5 May 2025

3 minute read

By: Chris Wright

South America

African swine fever (ASF) has re-emerged in the Americas and is now of global concern as a direct threat to the swine production and health. A study was done to simulate the potential spread dynamics of ASF in the Argentine swine industry and identify the principal pathways for transmission. A mechanistic, spatially explicit and stochastic simulation model was built/designed based on local conditions in the study region.

The model was created by Laura Alarcón, National University of La Plata, Argentina, and colleagues in Argentina and the US. The results of this simulation were presented at the 2024 Leman Swine Conference.

A daily time step and a real spatial representation of entities involved in the spread of disease were used: pig farms, markets, slaughterhouses, wild boar populations, landfills, ports/airports, international waste treatment plants and livestock trucks. Data was provided by the National Food and Animal Health Service (SENASA) based on official information for 2019.

The disease spread was simulated at two hierarchical levels: at the top level, ASF spread between farms or other spatial entities through direct pathways (movement of animals and vehicles between network nodes) and indirect pathways (contacts with wild boars, contaminated fomites, or human food waste). At the bottom level, once a pig farm was infected, a model was used to simulate ASF intra-farm dynamics. Most of the model parameters were calibrated based on field data and bibliographic sources.

A sensitivity analysis was then conducted to assess the role of five transmission pathways: large or small pig farms, wild boar populations, arrival of contaminated food at landfills, and contaminated trucks. Preliminary results suggested striking differences in ASF spread depending on each of the analyzed pathways. During the first 50-70 days from the initial infection events, the role of small farms, trucks and waste treatment plants increased the spread and prevalence of ASF, far outweighing the other routes of transmission. Moreover, the extent of potential contact (by sharing personnel, tools, equipment) between farms at local scale (10-35 km) and the size of small farms (10-50 sows) were key parameters controlling the dynamics.

In the full model including all routes, a farm-level prevalence of 61% was reached 60 days after the initial infection, whereas a reduction in indirect contact radius of 25 km reduced this prevalence to 12%. However, the mean intra-farm prevalence was 24.4% (SD = 31.8%) and showed considerable spatial heterogeneity depending on the type of farm.

Additionally, the prevalence of wild boar populations was estimated at 2%, while the percentage of contaminated trucks was estimated at 4% of the fleet. This model made it possible to weigh the role of the propagation pathways in the face of a potential ASF outbreak in the Argentine swine industry.

These results could guide the decision-making process for designing appropriate surveillance measures and preventive actions at both the national network and farm level. Management focus on small pig farms, waste treatment plants, and animal transport could help reduce the consequences of an ASF epidemic.