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Pune-based National Institute of Virology (NIV) has come out with a study on strains of Zika virus (ZIKV) that led to an outbreak in Rajasthan in 2018, and in Gujarat and Tamil Nadu in 2016 and 2017 respectively.
In the first such study, NIV said that while the mutation which causes microcephaly (birth defects) in children born to Zika positive pregnant women was not found in Rajasthan currently, it cautioned that the possibility of the same can’t be ruled out too.
The Union Health and Family Welfare Ministry had earlier ruled out this possibility saying that the known mutations of the virus does not cause microcephaly and large scale outbreaks.
The NIV paper, to be published in ‘Infection, Genetics and Evolution’, an Elsevier journal in April 2019, said that mutation (S139N) leading to increased infectivity in humans causing microcephaly was not present, but “once ZIKV establishes in the Indian ecosystem, its chances of mutation (to other forms) cannot be neglected. Despite the absence of the proposed mutation on the transmission and microcephaly, the chances of not finding such a clinical condition cannot be guaranteed.”
Replying to an email sent by Down To Earth, NIV’s Pragya D Yadav, one of the authors of the paper, said, “The paper clearly states that the mutation linked to higher transmission was not seen in the current Rajasthan strain. According to the study by Logan et al published in 2016 the mutation rate for zika virus is 12-25 bases per year. The presence of such high mutation rate increases the possibility of different mutations taking place in ZIKV genome and the above (microcephaly mutation) is not an exception.”
The paper further clarifies that a study done in Thailand by Wongsurawat et al. in 2018 found that despite the absence of this particular mutation, cases of microcephaly were reported.
The paper also says that presence of the reverse mutation (N139S) can cause milder neurovirulence, that is, it can affect the nervous system. Yadav confirmed its presence in Indian ZIKV sequence retrieved from Rajasthan.
More than 130 cases of Zika were reported in the last outbreak in Rajasthan, three in Gujarat and one in Tamil Nadu. More than 160 cases of Zika were reported in Madhya Pradesh as well, as reported by Down To Earth earlier. However, the sequencing of Madhya Pradesh virus is still going on.
In case of Gujarat, the study found that it was a pre-outbreak strain. Experts Down To Earth spoke to said it was a significant fact that the strain was present before the outbreak took place.
Hence, if there was better surveillance, the outbreak could have been minimised or prevented. The paper said that the ZIKV strain of Gujarat clustered with an Asian lineage belonging to Malaysian isolate of 1966.
“The sequence of ZIKV from Gujarat clusters with Malaysian strain, but that doesn’t mean it is Malaysian. It can be one of the route via which the virus has travelled to India. However, further comments on the route of ZIKV transmission can only be made later,” said Yadav.
Insofar as the sequencing of Rajasthan and Tamil Nadu ZIKV was concerned, the virus, too, formed cluster with an ‘old strain ancestor’ of Asian lineage, tracing its origin to Indonesia.
The study, therefore, confirmed that in all the three cases — Rajasthan, Gujarat and Tamil Nadu — it was Asian lineage at work, and not African. Asked if the former is more harmful than the latter, Yadav said, “Experimental studies need to be performed in order to comment on the extent of harmfulness of the Asian as well as African strain.”
The paper says that they hypothesised the presence of two different strains of Asian lineage coexist in the nearby regions of India. "Rapid diversification of these strains to other Asian ZIKV strains cannot be denied," it says.
This clearly points to the need for better monitoring and measures for the virus in India because of possible diversifications.
The paper, nonetheless, also says that besides surveillance, public awareness and vector control needs to be prioritised in India as it warns the there is an abundance of Aedes agegypti mosquito which carries the ZIKV.
It warns that the probability of future outbreaks in the country cannot be ‘overlooked’. Incidentally, several other studies issued similar warnings as well.
A paper authored by NIV’s director D T Mourya, published in 2017, said that the environment in India is conducive for ZIKV because of preponderance of the particular kind of mosquitoes given the highly humid and optimal temperature which supports their survival over many days.
Another paper authored by Amir S Siraj and T Alex Perkins, published in the British Medical Journal in September 2017, says India accounts for the highest population in Asia exposed to ZIKV.
“Around 465.7 million people in India could become infected by the virus in the event of a outbreak under circumstances which led to the outbreak in the Americas. At the country level, we project that India has the potential for the largest number of ZIKV infections by more than six-fold that of any other country, mainly due its large population and relatively high suitability for ZIKV transmission,” states the paper.
The US-based Centre for Disease Control had issued a level 2 (out of 3 levels) alert in December 2018. In its advisory, it advised pregnant women not to visit areas exposed to Zika in India as ZIKV was endemic (regularly found).
It had further said, “Public health officials in India have reported an unusual increase in the number of confirmed Zika cases in Rajasthan and surrounding states. Zika continues to be a risk throughout India.”
The Indian Council of Medical Research (ICMR), under the aegis of which NIV functions, raised ‘strong objection’ to this advisory, according to media reports. It said that there were “particular issues with the use of the word ‘endemic’ as it claimed the cases were localized”.
The fact that the current NIV study issues ‘alarm for India to enhance surveillance in other states and monitor the mutation’ indicates that the country’s vulnerability towards ZIKV is just not in localised areas. A detailed questionnaire sent by Down To Earth to ICMR director general Balram Bhargava remains unanswered.