The USDA’s surveillance of wild birds for possible highly pathogenic avian influenza infections has so far turned up no positive tests for the virus. And while this is encouraging, Dr. T.J. Myers of the USDA’s Animal and Plant Health Inspection Service (APHIS), cautioned not to read too much into that news.

During an interview with USDA Radio News, Myers said that since July, about 6,000 birds had been sampled. The agency has been testing wild birds in 43 states to get an early indication of what may be ahead as the weather gets cooler and wild birds make their southward migration through the United States.

“Absence of positive results doesn’t mean that the virus isn’t out there,” said Myers. “This group of viruses has quite an ability to mutate and to reassort with other influenza viruses, so it’s really difficult to predict what it may or may not do.”

Since December 19, 2014, APHIS has reported 223 detections of avian influenza, affecting more than 48 million birds. The agency’s last confirmed avian influenza case was reported on June 17. During that span of time, the virus was found in 21 states: Minnesota, Iowa, North Dakota, South Dakota, Nebraska, Wisconsin, Kansas, Missouri, Arkansas, Wyoming, Montana, Washington, Oregon, Utah, Idaho, California, Indiana, Nevada, Kentucky, Michigan and New Mexico.