Development And Evaluation Of Biosecure Static Composting Systems For Disposal Of Infectious Livestock Mortalities

Recent high-profile outbreaks of infectious animal diseases (e.g., mad cow disease, foot and mouth disease, and avian influenza) emphasize the need for safe methods of mortality disposal. Comparing to the conventional methods of burial, incineration and rendering, biosecure mortality composting could be a reliable and practical alternative for disposal of infectious carcasses and manure. However, the utilization of animal carcass composting was limited, which was partially caused by the lack of understanding of pathogen inactivation profile and animal tissue degradation efficiency. The present study aimed to develop a biosecure static composting system for disposal of cattle carcasses and manure in the event of an infectious disease outbreak, investigate the pathogen inactivation rate and cattle tissue degradation efficiency, and finally construct the theoretical and experimental foundation for the using of biosecure static composting system as a carcass biosafe disposal method in the event of animal disease outbreak.To develop a biosecure cattle mortality composting system, we construct duplicate biosecure structures in 2006 (Compost2006), containing 16 cattle (Bos taurus) mortalities (343 kg average weight) with carcasses placed on a 40 cm straw layer and overlaid with 160 cm of feedlot manure. The whole compost material was covered by the silage plastic all over the sides and built in a barley straw bale rectangle bin of 25 m length,5 m width,2.4 m height, and 1.2 m thickness. Temperature was monitored daily by the thermal couples and moisture content was determined at intervals at 80 and 160 cm depths (P80, P160) over 147 d composting period. At P80, compost heated rapidly, exceeding 55℃after 8 d and maintained temperatures of 55-63℃for>35 d. At P160, temperature failed to exceed 55℃, but remained above 40℃for>4 mo. Using manure of 68% initial moisture content could be one of the reasons that P160 failed to reach 55℃, as the downward pooling of water resulted in over-wet condition of 79% moisture content at P160 after 147 d and prohibited the oxygen transfer and the microbial activity. Although the cattle were deeply degraded after 147 d, raising carcasses higher and using manure with lower moisture content may improve heating in the area surrounding carcasses and enhanced the bovine tissue degradation. Thus, we built duplicate compost structures again in 2007 (Compost2007) with the modification of (1) carcasses were raised to 100 cm depth by placing them on an additional 60 cm layer of manure and (2) feedlot manure with 60% moisture content at construction was used. The following temperature profiles at depths of 40,100, and 160 cm (L40, L100, L160) all reached 55℃after 7 d, and maintained above 55℃for at least 70 d over 230 d biosecure composting. The moisture content at carcass depth varied between 54-67%, suitable to the microbial growing. Thus, placing carcass at middle depth and using manure of approximately 60% moisture content play important roles in the efficient compost heating.To investigate rates of microbial inactivation in Compost2006, Escherichia coli O157:H7, Campylobacter jejuni, and Newcastle disease virus (NDV) were inoculated in manure (E. coli O157:H7 and C. jejuni≈108 CFU/g; NDV,≈106 EID50/g), embedded at P80 and P160 and retrieved at intervals during composting. E. coli O157:H7 and NDV were undetectable after 7 d at both depths. The C. jejuni DNA was detected up to 84 d at P80 and >147 d at P160. Compost heat was the primary cause of pathogen inactivation, while factors other than heat also contributed. At the completion of biosecure composting (d 147 of 2006, and d 230 of 2007), levels of total coliforms were all below 1.0 log10 CFU/g dry wt at top, middle and bottom layers of structures, which sufficiently meet the requirements of USEPA (2003) and CCME (2005) as less than 3.0 log10 CFU/g dry wt.Real-time PCR SYBR Green Assay was developed to determine the concentration of bovine specific DNA in compost samples. A pair of specific and sensitive primers targeting 171 bp bovine mitochondrial (Mt) DNA fragment (Mt171) was designed. Compost DNA was extracted from freeze-dried and ground samples using DNA extraction kit. To eliminate the inhibition effects caused by the coextracted humid substances, the PCR system was optimized by adding bovine serum albumin (BSA) and diluting the DNA solution. The modification improved the real-time PCR efficiency and avoided the DNA purification process.Bovine DNA degradation in Compost 2006 and 2007 was studied using the optimized real-time PCR method. In Compost2006, bovine specific DNA degraded faster at P80 than P160. Compared to d 0, copies of the Mt171 fragment were reduced by 79% at P80, but only 20% at P160 after 147 d. In Compost2007, copies of the Mt171 fragment were degraded 75% by d 112 and 86% by d 230 at L100. Thus, the modification at construction in Compost2007 extremely improved the bovine DNA degradation at the carcass depth compared to Compost2006. Quantification of bovine specific DNA using real-time PCR assay could be an efficient method to evaluate the degradation efficiency of material of bovine origin.Bovine tissue decomposition ranked as brain>hoof>bone. More than 90% dry matter (DM) of brain disappeared after 7 d and 80% DM of hoof decomposed after 56 d, while only 15% DM of bone degraded after 147 d. The bovine Mt760 (a 760 bp bovine Mt-DNA fragment) and Mt171 fragments decomposed rapidly in bovine brain tissue (BT), and were degraded by 91%,84% by d 7, and 100%,99% by d 230, respectively. However, in the manure surrounded brain tissue (BM), copies of Mt760 and Mt171 increased substantially over d 7-28 but decreased gradually thereafter and reduced compared to d 0 by 94% and 75% after 230 d, respectively. The carcass degradation process was thus characterised by the primary cell decomposition within the decayed carcasses and the further infusion into the surrounding manure following the secondary deeper decomposition by the compost heterotrophic microbes.The present study developed and optimized a biosecure composting system degraded 16 mature cattle carcasses per structure in a period of 5-7 mo. The key points of high temperature composting include placing carcasses at middle layer surrounding with sufficient cattle manure, using feedlot manure of optimal moisture content (60%) and C/N ratio (>15), insulating compost matrix from cool environment by straw bales and retaining moisture content with plastic sheeting. As the biosecure composting system used inexpensive materials and was relatively simple to construct, it shows promise for use in disease outbreaks where emergency on-farm disposal of carcasses is required. The biosecure system could be sized to dispose of thousands of carcasses provided that sufficient space and stockpiles of manure were available.The rapid suppression of the model pathogens E. coli O157:H7, coliforms and NDV indicates the biosecure composting system can dispose of infectious carcasses and manure contaminated with similar bacteria or viruses that are sensitive to heat and the chemical properties of compost.