Zika Virus Explained

In Brazil, there were around 4,000 cases of babies born with microcephaly from October 2015 to January 2016. Before there were only 150 such cases per year. The suspected reason for the increase is outbreak of Zika virus. The government authorities in Colombia, Ecuador, El Salvador and Jamaica have suggested women to delay becoming pregnant. The pregnant women are also urged not to travel to about 24 countries, mostly in Caribbean and Latin America, where the outbreak is increasing. The World Health Organisation (WHO) has declared the outbreak of Zika virus as an international public health emergency.

What is the Zika virus?

Zika virus is not a new virus. The Zika virus was first discovered in the Zika forest in Uganda in 1947 in a rhesus monkey. And again in 1948, it was found in the mosquito Aedesafricanus, which is the forest relative of Aedesaegypti. The virus can be spread by both Aedesaegypti and Aedesalbopictus mosquitos. There are few cases suggesting that the virus can be transmitted through sex. 

The virus has a lot in common with dengue and chikungunya. The three are originated from same areas – West and Central Africa, and Southeast Asia and now they have expanded their range to much of the tropics and sub-tropics areas. The three are spread through same species of mosquitoes.

The Zika virus, although, was first discovered almost 70 years ago, its outbreak started only in 2007. Initially the outbreak occurred in Yap Island of Micronesia and six years later, it appeared in French Polynesia. Its outbreak occurred in Brazil in May 2015. By now, millions of people in tropical regions of the Americas might have been infected. Presently, the virus is known to be present in countries across Africa, the Americas, Asia and the Pacific.

How the virus outbreak suddenly started in Latin America?

The strain of the virus in Brazil was similar to one that had been circulating in the Pacific region. The virus could had been introduced in Brazil after the 2014 FIFA World Cup or following an international canoe event held in Rio de Janeiro in August of 2014, where competitors form various Pacific islands were participated. The other possible route for introduction could be from Chile, since there was a case of Zika disease in a returning traveller from Easter Island.

There are certain conditions that have favoured the outbreak of virus in Brazil. Firstly, continued deforestation followed by agriculture and growth of low-lying vegetation has provided a suitable condition for growth of mosquitos. Urbanisation and increasing poverty created favourable conditions for development of multiple breeding sites of dengue spreading mosquitos. Climate change with raising temperatures and/or humidity provided thriving grounds for mosquitos.

Secondly, in the past few decades, Aedesaegypti and Aedesalbopictus have spread to new geographical regions. Factors such as increased air travel and transportation, urbanisation, climate change, discontinued control efforts, etc. have led to spread of mosquitos to new areas and coming back to areas where they had previously been eradicated. For example, in Latin America, Aedesaegypti mosquito eradication campaigns were conducted successfully in the 1950s and 1960s. Following the success, the control efforts were gradually discontinued. By 2000s, the mosquito had made a full comeback.

Thirdly, availability of susceptible hosts is also a critical factor. Since the Zika virus is new to the Americas, there is a large population who are never infected with the virus before i.e. they haven’t developed antibodies to the virus. The large population easily became hosts for the virus. In large countries such as Brazil, the virus can continue circulating without running out of susceptible hosts for a long time.

All the above factors have formed a favourable environment for outbreak of Zika virus.

How does a mosquito transmit Zika virus?

The female mosquitoes pick up the virus from blood when they bite people. The virus then travels from their gut through their circulatory system to their salivary glands. The virus is injected to others when the female mosquito bites them. The mosquitoes bite during day time only.

What are the symptoms?

After its outbreak in Yap Island, majority of people were not aware that they were infected because they had no symptoms. Only few became ill with fever, rashes, red eyes, joint pains, muscle pain etc. No death cases had been reported. After the Polynesian outbreak, it was observed that Zika was associated with Guillain-Barre syndrome (GBS), a life-threatening neurological paralysing condition. In Brazil, initially, the virus had been detected in patients with fever. Later few cases of Guillain-Barre syndrome were reported. By late 2015, several cases of microcephaly were reported.

What is microcephaly?

Microcephaly is a condition in which an infant’s head size is much smaller than that of others of the same age and sex.  In around 15% of cases, the small head has no effect on the infant. But in other cases, the infant’s brain may not have developed properly during pregnancy or may have stopped growing during early years of infant’s growth. These infants face the developmental delays, intellectual deficits or hearing loss. The common cause for such defect is genetic abnormalities. There is no treatment for an unusually small head.

How the Zika virus might cause brain damage in infants?

The possibility of infants born with microcephaly was emerged only in October, 2015. However, there may be other factors, such as infection with other viruses. Presently, the link between microcephaly and infection of Zika virus is not yet confirmed. But in many cases the virus has been found in brain tissues. How exactly Zika virus affect the brain is unclear. Studies from 1970s found that the virus could replicate in neurons of young mice, causing neurological disorder.

Is there a test and treatment?

Earlier, Zika was not considered as a major threat because its symptoms are relatively mild. Only one in five infected with the virus develop symptoms include fever, rash, joint pains and red eyes. The infected usually do not have to be hospitalized. At present no widely available test for Zika infection. To detect the virus, a blood sample from the first week in the infection must be sent to an advanced laboratory. Since the symptoms are mild, it usually requires only rest, nourishment and supportive care.

New-borns should be tested for infection if the mother visited any country with outbreak of the virus or if the mother herself is tested positively to the virus. At present there are no vaccines against the Zika virus. Creating a new vaccine will take years of time and it also involves spending huge money.

The only way to reduce the possibility of infecting with the virus is to avoid travelling to those countries of virus outbreak. The pregnant women are advised to avoid or minimize mosquito bites by staying in screened or air conditioned rooms. Use of mosquito nets, wearing insect repellent, wearing long pants, long sleeves, shoes, and hats helps in preventing mosquito bites.

Why the WHO has raised alarm over Zika outbreak?

The WHO has raised alarm over outbreak of Zika virus mainly for four reasons:

1. The possible association of the virus infection with birth malformations and neurological syndromes.
2. The potential for further international spread given the wide geographical distribution of the mosquito vector.
3. The lack of population immunity in newly affected areas.
4. Absence of vaccines, specific treatments, and rapid diagnostic tests.

How vulnerable is India?

India is highly vulnerable to dengue and chikungunya, diseases which are carried by a mosquito that also carries the Zika virus. India with 1.3 billion people, where dengue and chikungunya are common, it is very difficult to find the prevalence of the Zika virus. The challenge lies in distinguishing Zika cases from dengue/chikungunya cases.

The Indian government has issued guidelines on the disease and asked the NCDC and the National Institute of Virology, Pune to take lead in studying the virus and finding required diagnosis. The union health ministry has asked the states to control the growth of Aedes mosquito, which is responsible for spreading deadly dengue and also Zika virus. The pregnant women are advised to either defer or cancel their travel to the affected countries. Travellers are asked to report to customs if they are returning from affected countries and suffering from febrile illness. Rapid Response Teams (RRTs) have been activated at central and state surveillance units with each team comprising an epidemiologist or public health specialist, a microbiologist and a medical specialist.