January 24, 2023


For the first time ever, researchers have proven through a controlled study that boars can infect gilts with African swine fever (ASF) through artificial insemination (AI). As the threat of the deadly ASF virus continues to inch closer to the U.S., more research is being conducted to help prevent and prepare for an ASF outbreak.

To date, little has been done to study the risk of boar semen.

“Most boar studs have 20 to 40 sow farms underneath them. The potential to infect a wide geographical region in one day is of big concern to me,” says Darwin Reicks, DVM, researcher and owner of Reicks Veterinary Research and Consulting based in St. Peter, Minn. He consults for many boar studs throughout the world and has done extensive research on boars, especially regarding transmission of disease.

Reicks and a team of researchers from around the globe wanted to find out when boars start shedding ASF virus.

“It was believed that boars shed ASF virus, but we needed to prove it, and do so using a collection technique that we could be confident would not allow environmental contamination of the semen,” he says. “Collecting semen is a process where there’s potential for cross-contamination. I did all of the semen collection using a technique that makes sure it’s only semen in the cup and nothing in the environment.”

The study, funded by the Pork Checkoff, was conducted at the Friedrich-Loeffler-Institut’s Biosafety level 4 (BSL-4) facility in Germany using four boars – two Large White boars and two Pietrain boars. In addition, 14 Large White-lineage gilts were part of the study.

Reicks and co-researcher Jane Christopher-Hennings of South Dakota State University’s Veterinary Diagnostic Laboratory traveled to Germany for two weeks to help complete the study with Virginia Friedrichs and Sandra Blome, an expert researcher on ASF.

“In a BSL-4 facility, there are many extra precautions like showering in and showering out, airlocks and more. The logistics were quite a bit more challenging with a virus that absolutely can’t leave that facility, too,” Reicks says.


The four boars were inoculated orally and intranasally with ASF virus. However, they did not get infected, Reicks says. So, on day 4 they made the decision to inject the boars with ASF virus so the boar semen would be ready for the already synchronized gilts.

Using the day of ASF virus injection as the new day 0, the researchers sampled boars on day 1 to make sure they did get infected. Once that was confirmed, they started semen sampling on day 2.

“We found ASF virus in the semen already on day 2 during the first day of semen collection. It was not in the semen on day 3 for whatever reason, but on days 4 and 5, all boars were shedding ASF virus in the semen.”

Using that semen, they bred the 14 gilts with infected semen. A week later, seven of the gilts were infected with ASF. A week after that, all 14 gilts were infected with ASF.

“We are not sure if the other half were infected from semen or their neighbors. Because of welfare requirements, we had to host the gilts in pens. For sure, the first half were infected on day 7 after insemination,” he explains.

The strain of ASF virus used, ‘Estonia 2014,’ was a milder strain that they specifically used with the hope they could get semen samples and possibly be able to collect from for three months.

“Unfortunately, the boars got fairly sick. We were working within certain specific welfare metrics, so we had to euthanize the boars once they reached a certain threshold of sickness,” Reicks explains. “All boars ended up being euthanized by day 25. The gilts were euthanized as well.”


One of the most interesting findings, Reicks says, was that there was no effect on semen quality until day 14.

“If boars are infected with a virus or are sick, you often don’t see anything with semen quality for 14 days. Sperm in the reservoirs and tail of the epididymis – the sperm that are close to maturity – are not really affected. Some of the boars in these trials had fevers over 104 degrees but we were still getting normal sperm up until 2 weeks after they were infected,” he adds.

That’s a little scary, Reicks says, adding that boar studs probably would not see anything early on in the first two weeks after an introduction of ASF as far as semen quality.

“We need to rely on testing and symptoms. It looks like ASF sheds early and could potentially be shedding in the semen before someone would be triggered that something really bad is going on,” he says.

Early on, the symptoms were fairly mild and they were able to get the boars collected, Reicks adds. However, there was already virus in the semen.

“On the good side of things, boar studs typically have good biosecurity, the vast majority have air filtration, air conditioning, are well located and sometimes have extreme procedures for staff and products to enter. So, the likelihood of a boar stud getting infected should be very low compared to other farms,” he says.


Early detection of infected boars is critical. Infection of a boar stud presents the risk of rapidly and widely distributing ASF virus within or between countries. If ASF were to get in the U.S., this study confirms the importance of routine testing of boars before distributing semen to sow farms. Reicks says the take-home message is clear: ASF is shed in the semen and it does infect gilts.

“ASF virus may be shed in the semen before obvious clinical signs,” he notes. “Testing is critical for preventing downstream infection.”

Plans are underway for a follow-up study funded by the National Pork Board where Reicks and other researchers will expose the boars orally again to ASF. He wants to better define those first few days and figure out how many days after oral exposure shedding might begin. He plans to have more boars so he can take a look at the range in responses like one might expect to see in a boar stud.




Other researchers in addition to Reicks and Christopher-Hennings include: Sandra Blome, Virginia Friedrichs, Tessa Carrau, Paul Deutschmann, Julia Sehl-Ewert, Hanna Roszyk and Martin Beer of the Friedrich-Loeffler-Institut; Tobias Hasenfuß and Elisabeth Gerstenkorn of Bundes Hybrid Zucht Programm in Germany; Jeffrey J. Zimmerman of the Department of Veterinary Diagnostic and Production Animal Medicine at Iowa State University; and Eric A. Nelson of the Animal Disease Research and Diagnostic Laboratory at South Dakota State University.