Poultry erysipelas – a case series from 1998–2023, highlighting diagnostic features, farmer interventions and molecular epidemiology

Erysipelas is caused by the bacterium Erysipelothrix rhusiopathiae and affects a wide spectrum of mammals and birds, including poultry such as turkeys, chickens, and waterfowl. This study retrospectively summarizes poultry outbreaks diagnosed at the Swedish Veterinary Agency from 1998–2023 with primary focus on laying hens during 2014–2023. For this purpose, case notes, bacterial culture results, farmer interviews, and whole genome sequencing were used. A total of 175 flocks were diagnosed, of which 155 (88.6%) outbreaks occurred in chickens (laying hens and parent breeders) and the remaining in turkeys, geese, and emus. Many of the outbreaks occurred in organic and free-range laying hen flocks. Multiple outbreaks occurred on 41.6% of the laying hen farms (range: 2–6 flocks/farm). In 2014–2023, 17.7% of all laying hen necropsy submissions were diagnosed with erysipelas (81 flocks). The disease was associated with a sudden onset of mortality and sometimes egg-drop. Newcastle disease or avian influenza was initially suspected in 16.1% of laying hen outbreaks (2014–2023). Bacterial co-infection was rare, and non-selective culture conditions were usually sufficient for diagnosis. Farmer interviews revealed that vaccination and euthanasia/slaughter were common interventions. Whole genome sequencing of isolates from laying hens identified serotypes 1a, 1b, 2, 5, and 6 of clade 2, 3, and i (intermediate). Multiple isolates within and between different flocks on the same farm were often closely related when outbreaks clustered in time (P = 0.0016). This is the hitherto largest erysipelas case series from poultry and provides useful information on an important emerging disease.RESEARCH HIGHLIGHTS Erysipelas is an important differential diagnosis of severe viral diseases.Most of the outbreaks occurred in organic and free-range laying hen flocks.Pre-enrichment or selective culture is useful in case of suspected co-infection.Isolates from temporally clustered outbreaks on a farm are often clonal.