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SHIC-FUNDED APP STUDY DEFINES OUTBREAK DYNAMICS AND ENVIRONMENTAL STABILITY OF BACTERIA

In response to a severe 2021 outbreak of Actinobacillus pleuropneumoniae (APP) serotype 15 in Iowa finishers, SHIC funded research to define the risk and mitigation of this emerging swine disease strain. Drs. Marcelo Almeida and Alyona Michael, Iowa State University, recently completed three objectives as part of the one-year research project, including 1) understanding the APP serological status of sows that supplied the finishers involved in the outbreak, 2) characterizing APP persistence in finishers who had recovered from the disease, and 3) comparing the environmental stability of two APP15 strains with an APP1 strain under laboratory conditions. This study provides information regarding the role of sows in epidemiology of APP, the recommended sample types for accurate diagnosis, and the bacterial stability across different temperatures and surfaces. Through responding to the APP15 strain emergence, this SHIC-funded study adds key information for improved APP surveillance and implementation of biosecurity practices on the farm.

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 US, 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.

Objective 1 of the investigation included cross-sectional sampling to determine the serologic status of sow farms supplying pigs to the APP15 affected finisher sites. 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, 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.

For 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. 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.

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