Porcine semen as a vector for transmission of viral pathogens-Part 2
Dominiek Maesa,*, Ann Van Sooma, Ruth Appeltanta, Ioannis Arsenakisa, Hans Nauwynckb
a Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium
b Department of Virology, Immunology and Parasitology, Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium
Semen, Pig, Artificial insemination, Virus, Review
Viruses in porcine semen which are in the OIE list
2.1.2. 伪狂犬病病毒Aujeszky’s disease virus (pseudorabies virus)
Table 4: Viral pathogens not on the OIE list (2015) that have been found in porcine semen: lesions in the genital tract of boars, impact on semen quality, and transmission to recipient sow on artificial insemination (AI).
Abbreviation: NI, not investigated.
Viruses in porcine semen which are in the OIE list
Classical swine fever virus
Classical swine fever virus is an enveloped RNA virus belonging to the family flaviviridae, genus Pestivirus. The virus has been eradicated from many countries or regions such as Western Europe, North America, and Australia . However, the virus has been periodically reintroduced into domestic pigs via contact with wild boars. During the CSF epidemic in the Netherlands in 1997 to 1998, two AI centers became infected and 1680 pig herds were declared CSF suspect . This highlights the importance of potential disease spread by AI centers. Boars experimentally infected with CSF virus have been shown to shed the virus in semen for up to 53 days after infection . The virus does not affect the semen morphology and motility, and the concentration is within the normal range. Sows that are inseminated with contaminated semen may show seroconversion, the virus may cross the placental barrier causing embryonic mortality, and the virus can be isolated from fetuses .
The virus will remain important, especially in areas in which it is endemic, because the virus is highly contagious, and in contrast to piglets, infection in breeding pigs is not always associated with prominent clinical and pathologic signs. If clinically normal CSF virus immunotolerant piglets are born, they can spread the virus for months without showing signs of disease or eliciting a serologic response .
Foot-and-mouth disease virus
Foot-and-mouth disease virus is a small nonenveloped RNA virus that belongs to the family of picornaviridae, genus Aphthovirus. The virus is endemic in Asia and some areas in Africa and South America . After natural infection, FMD virus has been recovered from semen from infected boars for up to 9 days, but AI with contaminated semen failed to transmit the disease to serviced sows . An effect on semen quality has not been described. Infection with FMD virus leads to viremia, with subsequent dissemination of the virus throughout virtually all tissues of the body, including the genital tract and the skin around the preputial orifice . The viral concentration in semen has been found to be low . Because infections are officially eradicated in many countries, and because boars are intensively monitored in other countries, the risk for transmission of this virus by the semen is low.
Japanese encephalitis virus
Japanese encephalitis virus is an enveloped RNA virus that belongs to the family flaviviridae, genus Flavivirus. It is a mosquito-borne human and animal pathogen. The virus represents an economically important reproductive pathogen of breeding pigs, especially in Asia and Northern Australia, and may cause infertility in Japanese pigs . Infection of susceptible boars resulted in edematous, congested testicles and semen with numerous abnormal spermatozoa, and significantly decreased total and motile sperm counts . These changes are usually temporary, and most boars recover completely. The virus has been isolated from the testicles of boars with orchitis and also can be shed in the semen for 5 weeks. The virus can be easily transmitted if gilts are inseminated with infected semen [13,57].
Viruses in porcine semen which are in the OIE list
Porcine reproductive and respiratory syndrome virus
The PRRSV is an enveloped RNA virus that belongs to the family arteriviridae, genus Arterivirus. Infections with PRRS virus are associated with reproductive failure in pregnant sows, mortality in suckling pigs, and respiratory disease in pigs of all ages . Usually, no or only mild symptoms are seen in mature boars. The virus is easily transmitted between animals. Infected animals can shed PRRSV in saliva, nasal secretions, urine, feces, and also in semen [19,20]. Vascular dissemination and replication of PRRSV in tissues of the reproductive tract has been shown. Also migration of infected monocytes and macrophages from the blood and lymph of the reproductive tract into the semen has been suggested [15,59].
The duration of shedding in semen samples of experimentally infected boars varies widely, ranging from 2  to 92  days after infection (mean 35 days; Table 1). In the latter study, PRRSV was isolated from the bulbourethral gland of a boar euthanized 101 days after infection. This marked variability in shedding may be due to different factors including individual boar variation [17,60], breed of the boar , the type of virus strain, and the technique used for detection of the virus (i.e., virus isolation, polymerase chain reaction [PCR], nested real-time PCR, reverse transcriptase PCR, swine bioassay). Using a bioassay, infectious virus has been shown in semen samples from 4 to 42 days after infection . The virus can be shed in semen, even in the absence of viremia and in the presence of neutralizing antibodies [17,60].
During acute illness, in addition to anorexia, lethargy, and respiratory clinical signs, boars may also lack libido. The effect of PRRSV infection on semen quality varies considerably between studies. In some studies, drastic changes in semen quality, e.g., reduced motility, abnormal acrosomes, morphologic alterations, and volume of the semen, were observed 2 to 10 weeks after infection, whereas no abnormalities were found in other studies [19,61].
已证实，处于急性感染期的公猪，可通过精液将PRRSV传播至种猪群内。不过，病毒传播力取决于精液中病毒的数量，所以病毒有可能不会传播。Prieto等人研究表明，感染公猪的单次射精精液内，PRRSV滴度为7×102 TCID50/mL。如果对单次射精精液进行稀释(15-30倍)，则病毒滴度约为4×101 TCID50/mL，相当于每80 mL稀释精液中PRRSV的滴度约为3×103 TCID50。母猪与感染公猪配种或输入实验感染的精液[19,21]，即使未出现病毒血症，也会发生PRRSV血清阳转。Prieto等人研究显示，使用含有PRRSV的精液，对血清阴性或未免疫的后备母猪进行授精，受胎率几乎不受影响，但可能出现早期胚胎感染和死亡。
Transmission of PRRSV from fresh semen of acutely infected boars into breeding herds has been shown . However, transmission does not always occur as it may depend on the amount of virus present in the semen . In a study of Prieto et al. , the PRRSV titer in an ejaculate of an infected boar was 7×102 TCID50/mL. If the ejaculate is extended (15–30 times), the virus titer will be approximately 4×101 TCID50/mL of extended semen, corresponding with a total amount of approximately 3×103 TCID50 of PRRSV in each dose of 80-mL extended semen. Sows bred by infected boars  or with experimentally contaminated semen [19,21] showed seroconversion to PRRSV, even in the absence of detectable viremia [17,60]. Prieto et al.  reported that insemination of seronegative or preimmunized gilts with semen containing PRRSV often has little or no effect on conception rates but may result in early embryonic infection and death.
Swine vesicular disease virus
The swine vesicular disease (SVD) virus is a small nonenveloped RNA virus of the family picornaviridae, genus Enterovirus. Outbreaks with SVD virus have been documented in some countries in Europe, Asia and Central America. Infection with the SVD virus may cause lesions similar to those on infection with the FMD virus. Therefore, SVD is important in the differential diagnosis of FMD. After natural infection, SVD virus has been isolated from infected boar semen for up to 4 days but AI with contaminated semen failed to transmit the disease to sows .
Viruses in porcine semen which are not in the OIE list
Porcine circovirus type 2
猪圆环病毒2型(PCV2)是一种无包膜小DNA病毒，属于圆环病毒科、圆环病毒属。猪同时感染PCV2并出现其他相关疾病，一般统称为PCV2相关疾病。感染PCV2病毒后，母猪会出现繁殖障碍，如晚期流产和死胎，但在实际生产中并不常见。感染PCV2的成年公猪通常不会出现临床症状及病变[24,26]。自然感染和实验感染的公猪精液中，已检测到该病毒。实验感染中，分别在感染后第2天和第5天，于血清和精液中检测到PCV2 DNA [24,26]。检测精液相关病毒之前，先进行PCV2病毒血症检测，但即使未出现病毒血症，精液也可能散毒。实验感染公猪后，一些研究发现公猪精液间断散毒[24,25]，而其他研究则发现精液持续散毒。McIntosh等人表明，自然感染的公猪在长达27周的时间内，精液偶尔散毒。这些数据表明，精液可能是PCV2的重要载体。病毒常污染精浆，但是在精浆和精子中均能检测到PCV2 [23,33]。精子很少发生形态、活力和浓度方面的变化。能繁母猪感染PCV2后也很少表现临床症状。
Porcine circovirus type 2 (PCV2) is a small nonenveloped DNA virus that belongs to the family circoviridae, genus Circovirus. Infections with PCV2 in pigs are associated with different disease conditions commonly referred to as PCV2-associated disease . Reproductive problems in sows such as late-term abortions and stillbirths have also been described but are considered to be rare under field conditions. Mature boars infected with PCV2 generally lack clinical signs and lesions [24,26]. The virus has been detected in semen of naturally and experimentally infected boars . On experimental infection, PCV2 DNA has been detected in serum and semen at 2 and 5 days after infection, respectively [24,26]. Detection of PCV2 viremia commonly precedes the detection of semen-associated virus, but semen shedding may occur in the absence of viremia. After experimental infection of boars, some studies found intermittent shedding in semen [24,25], whereas other studies observed continuous shedding of the virus . McIntosh et al.  showed that naturally infected boars shed the virus sporadically in semen for up to 27 weeks. These data suggest that semen may be a significant vehicle for PCV2. The seminal plasma is usually contaminated, but PCV2 is not only detected in the non–sperm cell fraction but also in the sperm one [23,33]. Changes in semen morphology, motility, and concentration are not commonly found . Clinical signs on PCV2 infection in breeding sows are rare .
Naïve sows inseminated with semen experimentally spiked with PCV2 exhibited reproductive failure, and their fetuses became infected . However, it is not known if the quantity of PCV2 naturally shed in semen is sufficient to infect sows or their fetuses. The virus is able to replicate in zona pellucida–free embryos, leading to embryonic death [65,66]. Delayed farrowing or pseudopregnancy is less frequently observed with PCV2-associated reproductive failure. An increased number of nonviable piglets may be present in PCV2-affected litters .
Porcine cytomegalovirus or suid herpesvirus type 2 is a DNA virus that belongs to the subfamily betaherpesvirinae of the family herpesviridae but is not assigned to any genus. Infection with porcine cytomegalovirus is usually subclinical in adults. After infection in boars, the virus was detected in the testis and epididymis . Shedding of virus in ejaculated semen has not been determined.
Porcine parvovirus (PPV) is a small nonenveloped DNA virus that belongs to the family parvoviridae, genus Parvovirus. The virus is ubiquitous in the pig population. The main transmission routes of PPV are oronasal and transplacental. The virus has been isolated many times from semen of naturally infected boars. Boars can shed the virus in semen during the acute phase of infection ; shedding beyond this phase has not been demonstrated, but the possibility of immunotolerant carriers of PPV as a result of early in utero infection has been suggested . Semen may also become contaminated by fecal particles containing virus, or within the male reproductive tract organs [45,46]. In boars, usually no clinical signs after infection are observed. On experimental infection in boars, no alternations in fertility or libido  or in sperm output, ejaculate volume, motility, or morphologic defects  were found. In breeding females, the major and often only clinical sign of PPV infection is reproductive failure: return to estrus, fewer pigs per litter, and increased numbers of mummified fetuses . The role of semen contaminated with PPV in creating clinical reproductive problems has not been clearly established.
Recently, a novel parvovirus namely PPV4, belonging to the family parvoviridae, genus Bocavirus, was detected in 5 out of 13 (38.5%) semen samples , and in one out of two semen samples in a study by Cságola et al. . The role of this virus in swine health has not been determined. Other novel porcine parvoviruses (PPV2, PPV3, porcine bocaviruses 1 and 2, other porcine bocaviruses labeled 6V and 7V, porcine boca-like virus) were found in pigs from different herds in Hungary, but they could not be detected in semen .
To be continued…