Diagnostic centres Medinova, Padma, Popular and Lab Aid, some private hospitals and the ICDDRB told The Daily Star they found malaria positive cases ranging from 5 to 50 percent of the number of tests for malaria they conducted in the last one year.

Experts studying blood samples of hundreds of patients infected with malaria parasites said many of the 'malaria positive cases' have either been misdiagnosed or do not have any symptoms, which could endanger their lives.

Investigations showed many doctors mistakenly diagnosed malaria as dengue, typhoid, and influenza and in some cases as jaundice because of their symptoms are similar to that of malaria --- high fever, headache and joint pains.

The deadly disease is caused by plasmodium parasite transmitted to humans through the bite of infected female anopheles mosquito, usually found in forests.

Dr Sheikh Md Abdur Razzak, chief of microbiology department at Sikder Women's Medical College in Dhaka, said, "I have evidence of thousands of blood slides which show presence of malaria parasite, amazingly most of the parasites are not readily detected under microscope."

Explaining this, he said, " The parasite in the red blood cells appears very light rose-colored, so it is not easily distinguishable. One has to carefully look for the light pink circle or often half circle-shaped parasite, but majority of malaria cases are mistaken for other diseases having almost identical symptoms."

"The problem is that since malaria is not commonly found in the city, when doctors suggest malaria test, technicians, often without appropriate knowledge, report a negative result," he pointed out.

Citing examples, the expert said, "A government employee aged over 50 had extreme pains in his limb joints for about a decade without a cure due to wrong diagnosis. Finally, we detected he had malaria."

Narrating his long suffering, the patient, Alamgir Noori, said he had given all up hopes for recovery. But now he is alright after taking drugs for malaria.

Another microbiologist, Prof Shamir Saha of Dhaka Shishu Hospital, also admitted detecting malaria parasites in many patients. "We found 15 malaria positive cases out of 72 last month and three out of 42 in June."

Dr Bena_Zir Ahmed, a microbiologist at the Institute of Epidemiology, Disease Control and Research (IEDCR), said, "Studies show the mosquito that transmits malaria exists in the city but I cannot comment on finding malaria positive cases since we do not conduct the relevant test at the IEDCR."

One expert said, "It is indeed mysterious that most of the patients tested positive for malaria parasite had no record of visiting areas like the Chittagong Hill Tracts forests, where mosquitoes that transmit malaria are found."

Entomologist (scientist who studies insects) Touhid Uddin Ahmed, who carried out a study last year to find if anopheles mosquitoes exist in the capital, said, "Finding mosquitoes that transmit malaria in a place like Dhaka city is quite an unusual affair. But we must remember epidemics of malaria in Araihazar and Raigonj

Experts mentioned malaria parasite enters the bloodstream and travels straight to the liver, where it replicates it replicates itself thousands of times while staying safe from any attack by the body's own defence mechanisms. Then they are released back into the bloodstream in a form that invade red blood cells. Once inside the red blood cell, they mature into a form that develops a tricky defence against the body's immune system.

In the red blood cell, the parasites feed on haemoglobin and multiply. After every 48 hours, the cell ruptures and new parasites invade more red blood cells and repeat the cycle.

Malaria kills about 3,000 children a day globally and the parasite that causes the disease is becoming harder to treat as it develops resistance to more and more drugs, a WHO study said.