Around Thanksgiving last year, outbreaks of Actinobacillus pleuropneumoniae serotype 15 (APP15) began popping up in the upper Midwest, resulting in a number of finisher sites experiencing respiratory disease with high mortality.
Pigs exhibited coughing, high fevers and respiratory distress which resulted in death loss of up to 51% within a matter of days post-onset, the Swine Health Information Center (SHIC) reports. This outbreak affected approximately nine otherwise unrelated production systems within a narrow geographic radius. Submissions from affected sites received at Iowa State University Veterinary Diagnostic Lab (ISUVDL) were diagnosed with APP15.
Additional laboratory analysis funded by SHIC of the APP strain involved in the outbreak follows.
The current APP15 outbreak is highly unusual in several respects.
1) The relatively rarity of APP15 isolation in the U.S.
2) The unusually high mortality for this strain in the U.S.
3) Epidemiologic evidence of a high rate of lateral transmission between systems within a narrow geographic radius.
Once the nature and impact of this outbreak were identified, ISUVDL, with funding support from SHIC, rapidly mobilized a tripartite investigatory effort with the following components:
- Field-based epidemiologic investigation to characterize outbreak dynamics
- Expansion of ISUVDL in-house serotyping capabilities to expedite both retro- and prospective identification of APP15 in ISUVDL submissions
- Genetic characterization of APP15 strains recovered from affected sites with comparison to historic isolates
“Laboratory work is centering on screening for any genetic shifts in virulence factors that might account for both the high pathogenicity and propensity for lateral transmission of APP15. Preliminary genetic sequencing was performed to characterize primary virulence factors, antibiotic resistance genes, and approximate genetic identity between APP isolates from affected herds,” says Alyona Michael, DVM, who provided SHIC with a preliminary report of results. “Analysis thus far has confirmed a high degree of homology between strains from the current outbreak but has yet to identify a common historical ancestor.”
Previously isolated APP15 strains suspected to be endemic to several Iowa systems appear genetically divergent from outbreak isolates, she explains. Expanded genetic sequencing will be performed on all 2021-2022 APP15 isolates, with comparison to historical North American strains made possible through collaborative efforts with Marcelo Gottschalk, DVM, at the University of Montreal.
SHIC Rapid Response Team epi investigations are aimed at identifying the most likely route of APP15 transmission between unrelated sites, a possible geographic and temporal origin for the prevalent strain, SHIC reports. More results will be reported when that information has been analyzed.
What Do You Need to Know About APP?
- APP is a non-zoonotic porcine bacterial pathogen.
- APP is considered an obligate parasite of the respiratory tract, and typically requires close contact between pigs for transmission, with poor environmental survival outside the animal host. In persistently affected herds, the bacterium is passed from subclinically infected sows to piglets.
- As colostrum-derived maternal immunity wanes post-weaning and pigs are exposed to stressors and coinfections, APP can spread from the upper respiratory tract to the lung. There, it produces toxins that can cause severe tissue damage, hemorrhage, and death. In animals that survive acute infection, production losses persist in the form of stunted growth and condemned cuts.
- Lateral transmission of APP via aerosol or human traffic between unrelated sites has been reported, but, is relatively infrequent.
- This bacterium can be subclassified into different serotypes (1-19), depending on the molecular composition of the cell wall. Serotypes vary in their pathogenicity, although disease severity varies geographically and depends on a herd’s existing immunity and concurrent health challenges.
In the future, ISUVDL and SHIC will be expanding both field and genomic investigations to determine the origin of this outbreak, and actively prospectively monitoring submissions for high-virulence APP15 persistence in the U.S. swine herd, SHIC reports.
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