Research into porcine reproductive and respiratory syndrome virus has researchers concerned that airborne virus may play a role in the spread of the disease. For the first time, scientists have shown that PRRSV can be transmitted through the air, at distances of at least 150 meters, and infect pigs. Dr. Scott Dee, an associate professor at the University of Minnesota's Swine Disease Eradication Center, presented this research finding at the recent Conference of Research Workers in Animal Diseases meeting in Chicago.
"Aerosol transmission of PRRS virus is a bit of a wild card, and probably one of the last routes of transmission that we haven't truly addressed in field conditions," Dr. Dee said.
According to him, PRRSV disease, which has plagued farmers since 1975, is the most economically important disease facing the swine industry today. Researchers are still sorting out how the virus is transmitted, how to stop transmission, and how the pig's immune system responds to infection. Transmission occurs primarily through pig-to-pig contact and infected feces, urine, and milk. The virus can also be transmitted to naïve pigs through contact with contaminated coveralls or boots, contaminated needles, and mosquitoes. Dr. Dee has even shown that PRRSV-contaminated snow can get wedged near a truck's wheel and transported to another farm, where it can be the source of infection for a new herd.
Previously, studies have shown that PRRSV can travel distances of one meter and infect pigs. Other studies, however, have indicated that aerosol transmission of the virus over long distances does not occur.
Dr. Dee has been interested in studying the virus' journey over long distances, because of the possibility of transmission between farms. He started to wonder whether their controlled aerosol experiments had been confounded by various environmental, physical, and biologic variables. There are many biologic unknowns regarding what is required for aerosol transmission. "What happens after the virus is emitted from the pig? How quickly does it inactivate? How quickly does it disperse?" said Dr. Dee, listing a few of the still unanswered questions. "What type of health status is required—do you need a mixed infection with a more pneumonic agent such as Mycoplasma hyopneumonia? What is the quantity of the virus exhaled by the pig?"
To neutralize these variables, Dr. Dee and his research team went back to the drawing board and developed what they call the Alaskan pipeline, 150 meters of piping sprawled across their Minnesota research farm. The scientists aerosolized the virus and transported it, using a spray gun and blower, through the piping to chambers containing PRRSV-naïve pigs.
A week after being exposed, the pigs were tested for the pathogen. In one experiment, one of two naïve pigs contracted the virus through the Alaskan pipeline; in another, two of five pigs contracted the pathogen.
"Under optimal conditions, the virus may be able to be transported by air," Dr. Dee said. He notes, however, that researchers "have a long way to go" in understanding aerosol transmission of the disease.