Montgomery County officials are spraying more mosquito-killing chemicals on parks, backyards and other areas after the county's first human case of the West Nile virus was confirmed this week. UTMB’s Alan Barrett studies West Nile in conjunction with Harris County. He said he is studying strains of this year's virus to see if it has mutated to become more prone to cause disease, which many feared was a possibility after 2012. But he said last year's virus had not become meaningfully more virulent than in 2012. Results of this year's test probably won't be available for several weeks, but so far the evidence indicates increased virulence isn't likely. Barrett said little is understood about what causes the virus to show up and proliferate. He said the virus's effects often are different from year to year — this year in Harris County, many mosquitoes have been found carrying the virus but no human cases have been found.