Letter to the Editor

High fever swine disease impacts in Asia

We write to inform our fellow AASV members of the current knowledge and opinions concerning a serious disease outbreak in mainland China, known locally as porcine high fever disease (PHFD). At least 23 provinces of China have been affected, including the peri-urban commercial farm sectors around Guangzhou, Shanghai, and Beijing, totaling about 300 million pigs. The vast extent of the outbreaks across the country and the impact on affected farms has made PHFD a major political issue, as these factors have apparently led directly to reductions in pig supply and hence rises in pork prices, additional to those due to rising input prices, particularly of feedstuffs. Different China-based media report 400,000 to over 100 million deaths during 2006 to mid-2007. Supermarket pork prices in eastern China rose from around $1 to $2.50 per kg (US$) between 2005 and mid-2007. Because China boasts a huge pig population of about 500 million, economic considerations suggest that the higher mortality figures may be more accurate. A farm outbreak is now viewed as a notifiable disease situation by China Ministry of Agriculture authorities, with imposition of quarantine, movement restrictions, and subsidized on-farm interventions. A budget of 3.8 billion yuan (approximately US$502 million) was allocated in mid-2007 for a pig-farmer support scheme, including free farm and sow insurance, pig transport, and vaccinations.¹

China was recently the world’s 5^th largest exporter of pork. The relatively unknown health status of Chinese pigs, occurrence of viral diseases such as classical swine fever (CSF) and foot-and-mouth disease (thought to exist widely but largely undeclared publicly) and now PHFD, are important issues for potential importers.

Representative cases of PHFD show high fever (40°C to 42°C), blotchy congested spots, and rashes, especially over the ears, face, and inner thighs. Pigs are depressed, anorexic, and lethargic and occasionally cough; pregnant sows abort. The disease course is usually 5 to 20 days. Many pigs die: the case fatality rate is estimated at 20%, with deaths occurring both in sick and apparently well-grown finishers and adult pigs. Pigs with PHFD are highly contagious – the whole adjacent pig population is generally affected within 3 to 5 days – so an affected farm faces a massive and sudden outbreak with major economic consequences. Mortality rates of up to 80% have been reported in rural pig-farm groups. Necropsy reveals widespread hemorrhages and edema in multiple organs, diffuse lung edema, diffuse petechiae in the kidneys, splenic infarcts, and hemorrhages into the bladder wall and local lymph nodes.

There is some dispute about the exact etiology of PHFD. Reliable first-hand reports and necropsy findings suggest co-infection by American-origin strains of porcine reproductive and respiratory syndrome virus (PRRSV) and virulent CSF virus. On many farms, other agents are likely to be involved, such as porcine circovirus type 2 genotype 2b and secondary bacteria. Classical swine fever may be very difficult to control, even with availability of excellent high-potency vaccines, due to maternal antibody interference with vaccinal uptake and early periweaning infection. In contrast to this suggested involvement of virulent CSF, Chinese government-backed presentations and articles have focused on the theory that PHFD and its high mortality are caused exclusively and primarily by a mutant atypical and highly pathogenic PRRSV strain spreading across China. Dr Gao’s laboratory in the Chinese Academy of Sciences in Beijing has produced a whole-genome sequence of this mutant PRRSV, which has a 30-amino-acid deletion in the NSP2 region.² However, mutations and deletions in the NSP2 region are also frequently noted in American strains of PRRSV.³ Whether any of these mutant strains in China (or the United States) can now initiate a high mortality syndrome without an accompanying pathogenic CSF infection and its accompanying immunosuppression and other deleterious effects requires independent verification. The concept of atypical PRRSV strains capable of causing deaths in adult pigs as part of an emerging disease has been disputed in past outbreaks of high-mortality syndromes in the United States,⁴ but the China situation suggests watchfulness is required. The Chinese-government-sponsored focus on a mutant PRRSV etiology for PHFD is consistent with the overall aim of enhancing international perceptions of pig farming in China, as it minimizes the possible involvement of undeclared, pathogenic CSF strains.

Pork accounts for 70% of meat consumption and 4% of the consumer price index in China. It is likely that PHFD-induced price rises have had a direct social impact, as the annual inflation rate has risen above the benchmark 3%. The very high costs of the PHFD outbreaks to pig farmers and wider Chinese society has therefore directly impacted the key Chinese government policies of enhanced rural employment, social cohesion, and food security. This prompted the premier of China, Wen Jiabao, to linger reassuringly and publicly at a supermarket pork counter in Xian on May 26, 2007. One can only hope that more world leaders will follow his example of personal time investment in the commercial pig industry.

-- Steven McOrist

-- Stanley Done

School of Veterinary Medicine and Science University of Nottingham; Sutton Bonington, England

References

1. People’s Daily Online. Chinese government to allocate 6.5 billion yuan to tether pork prices. June 23, 2007. Available at: http://english.peopledaily.com.cn/200706/24/eng20070624_387168.html. Accessed 30 August 2007.

2. Tian K, Yu X, Zhao T, Feng Y, Cao Z ,Wang C, Hu Y, Chen X, Hu D, Tian X, Liu D, Zhang S, Deng X, Ding Y, Yang L, Zhang Y, Xias H, Giao M, Wang B, Hou L, Wang X, Yang X, Kang L, Sun M, Jin P, Wang S, Kitamura Y, Yan J, Gao G. Emergence of fatal PRRSV variants: Unparalleled outbreaks of atypical PRRS in China and molecular dissection of the unique hallmark. PLoS ONE [serial online]. Published June 13, 2007. http://www.plosome.org. Accessed 2 September 2007.

3. Han J, Wang Y, Faaberg KS. Complete genome analysis of RFLP 184 isolates of porcine reproductive and respiratory syndrome virus. Vir Res. 2006;122:175–182.

4. Hurd HS, Bush EJ, Losinger W, Corso B, Zimmerman JJ, Wills R, Swenson S, Pyburn D, Yeske P, Burkgren T. Outbreaks of porcine reproductive failure: Report on a collaborative field investigation [published corrections appear in J Swine Health Prod. 2001;9:154]. J Swine Health Prod. 2001;9:103–108.