NEW DELHI: It's a virus that has already claimed over 100 lives but Indian and American scientists seem to have no clue about what it is. While the entire country is busy dealing with H1N1 influenza, a mystery virus is causing havoc in Uttar Pradesh.

Since January, the state has recorded 665 cases of Acute Encephalitis Syndrome (AES) -- a severe neurological condition associated with significant morbidity and mortality. Around 137 people have already succumbed to the viral infection.

Lab investigations have revealed that only 34 of the cases and four of the deaths were caused by Japanese Encephalitis -- a known scourge in the state. However, scientists have no clue what caused the rest of the 133 encephalitis deaths and 631 cases.

According to officials, this mystery virus has been causing brain fever and killing people in UP since last year but no breakthrough has been made regarding the virus's identity and nature.

Experts from Centre for Disease Control and Prevention (CDC) Atlanta had picked up 150 samples from UP to help identify the virus. But they too have failed to make any headway.

Speaking to TOI, director of National Institute of Virology (Gorakhpur) Dr Milind Gore said, "We are working on identifying the virus and its origin. At present we don't know what's causing these cases of AES and deaths. We suspect it could be a type of enterovirus."

Dr Gore added, "It is clear that JE alone isn't causing AES. We recently checked 450 stool swabs and found that 68% of them were positive for enteroviruses. However, we don't know exactly which one from this family of viruses is the dominant one behind the deaths. Even CDC experts haven't figured it out."

Officials said the search for the "real culprit" behind the AES cases had taken a beating in the last few months with scientists in NIV Pune drowned with H1N1 samples.

"H1N1 has proved to be a setback for the work on encephalitis in UP. Improved methods of diagnosis and new primers were to be tested against the unknown virus. But everybody at NIV Pune is busy with H1N1," an official said.

AES is characterised by inflammation of the brain, seizures and convulsions among children.

International NGO PATH's senior programme officer for JE Dr Pritu Dhalaria told TOI, "Usually, around four to five enteroviruses work in tandem, in clusters in particular areas. It is therefore difficult to identify exactly which virus is the deadly one. By this time last year, UP had recorded 435 AES cases of which only 15 were caused by JE. This year, the cases have increased, and we don't know which virus is behind it."

Non-polio enteroviruses are very common across the globe. They are second only to the common cold viruses, the rhinoviruses, as the most common viral infectious agents in humans.

Although infection often has no symptoms and goes unnoticed, these viruses are also associated with occasional outbreaks in which a larger-than-usual number of patients develop clinical disease, sometimes with fatal consequences.

Transmission of enterovirus infections is increased by poor hygiene and overcrowded living conditions. Measures that can be taken to avoid getting infected with enteroviruses include frequent hand washing.

Enteroviruses can be found in the respiratory secretions and stool of an infected person. Other persons may become infected by direct contact with secretions from an infected person or by contact with contaminated surfaces or objects.

According to some experts, UP has seen a nearly 40% increase in AES cases this year, majority of them not belonging to the enterovirus category.

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