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By Dr Carlo Maala and Dr Marika Genzow, Boehringer Ingelheim Animal Health

The first international awareness of a new disease in China became evident on the Internet platform Promed, where a report in April 2006 stated that 161 pigs were found dead in two areas in China near the Fujiang river in Zhejiang, Jiangsu province.
  Initially, the new phenomenon was referred to as ‘swine high fever disease’, or SHFD. The disease was observed and followed a pattern which started in backyard, small- and middle-size pig farms. Then, it usually spread to extensive pig farms. It caused high morbidity and mortality in pig herds, the animals having higher body temperature, rubefaction on the skin, blue ears, respiratory disorder and gastrointestinal symptoms. An infection with Porcine Respiratory and Reproductive Syndrome (PRRS) was suspected, but not yet confirmed.
  The change of season did not affect its spread – and soon identical clinical situations were recognised in neighbouring provinces in the middle and south of China, including Hunan, Hubei, Zhejiang, Fujian and Anhui. After August 2006, the disease spread to Henan, Shandong, Hebei, Beijing, Tianjin and Guangdong province, after which China reported the disease to the OIE. In April of the next year, there was a new wave of outbreaks in Sichuan and Henan and the northern provinces were affected in July 2007.The unprecedented large-scale outbreak resulted in a dramatic decline of pig numbers, especially in severe epidemic areas, and the bankruptcy of large numbers of small- and middle-size pig farms, and brought great economic loss to the Chinese swine industry. This disease was the major reason for sky-rocketing prices of pig and pork in 2007 in China. Pork was sold at prices up to 75% higher, even causing inflation in other parts of the Chinese economy.At last year’s Asian Pig Veterinary Society (APVS) Congress in Tsukuba, Japan, Dr Hanchun Yang, connected to China’s College of Veterinary Medicine and the Agricultural University of Beijing, spoke about the outbreak. In his presentation ‘High virulent PRRS in China – truth or false’, he said that due to no strict control measures at the beginning and no effective measures to restrict pig trade and movement among the different areas, the outbreak was pandemic all over the country.
  The disease did not stop at the borders of China. The disease spread rapidly to other countries including Vietnam, Laos, Cambodia, the Philippines and Bhutan, as can be seen in Figure 1. Even Thailand may have been infected. In total, more than 2,000,000 pigs were involved, causing great concern to the global swine industry and to public health.

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What type of viruses were at the bottom of the outbreak? Several laboratories in China have isolated the same PRRSV strain in the epidemic areas or pig farms of the outbreak, and completed the whole genomic sequencing of the viruses. Genomic analysis showed that all the PRRSVs isolated from the outbreaks of Porcine High Fever Disease belong to the North American genotype and share a unique discontinuous deletion of 30 aa in Nsp2.
  To confirm that the newly isolated PRRSV was the causative agent for the virulent outbreaks in China, an experimental infection was conducted. The clinical signs of ‘high fever disease’ could be re-produced in this experimental setting, which confirmed that disease is caused primarily by a highly pathogenic PRRSV strain, however common co-infections with other agents like PCV2, Classical Swine Fever (CSF) virus and Mycoplasma hyopneumoniae are contributing to the disease in the field.The origin of the virus is still mysterious. At the APVS, Yang speculated that the highly pathogenic (HP) PRRS in China experienced a gradual variation and accumulated genome changes from Chinese domestic viruses during its emergence. To date, it remained unclear to him, however, why the virus did exhibit high pathogenicity (fatality) for pigs compared to previous strains and other North American type strains.
  It is well known that PRRSV can cause immunosuppression and result in secondary bacterial infection in the affected pigs – and so did it happen with this strain. The pigs infected with the highly virulent PRRSV displayed severe secondary bacterial infection in the field. The major secondary infectious pathogens include Haemophilus parasuis, E.coli, Streptococcus Suis Serotype 2, Salmonella, Pasteurella multocida and eperythrozoon.

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