Design and evaluation of the 2009 national survey for disease freedom in the domestic pig population of South Africa

Abstract

The state of diseases in South African domestic pigs drastically changed when Porcine reproductive and respiratory syndrome (PRRS) was introduced in the Western Cape Province during 2004 and was followed by an outbreak of Classical swine fever (CSF), primarily in the Eastern Cape, in 2005. These outbreaks occurred mostly in rural and backyard pig populations. Subsequent to an intensive culling operation, it became necessary to implement country-wide surveillance to substantiate that freedom from these two diseases had been achieved. Samples for a country-wide cross-sectional study were collected from commercial and non-commercial pig establishments in a combined effort between the South African Pig Producers Organisation (SAPPO) and the National and Provincial Departments of Agriculture, Veterinary Services. The survey targeted the high-risk sector of the pig population and included rural pig populations, pigs from “grey” herds (commercial farms with inadequate bio-security) and commercial establishments. Samples were also tested for Swine influenza (SI), Transmissible gastroenteritis (TGE), Aujeszky’s Disease (Aujeszky’s), Foot-and-mouth Disease (FMD) and African swine fever (ASF). The first objective of this study was to evaluate the validity of the survey design. The second objective was to evaluate the results and, thereafter, make valid inferences regarding the disease status of the domestic pig population. This also included the confidence level of the overall survey. The third objective was to qualify and quantify sample losses to enable improved future survey design and execution. The outcome of the survey indicated that the results were sufficient to conclude that the non-commercial pig population is free from CSF, PRRS and SI at the expected minimum prevalence of 1%. The calculations also indicated that the results were adequate to conclude that the commercial pig population is free from CSF, PRRS and SI at the expected minimum prevalence of 5%. The outcome of the calculations done on the second phase test results allows South Africa to conclude that the non-commercial pig sector is free from FMD, ASF, Aujeszky’s and TGE, at the expected minimum prevalence of 2%. The sample sizes for Aujeszky’s, TGE, ASF and FMD tests in the commercial sector were too small to allow for a decision to be made and, as a result, it was not possible to declare this sector free of disease. However, the non-commercial sector is much larger and the risk of disease introduction higher, therefore, if the non-commercial sector tested negative for the disease, it is highly likely that the commercial sector will be free of the disease as well. It is recommended that passive surveillance be augmented and that a national pig survey be conducted every 3 years to provide proof of disease freedom. It is important to conduct a risk analysis to qualify and quantify the risk of CSF and PRRS introduction. This could improve future surveys by providing a scientific basis for targeted surveillance. Copyright