How an APP Outbreak Could Help Change the Pork Industry
When outbreaks of Actinobacillus pleuropneumoniae (APP) serotype 15 were detected in upper Midwest finishers 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.
In response to this severe outbreak, the Swine Health Information Center (SHIC) funded research to define the risk and mitigation of this emerging swine disease strain. Marcelo Almeida and Alyona Michael, both of Iowa State University, recently completed three objectives as part of the one-year research project, SHIC shared in a release. Not only does this study provide information regarding the role of sows for the epidemiology of APP, but it also recommended sample types for accurate diagnosis and deepened understanding of the bacterial stability across different temperatures and surfaces.
By responding to learn more about this APP15 strain emergence, this new knowledge will help improve APP surveillance and implementation of biosecurity practices on the farm.
What Happened?
In late November 2021, several Iowa finisher sites across multiple unrelated systems exhibited high mortality, reaching up to 51% in a matter of days due to an outbreak of virulent APP. Clinical signs associated with the outbreak included rapid onset of high fever, coughing and respiratory distress with mortalities exhibiting frothy, red discharge from both the nose and mouth. This outbreak challenged several assumptions about APP dynamics in the U.S., SHIC notes, such as the prevalence of virulent strains, the risk factors for APP lateral transmission, and environmental persistence. Preliminary sequencing efforts of APP isolates did not identify a clear source herd and further investigation into APP15 was warranted.
Three Objectives of the Outbreak Study
Objective 1: Cross-sectional sampling to determine the serologic status of sow farms supplying pigs to the APP15 affected finisher sites.
SHIC explained that serum samples, nasal swabs and tonsil scrapings were collected from 30 sows (parity 0 and 1) in each sow herd. Serum samples were tested for APP15 antibodies at the ISU Veterinary Diagnostic Lab, using routine procedures. For the sow farms that submitted samples, 15 of 19 farms were serologically negative for APP serogroup 3-6-8-15 suggesting the majority of sow farms providing pigs for sites experiencing outbreaks were free of APP15. The sites receiving pigs from those sources may have been laterally infected with APP15 during the post-weaning period. Unfortunately, SHIC points out that incomplete serologic screening of farms and lack of culture and sequencing follow-up made it impossible to confirm whether the isolate originated from an endemically infected sow farm, as opposed to originating from an alternative source of lateral transmission.
Objective 2: A prospective longitudinal study of individually identified finishers was conducted at one Iowa site following a confirmed recent APP15 outbreak.
Individually identified pigs were repeatedly sampled weekly to evaluate APP15 persistence in the nasal cavity and tonsils and monitor the development of humoral immunity, SHIC shared. These results were compared to oral fluids and environmental swab samples to understand population shedding dynamics and environmental persistence. Environmental swab samples included internal locations including feeders, water nipples and floors and external locations including rendering area and office door handles.
"During the finisher investigations, tonsil scrapings had a higher detection rate than nasal swabs or oral fluid, with positive detection in 95% of sampled pigs across six weekly sampling events ending at eight weeks post-outbreak. PCR of tonsil scrapings were overwhelmingly the most sensitive means of screening for individual carriers long-term. APP detection rate in oral fluids was over 10% by PCR up to eight weeks after the reported outbreak. These results are significant in that they suggest that oral fluids have a more temporally robust utility for post-outbreak population surveillance than previously reported," SHIC reported.
APP15 genetic material was primarily detected in avenues of human traffic (door handles, barn entry floor) and deadstock collection sites (rendering pile). Except for week eight, APP was not detected by PCR in any feeders or waterers, the article said. None of the PCR-positive environmental samples yielded isolates on culture.
"The environmental viability of APP could not be adequately determined due to the high degree of environmental bacterial contamination and further laboratory investigation of APP viability was performed," SHIC explained.
Objective 3: Compare the longitudinal viability of APP15, for outbreak and historical strains, to APP1 at different temperatures, surfaces and organic matrices.
Researchers evaluated survival of each strain on different surfaces from concrete to stainless steel to rubber. They also examined different substrates (water, fecal slurry and horse serum-NAD) at six time points post-inoculation at four temperature set points. For the laboratory comparison of APP serotypes, this study is the first to observe differences in stability across serotypes and the impact of solid substrates on stability when exposed to cooler or freezing conditions. Concrete surfaces exhibited the longest stability for APP15 and APP1 with all strains surviving up to seven days post inoculation at -20ºC, and up to 48 hours at 4ºC with APP1 and APP15h surviving up to 72 hours. Rubber and stainless-steel surfaces exhibited viability at 4ºC and 25°C, but not at 37°C. The results of this benchtop study support previous APP persistence research showing decreased stability in warmer conditions, with relatively prolonged persistence at 4°C and -20°C, SHIC reported.
"While the outbreak strain of APP15 did not exhibit enhanced persistence compared to other serotypes, contributing factors to the spread and geographic persistence of this bacterium could include PCR detection at rendering piles, increased survival on concrete and rubber surfaces under laboratory conditions, and cold ambient temperatures during the winter 2021 outbreak," SHIC said.
These studies are helping generate knowledge related to APP ecology and epidemiology that can have important implications for disease diagnosis, monitoring and surveillance, and biosecurity practices, SHIC highlighted.
Read More:
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