|This 400-liter rotation drum is called a dynamic aerosol toroid. It’s a virus holding tank, kept within a large refrigerated unit, it’s able to test temperatures as low as –4°F. Holding the virus at cold temperatures keeps it stable while researchers run experiments.|
Is aerosol transmission of porcine reproductive and respiratory syndrome virus a big deal?
Although researchers have learned much in recent years about PRRS virus transmission, they are still looking for practical answers regarding aerosol transmission, says Jeff Zimmerman, DVM, professor and researcher at Iowa State University’s College of Veterinary Medicine.
“We have been trying to break down the process of aerosol transmission into its smallest parts,” Zimmerman says. “Once we understand what is happening ‘by parts,’ we can reassemble everything into a model that predicts aerosol transmission.”
During the last several years, Iowa State University researchers have looked into four specific areas of aerosol transmission. These areas include the following:
Air samples/sampling protocols
This work was an attempt to understand the sensitivity of samples. “There is no standard sampling protocol for aerosols,” Zimmerman says. “In other words, there is nothing in place that says this is the best way to collect aerosol samples.”
Joe Hermann, an Iowa State graduate student and PhD candidate who recently accepted a position with USDA, started by comparing aerosol-sample-collection devices. From there, he studied how best to collect aerosol samples of the virus.
Much of Hermann’s work focused on “looking at how things fit together and attempting to answer basic questions, such as what is the least amount of virus that can be detected in an aerosol,” Zimmerman says.
Infectious PRRS virus stability
Once the basic information about aerosol sampling was collected, researchers went on to look at how temperature and relative humidity affected the stability of infectious PRRS virus in aerosols.
“This relates to how long virus could survive, which is the limiting step in terms of how far it could travel via aerosol,” Zimmerman explains.
The take-home point of this research is not surprising. “PRRS virus stability is very dependent on temperature. With our typical summer temperatures, we wouldn’t really expect much virus to survive for very long,” Zimmerman says.
As for humidity’s role, it’s irrelevant at high temperatures because the virus doesn’t survive that well. But at lower temperatures, the virus likes low relative humidity.
“People tend to think that just because it’s a humid morning, the PRRS virus is going to love it. But it’s been shown over and over that the viruses with envelopes, like PRRS and like swine influenza, don’t like high humidity levels; they ‘die’ faster at high humidity levels. This research confirmed that,” Zimmerman notes.
An important point to remember, he points out, is that virus stability is highly variable between pork production systems, depending on the geographic region.
“What’s frustrating is that, whether it’s in Texas or Mexico or Minnesota or Iowa, we still have these intrusions that can’t be explained. What you’d expect is that aerosol activity, if it occurs, will be much different between Sonora, Mexico, and Iowa or Minnesota,” Zimmerman says.
Shedding the virus
Iowa State researchers also have studied the rate of PRRS virus shedding by individual pigs in respiratory exhalations. This work shows that “in fact, they shed an extremely low level of virus,” Zimmerman says, “which is good news.”
According to the research, you’d have to say that the pigs are shedding virus, but at extremely low rates. What’s more, perhaps they are not shedding through the respiratory tract.
“We know that there’s some shedding in urine, for example,” Zimmerman says. “What about the possibility that they create droplet aerosols from moving about the pens? Pigs like to play, and if they’re bouncing around in their pens, they may be creating an aerosol.”
Inactivating airborne virus
The fourth area that Iowa State researchers considered is the probability of infection as a function of aerosolized PRRS virus dose exposure and methods to inactivate airborne PRRS virus.
“If we can determine such methods for PRRS virus, it will probably also help us with other airborne pathogens, such as swine influenza virus and Mycoplasma hyopneumoniae,” Zimmerman says.
This research is related to how much virus pigs are shedding and how long it survives in the environment. “If there is some circumstance under which a very low level of infectious virus could reach the next farm, there is some probability that a pig could become infected,” he says.
Researchers want to determine those probabilities. “If we have an estimate, maybe we could start explaining the sporadic nature of area spread or area transmission,” Zimmerman says. “Until we have those estimates, we will continue to struggle to understand what’s going on.”
So far, the methodology has been developed, equipment is in place and some early estimates have been made about probabilities. “We’re getting a proposal together now to continue this work,” he says.
FIELD STUDIES NEEDED
Nothing substitutes for field experiences, and field studies are needed to determine practical answers for porcine respiratory and reproductive syndrome virus aerosol transmission, as well as knowing more about other disease aspects, says Jeff Zimmerman, DVM, Iowa State University researcher.
“Descriptive, imprecise answers aren’t good enough anymore,” he says. “We need the quality of data that will tell us when, how and why things happen. Just to say, ‘yes, it happens,’ isn’t good enough.”
Zimmerman cites the complexities and severity of clinical PRRS outbreaks on farms. “It is complex because there are so many elements happening all at once,” he says.
Fall and early winter present many challenges. “When fall arrives, a lot of things happen at once. Typically, when it starts getting cold, the buildings’ curtains go up and we start concentrating airborne elements, such as dust, gas and other microorganisms.
“In those closed up buildings, you have much higher bacteria levels suspended in the air, along with other gases and particulates. So the pig is being assaulted by lots of things, not just PRRS virus,” Zimmerman says. “To select one element to blame is unrealistic.”
Field studies could provide some practical answers.
Although there has been an increase in funding for PRRS research during the last few years, “the simple truth is, very few field studies have been funded,” he says.
NECESSARY FUNDING SOURCES
Much of the porcine reproductive and respiratory syndrome virus aerosol-transmission research has been funded by the National Pork Board and USDA’s PRRS CAP grant.
But private industry also has provided assistance with some of the research, says Jeff Zimmerman, DVM, Iowa State University. For example, Boehringer Ingelheim Vetmedica provided a $25,000 piece of equipment for Iowa State researchers to dig into the aerosol question.