Finding PED disease in swine herds

How do you diagnose a disease that hasn’t been seen here before?

That’s the question that veterinary diagnostic labs, including ours here at SDSU, are grappling with. During the past couple weeks, a disease that previously had not been seen in the United States, Porcine Epidemic Diarrhea (PED), has been identified in Midwestern hog farms. Swine farmers in Asia and Europe are acquainted with this virus, but until now, it had never been seen here.

The PED virus can be life-threatening for a young pig. It infects the rapidly growing cells that line the intestine. These cells absorb fluids from the milk and water the pig drinks. When disrupted by this virus, these cells can no longer absorb the fluid, which now travels out of the pig in the form of diarrhea. Worse yet, the pig gets none of the benefit of the water or milk it drinks. Dehydration results, and if the pig is too small to have much body reserve, there’s a good chance he winds up dead.

In swine operations, PED causes signs similar to a well-known swine disease called Transmissible Gastroenteritis (TGE). They both are members of the same family – the coronaviruses – and they are both very transmissible from pig to pig. The virus leaves the infected animal in the manure and contaminates the environment so another pig can easily pick the virus up on its snout. The virus then can get into the pig’s digestive tract, completing what we call “fecal-oral” transmission. Diarrhea can occur in all ages of pigs, but it’s the small baby pigs before or just after weaning that are the most severely affected.

The bad news about PED is that-like TGE-there is no specific treatment. Herd immunity kicks in to limit the clinical signs after the virus blows through a herd. There are vaccines against TGE, but not for PED. If there’s good news, it’s that it’s that PED does not affect people nor does it affect other species. It is not a food safety concern.

So, back to our original question. If you happen across something that has not been identified in the US, how do you pin the problem to that particular virus? It’s not as easy as putting the pig poop into a machine that spits out an answer. You have to have some idea of what you are looking for. The first diagnosticians that got these cases did have an idea – since the clinical signs looked like TGE, they ran tests for TGE. When those tests came back negative, they had to dig deeper.

The diagnosticians then performed electron microscopy on the sample – in other words, taking a picture of the actual viruses present in the sample. Voila – there were coronaviruses present in the sample. Now the list of possibilities has shrunk down to the less common viruses. Using molecular techniques, the genetic code of the virus was amplified and compared to the millions of viral fingerprints stored on the internet. That’s how it became apparent that this was PED and not something else. This process would have taken much longer 20 years ago. We now have much more powerful molecular techniques and ability to analyze information sharing over the internet. This enables diagnosticians to respond significantly faster to this disease threat compared to diseases in the past such as PRRS.

How did the virus get here, and how is it going to spread? We don’t know those answers yet. But I do know that in order to answer those questions, we have to be able to reliably and quickly diagnose the presence of the virus first.

Over the past four months, I’ve been serving as the interim director of our diagnostic laboratory here at SDSU. It’s enabled me to learn even more about the way our diagnosticians here interact with veterinarians to solve the critical issues facing all of us today. Today it’s PED that’s causing losses in swine herds. But next month or next year it likely will be something else. Regardless of what comes up in the future, it’s great to know that we have a talented, experienced group of scientists with one foot in the lab and the other on the farm that are able to tackle these emerging issues.

Russ Daly, DVM, is the Extension Veterinarian at South Dakota State University. He can be reached via e-mail at russell.daly@sdstate.edu or at 605-688-5171.