South Korea study tracks three-phase decline in fowl typhoid

A newly published South Korean surveillance analysis suggests that reported farm-level fowl typhoid didn’t just gradually fade over time. Instead, the trend appears to break into three phases across 2000 to 2024: a decline after the early 2000s, a long period of relative stagnation, and then a more pronounced drop beginning around 2016. The paper, published this week in Animals, links those shifts in timing to two major developments in the country’s poultry health system: expansion of fowl typhoid vaccination, and later, the disruptive national response to the 2016–2017 H5N6 highly pathogenic avian influenza epidemic. (mdpi.com)

The study is based on 1,495 farm-level notification records from South Korea’s national livestock disease database. Using Bayesian information criterion to compare segmented regression models, the authors found that a two-breakpoint model fit best. Their interpretation is that the first decline was temporally consistent with the introduction of the live attenuated Salmonella Gallinarum 9R vaccine for commercial layer chickens in 2001, while the second breakpoint, estimated around 2016.5, coincided with the H5N6 HPAI crisis. Importantly, the paper emphasizes that this is an ecological analysis, so the overlap in timing shouldn’t be read as direct evidence that HPAI control caused the fowl typhoid decline. (mdpi.com)

That caution matters because South Korea’s 2016–2017 avian influenza response was unusually intensive. Published analyses of that epidemic report hundreds of infected farms, millions of birds culled, and broad use of movement restrictions, preemptive culling, disinfection, and surveillance. One PLOS One study, using official Korean outbreak data, reported 383 infected farms by April 4, 2017, and about 3.7 million poultry culled from 946 farms. Another review of the 2016–2017 H5N6 event described 343 poultry-holding cases and 51 wild bird cases, with the first poultry-farm case reported on November 16, 2016, after the first wild bird detection on October 28, 2016. (journals.plos.org)

The new fowl typhoid paper argues that those HPAI-era interventions may have done more than control influenza. Depopulation of infected premises, temporary sanitary downtime, shifts in production structure, and tighter controls on vehicles, people, and farm access could all plausibly reduce opportunities for S. Gallinarum persistence and spread. At the same time, the paper says the long middle phase should be viewed as a plateau, not a true rebound, because the estimated annual change was slightly positive but statistically imprecise. The authors also point to possible reasons fowl typhoid persisted at low levels during that period, including vertical transmission, infected breeder flocks, uneven vaccination, smaller poultry holdings, and incomplete biosecurity implementation. (mdpi.com)

There’s also reason not to overread the decline as eradication. Earlier Korean work found continued molecular diversification of S. Gallinarum field strains, and a separate study of South Korean chicken isolates from 2013 to 2018 reported increasing antimicrobial resistance among fowl typhoid isolates. In other words, fewer reported farm events doesn’t necessarily mean the pathogen disappeared or that transmission pressure was fully resolved. It may instead reflect a combination of vaccination, surveillance, structural industry change, and broader disease-control pressure. (mdpi.com)

For veterinary professionals, that’s the most useful takeaway. This paper is less about proving one intervention worked than about showing how disease surveillance curves can bend when multiple control systems overlap. Poultry veterinarians and animal health teams often evaluate vaccination programs pathogen by pathogen, but the South Korean experience suggests that emergency responses to HPAI may also have spillover effects on endemic bacterial disease through farm depopulation, network disruption, downtime, and biosecurity reinforcement. It also underscores the limits of national notification data: without farm-level vaccination coverage, denominator shifts, and management data, it’s difficult to separate true epidemiologic change from changes in reporting, farm structure, or exposure patterns. (mdpi.com)

The broader industry context supports that interpretation. South Korea has faced recurrent HPAI pressure for years, with more recent analyses describing continued poultry-farm outbreaks through the 2020–2026 seasons. That means the country’s poultry disease ecology has been shaped not only by endemic pathogens like fowl typhoid, but also by repeated waves of influenza-driven surveillance and control. For clinicians, diagnosticians, and poultry health managers, the study adds to a growing case for integrated surveillance that looks across diseases rather than in silos. (mdpi.com)

What to watch: The next step is whether researchers can pair these national trend data with farm-level information on vaccination uptake, flock type, regional production changes, and HPAI-control exposure to test which mechanisms actually drove the post-2016 decline. (mdpi.com)