Scientific Abstract
Increased global movement of people and animals increases likelihood of foreign animal diseases (FAD), like African Swine Fever (ASF), being introduced into the US swine population. Due to the risk of pathogen amplification in infected animals, swine on affected premises would need to be depopulated as soon as possible. The size of most US commercial swine operations makes rapid depopulation difficult, with large sow units being especially problematic due to number of large animals (>200 kg). Most current methods for swine depopulation and disposal are not good candidates for mass depopulation in sow units because they are either time consuming (delivered at the individual level), create an unacceptable risk of disease dissemination for other swine populations (e.g. pathogen dissemination during movement of carcass material to disposal sites), or are practically infeasible to facility designs. Rendering trailers are an ideal option for transportation of carcasses to disposal sites because their water-tight design means decreased release of the infectious agent in transit. Here, we combined the depopulation and disposal steps by using rendering trailers as depopulation chambers. With modification, rendering trailers can permit loading of live sows, which would allow sows to exit their housing facilities under their own power prior to depopulation. Depopulating sows after they have been loaded onto modified rendering trailers allows us to consider inhalation-based depopulation methods because we would overcome some of the inherent hurdles caused by housing facility designs (i.e. slatted floors and deep manure pits create large volume air spaces). The addition of modified rendering trailers to USDA’s National Veterinary Stockpile would mean rapid and efficient depopulation and disposal of sows could be performed in the US swine herd.
We evaluated the following depopulation agents: carbon dioxide gas (CO2), nitrogen gas (N2), compressed air foam (CAF), CAF made with nitrogen gas (CAF-N2), and aspirated foam (AF). We used a gated approach where we assessed our ability achieve success at each step before moving to the next step. First was the ability fill the container in a rapid manner. Second, each treatment was used in a small scale trial on anesthetized sows. Third, each treatment was used in a small scale trial on conscious sows. Fourth, one agent (aspirated foam) was used for a large-scale field trial. While all methods were successful, aspirated foam had the shortest mean time to the cessation of movement (186 seconds [95% CI 145 – 228], which was why it was selected for the large-scale field trial. Our results show that aspirated foam was comparable to the gold standard CO2, but had fewer potential pitfalls than CO2 (availability and sourcing CO2, vaporizing, delivery time, etc.). In the large-scale trial using aspirated foam, the average time to cessation of movement was 128 seconds (range 98-162; 95% CI 118 – 138) for groups of 45 sows.
Water-based foam (WBF) depopulation is a preferred method in poultry, leading to rapid death and relatively ease of application and USDA has WBF generating units in the National Veterinary Stockpile, which makes it an attractive agent. Historically, foam has not been feasible inswine due to their housing; however, use of a trailer provides new opportunities. Our study shows that WBF for depopulation of sows in modified rendering trailers is a viable option for mature swine (and likely younger pigs too).
When a FAD is introduced to the US swine herd, the way to lessen the economic impact to producers is by preventing further spread in the national swine herd. The ultimate objective of project was to establish an effective method to facilitate both the depopulation of sow units and the disposal of carcasses in the face of an FAD outbreak. We demonstrated that use of inhalant–based depopulation methods on sows in modified rendering trailers can be a cost-effective and efficient method for stamping-out FAD-affected sow farms. Using an inhalant-based depopulation method in rendering trailers will allow for containment of a FAD as quickly as possible to help maintain continuity of business for non-infected animals and non-contaminated animal products.
