In South-East Asia and China, a large proportion (up to 70%) of animals are still kept in traditional small-scale and backyard settings [8], and this leads to the close proximity of production systems of different sizes and biosecurity levels — important factors that can determine the scope for disease spread and the effect that diseases might have on pig populations. Since more than 70% of pig producers are smallholders, a major problem that prevails is the low level of knowledge and understanding among livestock producers regarding the benefits of disease control. Although commercial pig raisers are aware of the benefits of disease control, they are not as knowledgeable about the requirements for a cooperative national disease monitoring programme [3]. Medium- and small-scale pig farmers generally have the traditional farrow-to-finish systems, often with very close mixing of age groups. Replacement stock often comes from a variety of sources and of unknown health status with no adequate quarantine before entry. Hygiene application is usually very poor or does not exist with respect to the contact between farm workers and pigs outside their farms, or between outsiders and pigs housed within the farms. In small-scale hog-raising, disease control is normally done through vaccination. The use of high levels of combinations of antibiotics and anti-bacterial agents without adequate supervision or veterinary advice is very common [8].
Pork is one of the most important agricultural products in the region and pork consumption is highest in terms of total meat produced in some countries (Cambodia, China, Philippines and Vietnam). The increasing consumption of pork leads to an increase in commercial pig production. Statistical data reveal that HP-PRRS has been spreading mainly in areas with higher pig densities. In Vietnam, the Red River Delta forms the main area for intensive pig production in the northern part of the country and constitutes a prime location for pig diseases, including for the emergence of highly virulent PRRS. From here, there is a trade in live pigs, including piglets, fatteners and finishers to the south of Vietnam. Pig movements in these value chains also match and likely explain the rapid spread of PRRSV. The movement of pigs infected with HP-PRRS, even dead pigs, could also be responsible for the disease spreading between neighbouring countries [16].
Transboundary spread of HP-PRRSV from southern China to South-East Asia clearly suggests that biosecurity failures have occurred, including failure to control animal movements and trading among neighbouring countries at borders. Virus spread within a country also confirms the failure of biosecurity control, primarily via uncontrolled human movements within highly contaminated areas, especially at loading zones and slaughterhouses [R. Thanawongnuwech, personal observation]. Sharing such contaminated areas with other vehicles may increase the spread of the pathogen within the community.