The reappearance of chikungunya after 32 years and India's inability to prevent recurrent outbreaks of dengue year after year have raised the million dollar question: can India cope up with yellow fever (YF) if it ever strikes?
This is one of the questions likely to come up at the eighth international symposium on vector-borne diseases sponsored by the Indian Council of Medical Research (ICMR) at Madurai in Tamil Nadu that started on Friday.
After all, YF is also spread by the same aedes aegypti mosquito that transmits dengue and chikungunya. Moreover, India has a lot of monkeys that can act as a reservoir for the YF virus once introduced. And Africa, only a few hours by flight, is endemic for this virus and there is good chance of passengers with YF arriving in India.
"Despite these favourable factors, YF is absent in India and it is a mystery," said Kalyan Banerjee, former director of the National Institute of Virology in Pune. He, however, cautioned that YF would come "some day" if India failed to shore up its crumbling vector control programme.
According to Banerjee, recurring dengue outbreaks are due to unplanned urbanisation and crumbling mosquito control programme. Environmental management to prevent mosquito breeding is key to protecting the country from yellow fever - occurring naturally or introduced deliberately.
Sriram Prasad Tripathy, former chief of ICMR, echoed him. "No one has excluded the possibility of YF coming to India," said Tripathy from Pune. "An effective vaccine against YF is there, but the cost of vaccinating the entire population would be prohibitive."
Some public health experts believe that the absence of YF in India has to do with the genetic makeup of Indians or the Indian strain of aedes aegypti mosquitoes are incapable of transmitting the virus.
But virologist Pradeep Seth, formerly with the All India Institute of Medical Sciences (AIIMS) in New Delhi, does not think so. "We will be inviting trouble if we neglect mosquito control thinking that Indians are immune to YF," he said.
As early as in 1968, an expert committee report submitted to the UN warned that "importation of this disease is possible wherever a suitable environment and susceptible animal and mosquito hosts exist".
The report cited the example of how previously uninfected areas of Ethiopia were invaded by yellow fever and an epidemic resulted in 15,000 deaths in 1960.
The report had also warned "it might be extremely serious if the virus were introduced into Asia or the Pacific islands where the disease appears to have never occurred but where local species of mosquitoes are known to be able to transmit it".
India had been taking the warning very seriously, unwilling to take any risk of introducing the disease. In 1975, the government led by late Indira Gandhi terminated a US-funded project in Delhi that was experimenting with genetically modified aedes aegypti mosquitoes and was planning a large-scale release in Sonepat, Haryana.
"The situation in India is ideal for the spread of the disease and that is why we are so worried and want to see that no focus is established in any part of he country," former director general of health services JB Srivatsav told a parliamentary committee in 1975.
It is generally believed that the YF virus has not been introduced in India due to stringent and successful quarantine measures.
But as a new book on yellow fever (Prometheus books, Amherst, New York) by award winning investigative journalist James L Dickerson says, "There is the potential threat of YF as a biological warfare agent in the hands of terrorists".
Scientists know that YF virus can be incubated in chicken eggs, preserved for as long as five months in tissues of infected monkeys and delivered by aedes mosquitoes.
Aedes aegypti has a remarkable ability that not all mosquito species have: it can transmit viruses to its eggs. In other words, eggs of YF infected mosquitoes can be dried and put on a piece of paper in an envelope and mailed to any part of the country. The baby mosquitoes hatched from the eggs will carry the virus and spread it to people.
India's vulnerability to biological warfare attack by YF has been known since World War II. According to CG Pandit, former director general of ICMR, "the Indian government, early in 1940, received confidentially the information that in the event of war breaking out in the Far East, there is the possibility of Japan resorting to biological warfare with YF virus".