Epidemiological investigation of cases of senecavirus A in United States swine breeding herds

The objective of this report was to describe the findings from epidemiological investigations to characterize clinical features and identify likely routes by which senecavirus A (SVA) was introduced into sow herds in the Midwest USA.

Sow farms investigated were enrolled from cases submitted to the Iowa State University Veterinary Diagnostic Lab (ISU VDL) where SVA was confirmed by PCR. A standard Epidemiologic Investigation Form was used to collect information in a manner suitable for aggregate analysis. The form was adapted from one developed for the PRRS Outbreak Investigation Program, funded by the Iowa Pork Producers Association (IPPA). Information was collected about the herd, premises, outbreak and a comprehensive list of risk events, such as swine movements and entry of people, and the carrying agents associated with them. Risk events were assigned a risk level of high, medium, or low based on the frequency of the event, likelihood that carrying agent(s) associated with the event were contaminated or infected with SVA and the likelihood that SVA was transmitted from carrying agent(s) to pigs in the herd.

Investigations were conducted on a farrow-to-finish herd in Iowa (1) and farrow-to-wean herds in Illinois (1), Iowa (1), Minnesota (1), and Nebraska (2). All 6 farms reported increases in pre-weaning mortality (PWM) that resolved within 3 weeks. All 6 reported that sows were anorexic during the acute outbreak. A mild to moderate piglet scour was reported in (3/6) cases. Vesicular lesions on the nasal and coronary bands of sows were reported in 4/6 cases. The prevalence of vesicular lesions ranged from 10 to 70% of sows in the herd. In one case, 90% of the sows were severely lame. The frequency of risk events varied, from 132 to 757 in the 4 week period before the onset of clinical signs, depending on the size of the farm and farrowing system (continuous versus batch) used. Events rated as high risk for SVA introduction by the investigation team were: on-farm employees (4/6 farms), dead disposal (4/6), cull sow removal (3/6) and replacement gilt entry (2/6).

The clinical presentation was variable. Outbreaks occurred on farms of all sizes in swine-dense and non-dense areas. Of the farms investigated, 3 had relatively low levels of biosecurity, including the absence of showering procedures. One had relatively good biosecurity, but a high frequency of risky events due to the large size of the farm. For the other 2 farms, the biosecurity was average but operational connections with other positive farms or major deviations from protocols were identified as likely responsible for the outbreaks.

Disclosure of Interest: None Declared.