The researchers said they also found bees in colonies with the collapse disorder are infected with a greater number of pathogens than their non-infected counterparts. However, no individual pathogen can be identified as the cause of the collapse disorder.
Colony collapse disorder is described in the online encyclopedia Wikipedia.org as a phenomenon involving the abrupt disappearance of worker bees in a beehive or a honey bee colony.
The international team of researchers -- from Penn State University, North Carolina State University and the U.S. Department of Agriculture, along with the University of Liege and Gembloux Agricultural University in Belgium -- collected samples of adult bees, wax comb, pollen and developing larvae from 91 colonies in 13 apiaries in Florida and California.
The scientists quantified more than 200 variables, including the presence of parasites such as varroa and tracheal mites; infection by bacteria, viruses and fungi; pesticide levels; nutritional factors and bee physiology.
No single factor was found consistently only in colonies suffering from the disorder.
"Our results suggest that this condition may be contagious or the result of exposure to a common risk factor that impairs the bees' immune systems, making them more susceptible to pathogens," Penn State entomologist Dennis vanEngelsdorp, the study's lead author, said.
The study's findings were reported in the Aug. 3 edition of the online journal PLoS One.