非洲猪瘟在中国的经验和教训 - AASV 2020
African swine fever what’s working and not working in China - AASV 2020
Scott Dee, DVM, MS, PhD, DACVM
Gordon Spronk, DVM
Joseph Yaros, DVM
African swine fever virus (ASFV) was officially reported by the Ministry of Agriculture and Rural Affairs (MARA) in the domestic swine herd in China on August 3, 2018.1 Pipestone staff have been visiting swine herds throughout China and Asia for over 20 years as requested by Chinese producers, vendors to the Chinese industry, governmental agencies, and non-governmental agencies (NGOs). These visits have given our staff extensive field observations of clinical disease in swine herds in Asia both prior to and after the official reporting of ASFV. It may be of interest that swine disease presentation in Asia in the field typically results in the same field observations of the same pathogens as in North American herds and the rest of the world. In the majority of cases, the pathogen presentation is as widely described and published. In a small subset of field observations however, the presentation may be different and vary widely due to co-infections, pathogen isolate differences, operational differences, and implemented interventions or medications. Additionally, several pathogens (described as Foreign Animal Diseases, FADs) including foot and mouth disease (FMDV), classical swine fever (CSFV), pseudorabies virus (PRV) that have been eliminated from swine herds in North America and Europe have been commonly observed in China swine herds. These field observations of isolated pathogens (FADs) or with co-infections may provide opportunity to educate future global veterinarians and health officials allowing improvement in the health of the global swine herd. This paper will focus on the field observations of ASFV in commercial operations in Asia.
ASFV in the world swine herd<<
Prior to official reports of ASFV in the largest swine herd in the world, ASFV was commonly known to have spread widely over the past nearly 100 years from Africa to the Iberian Peninsula, to eastern Europe, across Russia, and finally into Asia. Movement of the virus is commonly understood to be the result of “people or human” movement of the pathogen; the virus movement speculated to be by movement of either infected pigs or meat by producers or staff.
ASFV is known and reported to be one of the deadliest viruses (if not the deadliest as measured by mortality) to infect swine of all ages. In small stakeholder operations in Asia, the virus may result in death loss of 100% of the pigs in the herd (note that any surviving pigs are commonly sold prior to death resulting in under reporting of true mortality). In larger operations, a small percentage of pigs have been observed and reported to survive an ASFV episode. Clinical presentation of ASFV infection is widely published and commonly understood to include a hemorrhagic fever, slow spread in a population, and high mortality. There are variations from these published reports under large field conditions in Asia and will be reviewed during this session.
In China, from August 2018 until August 2019, there were official reports of 160 cases in 32 of the 33 provinces in China. The virus has spread throughout China (and now in 10 countries in Asia), affecting pigs of all ages and genetics. Additionally, all types of production systems have been infected and observed under field conditions including smaller traditional farms (less than 500 head) to modern western style barns (over 8,000 sows) of all designs and operational practices.
Currently, the pig population in China has been unofficially reported to be dramatically reduced with common concern that total losses documented by official reporting starting in August 2018 is understated. While official reports do report a decline in the size of the China herd, it is widely speculated that official reports are under-estimating the full extent of total loss due to ASFV. There is no accurate or commonly accepted method to determine the exact amount of loss, with various authors speculating a minimum of 30% reduction in herd size to many sources reporting losses at various levels approaching higher than 50% loss of the China sow herd. Regardless of the exact loss, field observations confirm the severe impact of this virus is as widely published.
Reports that vaccines for ASFV will be (or already are) available in China are widely described. The first tool that veterinarians and producers traditionally implement to control any pathogen is a vaccine which successfully results in both reduction of mortality in an affected population and stops the spread of the pathogen between naïve animals. One additional feature of a good vaccine is the ability to discern the difference between the vaccinated pig and a pig exposed and infected with field virus. Researchers have struggled for decades to develop an effective ASFV vaccine and as of this writing, there is no officially approved vaccine for ASFV to prevent infection of naïve pigs.2
ASFV的流行病学信息在受控的研究条件下被广泛发表：该病毒会通过猪的所有体液散毒，其中，血液中病毒浓度最高。猪可通过鼻对鼻接触、精液、母乳、粪/口、含有病毒颗粒的皮下注射以及摄入受感染猪的组织而感染。该病毒的潜伏期为3-14天，临床症状包括无精打采、皮肤红斑、猪扎堆、发热、血痢疾和猝死。The epidemiology of ASFV is widely published under controlled research conditions: the virus is shed from all porcine bodily fluids, with blood containing the highest virus concentrations. Pigs can be infected via nose to nose contact, semen, milk, fecal/oral, injections under the skin containing viral particles, or ingestion of tissue from infected pigs. The incubation time of the virus is 3-14 days, with clinical signs including lethargy, erythema, piling of pigs, febricity, bloody scours, and sudden death.因为来自中国的存在潜在混合感染的大规模商品场的田间报告与已知的某些报告结果不符，因此还需要更多的流行病学研究和文献。其中一些田间报告的差异可能是由不同毒力的病毒分离株导致的，毒力通常报告为高、中、低毒力。这些ASFV分离株的田间表现和发病机制也可能不同，分为极急性、急性、亚急性和慢性。田间观察支持这些报告的临床表现差异。
More epidemiological research and publication is needed as field reports from China in large commercial operations with potential co-infections dispute some of these reported results. Some of the variation in field reports may be explained by differences in virus isolate commonly reported as High, Moderate and Low virulence. These ASFV isolates also may have different field presentations and pathogenesis described as Peracute, Acute, Subacute and Chronic. Field observations support these reported differences in clinical presentation.
Differential diagnosis for ASFV include, but are not limited to, classical swine fever, porcine reproductive and respiratory syndrome, pseudorabies virus, salmonella, and post-weaning multisystem wasting syndrome and other bacterial pathogens including Clostridium and Streptococcus.
ASFV is highly pathogenic, but not reported to be highly transmissible. Some authors describe this as “contagiosity” (there may be high mortality with low infection rate when compared to other pathogens like FMDV).3 In clinical field cases in large populations of commercial pigs where the virus has been left uncontrolled and epidemiological data has been monitored, total mortality can more than double in the matter of days until the majority of the population is deceased.
Investigations into the source of the infections into a population in Asia have been conducted to understand the routes of entry which may allow for future prevention of movement of the virus to other area farms. In general, field observations in Asia support rapid area spread with speculated routes of movement of the virus attributed to movement of infected pigs, movement of infected meat, contaminated fomites, rats, flies, mosquitoes, staff, feed, and lack of implementation of proper biosecurity protocols. The role of contaminated feed cannot be overlooked in the transmission dynamics. Improper disposal of the carcasses and movement of infected pigs are high on the list of methods that may have moved this virus across all of China. Pigs that have died from ASFV require proper disposal and enhanced biosecurity practices to limit the spread of ASFV to naïve populations since carcasses are shedding high levels of virus.
Site depopulation and cleanup protocols have been implemented and observed under field conditions and vary widely. It is critical that following depopulation of a site, a cleaning and disinfection protocol that removes all organic material and viral particles is completed to prevent viral transmission to the next group of pigs and for the site to remain healthy and free from ASFV. Commonly accepted protocols for other viruses (including but not limited to PEDV or PRRS) have been successfully used in the field. Attempts to eliminate ASF virus from sow populations using test and removal techniques are being reported in China.4 This elimination technique using herd closure, partial depopulation and whole herd sampling methods may hold promise and needs further research and documentation to understand if effective for this pathogen.
To ensure that ASFV does not enter other sites, a full biosecurity evaluation should be conducted to ensure that protocols regarding personnel entry, trucking, loading, feed, downtime, and other possible routes of entry are properly written, staff are educated on the topics, and standard operating procedures are properly implemented. A critical step is to be sure daily monitoring is also completed to maintain daily review and correction of any deficiencies in implementation and education by staff on the farm.
Additional capital expenditure projects to improve biosecurity are often necessary to prevent the infection and loss of more sites and farms. The only country that has successfully eliminated ASFV is Spain, in 1995, after years of unsuccessful attempts to eliminate ASFV from their national herd.2Asia now faces this decision as the virus has not only spread through China, but also into adjourning countries and may continue to spread without careful implementation of interventions to stop viral spread between farms and additional countries. All across the world it is the responsibility of the individuals with the greatest expertise in the swine industry to provide assistance and knowledge on preventing the spread and circulation of ASFV. In the United States, with leaders in the swine industry leading the charge to keep ASFV out of the country, the government will follow.
1. 2019. ASF Situation in Asia Update. FAO ASF Situation Update - African Swine Fever (ASF) - FAO Emergency Prevention System for Animal Health (EMPRES-AH). Food and Agriculture Organization of the United Nations.
2. Sanchez-Vizcaino, Jose M., et al. 2019. African swine fever virus. Diseases of Swine. 11:443-452.
3. Depner, K. 5 Aug. 2018. The World of ASF A World Apart. 7th ECPHM Resident Workshop.
4. Yan Jason 2019. Why New Generation Testing and Removal is Winning ASF Control in China. Proceedings, 2019 Leman China University of Minnesota. Zhengzhou, China. October, 2019.