* MERS virus first emerged in humans two years ago * Saudi Arabia has been criticised for slow response * Chief scientist says 16 of 25 studies **w underway * Case-control study should answer key questions By Kate Kelland, Health and Science Correspondent LONDON, June 30 (Reuters) - Saudi Arabia says it has recruited Patients for a crucial study on the source of the deadly MERS virus, ack**wledging it is late but pledging more work on the epidemic after international criticism of its slow response. Scientists and global public health experts have faulted Saudi Arabia's response for allowing the spread of the Middle East Respiratory Syndrome (MERS) virus, which has **w killed nearly 300 people inside the kingdom. The kingdom's chief scientist, Tariq Madani, said the study was **w under way, having so far enrolled the first 10 "cases" - people who had the disease and either died or recovered - alongside 40 "controls" to compare them with. CRITICS The World Health Organisation (WHO) and other critics say the Saudi failure to properly investigate the causes of the disease, including the absence of a case-control study, contributed to the virus taking hold there and spreading via travellers to some 20 countries around the world.