豬精液-病毒傳播的載體（三）
Porcine semen as a vector for transmission of viral pathogens-Part 3
Dominiek Maesa,*, Ann Van Sooma, Ruth Appeltanta, Ioannis Arsenakisa, Hans Nauwynckb
a比利時，梅勒爾貝克，根特大學，獸醫學院，畜群健康與產科學，繁殖科
a Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium
b比利時，梅勒爾貝克，根特大學，獸醫學院，病毒學實驗室，免疫與寄生蟲學，病毒學科
b Department of Virology, Immunology and Parasitology, Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium
關鍵詞 Keywords：
豬精液、豬、人工授精、病毒、回顧/檢查
Semen, Pig, Artificial insemination, Virus, Review
接上文...
2.2 存在豬精液中，未列入OIE名錄的病毒
Viruses in porcine semen which are not in the OIE list
2.2.4. 非典型豬瘟病毒
Nonclassical swine fever pestiviruses
豬對非典型豬瘟病毒易感，此類病毒包括牛病毒性腹瀉病毒（BVDV）和邊界病病毒；這些RNA病毒會導致牛羊罹患相關疾病。先天感染這些病毒的豬可能會大量散毒。Terpstra和Wenswoort [35]從先天感染及不育公豬的口咽液、尿液和精液中分離出BVDV。
Pigs are also susceptible to non-CSF pestiviruses including BVDV and border disease virus; these are RNA viruses which are associated with disease in cattle and sheep, respectively. Pigs congenitally infected with these viruses may shed large amounts of virus. Terpstra and Wenswoort [35] isolated BVDV from oropharyngeal fluid, urine, and semen of a congenitally infected and infertile boar.
Bungowannah病毒與豬瘟（CSF）病毒的基因不同，但是由於其分子特性，可將其納入瘟病毒屬。2003年，該病毒導致澳大利亞兩個豬場爆發嚴重疾病，主要是死胎、木乃伊胎增多、斷奶前死亡率升高[72]。尚未明確該病毒通過胚胎或精液傳播的風險。
Bungowannah virus is genetically distinct from CSF virus, but molecular characterization supports its inclusion in the genus Pestivirus. The virus caused a severe disease outbreak characterized by an increase in stillbirths, preweaning mortality, and mummified fetuses in 2003 in two pig herds in Australia [72]. The risk of transmission by embryos or semen has not yet been evaluated.
2.2.5. 豬腸道小核糖核酸病毒
Porcine enteric picornaviruses
豬腸病毒(PEV)和豬捷申病毒均為無包膜RNA病毒，屬於微小核糖核酸病毒科。這兩類病毒在環境中存活力很強。病毒在豬之間的傳染，主要是通過口腔接觸受汙染的糞便。精液有可能在采精過程中受到汙染。公豬感染PEV可能導致精囊炎、精子異常、性欲下降[32]。除了妊娠期間發生感染，通常觀察不到母豬表現出臨床症狀。若妊娠期間發生感染，可能會導致胚胎死亡、木乃伊胎、死胎和弱仔[73]。
Porcine enterovirus (PEV) and porcine teschovirus are nonenveloped RNA viruses belonging to the family of Picornaviridae. They are highly resistant in the environment. Infections are transmitted between pigs mainly by oral exposure to contaminated feces. Contamination of semen during collection is possible. Infections with PEV in boars may lead to seminal vesiculitis, sperm abnormalities, and a decreased libido [32]. In the sow, usually no clinical signs are observed, except if infections occur during pregnancy. In the latter case, infections may cause fetal death and mummification, stillbirth, and weak-born piglets [73].
已從公豬生殖道中分離出豬捷申病毒[32]，使用帶毒精液對後備母豬授精，不會影響受精[74]。但Dunne等[44]提出，被PEV和豬捷申病毒感染的精液會導致胚胎和新生仔豬死亡。尚無後續研究證實這一發現。
Porcine teschovirus has also been isolated from the male genital tract [32], but insemination of gilts with contaminated semen did not affect fertility [74]. Dunne et al. [44] however suggested that semen contaminated with PEV and porcine teschovirus could cause embryonic and neonatal death. There are no recent reports that confirmed this finding.
2.2.6. 細環病毒
Torque teno virus
已經在豬身上發現兩種細環病毒（TTV）：細環病毒1型（TTsuV1）及2型（TTsuV2）。 均為DNA病毒，屬於新創的指環病毒科、細環病毒屬。據估計，豬細環病毒（TTsuVs）在全球豬群中普遍存在 [75]。目前，感染豬細環病毒（TTsuV）無相關臨床症狀出現。但TTsuV與豬圓環病毒（PCV）相關疾病有關[76,77]。細環病毒呈垂直和水平傳播。可在胎兒組織和血液、精液和初乳中檢測到該病毒[36]，由此證明其存在垂直傳播的可能。從人工授精中心收集100頭公豬的獨立精液樣本，用常規PCR檢測TTsuV1和TTsuV2[37]。其中四份樣本呈TTsuV1 PCR陽性，五份呈TTsuV2陽性，合並感染後，兩種TTsuVs均被檢測到。胎兒感染的途徑尚不明確，但相關顯示證明可能是由於精液帶毒或者胎盤傳播。
Two species of TTV have been described in pigs: torque teno sus virus types 1 (TTsuV1) and 2 (TTsuV2). They are DNA viruses that belong to the newly created family anelloviridae, genus Iotatorquevirus. It is considered that TTsuVs occur in pig populations worldwide [75]. At present, no clinical signs are associated with TTsuV infection. However, TTsuVs have been linked to PCV-associated disease [76,77]. Torque teno sus viruses are apparently transmitted both vertically and horizontally. They can be detected in fetal tissues and blood, semen, and colostrum [36], indicating the potential for vertical transmission. Single semen samples from 100 boars from an AI center were analyzed by conventional PCR for TTsuV1 and TTsuV2 [37]. Four samples were PCR positive for TTsuV1 and five for TTsuV2, whereas in a single coinfection, both TTsuVs were detected. The route of fetal infection is uncertain but may reflect virus-contaminated semen or transplacental transmission.
2.2 存在豬精液中，未列入OIE名錄的病毒
Viruses in porcine semen which are not in the OIE list
2.2.7. 埃博拉病毒
Ebolavirus
雷斯頓埃博拉病毒是一種有包膜RNA病毒，屬於單分子負鏈RNA病毒目、絲狀病毒科。 2008年，菲律賓豬群中爆發嚴重疾病，據報導疑似由藍耳病毒（PRRSV）引起。但是，經詳細的診斷後發現，並非PRRSV而是雷斯頓埃博拉病毒（REBOV）引起該病，而且在豬場工人身上檢測到REBOV特異性抗體[30]。此次REBOV疫情爆發的深遠影響尚待確定。隨後的實驗表明，盡管感染REBOV後，病毒在豬體內複製，但可能不表現症狀。豬群間REBOV的傳播機製尚不明確。也不清楚REBOV的散播途徑。在非人靈長類動物模型中，REBOV會在包括精液在內的所有體液中排散[29]。尚不清楚REBOV是否可通過精液傳染給母豬。與REBOV相比，已證實紮伊爾埃博拉病毒可導致5至6周齡豬產生嚴重肺部疾病，引起呼吸困難[42]。
Reston ebolavirus is an enveloped RNA virus, classified in the order mononegavirales, family filoviridae. In 2008, severe disease outbreaks suspected to be caused by PRRSV were reported in swine herds in the Philippines. However, detailed diagnostic work revealed that not PRRSV but REBOV had caused the disease and that REBOV-specific antibodies were found in farm workers [30]. The broader significance of this outbreak of REBOV remains to be determined. Subsequent experimental challenge suggested that REBOV infection in pigs may be asymptomatic despite replication of the virus. The mechanism of transmission of REBOV to pigs or between pigs is unknown. Routes of REBOV shedding by pigs are also not known. In nonhuman primate models, REBOV is shed in all body fluids, including semen [29]. Whether REBOV is shed by boars in semen or transmitted to recipient females is unknown. In contrast with REBOV, Zaire ebolavirus has been shown to cause severe lung pathology in 5- to 6-week old pigs leading to respiratory distress [42]. 2.2.8. 豬內源性反轉錄病毒
Pig endogenous retroviruses
豬內源性反轉錄病毒是一種有包膜RNA病毒，屬於反轉錄病毒科、丙型反轉錄病毒屬。高達8%的哺乳動物基因組DNA來源於反轉錄病毒[78]。在豬身上，已經發現了三種反轉錄病毒（PERV）亞型：PERV-A，PERV-B和PERV-C。PERV-A和PERV-B普遍存在於豬群中，而PERV-C在種間、種內分布程度各異 [38]。
Pig endogenous retroviruses are enveloped RNA viruses of the genus Gammaretrovirus in the family retroviridae. Up to 8% of mammalian genomic DNA is believed to be retroviral in origin [78]. In pigs, three PERV subtypes have been identified: PERV-A, PERV-B, and PERV-C. Subtypes A and B are ubiquitous in all pig breeds, whereas PERV-C is variably distributed between and within breeds [38].
尚未在豬中發現呈外源性水平傳播的反轉錄病毒。然而，已經發現PERV-A和PERV-C之間的重組。PERV A/C重組體不以原病毒形式存在於宿主種係中，因此被認為是潛在外源病毒[79]。一些豬的體細胞中存在完整的A/C原病毒基因組，由此表明其可進行複製。尚未完全了解PERVs在豬之間傳播的可能。然而，由於PERVs被嵌入基因組中，因此表明它們是通過精液傳播的。
Exogenous horizontally transmissible retroviruses have not been found in pigs. However, a recombination between PERV-A and PERV-C has been found. The PERV A/C recombinant is not present in proviral form within the host germ line and is therefore considered a candidate exogenous virus [79]. The presence of the full genomic A/C provirus in the somatic cells of some pigs indicates its potential for replication. Little is known regarding the potential for transmission of PERVs between pigs. However, as PERVs are embedded in the genome, they are transmitted via semen.
2.2.9. 風疹病毒（藍眼病）
Rubulavirus (blue eye disease)
豬風疹病毒是一種有包膜RNA病毒，屬於副黏病毒科、腮腺炎病毒屬。該病毒可導致藍眼病[80]。此病毒流行於墨西哥，會造成
種豬繁殖障礙。感染後，與其他成年動物一樣，公豬會罹患嚴重附睾炎和睾丸炎，性欲減退，通常不表現臨床症狀。尚未通過實驗證明該病毒通過精液傳播；但是，在接種後長達49天之內，已經從精液、睾丸和生殖道的其他組織中檢測到該病毒[39]。近期數據表明，在接種後5至48天可從精液中分離出該病毒，接種後64至142天可從睾丸和附睾中分離出該病毒。接種後2到64天，血清中可檢測到病毒RNA，接種後142 天，仍可在精液中檢測到此病毒[40]。由此證明，豬風疹病毒RNA持續存在於精液中，且長時間感染生殖道。精液檢查顯示，感染病毒的豬群中，約30%的公豬出現暫時性或永久性不育、精子濃度下降、形態異常增加、精子的活力和存活能力降低。一些公豬出現無精症[40]。
Porcine rubulavirus is an enveloped RNA virus that belongs to the genus Rubulavirus in the family paramyxoviridae. The virus is associated with blue eye disease in pigs [80]. It is an important pathogen in Mexico and causes reproductive problems in breeding pigs. Boars, like other adult animals, generally do not show clinical signs, except for epididymitis and orchitis and in severe cases, loss of libido. Transmission of the virus through semen has not been proven experimentally; however, virus has been recovered from semen, the testis, and other tissues of the reproductive tract for up to 49 days after inoculation [39]. More recently, isolation of the virus from semen was achieved between 5 and 48 days post inoculation (DPI) and from the testicles and epididymides between 64 and 142 DPI. Viral RNA was detected in the serum between 2 and 64 DPI and in the semen until 142 DPI [40]. These results confirm that the RNA of the porcine rubulavirus persists in the semen and that this virus remains in the reproductive tract for prolonged periods of infection. Semen evaluation demonstrated that about 30% of boars in herds infected with the virus showed temporary or permanent infertility, with a decrease in concentration, an increase in abnormalities in morphology, and a decrease in motility and viability of spermatozoa. In some boars, there was azoospermia [40].
3.人工授精將病毒傳染給母豬的風險
Risk for transmission of viruses by AI to the recipient sow
通常，在發病時期，精液中存在病毒的可能性最高。但是，現實情況中，不會從臨床受影響的公豬中采精，這樣可以降低病毒傳染母豬的風險。無臨床症狀、出現輕微臨床症狀之前，散毒就已經開始，加之嚴重感染豬隻未被發現，在這些情況下，如果不采取特別控製措施，會增加病毒傳播風險。
In general, the risk of virus to be present in semen is the highest during the stage of clinical disease. Under practical circumstances, however, no sperm collection will take place from clinically affected boars, and consequently, the risk of pathogen transmission to the sow is low in this case. Although virus shedding can start before the development of clinical signs, which can be mild or absent, acutely infected boars can remain unnoticed. In these situations, the risk of virus transmission is much higher as no special control measures will be taken.
實驗證明，CSFV [10] PRRSV [61]可通過精液傳染母豬，在精液中加入PPV [45]、ADV [52]、PRRSV[18,21]和PCV2[41]後，亦可。Habu等 [13]，Guérin和Pozzi [57]提出，雖然沒有找到科學的文章證實，但是如果使用含有日本乙型腦炎病毒的精液進行授精，則後備母豬極易被感染。若將被病毒汙染的精液應用於人工授精，母豬乃至母豬群都會有病毒傳播的風險。Nathues等[81]以瑞士引入公豬精液導致PRRSV爆發為例，對此加以說明。
Transmission of viral pathogens by semen to the sow has been clearly proven for CSF virus [10] and PRRSV [61] on experimental infection of boars and for PPV [45], ADV [52], PRRSV [18,21], and PCV2 [41] after inoculation of the virus in the semen. Habu et al. [13] and Guérin and Pozzi [57] reported that Japanese encephalitis virus is easily transmitted to gilts if they are inseminated with contaminated semen, although we did not find a scientific article confirming these statements. Semen contaminated with viral pathogens used for AI does indeed poses a risk for transmission to the recipient sow and by extension the sow herd. This was illustrated by Nathues et al. [81] who described an outbreak of PRRSV in Switzerland after import of boar semen.
但是，病毒並不總是能夠形成傳播（例如PRRSV）[18,61]。母豬被感染所需的條件很複雜，未能傳播可能是由於母豬免疫、病毒特點以及未達到最小感染劑量。因此，針對精液傳播PRRSV的風險，以及母豬感染病毒所需的最小劑量，開展了更多的研究。那麼，與常規宮頸輸精相比，小劑量精液的深部輸精（40–50 mL對比80–90 mL），可降低病毒傳染母豬的風險[82]。
However, transmission will not always be successful (e.g., PRRSV) [18,61]. The conditions required for establishment of infection in the sow are complex, and lack of transmission might be explained by sow immunity, virus characteristics, and failure to reach the minimum infectious dose. In this regard, much research has been directed toward the risk of transmission of PRRSV by semen and the minimum dose necessary to establish infection in the sow. In this regard, it can be expected that postcervical insemination with smaller semen doses than conventional cervical insemination (40–50 mL compared to 80–90 mL) is associated with a lower risk for pathogen transmission to the sow [82].
就群體而言，使用帶毒精液進行配種的母豬數量，也決定了形成傳播的可能性。此類母豬數量越多，形成病毒傳播的可能性越高。其他動物，如牛，人工授精時主要使用凍精，而非鮮精。這樣，就有更多的時間和機會，檢測公牛和/或精液是否含有特定病原，進而降低通過人工授精傳播病原的風險。
At a population level, also the number of sows inseminated with contaminated semen determines the likelihood of successful transmission. The more sows are inseminated with contaminated semen, the higher the likelihood of successful transmission of the virus. In other animal species such as cattle, AI is mostly practiced with frozen semen and not with fresh semen as in pigs. This allows more time and opportunities to verify whether the bulls and/or semen are free from specific pathogens, and consequently, the risk for pathogen transmission via AI is lower.
4.病毒通過人工授精對母豬的影響
Impact of viruses by AI to the recipient sow
使用帶毒精液對母豬進行人工授精，會出現多種結果。精子質量下降、胚胎早期死亡和/或子宮內膜炎、母豬群中的臨床疾病，和/或感染有害病原導致母豬健康水平下降、阻礙病原淨化或幹擾監管措施，這些均可能導致受胎率降低[57] 。病原體可穿過透明帶直接侵入胚胎，和/或病原體（例如，感染PPV）引起子宮上皮病變，造成胚胎早期死亡[83]。妊娠後的6至7天，透明帶作為不可滲透屏障，圍繞並保護胚胎，有助於其避免受諸如PPV，PCV2，ADV和PRRSV等病原體的入侵[65,84,85]。但是，病原體（如ADV）在囊胚期可穿過透明帶，導致胚胎更容易感染[84]。
The consequences of AI with semen contaminated with viral pathogens for the sow can be quite variable. It can result in reduced conception rates because of reduced semen quality, early embryonic death and/or endometritis, clinical disease in the sow herds, and/or infections with unwanted pathogens leading to reduced health status, stamping out, or regulatory measures [57]. Early embryonic death may result from direct invasion of the embryo by the pathogen after it has hatched from the zona pellucida, and/or by uterine epithelial alterations in response to the pathogen (e.g., with PPV) [83]. Until 6 to 7 days after conception, embryos are surrounded and protected by the zona pellucida, an impervious barrier, which helps the embryo to avoid invasion of pathogens such as PPV, PCV2, ADV, and PRRSV [65,84,85]. After hatching, however, blastocyst stage embryos may become susceptible to the infection as is the case, e.g., with ADV [84].
5.診斷 Diagnostics
可以采用不同的診斷方法，如病毒活性檢測、病毒核酸檢測或病毒抗體檢測，來判斷人工授精中心的公豬是否感染病毒和/或精液中是否帶毒。九十年代初期至今，在提高敏感性和特異性，以及同時檢測不同病原和檢測速度方麵取得了重大進步。
The presence of viral infections in boars of AI centers and/or the presence of viral pathogens in semen can be assessed using different diagnostic methods such as demonstration of viable virus, nucleic acid of the virus, or antibodies against the virus. From the early 1990s onward, major improvements have been made in terms of increased sensitivity and specificity, simultaneous testing of different pathogens and speed of testing.
5.1. 病毒活性檢測
Detection of viable virus
通常，公豬精液采集後幾天內，即會被稀釋使用，應在短時間內拿到檢測結果。檢測精液中病毒的常規方法，如病毒分離鑒定或生物鑒定，靈敏度不高，耗時且昂貴。由於細胞毒性、細菌汙染，以及會幹擾細胞培養係統、抗病毒因子會非特異性中和病毒，現已很少在精液內使用病毒分離
技術[59]。基於現實條件和動物福利限製，無法對大量動物進行樣本接種檢測[64]。限於以上因素，病毒分離和豬生物鑒定於常規臨床診斷而言並不實用。
As diluted boar semen is mostly used within a few days after collection, the outcome of analysis should be available within a very short time. Conventional methods for virus detection in semen, such as virus isolation or conducting bioassays, are not very sensitive; they are time-consuming and very expensive. The application of the virus isolation technique is markedly reduced for semen because of cell toxicity, bacterial contamination, resulting in interference with cell culture systems, and antiviral factors that nonspecifically neutralize the virus [59]. Animal inoculations cannot be performed for testing large numbers of samples because of practical reasons and also because of animal welfare reasons [64]. Given these limitations, virus isolation and swine bioassays are not useful techniques for routine diagnosis.
5.2. 病毒核酸檢測
Detection of viral nucleic acid
PCR是一種靈敏度高、特異性強，能夠快速檢測基因組序列（如，病毒）的檢測
技術。已經有多種PCR
技術可用於檢測公豬精液中存在的病毒。Van Rijn等[86]研發出的實時熒光定量PCR，可以對精液中5種能夠嚴重影響經濟效益的病毒（ADV，CSFV，FMDV，SVDV，PRRSV）進行檢測。實時PCR
技術是將擴增和擴增產物的檢測合二為一。從而來自環境中的汙染可能大大降低。通過實驗發現，相比病毒分離，采用實時PCR檢測帶毒精液，可以在感染後更早、更快發現病毒。 Rovira等[87]研究了反轉錄酶PCR針對不同生物樣品的靈敏度，這些樣品分別為3個樣品為一池和5個樣品為一池。用低毒PRRSV分離株接種了29頭公豬。2周內，每2到3天從每頭公豬獲取血清、血液拭子和精液樣本。得出的數據顯示，急性感染期間，血清是檢測PRRSV的最佳樣品，血液拭子亦可。但是大多數精液樣本中，未能檢測到PRRSV感染。以3個和5個樣品為一池會造成反轉錄酶PCR的靈敏度下降。當分別以5個樣品為一池檢測血清或血液拭子，其靈敏度分別降低6％和8％。
The PCR technique is known as a sensitive, specific, and rapid tool for the detection of genomic sequences of, e.g., viruses. Different PCR techniques have become available for detecting viral pathogens present in boar semen. Van Rijn et al. [86] developed quantitative real-time PCR tests for detecting five economically important viruses in semen (ADV, CSFV, FMDV, SVDV, PRRSV). The real-time PCR technique combines amplification and detection of amplified products in one closed tube. Therefore, possible contamination from the environment is strongly reduced. In semen of experimentally infected boars, viruses were detected much earlier after infection and more frequently by real-time PCR tests than by virus isolation. Rovira et al. [87] investigated the sensitivity of reverse-transcriptase PCR on different biological samples run individually, in pools of 3 and in pools of 5. Twenty-nine boars were inoculated with a low-virulent PRRSV isolate. Serum, blood swab, and semen samples were obtained from each boar every 2–3 days for 2 weeks. Data showed that serum was the best sample to detect PRRSV during acute infection, with the blood swab sample performing almost as well. Semen samples failed to detect PRRSV infection in most of the cases. Pooling samples at pool sizes of 3 and 5 resulted in a decrease in the sensitivity of reverse-transcriptase PCR. Sensitivity was reduced by 6% and 8%, respectively, when serum or blood swab samples were run in pools of 5.
5.3. 抗體檢測
Detection of antibodies
常用血清學方法來調查公豬站中是否存在針對不同病原體的血清抗體。這是一種簡單且經濟的檢測方法。檢測結果適用於解釋群體或公豬站內的健康水平，而非單個豬隻。但是，此種檢測方法重要的缺點是，病原體暴露與檢測到抗體水平之間的時間間隔。許多血清檢測間隔一般為一到幾周，這意味著在此期間，未被確定感染的公豬，也可能散毒。成年公豬的頸靜脈穿刺，對操作者的安全而言，有一定的局限性。因此，血液拭子更常用。為了采集血液拭子，在收集精液時用針刺其耳朵，並使用棉簽收集血滴。檢測PRRSV時，與精液樣本相比，血液拭子中病毒量更多，可更快檢出PRRSV[88]。
Serology is frequently used to investigate the presence or absence of serum antibodies against different pathogens in boar studs. It is an easy and rather inexpensive method that is suitable for monitoring. The results should be interpreted at group or stud level, not at the individual animal level. An important disadvantage is the time period between pathogen exposure and detectable levels of antibodies. For many serologic tests, this time period ranges from one to several weeks, which implies that during that period, boars may shed the virus although they are not identified as being infected. Puncture of the jugular vein in adult boars has some practical limitations in terms of safety of the handler. Therefore, blood swabs have become popular. For blood swab collection, the ear is pricked with a needle during semen collection and a cotton swab is used to absorb the blood droplet. For PRRSV, increased amounts of PRRSV were shown in the blood swab as compared to semen samples, and infections could be detected earlier [88].
5.4. 臨床診斷的說明
Interpretation of diagnostic testing
顯然，無法直接將精液歸類為無病毒或帶毒精液。許多病毒，如PRRSV，ADV，PCV2，在病毒血、精液中病毒的存在和血清中的抗體之間存在時間不一致性。就PRRSV而言，如成年公豬的病毒血症持續時間很短，且病症的結束早於精液散毒[59]。此外，在感染的初期，盡管精液已在散毒，但血清學結果仍為陰性。最後，當精液停止散毒後很長一段時間，公豬仍可能為血清陽性。因為精液通常是間斷散毒，尤其是在感染後期，所以即便檢測精液樣品陰性，病毒隨後可能還會繼續排散。陰性結果僅表示所檢測的精液樣品不包含病毒，且僅指單次射精精液中可能不含病毒。並不能保證公豬隨後射出的精液不帶毒。因此，對待血清陽性，但精液檢測陰性的公豬應更加謹慎。
It is clear that categorizing semen as either virus free or virus contaminated is not straightforward. For many viruses, e.g., PRRSV, ADV, PCV2, temporal inconsistencies exist among viremia, the presence of virus in semen, and antibodies in serum. In the case of PRRSV, e.g., viremia in adult boars may be of short duration and end before virus shedding in semen ends [59]. Furthermore, in the initial phases of the infection, serologic results will be negative although the virus can be shed in the semen. Finally, boars may remain serologically positive long after the virus is no longer shed in the semen. Because shedding of virus in semen is often intermittent, especially in the later phases of the infection, a negative test result on a semen sample does not preclude subsequent shedding of the virus. A negative result only means that the tested sample of the ejaculate does not contain virus and that the particular ejaculate is likely to be virus free. It does not guarantee absence of contamination in subsequent ejaculates of a boar. Consequently, a negative semen test from a serologically positive boar should be interpreted with caution.
未完待續...
To be continued…
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