对虾养殖的棘手难题科普实践

对虾养殖的棘手难题

Collaboration article with Stony_Brook

对虾是一种美味的高蛋白海鲜，养殖规模大，在甲壳类水产养殖中占据首要地位，也备受消费者的喜爱。凡纳滨对虾（Penaeus vannamei）、罗氏沼虾（Macrobrachium rosenbergii）和斑节对虾（Penaeus monodon）是世界范围内产量最高的三种优质对虾。其中，凡纳滨对虾富含蛋白质、矿物质和不饱和脂肪酸，并且含有多种对人体有益的生物活性成分，是一种质量优良、营养均衡的食源性营养源。因此，凡纳滨对虾在保健品、医药、化妆品、食品添加剂以及水产养殖等方面都具有广阔的应用前景，有较高的经济价值和医用价值。

Shrimp, a delicious seafood high in protein, currently plays a leading role in crustacean aquaculture production with a fast-growing market and farming size. Penaeus vannamei, Macrobrachium rosenbergii, and Penaeus monodon are known as the three best-quality shrimps with the highest production rates in the world.Penaeus vannamei in particular, is especially rich in proteins, minerals, unsaturated fatty acids, and contains functional biological components that have various beneficial effects for the human body. Overally, it is a high-quality and balanced source of nutrition. Therefore, this kind of shrimp has broad applications in health products, medicine, cosmetics, food additives, and aquaculture, and has a high economic and medicinal value.

凡纳滨对虾还具有生长速度快，抗病能力强，易粗养易运输以及产肉量高等优点。随着中国经济的发展和人民生活水平的提高，目前的对虾产量已不能满足快速增长的市场需求。凡纳滨对虾的产量和养殖规模在中国虾蟹类人工养殖业中均居首位。根据《中国渔业统计年鉴》的数据，在2013年至2020年间，中国对虾产量总体呈上升趋势，到2020年达到630.73万吨。目前全球对虾产量超过800万吨，由此可见，在中国乃至全世界，对虾已经成为一种交易量很大的海鲜。

Penaeus vannamei has the advantages of fast growth, strong disease resistance, easy rough culture and transportation, and high meat yield. With the development of China's economy and the improvement of people's living standards, the current yield of shrimp is unable to satisfy the fast-growing market demand. Among the artificially cultured shrimp and crabs in China, the yield and culture of Penaeus vannamei rank first. According to the data from China Fishery Statistical Yearbook, Chinese shrimp production showed an overall upward trend from 2013 to 2020, reaching 6.3073 million tons (MT) by 2020, while the whole world currently exceeds 8 MT of high value. This proves that shrimp has emerged as a highly traded seafood product in China and around the world.  

水产养殖业在亚洲甚至是全球水产行业中占据主导地位，但目前正面临着严峻的考验。据世界粮农组织估计，由疾病造成的经济损失每年超过90亿美元，约达世界鱼类和贝类养殖行业产值的15%。疾病的威胁不仅阻碍许多水产养殖业的发展，甚至导致一些养殖企业倒闭。一些国家的对虾产业也因病害而遭受重创。

Aquaculture dominates aquatic food production both in Asia and globally, and is now facing a significant challenge. In this industry, economic losses due to disease outbreaks have been estimated by the Food and Agriculture Organization (FAO) to be over $9 billion per year, which is approximately 15% of the world's farmed fish and shellfish production value. The threat of disease has become a barrier that prevents numerous aquaculture industries from developing, and has even led to the collapse of some. Because of this, shrimp production has been seriously damaged in several countries.

例如对虾急性肝胰腺坏死病（AHPND），最初被称为早期死亡综合征（EMS），对对虾养殖业就造成了毁灭性的打击。这种疾病发病快，从放养后约8天开始传染，在20-30天内对虾出现大量死亡（死亡率高达100%）。据估计，疾病死亡率每增加1%，经济损失每公顷增加25.5美元，损失概率增加1.4%。

Acute hepatopancreatic necrosis disease (AHPND), originally known as early mortality syndrome (EMS), has a devastating impact on the shrimp aquaculture industry. This disease develops quickly, starting about 8 days post-stocking, with severe mortalities (up to 100%), occurring within 20-30 days. It is estimated that these losses raised prices by $25.5 per hectare, and loss probability raised 1.4% for every 1% increase in disease mortality. 

对虾急性肝胰腺坏死病自2009年首次暴发以来，迅速在世界各地蔓延，不仅导致对虾大量死亡，给对虾养殖业造成严重经济损失，也对食品安全构成了威胁。自2013年底南亚国家的养殖行业崩溃以来，该疾病危险性不断升级。在2017年，世界动物卫生组织（OIE）通过了修正提案，在《水产守则》和《养殖指南》中为该疾病设立了专门的章节，进一步强调了该疾病的严重性。

Since the outbreak in 2009, AHPND has spread rapidly around the world, causing massive shrimp mortality and severe economic losses to the shrimp farming industry, as well as posing a threat to food safety. Moreover, it escalated in late 2013, when the industry collapsed in South Asian countries. In 2017, the World Organization for Animal Health (OIE) adopted amendments to create a specific chapter in both the Aquatic Code and Manual for AHPND, which demonstrates that the authorities have acknowledged its seriousness.

对虾急性肝胰腺坏死病的主要病原是一种携带pVA1质粒的副溶血性弧菌（Vibrio parahaemolyticus），该质粒编码靶向虾的肝胰腺细胞的PirA和PirB二元毒素。这些毒素会结合对虾的肝胰腺上皮组织并造成穿孔，最终导致细胞的死亡。

The major pathogen of AHPND is Vibrio parahaemolyticus which carries a unique plasmid, pVA1. The plasmid encodes the binary PirA and PirB toxins which target the digestive gland of shrimp. These toxins bind with hepatopancreatic epithelial tissue, forming pores and leading to subsequent cell death.

pVA1质粒可从副溶血性弧菌转移到其他菌种，如朋氏弧菌（V. punensis）、哈维氏弧菌（V. harveyi）、欧氏弧菌（V. owensii）、坎贝氏弧菌（V. campbelli）、希瓦氏杆菌（Shewanella sp.）等，这使宿主病菌具有更强的致病能力。致病菌传播后会定居在对虾的消化道内，对组织造成严重破坏。感染AHPND的对虾的主要临床症状是食欲减退、嗜睡、生长缓慢、外壳软化和空肠。由于AHPND的发展迅速，一旦发病，就将导致无法挽回的大规模对虾急性死亡。

The pVA1-type plasmid can also be transferred from V. parahaemolyticus to another species, such as V. punensis, V. harveyi, V. owensii, V. campbelli, and Shewanella sp., giving the host bacteria a stronger pathogenic ability. The bacteria then spread and colonize the shrimp's digestive tract, causing severe damage to tissues. Moreover, the primary clinical AHPND symptoms in infected shrimps are anorexia, lethargy, slow growth, soft shelling, and empty digestive tract. Due to the rapid development of AHPND, once the disease occurs, it will cause an irreparable large-scale acute shrimp death.

传统AHPND的防治方法存在收效甚微或者治疗效果不可持续的缺点。例如，将抗生素添加进对虾饲料或养殖水体中是对抗对虾细菌感染的有效方法之一，但使用抗生素会对公众健康和环境构成严重威胁。在对虾养殖场中，抗生素的持续使用将促进抗生素耐药细菌（ARB）菌株的发展，从而使这些药物渐渐不再有效。ARB菌株更加耐受温和的热处理方法，如食品加工中的巴氏杀菌法，这将严重威胁到食品安全。此外，由于微生物组多样性对宿主健康有显著影响，而竞争排斥原理认为肠道微生物多样性越高，病原菌定植的可能性就越低。在孵化场和养虾场使用抗生素会破坏肠道微生物的动态平衡，因此这也可能导致情况恶化。

The conventional approaches for preventing and curing AHPND either have limited success, or are unsustainable in the long-term. Mixed in the shrimp diet or added to rearing water, antibiotics are used as powerful medicines to fight bacterial infections in shrimp production. However, their use presents a serious threat to public health and the environment. Additionally, there is growing antibiotic resistance which is curbing the effectiveness of these drugs. Continuous application of antibiotics by shrimp farms facilitates the development of antibiotic-resistant bacterial (ARB) strains. Antibiotic-resistant strains can be more tolerant to mild heat treatments, such as pasteurization in shrimp processing, which will threaten food safety. Further, it has been proved that microbiome diversity significantly impacts host health. The competitive exclusion principle, which is also known as Gause's law of competitive exclusion, states that the higher gut microbial diversity, the lower the possibility for pathogenic colonization. The application of antibiotics at hatcheries and shrimp farms would disrupt the microbiota homeostasis.

减轻养虾场弧菌感染的另一种方法是改善水质。由于弧菌感染是概率性事件，而对虾对致病菌的敏感性往往受到有利于疾病爆发的水质条件的影响，因此对对虾养殖场的水质进行监管以防止大规模感染是很重要的。

An alternative strategy to mitigate the effects associated with Vibrio infections in shrimp farms, is to improve water quality. As Vibrio species are opportunistic, the susceptibility of Vibrio infection to shrimp is often affected by water conditions that favor a disease outbreak, which raises the urgency to manage the water quality of shrimp farming to prevent mass infection.

此外，维持池塘和对虾胃肠道中藻类和细菌之间的生物平衡也是减少AHPND感染影响的有效途径之一，使用益生菌可以抑制对虾的某些细菌感染。然而，实际上益生菌或其天然产物是否真的抑制了水产养殖池塘中的某些诸如弧菌的病原菌目前尚不清楚，具体的抑制途径与机理也有待进一步探究。而关于不同益生菌在水产养殖中对对虾的协同保护作用也尚不清楚。

Besides, maintaining the biological balance among algae and bacteria in ponds and the gastrointestinal tract of shrimps is also one of the ways to reduce the effects of AHPND infection. The use of probiotics to inhibit certain bacterial infections in shrimps and enhance water quality has proved to be valid for aquaculture purposes. However, it is unclear whether the actual probiotics or their natural products actually inhibit certain pathogenic bacteria like Vibrio in aquaculture ponds. There is limited knowledge on the synergistic protective effects of different probiotics on shrimp in aquaculture.

最近的研究发现，对虾免疫系统会对AHPND作出响应，分泌抗菌肽（AMP）（如penaeidin和crustin）。此外还发现多种免疫相关因子在对虾体内具有抗菌作用，注射这些因子的重组蛋白可以有效预防AHPND。同时，一些植物提取物以及噬菌体也可作为潜在的抗菌剂来抑制副溶血性弧菌的生长。它们已经被应用到对虾饲料中，以最大程度地减少病原体的影响，提高对虾的存活率。

Recent research has found that, in response to AHPND, the shrimp immune system expresses antimicrobial peptides (AMPs) such as penaeidins and crustins, which provide some protection against AHPND-causing V. parahaemolyticus. Several immune-related factors have been discovered to be antibacterial in shrimp, and the injection of recombinant proteins of these factors can be an effective treatment to prevent AHPND. While some plant extracts and phages (bacterial virus) can serve as a potential antimicrobial agent to inhibit the growth of V. parahaemolyticus, they have also been successfully utilized as feed supplements in the shrimp diet to minimize the effect of pathogen and improves the survival of shrimp species.

尽管与AHPND相关的研究取得了许多进展，但PirA/B引起AHPND的潜在机制仍不清楚。pVA1毒力质粒的关键信息，包括每个细菌细胞内的质粒拷贝数的变化及其对AHPND发病机制的影响仍是未解之谜。因此，AHPND的发病机制还需要进一步的研究。

Despite the progress in AHPND-related research, the potential mechanisms of PirA/B that cause AHPND in shrimp remain unknown. Although many reports related to the pVA1 virulent plasmid, the critical information remains a mystery, including variability of the plasmid copy number per bacterial cell and how this influences the pathogenesis of AHPND, indicating further investigatory for the pathogenesis of AHPND is also needed.

目前AHPND的诊断主要通过观察患病对虾的临床症状，此外也有对病原菌利用包括PCR扩增检测、分子诊断工具等方法进行检测。但目前尚无可用于现场检测且有效快速的检测方法。综上所述，我们仍然迫切需要针对该病的有效的诊断、治疗、预防和管理手段，以应对未来可能出现的大规模疾病暴发。

The current diagnosis of AHPND includes the examination of the affected shrimps through the clinical symptoms of AHPND, PCR amplification detection, molecular diagnostic tools, etc. However, effective methods, used on-site and in-time, have not been established to help diagnose AHPND. In summary, effective prevention, treatment, diagnosis, and management of this disease are still in urgent need to take precautions and deal with further outbreaks.