从古典猪瘟的存在看非洲猪瘟的防控
2019-03-11TK康博士 曾容愚
1.古典猪瘟和非洲猪瘟的临床症状最相似，是早发现ASF疑似猪群，早隔离处理措施的最大干扰项。净化猪瘟，就可以更好地感知ASF的入侵。
1.1在猪群非洲猪瘟已经呈现地方性流行的地区，其特征已从死亡率接近100%的特急性疾病转变为死亡率较低的疾病，但是，急性、亚急性或慢性的感染率会增加（SanchezBotija和Polo Jover，1964年；Scott，1965a）。通常不可能根据临床症状和病变来区分这些形式的非洲猪瘟和古典猪瘟。更令人担忧的是，带毒猪只在家猪中维持和传播非洲猪瘟的作用变得越来越重要(W.R.Hess,1971）
In areas where ASF has become enzootic in domestic swine, its character has changed from a peracute disease with mortality approaching 100 per cent to a disease of lesser mortality with an increased incidence of acute, subacute, and chronic infections (SANCHEZ BoTIJA and PoLo JovER, 1964; ScoTT, 1965a). It is often impossible on the basis of clinical signs and gross lesions to distinguish these forms of ASF from classical swine fever. Of even greater concern is the fact that carriers have become increasingly important in the maintenance and spread of the disease in domestic swine (W.R.Hess,1971).
1.2ASF和CSF感染的细胞亚群高度一致
非洲猪瘟病毒和猪瘟病毒都对免疫系统的细胞，主要是那些来自单核巨噬细胞谱系（属于白细胞），有感染嗜性。利用这些细胞特有在全身的游走能力，实现病毒的扩散和持续感染，是这两个病毒在体内得以复制和产生致病性的重要手段。非洲猪瘟感染出现发热反应，在此期间，白细胞总数可能降至正常值的40%左右（Detrayand Scott，1957年）。此外，ASF感染的猪血小板降低30倍（临床发现ASF感染猪血凝不良）（Hovakim Zakaryan，2014）
Both ASFV and CSFV sharea tropism for immune system cells, mainly those that are derived from themonocytemacrophage lineage.It is assumed that theinfection of these cells plays an important role in virus replication andpathogenesis by exploiting their migratory ability, which promotes viral spreadand persistence in the host for both ASFV and CSFV. During this period thetotal leukocyte count may fall to about 40 per cent of normal (DETRAYand ScoTT,1957). By the last day of infection the number of platelets in the peripheral blood was 30-fold reduced compared with control (Hovakim Zakaryan，2014).
1.3ASF和CSF感染破坏的组织和器官高度一致
如CSF一样，ASF几乎专门侵害网状内皮组织。事实上，每个器官和组织都可能显示出一些由血管损伤引起的变化。
The virus acts almostexclusively on reticuloendothelial tissues. In fact, every organ and tissue mayshow some changes that are attributable to vascular damage.
2.在非洲猪瘟疫苗上市之前，猪体健全强大的免疫系统是猪感染ASF后减少排毒，减少扩散，实现“早发现，早处理,'拔牙式'”定点清除策略的唯一武器。猪瘟是破坏免疫系统的头号疾病，导致猪体最广泛的免疫抑制。所以，非瘟面前，做好古典猪瘟的深度防控和场内净化，尤其重要。
2.1猪瘟病毒感染加剧非洲猪瘟病毒感染损失的免疫学机理
2.1.1 从ASFV感染中康复的猪通常可以抵御同种病毒下一次的感染，但一般情况下没有对异种病毒的交叉保护能力。总的来说，是否存在抗体介导的保护，即中和抗体，仍有争议。注射高免血清可能有一定的缓解作用。然而，好几个报道认为根本不存在中和抗体，其他报道在体外实验时发现抗体可以降低病毒滴度或在一定程度上中和ASF病毒。
虽然抗体的作用仍存争议，细胞毒性T细胞似乎是抗病毒免疫保护的主力军。有报道证明，CD8+T细胞的减少将会导致保护作用的消失（Schulz et al,2017, Vet Res）。
2.1.1 Pigs recoveringfrom ASFV infection are usually protected against homologues challenge, butcross-protection against heterologous strains is often missing. Generally,theexistence of an antibody-mediated protection, i.e. virus neutralization, iscontroversially discussed. It is possible to confer a certain level ofprotection by passive transfer of hyperimmune sera [23]. However, severalauthors suggest the complete absence of neutralizing antibodies [24], others foundthat antibodies could reduce virus titers or neutralize ASF virus to a certainextent in vitro [25–27]. While the role of antibodies is controversiallydiscussed, cytotoxic T-cell responses seem to play a major role in mediatingantiviral protection. It was demonstrated that depletion of CD8+ cells leads toabrogation of protection [31].
2.1.2 猪瘟病毒导致的各个白细胞亚群的数量减少程度不同，其中B淋巴细胞，辅助性T细胞和细胞毒性T细胞的数量减少最明显，这会导致猪体出现广泛性的免疫抑制，这种免疫抑制不仅仅是针对猪瘟病毒感染本身，而且针对与猪瘟病毒同时出现的混合感染或者随后出现的继发感染（Summerfield等，2001）。
2.1.2 CSF leukopeniaaffects leukocyte subpopulations unequally, with B-lymphocytes, helper T cells,and cytotoxic T cells the most affected. This would have consequences in termsof the immuno-compromisation of the animals,not only in the face of CSFV, butalso with respect to other concomitant or secondary infections（Summerfield et al,2001）.
综上所述，猪瘟感染会导致猪体白细胞的减少，尤其是细胞毒性T细胞的减少，而细胞毒性T细胞又是目前知道的猪体对非洲猪瘟产生保护的主要力量，因此，小编推测，猪瘟感染猪只一旦继发非洲猪瘟感染，其原本低下的免疫保护力将会变得更加不堪一击，临床损失会加剧。也就是说猪瘟病毒感染可能会加剧非洲猪瘟病毒感染的损失（下面试验刚好证这推测）。
2.2双重感染的案例
亚临床感染经典猪瘟的野猪再感染非洲猪瘟的研究
African swine fevervirus infection in Classical swine fever subclinically infectedwild boars Cabezón et al. 2017,BMCVeterinary Research
2.2.1 有趣的是，感染猪瘟的野猪（A组）表现为典型的渐进性急性出血性疾病。然而，没有感染猪瘟病毒的野猪（B组）没有一头表现出血性疾病的临床特点。就临床症状的严重性而言，两组在感染非洲猪瘟后4天和7天之间差异达到统计学意义上显著的水平（p<0.05）。
2.2.1 Interestingly, theanimals infected with a CSF persistent form (Group A) showed a progressiveacute haemorrhagic disease after ASFV infection. However, none of thepestivirus-free-ASFV infected wild boars (Group B) developed the haemorrhagicclinical form of the disease. Statistical significant differences (p < 0.05)in terms of clinical signs between both experimental groups were found from 4dpi to 7 dpi.
2.2.2 IFN-α是猪先天性免疫应答的支柱力量，猪瘟病毒持续感染的猪一直保持IFN-α阴性，这也就是为什么猪瘟病毒亚临床感染的猪一直能够在体内维持高而恒定的猪瘟病毒载量[28，42]。本试验再次令人吃惊地发现在猪瘟亚临床感染的野猪中，IFN-α的应答受到了抑制，即使这些猪后来感染了非洲猪瘟病毒，这样一个能对先天性免疫系统产生强烈刺激的病毒，IFN-α的应答一直没有增强。
2.2.2 The CSFV PIanimals remained IFN-α negative, a cornerstone in the innate immunemechanisms; this fact might promote the maintenance of a high and constant CSFV load [28, 42]. Strikingly, once again, the IFN-α response seemed to beimpaired in CSFV PI animals,even after infection with the ASFV virus, whichinduces a potent effect on the innate immune system.
在两种疾病都流行的国家，考虑到ASF传播的增加和CSF亚临床形式的长期共存，不能排除猪同时感染ASFV和CSFV的可能性，需要更深入地研究。本试验提示猪瘟亚临床感染可能会使猪只更易感非洲猪瘟病毒，并加重其病程（国内临床已有双重感染的发现，私人通讯）。
Considering theco-existence of the increasing spread of ASF and the presence of CSF subclinical forms in endemic countries for both diseases, the possibility of ASFV and CSFV co-infection in swine cannot be ruled out and needs to be studiedin greater depth. The CSFV subclinical infection may also predispose animals toASF disease and aggravate its progression.
3.ASF面前，减少免疫次数就是减少猪群里面ASF传播的风险。2018年天康猪瘟E2疫苗大量使用数据证明，母猪一年只需免疫2针，仔猪只需免疫一针就能上市；进一步减少猪群的应激和人和猪的接触，降低猪群感染ASF的风险。过去的2018年，我们成功地解放了猪瘟超免用户，2019年我们将继续解放猪瘟首免用户。
感染基因II型ASF毒株后第三天出现临床症状，第二天就出现毒血症。这个信息提示我们，对外表健康的猪免疫疫苗，也有散毒风险（针头散毒是必然的）。
Although the firstclinical signs of infection appeared at 3 dpi,viremia was observed after the 1st day of infection.
3.1 母猪连续免疫2次E2后，抗体持续280天。
3.2 母猪第一年免疫3次每年，第二年免疫2次每年，母抗持续10周以上。
3.3 仔猪8周-10周免疫一次E2，有效抗体持续到免疫后5个月，实现一针上市的目的。
