When outbreaks of Actinobacillus pleuropneumoniae serotype 15 were detected in the upper Midwest in late November 2021, experts agreed they were unusual in several aspects.
Not only was there relative rarity of APP15 isolation in the U.S., but there was also unusually high mortality for this strain and epidemiologic evidence of lateral transmission between systems within a narrow geographic radius.
That’s why the Swine Health Information Center (SHIC) funded work led by Derald Holtkamp of Iowa State University (ISU) following the outbreak to evaluate possible sources of APP15 transmission between company-owned sites with outbreaks. In parallel, SHIC also funded work with Alyona Michael and Marcelo Almeida of the ISU Veterinary Diagnostic Lab, to rule out endemic maintenance of APP15 in sow herds that had supplied finishers impacted in 2021 and to characterize post-outbreak ecology for identification of risk factors for lateral transmission.
Their interim report reveals several key findings, SHIC reports in its latest newsletter.
Although APP has been in the U.S. for a long time, it was not a problem herd veterinarians regularly faced due to infrequency of APP-associated disease. SHIC credits this to a significant effort by swine genetics companies in the 1990s to successfully eliminate APP from their farms and the rest of the pork production industry followed suit.
“When APP outbreaks did occur, they were not large scale nor with the high level of mortality seen in the November 2021 outbreak in Iowa. In 2021, however, when more than usual cases of APP were seen at the ISU VDL, staff talked with herd veterinarians and learned that cases were from a certain geographic area and had high mortality reaching 50% in some instances,” explains Paul Sundberg, SHIC executive director.
In response, researchers began investigating the shedding patterns of APP15 in nasal swabs, tonsils and oral fluids to help future diagnostic investigations. They also looked for unique characteristics about this specific serotype/strain compared to other historical APP isolates.
A Look at the Study
Serum was collected from sows for serological evaluation in the study. Of the 16 farms tested, four were positive for APP antibodies. In the positive farms, antibodies were detected by ELISA against APP serotypes 3, 6, 8 and 15, SHIC reports. In the majority of farms that tested negative for APP antibodies (12/16), it was assumed that pigs supplied to farms that later had outbreaks were APP-negative at placement. Experts say this provides evidence for lateral APP15 introduction for finisher sites receiving pigs from serologically negative sow farms. For farms testing positive for the serogroup APP15, the next step was to sequence isolates from that farm.
Sows positive for antibodies against APP serotypes 3, 6, 8 and 15 underwent further testing to identify which sows were positive only for APP15. This is accomplished by collecting tonsil scrapings from serologically positive sow farms to attempt bacterial isolation, PCR serotyping and whole-genome sequencing, SHIC explains.
“Whole-genome sequencing will be essential to match an APP15 isolate from sow farms to the strain circulating in the outbreak, as not all APP15 isolates are the same,” Sundberg says.
Next, an affected site was sampled and tested post outbreak to understand shedding dynamics and environmental persistence. On a site with three barns, 67 pigs identified by ear tags were tested weekly over a six-week period. The goal was to determine if the same pigs were continually or intermittently shedding over that period of time, SHIC reports.
The Results
In the first sampling, 50% of the pigs were positive in both nasal swabs and tonsil scrapings. Incomplete agreement between those two sample types occurred and over time, the percentage of positive results increased in tonsil scrapings while nasal swabs decreased, SHIC reports. Researchers determined tonsil scrapings were more sensitive samples than nasal swabs as time passed after outbreaks.
In all three barns, oral fluids sampling took place in every pen with animals. In the second and third samplings, over 50% of samples were positive.
“This was surprising, as oral fluids have traditionally been considered a poor sample type for APP diagnostics. This may be associated with differing dynamics of experimental vs field APP challenge and transmission. Another key finding was that oral fluids were successfully used for detection of bacteria by PCR,” Sundberg says.
In the end, SHIC reports that researchers could detect APP up to the last sampling point of the study in tonsil scrapings and oral fluids, at lower detection rates, nine weeks after the reported outbreak.
“This is significantly longer than literature reports of seven days post infection,” Sundberg points out.
During the environmental sampling, only the rendering box and the office door handle tested positive for APP by PCR at the first sampling point. All samples tested negative for APP by PCR on the second sampling. The rendering box was positive again at the third time point and the door handles of all three barns on site also tested positive, SHIC notes.. The office door handle, and the floor by the entrance door of two barns, tested as suspect. At the last sampling, 11 samples tested positive for APP by PCR at the rendering box, feed plates, and floor samples collected in front of feeders in all barns as well as floor by the entrance door of one barn.
Read more from Farm Journal’s PORK:
APP Strikes with a Vengeance in Upper Midwest Pig Farms
The Elephant In The Barn: Why We Can’t Ignore This Risk on the Pig Farm
Every Decision is an Investment
APP Serotype 15: A Bizarre Situation, Swine Expert Says
Feed and Biosecurity: What You Need to Know
Before Emergency Strikes: Ohio State University Takes an Important Message on the Road
