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不可忽视的奶业隐形杀手——霉菌毒素返回 X

2020-05-21

不可忽视的奶业隐形杀手——霉菌毒素


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世界范围的奶牛单产都在不断提高,一定程度上源于现在的奶牛干物质采食量(DMI)比以往更高。较高的DMI使得霉菌毒素的摄入量也有所增加。反刍动物摄入霉菌毒素会导致其生长性能下降、拒食、饲料转化率低、增重减少、免疫抑制、繁殖障碍以及产品中存在霉菌毒素残留。


在单胃动物上,霉菌毒素的毒性影响通常更明显且更容易观察到。而在反刍动物上,人们关注霉菌毒素及其影响较少。实际上反刍动物通过瘤胃微生物群的作用解除霉菌毒素毒性的能力有限,一旦瘤胃解毒能力达到饱和,任何过量的霉菌毒素都会对动物造成负面影响。直到近些年人们才更加关注霉菌毒素对反刍动物的生产、健康状况和繁殖的影响。

 

霉菌毒素的发生条件与主要种类


曾有项研究调查了欧洲饲料样品中的霉菌毒素,结论是82%的样品被霉菌毒素污染,这表明霉菌毒素的存在是很广泛的。在某些环节上,当条件适宜的情况下霉菌毒素就会产生:不论是在生长的植物上,还是在收割期,或在以后的存放过程中和运输过程中。一般而言,许多霉菌产生霉菌毒素的适合温度范围介于20-30℃之间。而所谓的“贮藏期真菌”(收割后引发霉菌毒素问题的真菌)包括曲霉菌和青霉菌,即使在水分含量仅为14%-18%之间,且温度范围为10-50℃的条件下都会生长并产生霉菌毒素。


应激因子通常也会促进真菌定殖和毒素形成,如干旱、施肥差、作物密度高、杂草竞争、机械损坏等,这将削弱植物的自然防御能力。此外,各种原料混合在配合饲料中会增加饲料被几种毒素污染的风险。



反刍动物的日粮组成复杂,包括粗饲料、精饲料和青贮饲料,不同种类霉菌毒素单一污染的饲料原料或因通过混合而成为复合污染。各种不同霉菌毒素的组合也可能导致毒性互作。


迄今为止已鉴定出的潜在的有毒霉菌毒素中,黄曲霉毒素、伏马菌素(代表:伏马菌素B1[FB1])、单端孢霉烯(代表:脱氧雪腐镰刀菌烯醇[DON])、棒曲霉素、赭曲霉毒素、麦角生物碱和玉米赤霉烯酮(ZEN)在精饲料中是直接主要的种类,而存储性饲料,特别是青贮饲料主要含有棒曲霉素、霉酚酸和异烟棒曲霉素。


青贮前形成的霉菌毒素可能感染田间处于生长期的作物,或作为共生体寄生在植物内,如谷物作物的茎叶(田间霉菌毒素)。田间霉菌毒素包括单端孢霉烯毒素、玉米赤霉烯酮、伏马菌素、黄曲霉毒素和麦角生物碱。典型的镰刀菌毒素受天气影响很大。镰刀菌感染一般通过籽粒、叶子、茎或被感染的种子传播,土壤和田间腐烂的植物残渣是镰刀孢子和分生孢子的主要来源。


通常,三类真菌被认为是对家畜(特别是奶牛)有明确影响的霉菌毒素的重要来源:曲霉菌、镰刀菌和青霉菌。以下讨论将仅限于占主导地位的这三类。表1列出了这三组真菌产生的主要霉菌毒素。与一般的看法相反,新鲜的牧草并非没有霉菌毒素,而是有其自身的一类特异性霉菌毒素,如黑麦震颤素、麦角缬碱、雀裨麦角生物碱和相关的麦角生物碱和单端孢霉烯毒素


表2列出了美国食品和药品管理局(FDA)及欧盟(EU)当前对于黄曲霉毒素的法律限制以及关于其他主要霉菌毒素最大限量的指南。这两个指南要求基本一致,但需要注意的是,FDA重点严格限制伏马菌素,而欧盟则重点严格限制饲料中的黄曲霉毒素。不过,欧盟允许个别饲料中黄曲霉毒素B1的最高含量可达20 ppb。欧盟也提供了关于OTA、ZEN、T-2毒素和HT-2毒素的限量指南。


 

霉菌毒素对奶牛健康与繁殖的影响


每种霉菌毒素都会对动物的健康造成特定影响并出现特征性的症状。霉菌毒素对健康的许多影响是基于酶和免疫系统的变化。这些变化具有明显特点,足以将病因和特定的一类霉菌毒素关联起来。当某种霉菌毒素对免疫系统产生影响时,还会引起一些其他机会性感染。表3概述了与各种霉菌毒素有关的明显中毒病例的具体症状及其对繁殖功能的影响。


霉菌毒素的影响取决于摄入量、毒素剂量、接触霉菌毒素的时间和动物敏感性。然而,导致健康出现问题或繁殖功能不正常的霉菌毒素水平设定并不是很明确(某些霉菌毒素根本没有相应的剂量标准),即使有些提供的剂量也大多数是在实验条件下观察到的。列出的症状可能以非典型的形式在霉菌毒素水平较低的情况下出现,特别是在实际生产条件下,饲料中含有一种以上的真菌或霉菌毒素时更是如此。同样地,较低浓度的霉菌毒素也可能对敏感性动物诸如高产奶牛或犊牛产生明显的影响。


虽然关于某些霉菌毒素对动物繁殖功能的影响仍然存在很大的不确定性,但黄曲霉毒素、单端孢霉烯毒素和玉米赤霉烯酮的影响似乎已经比较明确。表3中的列表并不详尽,并且指的是会产生显著健康或繁殖影响的霉菌毒素水平。这些水平明显高于导致DMI和牛奶产量初始下降的霉菌毒素水平。

霉菌毒素对产奶量和牛奶质量的影响


急性和高含量霉菌毒素会抑制产奶量并导致乳成分发生变化。所有霉菌毒素都有此作用,包括那些通常不被认为会对反刍动物构成威胁的霉菌毒素。但是,这是否主要是由于DMI或代谢变化造成的还不是很明确。一篇关于已发表试验的综述表明,相对较低的霉菌毒素污染对产奶量和乳成分的影响并不总是立刻体现。表4总结了出版文献的结果,评估了相对较低水平的AfB1、DON和ZEN对产奶量或乳成分的影响。


除Keese进行的试验外(2008)[23],其他试验没有发现霉菌毒素对DMI、产奶量或乳成分有明显的影响。在[24,25]进行的研究中使用的黄曲霉毒素水平远高于法律规定或建议的水平(表2),此时肝脏脂肪会发生沉积(表3)。尽管他们黄曲霉毒素的用量低于1600ppb,但仍然发现产奶量似乎受到了影响。


表4所示的DON试验使用的毒素水平通常接近允许的最高霉菌毒素水平。但是,它们并没有导致产奶量或乳成分发生显著变化,即使含量增加到12ppm时[26,27]。另一方面,在另一项涉及两个更大奶牛群体的相关实验中,尽管日粮类型相似,但是DMI、产奶量和乳成分都发生了显著变化[23]。在[28]的研究中,DON水平接近欧盟的指导要求,但却发现DMI和产奶量增加了(P <0.05),不过乳脂和乳蛋白仍然受到了显著的负面影响。[28]中还发现与实际情况一致,体细胞数(SSC)随着DON水平升高而增加。[27]研究中也观察到这样一个相似但并未受重视的结果。


另外还有一项研究发现,相对于饲喂无污染饲料的对照组,日粮伏马菌素含量为100ppm时,在泌乳的前70天,牛群的产奶量减少。据报道,在这种霉菌毒素水平下,奶牛伴有轻度肝病,产奶量平均每天减少6公斤/头。研究人员认为主要是DMI减少的结果。


霉菌毒素的免疫反应及附带影响


霉菌毒素会激发免疫反应发生变化。AfB1、单端孢霉烯毒素、OTA及其代谢物作用于细胞免疫系统后具有免疫毒性和/或抑制作用。即使是低水平的霉菌毒素也可能会导致免疫抑制,引起传染病发生率增加,但是却并不表现明显的霉菌毒素中毒症状。


霉菌毒素对奶牛健康产生明显影响的关键阶段是围产期和随后的泌乳早期,如果这时免疫状态受到损害,发生代谢疾病的风险将增加。皮质类固醇释放增加,加上霉菌毒素的免疫抑制作用,会增加奶牛发生外部感染和机会性感染的敏感性。这反过来将使霉菌毒素中毒的诊断复杂化,特别是当低水平霉菌毒素影响免疫状态时。


采食饲料被镰刀菌毒素自然污染的奶牛的免疫反应发生的变化(主要是3.5ppm DON 和痕量 ZEN)已有报道。在如此低的污染水平下,对产奶量或乳成分不会产生任何影响,但免疫参数受到显著影响(表5)。



低水平的单端孢霉烯类污染会降低中性粒细胞的吞噬作用,这会使日粮受到污染的奶牛产生非特异性免疫反应的能力降低。初次抗体应答(主要是IgG和IgA)同时升高但二次应答没有变化,表明白细胞内的信号传导受到干扰。


还有研究表明,天然、低剂量的ZEN和DON诱导了奶牛的急性自身免疫反应。在同一研究中,在受感染动物的血浆中检测到高水平的DON和低水平的ZEN。


霉菌毒素也可能对关键瘤胃细菌产生不利影响,但这通常会被忽略。然而,研究发现,即使AfB1浓度很小,也可能导致采食量显著减少,瘤胃动力降低,而摄入T-2毒素可能引起皱胃溃疡和瘤胃乳头脱落。




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