Pig movements across eastern Indonesia and associated risk of classical swine fever transmission

Abstract

Classical swine fever (CSF), a highly contagious Pestivirus, has caused substantial socioeconomic loss for pig farmers in Indonesia since its introduction to this country in the mid 1990s. Live pig movements are believed to have facilitated the introduction of CSF into Nusa Tenggara Timur (NTT) province, in eastern Indonesia. This province has the largest pig population, with approximately 85% of households owning at least one pig and the smallholder sector being the dominant industry. Pigs have high cultural and economic importance—being used as an income source, financial security and playing an important role in traditional and religious ceremonies. The number of reported CSF cases in NTT is still increasing, with newly infected islands as recent as 2011. This study was conducted from 2009 to 2012 that investigated live pig movements and the role of markets and villages in CSF transmission across NTT province. The research aimed to identify areas where mitigation measures could be implemented to assist in the control of CSF to reduce its spread in NTT and reduce the potential risk to Australia. Six phases were incorporated into the study to produce findings that could inform decision making. The first phase of the research was to investigate formal pig movements (farm to market) by conducting a market survey where interviews were conducted at nine live pig markets on West Timor, Flores and Sumba islands during September and November 2009, with 292 pig seller and 281 pig buyer respondents. Information was collected by questionnaire on pig movements, pig management, biosecurity, and knowledge on pig health and CSF. Grower and fattener pigs were most commonly sold at market with high annual demand periods identified from August to October. Understanding of CSF and biosecurity was limited, with 85% of sellers and 83% of buyers stating they had no prior knowledge of CSF. Observations were also conducted at each market site which provided baseline information on live pig markets and confirmed an extreme lack of biosecurity with high risk practices having the potential to influence CSF transmission. To investigate the potential role of informal movements of pigs (farm to farm) in CSF transmission, a survey of smallholder pig farmers was conducted from March to May 2010. Eighteen villages were selected across West Timor, Flores and Sumba, and 289 pig farmers were interviewed. Information was collected by questionnaire on pig movements, pig management, biosecurity, and knowledge on pig health and CSF. Most (73.0%) farmersstated they purchased pigs in order to raise the animal on their farm. Over half of the respondents (65.0%) purchased pigs from another farmer and not through a market, and 35.6% reported purchasing at least one pig within the last year. Pigs were sold or left their herd most commonly during the months of January, August, September and October. Information obtained from the market and farmer surveys enabled a social network analysis (SNA) to be conducted on formal and informal movements using information on trading practices, source and destination locations, and the number of pigs being moved. Both inter- and intra-island movements were found to occur, however inter-island movement was only observed between Flores and Sumba islands. West Timor and Sumba had highly connected networks where large numbers of villages were directly and indirectly linked through pig movement. Pig movements were identified from Kota Kupang to the border of East Timor in West Timor connecting all five districts. The pig-movement network on Sumba had a higher potential for pigs to move a greater number of sequential locations across the entire island. Flores was found to have a more fragmented network, with pig movements concentrated on its eastern or western regions, influenced by terrain. Markets were considered high-risk locations for the introduction and spread of disease, having over 20 contacts (based on in and out-degree values) with different villages on an operational day. Of the markets investigated, Detusoko and Mbay markets on Flores and Waikabubak Market on Sumba represented the highest-risk market locations for the potential to spread disease through the network. For informal movement among the villages investigated, Rindi Village in Sumba represented a high-risk location for CSF spread via informal movements with the greatest exiting pig volume (78 pigs) from 2009–2010. Nunbaun Delha Village in West Timor was a high-risk location for CSF introduction due to external contact with 13 other villages. A quantitative risk assessment was conducted on formal and informal pig movements across NTT province to assess the likelihood of classical swine fever virus (CSFV) transmission along the market chain. This risk assessment sought to identify pathways in the live pig market chain with the greatest risk of CSFV transmission and to assess the effect of mitigation measures to reduce CSFV transmission. Two modular risk models were developed, one to assess formal pig movements and the other to assess informal pig movements, with only live pig to pig transmission considered. Data obtained from the market and farmer surveys, published literature and expert opinion were utilised. A Monte Carlo simulation was performed with @Risk (Palisade) with 10,000 iterations. Modules were divided based on market chain movement processes from village to market/village. Outputs included the number of infected and clinical pigs at market/village and the probability a market/village was infected with CSFV. Mitigation strategies assessed were vaccination and pre-entry market or village inspection. The baseline model demonstrated that markets and villages in West Timor and Sumba had the highest number of infected pigs entering a premise. The process of inspection at the market or village needed to be strict for it to be effective and to reduce the probability that a market or village was infected. Increased vaccination coverage reduced the number of infected and clinical pigs arriving at a market or village. However, CSFV was not eliminated from the environment. Due to the risk posed by eastern Indonesia for the introduction of CSF into northern Australia, a simulation model was used to investigate the most appropriate surveillance techniques to detect and delineate the extent of infection among wild pigs for use following a CSFV incursion in the Kimberley region, north-western Australia. Due to the complexity of wildlife population dynamics and herd behaviour, it was concluded that a targeted approach to surveillance needed to be implemented. The use of simple random sampling was suboptimal, although disease was detected. The detection and containment of an outbreak must be as early and rapid as possible. The best way to approach the selection of an appropriate surveillance strategy is to use a more situation-based surveillance approach that accounts for disease distribution and the time period over which an epidemic has occurred. Radial and leapfrog surveillance were demonstrated to improve the effectiveness of infection delineation at various stages of a disease outbreak. These findings have provided baseline information on pig movements and the pig market chain, allowing further analyses to be conducted to assist in guiding decision making for the development of mitigation strategies to control CSF in NTT. The identification of higher-risk practices can now be addressed through farmer education, and the detection of higher-risk village and market locations has suggested potential sites for mitigation measures to be trialled. Moreover, the quantitative risk analysis has provided suggestions for potential control measures to reduce the risk of CSF transmission along the market chain, and the simulation model output has provided information on effective surveillance techniques for CSF detection and delineation in northern Australia.