecticide-treated mosquito nets to provide protection. Mosquito coils or other insecticide vaporizers may also reduce the likelihood of being bitten.

During outbreaks, space spraying of insecticides may be carried out periodically to kill flying mosquitoes. Suitable insecticides (recommended by the WHO Pesticide Evaluation Scheme) may also be used as larvicides to treat relatively large water containers.

Basic precautions for protection from mosquito bites should be taken by people traveling to high risk areas. These include use of repellents, wearing light colored, long sleeved shirts and pants and ensuring rooms are fitted with screens to prevent mosquitoes from entering.

Disease outbreaks

The first documented outbreak of Zika virus in the South Pacific occurred on Yap Island in the Federated States of Micronesia in 2007. (1) This outbreak affected 180 (confirmed, probable and suspected) people and was characterized by rash, conjunctivitis and joint pain.

In October 2013, French Polynesia reported its first outbreak, which was estimated to affect around 11% of the population. (2) This particular outbreak spread to other Pacific Islands including New Caledonia, Cook Islands, and Easter Island. As most cases of Zika virus infection present with mild illness similar to other circulating arbovirus infections, and there was limited laboratory capacity during this outbreak for the detection of Zika virus, it is likely that many cases of infection were not identified.

More about disease vectors

Both Ae. aegypti and Ae. albopictus have been implicated in large outbreaks of Zika virus. Ae. aegypti is confined to tropical and sub-tropical regions, while Ae. albopictus can be found in tropical, sub-tropical and temperate regions. Ae. albopictus has spread from Asia and become established in areas of the South Pacific, Africa, Europe and the Americas in recent decades. In the South Pacific, Ae. hensilli was implicated in the spread of Zika virus on Yap Island in 2007, (1,3) while Ae. polynesiensis was suspected to spread Zika virus in French Polynesia in 2013. (2) Neither of these endemic species had been recognized as a Zika virus vector before, indicating that as this emerging disease spreads to previously unaffected countries, the potential exists for other endemic Aedes species to play a role in transmission.

  • Ae. aegypti is closely associated with human environments and can breed in indoor (flower vases, concrete water tanks in bathrooms), and artificial outdoor (vehicle tyres, water storage vessels, discarded containers) environments.
  • Ae. albopictus thrives in a wider range of water-filled breeding sites than Ae. aegypti, including coconut husks, cocoa pods, bamboo stumps, tree holes and rock pools, in addition to artificial containers such as vehicle tyres and plant pot saucers. This diversity of habitats explains the abundance of Ae. albopictus in rural as well as peri-urban areas and shady city parks.
  • Ae. hensilli breeds in coconut shells, tins, plastic containers, vehicle tyres, tree holes, canoes and metal drums. (4)
  • Ae. polynesiensis breeds in tree holes, coconut shells and crab holes.

WHO response

WHO responds to Zika virus infection by:

  • Providing technical support and guidance to countries for the effective management of cases and outbreaks;
  • Supporting countries to improve their surveillance systems;
  • Providing training on clinical management, diagnosis and vector control including through a number of WHO Collaborating Centres;
  • Publishing guidelines for vector control; and
  • Encouraging countries to develop and maintain the capacity to detect and confirm cases, manage patients, and implement social communication strategies to reduce the presence of the mosquito vectors.

References

  • Duffy MR, Chen, Hancock WT, Powers AM, Kool JL, Lanciotti RS, Pretrick M, Marfel M, Holzbauer S, Dubray C, Guillaumot L, Griggs A, Bel M, Lambert AJ, Laven J, Kosoy O, Panella A, Biggerstaff BJ, Fischer M, Hayes EB. Zika Virus Outbreak on Yap Island, Federated States of Micronesia. 2009 June. The New England Journal of Medicine. 360:2536-43
  • Musso D, Nilles EJ, Cao-Lormeau VM. Rapid spread of emerging Zika virus in the Pacific area. 2014. Clinical Microbiology and Infection. 20(10):O595-6
  • Roth A, Mericer A, Lepers C, Hoy D, Duituturanga S, Benyon E, Guillaumot L, Souares Y. Concurrent outbreaks of dengue, chikungunya and Zika virus infections - an unprecedented epidemic wave of mosquito-borne viruses in the Pacific, 2012-2014. 2014. Eurosurveillance. 19(41)
  • Shinichi N. Mosquito Fauna in the Federated States of Micronesia: A Discussion of the Vector Species of the Dengue Virus. 2014. South Pacific Studies. 34(2):117-27
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