Individuals’ immunity to other strains is most likely the reason for emergence of this rare serotype, believe experts
The dengue virus serotype DENV-4 has resurfaced in Maharashtra’s Pune city after a period of three decades. Scientists fear that this virus may cause a rise in cases of dengue in the city and the state as even individuals who have been infected with serotypes DENV1, 2, or 3 can be re-infected with this serotype. This is known as secondary infection.
Those already been infected with one of the serotypes in the past are at higher risk than those who have never had dengue before. The serotype is known to cause mild symptoms in primary infection.
Cecilia Dayaraj, dengue team leader at National Institute of Virology (NIV), Pune, tells Down To Earth that out of 93 samples tested by the institute so far, 11 have tested positive for Dengue-4 while 15 others have tested positive for other serotypes.
Why has the strain emerged now?
DENV-4 has not been detected in Maharashtra for the past 30 years. However, since 2009, NIV has identified two to three cases each year. This year, there is a dramatic increase in the number of identified cases. One case has also been identified in Thane, says Dayaraj.
Talking about the reasons for its re-emergence at this time, Dayaraj says, “We have seen that DENV-1, 2 and 3 have been circulating in Pune for the past nine years. The fact that many individuals are immune to the three serotypes most likely has something to do with the emergence of this rare serotype.”
She said that more research is needed to understand the reasons behind the phenomenon. “Our virus genome related analysis shows that the DENV-4 viruses detected in recent years are different from the viruses that were circulating in India in the 1960s. However, we need to sequence many more viruses if we want to understand if there is any genetic basis to the emergence of DENV-4 in recent years,” says the virology expert.
She confirms that higher rates of infection are possible due to the appearance of this serotype. “Considering only a few cases of DENV-4 have been detected in the past, it follows that a large population in the city is not immune to DENV-4 and therefore can be infected by it.” The doctor adds, “However, secondary infections may be caused by other strains too.”
According to her, the intensity of infection varies from one person to the next. The doctor explains that according to most reports, DENV-2 is dominantly associated with severe secondary infections. DENV-4 primary infections are known to be mild but secondary infections have been associated with severe disease. A study in Thailand had reported that despite the low prevalence of DENV-4, it was responsible for 10 per cent of Dengue Hemorrhagic Fever cases in children.
Several parts of Maharashtra and neighbouring states are seeing high incidence of the fever this year. In Maharashtra, the eastern Vidarbha districts of Nagpur, Wardha, Bhandara, Gondia, Gadchiroli and Chandrapur have till date reported 37 dengue outbreaks, with 323 cases testing positive. This is much higher than last year’s figure of 268. Milind Ganvir, assistant director malaria at Nagpur circle, says that the high incidence can be attributed to monsoon failure and the region seeing alternate periods of rain and humidity that encourages mosquito breeding.
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