The year began with the US Centers for Disease Control (CDC) and the government of Canada issuing a level-two warning for those traveling to Caribbean countries, after a resident of Puerto Rico tested positive for Zika virus on December 31. While warning travellers to protect themselves from the vector, mosquitoes, the advisory specifically advised pregnant women and those trying to conceive to avoid travelling to these regions till the transmission of the virus is contained. On Monday, the World Health Organisation (WHO) said the virus is expected to spread to all countries in the Americas, except for Canada and Chile.
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Given this rapid spread of the virus, does India have reason to worry? Zika virus has never been isolated in India. The only time any trace of the virus was recorded in India was in 1952-53. A study titled ‘Neutralising antibodies against certain viruses in the sera of the residents of India’, published in a journal called The Journal of Immunology, recorded among other viruses, antibodies to the Zika virus. The study, which went to 38 localities in six states in India, tested 15 vector borne viruses. “Significant numbers of the sera neutralised Zika,” noted the study authored by “KC Smithburn of the Rockefeller Foundation in New York, JA Karr, director, virus research centre in Poona and PB Gatne, Medical offer, Bombay State India.”
Dr T Mourya, director, National Institute of Virology, Pune, said in an email response, “There have been no outbreaks in India but passive immunity has been traced in Indian samples in the past. The last Indian study on this was conducted between 1952-53. So far, other than this, there are no reports on this virus in India.”
Passive immunity refers to the process of acquiring immunity against a foreign body-antigen without actual exposure to the virus. To actively acquire immunity, the body’s immune system has to be affected by the virus and fight it off. Passive immunity can be acquired naturally, from the mother during fetal development or from breast milk, or artificially through vaccines. For Indians who acquired immunity against Zika, despite the virus never having been isolated, the process of acquiring passive immunity is likely to have been a more complicated process.
The Zika virus has been widely described in the scientific community as a “cousin” of India’s endemic dengue virus. Dengue has by far been accepted as the more virulent, stronger cousin of zika. Dengue, zika and chikungunya are spread through a common vector, the Aedes aegypti mosquito. To put it simply, exposure to similar viruses is likely to have resulted in the formation of antibodies which are similar to that of the Zika virus.
Dr Lalit Dar, professor of microbiology in AIIMS, describes this as “cross reactive immunity” to the virus. “When the proteins of the virus are similar, so are the antibodies that develop against them. Since Indians are exposed to all four strains of dengue circulating in the atmosphere since birth, the body develops antibodies against them. Since Zika virus is similar in composition, these antibodies may be cross reactive with the Zika virus,” Dar said.
The US-based CDC also identifies India as one of the countries where antibodies to Zika virus have been identified only in healthy people. In other words, though people in India have never actually fallen sick from the Zika virus, studies have shown some people have developed immunity to the virus. “These studies cannot determine where the people were infected or if they were infected with Zika virus because the antibodies may have resulted from infections with other closely related viruses, such as dengue,” the CDC has stated in its report on the Zika virus published in the first week of January.
Interestingly, Zika virus has generally been associated with relatively mild symptoms that are known to last 2-7 days, according to WHO. The virus, which got its name in 1947 from the Zika forests of Uganda where it was first reported, has been confined mainly to Africa, according to the WHO, with “small and sporadic outbreaks in Asia”. Previous outbreaks of the virus have shown that when Zika affects adults, it causes symptoms like any other virus – fever, body ache, rashes, joint pain, reddening of eyes. The association with microcephaly or abnormally small brain development in newborns, which is being attributed to the present Zika outbreak in Brazil, also marks a new chapter in the understanding of the Zika virus.
Scientists reckon this is probably the first time the virus is presenting neurological symptoms. The WHO goes a step further and says there is limited evidence on two fronts – the association of the virus with neurological symptoms and its transmission from mother to child during pregnancy or birth. “Neurological and autoimmune complications are infrequent but have been described in the outbreaks in Polynesia and, more recently, in Brazil,” the WHO notes in its latest report.
As Dr Dar of AIIMS explains, “This is the first time the Zika virus has been linked to microcephaly when it affects pregnant women. Usually, it has been very manageable with symptomatic treatment. The fact that microcephaly in Brazil showed a 20 per cent jump during a Zika virus outbreak warranted investigation.”
The CDC also said in their advisory that “knowledge of the link between Zika and these outcomes is evolving”. The CDC has, however, said that “until more is known, CDC recommends special precautions” for pregnant women and women who are planning to conceive.
According to the WHO, while viruses related to Zika, such as dengue and chikungunya, have been known to spread from mother to newborn, so far, Zika was known to nearly always need the vector mosquito for transmission. “As the virus spreads in the Americas, giving us more experience with its symptoms and complications, it will be possible to characterise the disease better,” the WHO said in its report.
Recent reports have also warned that the virus may be sexually transmitted, after strains were isolated in semen samples. The WHO has said this potentially means it may even spread through blood transfusion.