A new blood test is quickly changing the way boar studs and possibly others test semen for porcine reproductive and respiratory syndrome (PRRS) virus.

While working on ways to increase the effectiveness of pooling semen samples to reduce the cost of PRRS testing, Minnesota swine veterinarian Darwin Reicks made a startling discovery.

He found that swine semen represents a poor model for early detection of the PRRS virus in boars.

“It was a real eye opener to me. I had 40 boars that we had infected six days earlier, and we only found two of them with semen PCR (polymerase chain reaction). It made me realize that with a typical boar stud doing statistical sampling, the odds are you are going to miss infection for a few weeks after it enters the boar stud. By the time you've figured it out, the boar stud could have already infected farms downstream,” he observes.

His research was supported by a grant from Boehringer Ingelheim Vetmedica, Inc.

PRRS Virus Infection

Initial PRRS virus infection moves slowly through a group of animals. When a boar stud first gets hit with PRRS, typically only a few animals are infected.

“The virus lays below the radar for several weeks before you really see any clinical signs. It spreads extremely slowly at first, before it builds up to a fairly high level in a few animals and spreads to others,” says Reicks, Swine Vet Center at St. Peter, MN. He coordinates swine health programs for a number of Midwest boar studs and consults internationally.

Finding A Better PRRS Test

The disappointing test results of finding PRRS in semen made it pretty clear to Reicks that a method of testing using blood had to be developed.

“Semen PCR is a very good test, but the problem is, unless you test every single ejaculate by individual semen PCR, by the time you pick up the virus in one boar, the odds are that other boars have already shed it in the semen,” he suggests.

Reicks also wanted to find a system that would be safer and faster than the standard serum collection procedure. That procedure calls for boars to be snared and a serum sample taken from the jugular vein, which is then sent to the lab to be spun down in a centrifuge and analyzed. It isn't practical to do this on a boar every week or twice a week, he explains.

In an experiment funded by Pork Checkoff, Reicks infected 20 boars with the PRRS virus and then tested them using semen, serum and a new blood swab test.

“Out of 60 samples, we had 59 of 60 samples test positive for PRRS with blood swab, and 60 out of 60 with serum. In only 27 out of 60 samples was PRRS virus detected in semen,” he points out.

And it took a lot longer for the virus to show up in semen than in serum.

The other issue with detection of the PRRS virus in semen is that the quantity of virus is so low, it barely registers on the PCR test in the early stages of an infection, says Reicks.

Plus, if you dilute or pool samples to cut testing costs, “you can basically just water it down to the point where the test won't pick up the virus anymore.”

In contrast, boars shedding virus in semen averaged 66 times more virus in blood swab tests than in semen, according to tests Reicks conducted last summer.

New Blood Swab Test

For the new blood swab test, while the boar is on the dummy being collected at the point of ejaculation, a very small needle is used to puncture one of the veins in the boar's ear, he explains. The blood drips out and it is wiped up with a swab, which is placed in saline solution, and the shaft containing swab and saline are submitted to the University of Minnesota Veterinary Diagnostic Laboratory for analysis.

Besides being safer than snaring a boar or collecting semen, it is also faster, remarks Reicks. To analyze its impact on the boar, flinch rates are measured. Flinch rate is whether the boar flicks its ear or not when it is poked with the needle.

“We found that there was typically about a 10% flinch rate, so we think collection time is a good, safe time for people to be sampling boars for PRRS,” he adds.

Blood swab samples received at the Minnesota diagnostic lab by noon can yield results by 7-8 p.m the same day. This means that a semen shipment only has to be held until later that night or early the next morning before it can be used with confidence, explains Reicks.

“But more importantly, the odds of detecting the virus sooner are so much better. You can more easily justify doing pooling of samples, because there is more virus in the blood early on than there is in the semen,” he stresses.

Studs Switch to New Test

Since mid-August, when Reicks first used blood swabs, all of the studs he works with have switched over to the new test.

“This has been a pretty big deal for the boar stud industry, and it may take another six months or so, but I don't think we'll have anyone doing semen PCR for PRRS testing anymore on the PRRS-negative studs,” he says.

Fast Test Results Saves Studs

In short, the blood swab normally detects infection within 24 hours of exposure when doing an individual PCR blood swab test, and results can be obtained within 12 hours.

Early detection of the PRRS virus can save boar studs from expensive depopulation.

Reicks observes: “We did that on a stud this past fall. We detected virus with the blood swabs and found there were three animals infected. We eliminated a circle of boars around them and were able to save the boar stud from depopulation.”

Reicks is working on refinements that would allow blood swab samples to be pooled and further improve the sensitivity of results.

Currently, it costs about a dollar per dose to do an individual blood swab PCR test on a boar.

The only justification for semen PCR testing for PRRS in the future will probably be in PRRS-positive boar studs that want to assure that semen they distribute is PRRS negative. Reicks says there are a few boar studs that maintain a PRRS-positive status because they don't feel they could stay negative if they depopulated.

Reicks says interest has been strong in learning more about the blood swab test. He will speak on its development at the American Association of Swine Veterinarians annual meeting in early March in Toronto, Canada.

Remarkable Advancement

The number of blood swab samples submitted to the University of Minnesota Veterinary Diagnostic Laboratory for testing of the porcine reproductive and respiratory syndrome (PRRS) virus is quickly rising, reports pathologist Kurt Rossow, DVM.

In 2004, only 6% of PRRS test samples were of the blood swab category, compared to 21% so far in 2005, he says.

“The blood swab method is safer, and you get a better sample as compared to semen,” explains Rossow.

PRRS virus shows up from a day to several days sooner in blood compared to semen, and in much greater quantities, he confirms.

Rossow notes the boar swab collection method is a “remarkable advancement in PRRS virus detection strategies and plays an important first step in the control and eventual eradication of this disease.”