KOLKATA: An unknown fever, which has symptoms similar to dengue, has laid thousands low just when the civic body is walking the extra mile to keep vector-borne diseases in check this monsoon. At least two south Kolkata private hospitals have received a dozen patients each in July and are still treating several. There could be several undetected viruses or bacteria behind the spate of fever that have become difficult to detect, experts believe.“This happens every year but this time the number of affected is unusually high. The symptoms of the unknown fever have been high temperature, joint pain and redness in the eyes. It has been taking about a week to recover. Thankfully, dengue cases have been few so far or else the situation may have turned quite unmanageable,” said Belle Vue Clinic consultant Samarjit Naskar. The hospital had received around 10 unknown fever patients in July.Even though undetected viruses rarely turn fatal, children, elderly and those with low immunity were at risk, pointed out AMRI Hospital consultant Debashish Saha. “Since we are not aware of the virus, patients need to be monitored closely if the fever and the symptoms persist. Generally, these viruses lose their sting in a few days but are extremely contagious and end up affecting thousands. In some cases, the fever has led to more serious ailments like septic myocarditis , which is triggered by infection in the blood stream. It leads to a heart failure. Unknown viruses may also cause hepatic encephalopathy or loss of brain function,” said Saha.If tests are done at the right time, every fever can be traced to a virus, according to consultant Arindam Biswas. “To be termed a fever of unknown origin, a patient needs to suffer from high temperature for at least three days without a cause being detected. In most cases of unknown fever, patients either don’t get the necessary tests done or don’t care to consult a doctor,” said Biswas.But there are instances where tests fail to identify the trigger of a fever, pointed out Saha. “Over the last one month, I have come across several cases of fever which had not been triggered by typhoid, dengue or malaria . These are loosely termed unknown fever. A small number of these patients suffer from sepsis or infection. Again, the origin of a mere 40% of these infections are identified,” said Saha.Children have been among the worst sufferers due to their lower immunity. Scores have been admitted to hospitals with dengue-like symptoms — high fever, joint pain and diarrhea . “It has been spreading fast through schools. Many have had high temperature for 4-5 days, leading to convulsion or shock. Even though none of the cases I came across has been fatal, some have been very critical,” said paediatrician Santanu Ray.